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UBC Theses and Dissertations

Land degradation in Mexican maize fields Sancholuz, Luis Alberto 1984

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LAND DEGRADATION MEXICAN MAIZE FIELDS by LUIS ALBERTO SANCHOLUZ L i c e n c i a d o en Z o o l o g i a U n i v e r s i d a d N a c i o n a l de La P l a t a , A r g e n t i n a 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES ( Resource Management Sc i ence ) We accept t h i s t h e s i s as conforming to the r e q u i r e d s t andard THE UNIVERSITY OF BRITISH COLUMBIA 28 March 1984 © L u i s A l b e r t o S a n c h o l u z , 1984 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements fo r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s t h e s i s f o r scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or p u b l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 i i ABSTRACT T h i s s tudy seeks answers for two s imple yet e l u s i v e q u e s t i o n s : a) i s l a n d d e g r a d a t i o n a r e a l t h r e a t to the p r o d u c t i v i t y of the Mexican maize f i e l d ? and b) can the a p p l i c a t i o n of f e r t i l i z e r compensate for the l o s s e s of p r o d u c t i o n due to f e r t i l i t y d e p l e t i o n by s o i l e r o s i o n ? The t h e s i s i s based on an i n t e g r a t e d examinat ion of e m p i r i c a l e v i d e n c e . I n t e r n a t i o n a l l i t e r a t u r e , n a t i o n a l s t a t i s t i c s , r e g i o n a l s u r v e y s , and greenhouse and f i e l d exper iments are pursued i n o rder to answer the above q u e s t i o n s . In each case c o n c l u s i o n s are drawn, but these c o n c l u s i o n s vary w i t h the l e v e l of a n a l y s i s . S t a t i s t i c s of maize p r o d u c t i o n i n Mexico show net ga ins i n p r o d u c t i v i t y i n the l a s t t h i r t y y e a r s . A f t e r c o r r e c t i n g for the t e c h n o l o g i c a l improvements i n tha t p e r i o d , i t appears tha t the i n t r i n s i c p r o d u c t i v i t y of the l a n d has not d e c l i n e d . T h i s i s c o n t r a r y to p r e d i c t i o n s i n the l i t e r a t u r e on s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n , p a r t i c u l a r l y i n the t r o p i c s . A c c o r d i n g to t h i s l i t e r a t u r e , these maize f i e l d s are not o n l y t h r e a t e n e d , but they should a l r e a d y e x h i b i t s i g n i f i c a n t l o s s e s i n p r o d u c t i v i t y . To examine t h i s c o n f l i c t i n g e v i d e n c e , a case study on three c o n t r a s t i n g s o i l types was conducted i n c e n t r a l V e r a c r u z . Greenhouse exper iments w i t h e r o s i o n and f e r t i l i z a t i o n of these s o i l s suggest that f e r t i l i z e r s can compensate for l o s s e s of p r o d u c t i v i t y r e s u l t i n g from e r o s i o n . F i e l d exper iments l eave no doubt tha t the o p p o s i t e i s t r u e : e r o s i o n d r a m a t i c a l l y reduces maize p r o d u c t i v i t y and f e r t i l i z e r s do not compensate. In c o n c l u s i o n , the t h e s i s o f f e r s an e x p l a n a t i o n for t h i s paradox . As l e v e l s of a n a l y s i s are a b s t r a c t e d from the f i e l d to the n a t i o n a l l e v e l , or p r o j e c t e d from the greenhouse to the f i e l d , c r i t i c a l i n f o r m a t i o n i s l o s t . Measures of l a n d p r o d u c t i v i t y are too aggregated at the n a t i o n a l l e v e l and too d i s a g g r e g a t e d in the greenhouse . T h i s confuses the assessment of l a n d d e g r a d a t i o n which r e q u i r e s the d e t e c t i o n of smal l changes i n l a n d p r o d u c t i v i t y . When l a n d i s p r o p e r l y c o n s i d e r e d , as i n the l i t e r a t u r e reviewed and the f i e l d exper iments i n c l u d e d i n t h i s t h e s i s , the r e s u l t i s c l e a r . The p r o d u c t i v i t y of the Mexican maize f i e l d w i l l s u f f e r from c o n t i n u o u s l a n d d e g r a d a t i o n , and t h i s n o t w i t h s t a n d i n g b e t t e r management i n p u t s . TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES ix LIST OF FIGURES x i ACKNOWLEDGEMENTS x i i i Chapter I : INTRODUCTION 1 Chapter I I : MAIZE PRODUCTION IN MEXICO 6 1 1 . 1 . Supply and demand 6 I I . 1.1 Trends i n p r o d u c t i o n 6 1 1 . 1 . 2 . Area h a r v e s t e d and y i e l d s 10 1 1 . 1 . 3 . Trends i n consumption 12 I I . 1 . 3 .1 . E f f e c t i v e demand 18 1 1 . 1 . 4 . F o r e i g n t r ade 20 1 1 . 2 . P r o d u c t i o n i n the maize f i e l d 22 11.2 .1 Maize f i e l d s management 25 11 .2 .2 Maize f i e l d s p r o d u c t i v i t y 27 1 1 . 2 . 2 . 1 . E f f e c t s of management i n p u t s on p r o d u c t i v i t y 27 1 1 . 2 . 2 . 2 . V a r i a b i l i t y on l a n d p r o d u c t i v i t y e s t imate s 29 1 1 . 3 . A t e s t of p r o d u c t i v i t y t r a j e c t o r i e s 34 11 .4 . Summary 40 Chapter I I I . SOIL EROSION, FERTILITY DEPLETION, AND PRODUCTIVITY: A REVIEW OF THE LITERATURE 42 111 .1 . I n t r o d u c t i o n 42 111 .2 . S o i l e r o s i o n 42 1 1 1 . 2 . 1 . Determinant s of s o i l e r o s i o n 44 1 1 1 . 2 . 1 . 1 . R a i n f a l l and runof f 44 1 1 1 . 2 . 1 . 2 . S o i l e r o d i b i l i t y 46 1 1 1 . 2 . 1 . 3 . V e g e t a t i v e cover 47 1 1 1 . 2 . 1 . 4 . S o i l e r o s i o n c o n t r o l 49 1 1 1 . 2 . 2 . Ev idence of s o i l e r o s i o n i n maize f i e l d s . . . 50 1 1 1 . 2 . 2 . 1 . Surveys 51 1 1 1 . 2 . 2 . 2 . E r o s i o n r a t e s i n t r o p i c a l maize f i e l d s 52 1 1 1 . 2 . 2 . 3 . E r o s i o n r a t e s i n Mexican maize f i e l d s . 54 111 .3 . S o i l e r o s i o n and p r o d u c t i v i t y 56 I I I . 3 . 1 S o i l e r o s i o n e f f e c t s on s o i l p r o p e r t i e s 56 I I I . 3 .2 . S o i l e r o s i o n impact on p r o d u c t i v i t y 58 111 .4 . S o i l f e r t i l i t y d e p l e t i o n through c o n t i n u o u s c u l t i v a t i o n 65 111 .5 . Summary 70 Chapter I V : ENVIRONMENTAL FRAMEWORK FOR A CASE STUDY IN CENTRAL VERACRUZ 71 IV. 1 . I n t r o d u c t i o n 71 I V . 2 . G e o g r a p h i c a l l o c a t i o n 72 I V . 3 . S o i l s 76 I V . 3 . 1 . Andoso l s 77 I V . 3 . 2 . Tepe ta te s 80 I V . 3 . 3 . C a l i c h e s 82 I V . 4 . C l i m a t e 85 I V . 4 . 1 . Growing season 85 I V . 4 . 2 . R a i n f a l l p a t t e r n s 87 v i I V . 5 . Maize f i e l d management 90 I V . 5 . 1 . C u l t i v a t i o n 90 I V . 5 . 2 . P l a n t breeds 93 I V . 5 . 3 . Harves t 94 I V . 5 . 4 . Maize y i e l d s 94 I V . 6 . An e v a l u a t i o n of s o i l e r o s i o n r i s k s fo r the t e s t s i t e s 96 I V . 6 . 1 . R a i n f a l l e r o s i v i t y , R 96 I V . 6 . 2 . S o i l e r o d i b i l i t i e s , K 97 I V . 6 . 3 . S lope e f f e c t , LS 98 I V . 6 . 4 . Crop management, C 100 I V . 6 . 5 . S o i l c o n s e r v a t i o n p r a c t i c e s , P 103 I V . 6 . 6 . S o i l l o s s e s t i m a t e s , A 103 I V . 7 . A f i e l d t e s t on s o i l l o s s e s 105 I V . 8. Summary 110 Chapter V : AN EXPERIMENTAL APPROACH 112 V . 1 . 1 n t r o d u c t i o n 112 V . 2 . E r o s i o n and p r o d u c t i v i t y i n the greenhouse 114 V . 2 . 1 . M a t e r i a l s and methods 115 V . 2 . 1 . 1 . Greenhouse f a c i l i t i e s 117 V . 2 . 1 . 2 . S o i l s 118 V . 2 . 1 . 3 . F e r t i l i z a t i o n 120 V . 2 . 1 . 4 . Water ing procedure 121 V . 2 . 1 . 5 . Seeds and c u l t i v a t i o n methods 122 V . 2 . 1 . 6 . Weed and pest c o n t r o l 123 V . 2 . 1 . 7 . H a r v e s t i n g procedures 123 V . 2 . 2 . R e s u l t s and d i s c u s s i o n 123 v i i V . 2 . 2 . 1 . Experiment 1, e r o s i o n e f f e c t s on maize growth and y i e l d s 124 V . 2 . 2 . 2 . Experiment 2, e r o s i o n , f e r t i l i z a t i o n , and water e f f e c t s on y i e l d s 130 V . 2 . 3 . P r e l i m i n a r y c o n c l u s i o n s : greenhouse 138 V . 3 . S o i l e r o s i o n and p r o d u c t i v i t y i n the f i e l d 140 V . 3 . 1 . M a t e r i a l s and methods 140 V . 3 . 1.1. S o i l s 141 V . 3 . 1 . 2 . F e r t i l i z a t i o n 143 V . 3 . 1 . 3 . Seeds and c u l t i v a t i o n p r a c t i c e s 143 V . 3 . 1 . 4 . Weed and pest c o n t r o l . . . 145 V . 3 . 1 . 5 . Weather 145 V . 3 . 1 . 6 . H a r v e s t i n g procedures 146 V . 3 . 2 . R e s u l t s and d i s c u s s i o n 147 V . 3 . 2 . 1 . G e rmina t io n and s u r v i v a l p a t t e r n s 147 V . 3 . 2 . 2 . Maize growth 148 V . 3 . 2 . 3 . G r a i n y i e l d s 148 V . 3 . 3 . P r e l i m i n a r y c o n c l u s i o n s : f i e l d exper iment . . . . 1 5 3 V . 4 . Summary 155 Chapter V I : SUMMARY AND CONCLUSIONS 157 BIBLIOGRAPHY 162 APPENDIX 1. S t a t i s t i c s of maize p r o d u c t i o n and consumption i n Mexico 175 1.1. Sources of maize p r o d u c t i o n and consumption d a t a ' . . 1 7 5 1 .1 .1 . P r o d u c t i o n s e r i e s of data 175 1 .1 .2 . Consumption s e r i e s of data 177 1 .1 .3 . Surveys of p r o d u c t i o n 177 v i i i 1.2. Data 179 APPENDIX 2. S o i l a n a l y s i s 186 2 . 1 . Methods of s o i l a n a l y s i s 186 2 . 1 . 1 . F i e l d methods 186 2 . 1 . 2 . L a b o r a t o r y methods 186 2 . 2 . S o i l data 189 LIST OF TABLES T a b l e I I . 1 r a t e s of growth for maize p r o d u c t i o n , a r e a , and y i e l d s by decades 11 Tab le I I . 2 A g r o e c o l o g i c a l c h a r a c t e r i s t i c s of the Mexican maize f i e l d s 23 Tab le I I . 3 Land , water , and energy management i n Mexican maize f i e l d s 26 Tab le I I . 4 E f f e c t s of i r r i g a t i o n , f e r t i l i z e r s , and improved seeds on the p r o d u c t i v i t y of the maize f i e l d s 28 Tab le I I . 5 E f f e c t s of the use of t r a c t o r s , c r e d i t , i n s u r a n c e , and t e c h n i c a l a s s i s t a n c e on maize f i e l d s p r o d u c t i v i t y 30 T a b l e I I . 6 H a r v e s t e d and c u l t i v a t e d maize y i e l d s i n Mexico 31 T a b l e I I . 7 Maize y i e l d s e s t i m a t e s a c c o r d i n g to d i f f e r e n t sources 33 T a b l e I I I . 1 E s t i m a t e d maize y i e l d r e d u c t i o n s i n t o p s o i l removal exper iments 66 Tab le IV.1 E x t e n s i o n , a l t i t u d e , and s lope s for the three s o i l s of t h i s study 78 Tab le I V . 2 C l i m a t i c parameters of s tudy area .' 86 T a b l e I V . 3 Ca lendar of c r o p p i n g a c t i v i t i e s 91 T a b l e I V . 4 Maize y i e l d s e s t i m a t e s for the three s o i l s of t h i s s tudy 95 T a b l e I V . 5 U S L E - S o i l e r o d i b i l i t y parameters and K v a l u e s for the t h r e e s o i l s of t h i s s tudy 98 X Table I V . 6 S lope c h a r a c t e r i s t i c s for the three s o i l s of t h i s s tudy and USLE-LS v a l u e s 100 Tab le I V . 7 H i s t o r y of l a n d use i n maize f i e l d s for the three s o i l s of t h i s s tudy 101 Table I V . 8 USLE-crops tage p e r i o d s for the three s o i l s of t h i s s tudy 103 Tab le I V . 9 . USLE f a c t o r s and s o i l l o s s e s t imate s for the three s o i l s of t h i s s tudy 104 Tab le I V . 1 0 . Expected and observed s o i l l o s s e s for the three s o i l s of t h i s s tudy 109 Tab le V . 1 ANOVA for y i e l d s of experiment 1 127 Tab le V . 2 ANOVA of y i e l d s for experiment 2 133 Table V . 3 Per cent r e d u c t i o n i n y i e l d s due to e r o s i o n i n two exper iments 139 Tab le V . 4 Average depths of s o i l removed i n e x p e r i m e n t a l p l o t s 142 Tab le V . 5 Maize g e r m i n a t i o n i n f i e l d exper iments 147 Tab le V . 6 ANOVA for g r a i n y i e l d s i n f i e l d experiment 154 Tab le 1.1. Y i e l d s , h a r v e s t e d a r e a s , and t o t a l p r o d u c t i o n of maize i n M e x i c o : 1895-1982 180 Tab le 1.2. D e c e n i a l s t a t i s t i c s of maize p r o d u c t i o n i n M e x i c o : 1900-1980 182 Tab le 1.3. P o p u l a t i o n , maize consumpt ion , and maize ba lance of t r a d e i n M e x i c o : 1895-1982 183 Tab le 1.4. Censuses data on Mexican maize p r o d u c t i o n : 1950-1970 186 LIST OF FIGURES F i g u r e 2 . 1 . Trends i n maize p r o d u c t i o n in Mexico 7 2 . 1 a . Trends i n t o t a l p r o d u c t i o n and consumption 8 2 . 1 b . Trends of h a r v e s t e d areas 8 2 . 1 c . Trends of maize y i e l d s 8 F i g u r e 2 . 2 . Consumption of maize and p o p u l a t i o n i n Mexico (1895-1981) 13 F i g u r e 2 . 3 . Per c a p i t a consumption Qf maize (1895-1981) . . . 15 F i g u r e 2 . 4 . Maize y i e l d s , maize p l a n t i n g d e n s i t i e s and N f e r t i l i z a t i o n r a t e s i n the U . S . and Mexico 36 F i g u r e 4 . 1 . L o c a t i o n of the study areas 73 F i g u r e 4 . 1 a . R e g i o n a l map showing s o i l s s t u d i e d 74 F i g u r e 4 . 1 b . R e g i o n a l p r o f i l e showing s o i l s i t e s 74 F i g u r e 4 . 2 . Cl imodiagrams for the three s o i l - c l i m a t e zones 88 F i g . 4 .2a Humid warm temperate 89 F i g . 4 .2b Humid s u b t r o p i c a l 89 F i g . 4 .2c Subhumid t r o p i c a l 89 F i g u r e 4 . 3 . R e l a t i o n s h i p between t o p s o i l depth and s lope for the three s o i l s of t h i s s tudy . . . . 1 0 6 F i g u r e 5 . 1 . Maize growth p a t t e r n s i n experiment 1 125 5 . 1 a . C o r r e l a t i o n of biomass and h e i g h t 126 5 . 1 b . C a l i c h e , s i t e #8 126 5 . 1 c . A n d o s o l , s i t e #3 126 5 . 1 d . T e p e t a t e , s i t e #5 126 5 . 1 e . C a l i c h e , s i t e #7 126 5 . I f . A n d o s o l , S i t e #2 126 5 . 1 g . C a l i c h e , s i t e #6 126 5 . 1 h . A n d o s o l , s i t e #1 126 5.1 g . T e p e t a t e , s i t e #4 126 F i g u r e 5 . 2 . Y i e l d s in pot experiment 1 128 F i g u r e 5 . 3 . I n t e r a c t i o n between s o i l t y p e , f e r t i l i z e r and e r o s i o n , experiment 2 . . . . 1 3 1 F i g u r e 5.3a Andoso l s 132 F i g u r e 5.3b Tepe ta te s 132 F i g u r e 5.3c C a l i c h e s 132 F i g u r e 5 .4 . Second order i n t e r a c t i o n s for experiment 2 . . . . 1 3 5 F i g u r e 5.4a Y i e l d s , e r o s i o n , and s o i l type 136 F i g u r e 5 .4b Y i e l d s , f e r t i l i z a t i o n , and s o i l type 136 F i g u r e 5.4c Y i e l d s , f e r t i l i z a t i o n , and e r o s i o n 136 F i g u r e 5 .4d Y i e l d s , water s t r e s s , and s o i l type 136 F i g u r e 5.4e Y i e l d s , water s t r e s s , and e r o s i o n 136 F i g u r e 5 . 4 f Y i e l d s , water s t r e s s , and f e r t i l i z a t i o n . . . . 1 3 6 F i g u r e 5 . 5 . Growth p a t t e r n s of maize p l a n t s i n f i e l d exper iments 149 F i g u r e 5 . 5 a . Andoso l s 150 F i g u r e 5 . 5 b . Tepeta te s 150 F i g u r e 5 . 5 c . C a l i c h e s . . . . 1 5 0 F i g u r e 5 . 6 . G r a i n y i e l d s fo r d i f f e r e n t t rea tments i n f i e l d experiment '. 151 ACKNOWLEDGEMENTS Many peop le and i n s t i t u t i o n s have made t h i s t h e s i s p o s s i b l e . My longterm a d v i s o r , A l a n Chambers, he lped me to b r i n g the work to an end, and I am very t h a n k f u l for tha t as w e l l as for h i s i n d e f a t i g a b l e f r i e n d s h i p . I am e s p e c i a l l y g r a t e f u l to the f o l l o w i n g persons for the s t i m u l a t i o n they gave me from the b e g i n n i n g : J aures M a u r i , G e r a l d M a r t e n , P a u l Z i n k e , E f r a i m Hernandez X o l o c o t z i and W i l l i a m Rees . L e s l i e L a v k u l i c h , James Kimmins and Laurence Van V l i e t s u p e r v i s e d my work and made i n v a l u a b l e s u g g e s t i o n s for the p r e p a r a t i o n of the f i n a l m a n u s c r i p t . Jaime V e l e z , Lourdes Guzman and Eduardo Johnson he lped me w i t h the greenhouse and f i e l d e x p e r i m e n t s . ThuDung Nguyen and P a t r i c i a C a r b i s a s s i s t e d me wi th s o i l l a b o r a t o r y a n a l y s e s . The I n s t i t u t o N a c i o n a l de Recursos B i 6 t i c o s (Mexico) deserves my warmest thanks for l o g i s t i c s u p p o r t ; i n p a r t i c u l a r , A r t u r o Gomez Pompa, S i l v i o O l i v i e r i and E n r i q u e Pardo-Te jeda f a c i l i t a t e d my f i e l d work. The A g r i c u l t u r a l Economics D i v i s i o n of the Mexican government (DGEA-SARH) c o u r t e o u s l y p r o v i d e d computer tapes c o n t a i n i n g four yea r s of f i e l d survey d a t a . S c h o l a r s h i p s from the U n i v e r s i t y of B r i t i s h Columbia and the F o r d Founda t ion a l l o w e d me to pursue these s t u d i e s . Thank you M a r g a r i t a and Pedro fo r many year s of p a t i e n c e and l o v e . The f i n a l work, I wish to d e d i c a t e to the l o v i n g s t u r d i n e s s of M a r i a Raquel I p a r r a g u i r r e , my mother . 1 CHAPTER I : INTRODUCTION Mexican maize p r o d u c t i o n i s geared to feed her own p o p u l a t i o n . Every day, Mexicans eat a lmost h a l f a k i l o of t h i s g r a i n i n t o r t i l i a s , t amales , a t o l e s , g o r d i t a s , p o z o l e , e n c h i l a d a s or i n any o ther of the s i x hundred maize d i s h e s known to Mexicans ( A n o n . , 1982a). Maize c o n t r i b u t e s h a l f the volume of a l l f o o d s t u f f s consumed i n M e x i c o . I t p r o v i d e s almost h a l f of the c a l o r i e s and a t h i r d of the p r o t e i n s of the average d i e t (Chavez, 1973; CECODES, 1980). Maize f i e l d s can be found almost everywhere i n M e x i c o : from n o r t h to south and a c r o s s the c o u n t r y ; from h i g h i n the mountains to low i n the c o a s t a l p l a i n s ; from the b a c k - c o u n t r y to the back-yards of human s e t t l e m e n t s ; winter or summer and i n a l l c l i m a t e s and s o i l s . There are a p p r o x i m a t e l y two and a h a l f m i l l i o n maize f i e l d s throughout Mexico (CDIA, 1980, p . 44) t o t a l l i n g seven to e i g h t m i l l i o n h e c t a r e s . The average maize f i e l d measures c l o s e to three h e c t a r e s i n s i z e . C o n s i d e r i n g an average maize y i e l d of 1.4 tonne per hec ta re per y e a r , the average f i e l d produces 4.2 tonne per y e a r . Ten and a h a l f m i l l i o n tonnes of maize c o u l d feed s i x t y m i l l i o n Mexicans a d e q u a t e l y , but today the c o u n t r y has seventy m i l l i o n people and they are i n c r e a s i n g at the r a t e of two m i l l i o n per year (see T a b l e 1.3, Appendix 1) . Not s u r p r i s i n g l y , the e q u i l i b r i u m between supp ly and demand of maize i s one of the most s e n s i t i v e i s s u e s of Mexican 2 p o l i t i c s . Recent governments have been concerned wi th s e c u r i n g adequate s u p p l i e s of maize to meet the n a t i o n ' s demands. To t h i s end , a number of p o l i c i e s have been d e s i g n e d : p r i c e c o n t r o l s to p r o t e c t consumers ; a g r i c u l t u r a l development programs to s t i m u l a t e p r o d u c e r s ; and imports to b r idge the gaps . I t i s d i f f i c u l t , however, to s o l v e such a m u l t i f a r i o u s problem i n a matter of y e a r s . An almost p e r f e c t c o o r d i n a t i o n of government, m i l l i o n s of p roducer s and m i l l i o n s of consumers w i l l be r e q u i r e d to s i m u l t a n e o u s l y r a i s e the p r o d u c t i v i t y of the maize f i e l d , upgrade the n u t r i t i o n a l s tandards of the p o p u l a t i o n , and o b t a i n f o r e i g n c u r r e n c y from a badly shaken ba lance of payments (Walsh, 1983). But concern for the p r o d u c t i o n of maize i s h a r d l y a new phenomenon i n M e x i c o . E n t e r the Mexicans of o l d . They were, most p r o b a b l y , the very people t h a t domes t i ca t ed ( invented? ) maize some seven thousand year s ago ( l i t i s , 1983). Moreover , the Mexica knew tha t c o n t i n u o u s c u l t i v a t i o n on s l o p i n g t e r r a i n was bound to degrade the p r o d u c t i v i t y of the l a n d . There i s ample h i s t o r i c a l and a r c h a e o l o g i c a l ev idence of t e r r a c e s i n Mexico before the Spani sh conquest (Donkin , 1979). Check-dam t e r r a c e s , maguey hedges , s l o p i n g - f i e l d t e r r a c e s , and bench t e r r a c e s were widespread over a l l r e g i o n s of the c o u n t r y . Today, many of them have been abandoned or c o m p l e t e l y d e s t r o y e d (Denewan, 1980). R e c a l l the Mexican R e v o l u t i o n of the second decade of t h i s c e n t u r y . I t was war between peasants and l a n d l o r d s . In the end , the peasants got a b e t t e r share of the l a n d , but the l o s s e s i n human l i f e , economic i n f r a s t r a c t u r e , and even i n maize 3 p r o d u c t i o n were huge and f e l t for many year s to come. C o n s i d e r the f o l l o w i n g words w r i t t e n t h i r t y year s ago: " . . . c o r n (maize) i s the ' s t a f f of l i f e 1 to the Mexican people and i t w i l l grow and g ive some r e t u r n s under a tremendous v a r i e t y of c o n d i t i o n s of c l i m a t e and s o i l . But i t i s a s o i l d e p l e t i n g c r o p , and i t s c u l t i v a t i o n induces e r o s i o n even on g e n t l e s l o p e s . Thus corn c u l t u r e i s both a b l e s s i n g and a c u r s e to the c o u n t r y ; b u t , b l e s s i n g or c u r s e , i t i s a n e c e s s i t y . " 1 F i n a l l y , n o t i c e that the Green R e v o l u t i o n was modeled i n Mexico some t h i r t y year s ago. The I n t e r n a t i o n a l Maize and Wheat Improvement- C e n t e r , near Mexico c i t y , deve loped the wonder seeds which y i e l d e d unprecedented amounts of g r a i n per h e c t a r e of l a n d . B u t , even one of the f a t h e r s of t h i s r e v o l u t i o n has acknowledged that these seeds o n l y f i t t e d l a r g e commercia l farms, not the s m a l l and numerous maize f i e l d s (Wel lhausen , 1976). T h i s t h e s i s examines the f o l l o w i n g q u e s t i o n s : Is l a n d d e g r a d a t i o n a r e a l t h r e a t to the p r o d u c t i v i t y of the Mexican maize f i e l d ? Can the a p p l i c a t i o n of f e r t i l i z e r compensate for the l o s s e s of p r o d u c t i o n due to f e r t i l i t y d e p l e t i o n by s o i l e r o s i o n ? To approach these q u e s t i o n s , the t h e s i s takes a t w o f o l d view of the p r o d u c t i v i t y of the Mexican maize f i e l d : 1) the s tudy examines the c u r r e n t p r o d u c t i v e s t r u c t u r e as w e l l as r ecent FAO, 1954, p . 161 4 t e c h n o l o g i c a l improvement s ; 2) i t a s s e s s e s t h e impact of s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n on t h e p r o d u c t i v i t y o f t h e l a n d . E v i d e n c e i s p r e s e n t e d from v a r i o u s l e v e l s of a n a l y s i s : n a t i o n a l , r e g i o n a l , f i e l d , and g r e e n h o u s e , and t e s t s f o r t h e above q u e s t i o n s w i l l be p r o p o s e d i n each c a s e . Such an encompas s ing v i e w i s needed f o r e f f e c t i v e c o m m u n i c a t i o n among t h e many p e o p l e , i n s t i t u t i o n s , and d i s c i p l i n e s i n v o l v e d i n l a n d use p l a n n i n g . The t h e s i s i s o r g a n i z e d as f o l l o w s : a f t e r t h i s g e n e r a l i n t r o d u c t i o n , C h a p t e r I I d e a l s w i t h t h e p r o d u c t i o n of m a i z e i n M e x i c o . I t c o n s i d e r s two main a s p e c t s : n a t i o n a l p r o d u c t i o n t r e n d s and c u r r e n t p r o d u c t i o n i n the maize f i e l d . The c h a p t e r c o n c l u d e s w i t h a t e s t of. l o n g t e r m p r o d u c t i v i t y d e c l i n e f o r t h e M e x i c a n ma ize f i e l d s . A p p e n d i x 1 c o n t a i n s raw d a t a used i n t h i s c h a p t e r . C h a p t e r I I I o f f e r s a r e v i e w of t h e l i t e r a t u r e on e r o s i o n and f e r t i l i t y d e p l e t i o n i n c o n t i n u o u s c u l t i v a t i o n s y s t e m s , w i t h emphas i s on ma ize c u l t i v a t i o n i n t r o p i c a l s o i l s . I t a l s o p r e s e n t s d a t a on t h e impac t of s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n on maize y i e l d s . C h a p t e r IV i n t r o d u c e s a case s t u d y i n c e n t r a l V e r a c r u z c o n c e r n i n g t h r e e d i f f e r e n t s o i l s . C l i m a t e and ma ize f i e l d management i n t h e r e g i o n a r e a l s o d e s c r i b e d . The c h a p t e r c o n c l u d e s w i t h an e s t i m a t i o n of the r i s k s of e r o s i o n f o r a l l t h r e e s o i l s . Raw s o i l d a t a a r e p r e s e n t e d i n A p p e n d i x 2. C h a p t e r V d e s c r i b e s greenhouse and f i e l d e x p e r i m e n t s w h i c h 5 t e s t the main q u e s t i o n s of the t h e s i s . Through s i m u l a t e d e r o s i o n and f e r t i l i z a t i o n , maize y i e l d s are s t u d i e d i n the s o i l s r e f e r r e d to i n Chapter I V . Chapter VI c o n c l u d e s the t h e s i s w i t h a review of the main f i n d i n g s and a d i s c u s s i o n of t h e i r i m p l i c a t i o n s for maize f i e l d management i n M e x i c o . 6 CHAPTER I I : MAIZE PRODUCTION IN MEXICO They s h a l l never take from our p a n t r i e s the produce of our land, maize, our sustenance, the bearer of l i f e 1 In order to get a general p i c t u r e of Mexican maize p r o d u c t i o n , i t i s necessary: 1) to review trends i n production and consumption of maize i n Mexico and 2) to d e s c r i b e the f u n c t i o n of the u n i t of p r o d u c t i o n , the maize f i e l d . In c o n c l u s i o n , the chapter d i s c u s s e s long-term trends i n maize f i e l d p r o d u c t i v i t y . I_I. J_. Supply and Demand T h i s s e c t i o n c o n t a i n s a review of the modern h i s t o r y of maize pr o d u c t i o n and consumption in Mexico. The o r i g i n a l s e r i e s of data i s i n c l u d e d in Appendix 1. I I . 1.1 Trends i n pro d u c t i o n T h i s century has seen two p a t t e r n s of maize pr o d u c t i o n i n Mexico: s t a n d s t i l l and growth. F i g u r e 2.1a shows that t o t a l p r o d u c t i o n , d u r i n g the f i r s t f o r t y years of t h i s century, o s c i l l a t e d around two m i l l i o n tonnes per year. In t h i s p e r i o d Anonymous, near T e n o c h t i t l a n , 1528. 7 F i g u r e 2 . 1 . Trends i n maize p r o d u c t i o n i n M e x i c o . 2 . 1 a , p r o d u c t i o n and consumpt ion ; 2 .1b area h a r v e s t e d ; 2.1c maize y i e l d s . Data are 10 year a v e r a g e s . Source : DGEA,USDA s e r i e s ( t a b l e s 1 and 2, Appendix 1) . Bars i n d i c a t e d i s p a r i t y between e s t i m a t e s . 8 D E C A D E S 9 i n t e r n a l armed c o n f l i c t s (1910-1917) and the wor ld-wide economic d e p r e s s i o n (1930) marked the ups and downs of p r o d u c t i o n . Quite a d i f f e r e n t s t o r y began i n the 1940 ' s . P r o d u c t i o n s t a r t e d a c o n t i n u o u s upward swing which l a s t e d at l e a s t twenty-f i v e y e a r s . Between the 1930's and the 1960's the n a t i o n a l harves t q u a d r u p l e d . In the 1950's and 1960's , p r o d u c t i o n was i n c r e a s i n g c o n s i s t e n t l y at f i v e to s i x per cent a n n u a l l y (Appendix 1, Tab le 2, C o l 5 & 6 ) . These were the golden year s of Mexican a g r i c u l t u r e (Hewitt de A l c a n t a r a , 1980; Lamart ine Y a t e s , 1981). Much of what has been s a i d about post-war a g r i c u l t u r a l development i n the T h i r d World i s based on these i m p r e s s i v e achievements of the Mexican f a rmers . The p r o d u c t i o n of f o o d s t u f f s - - a n d maize was no excep t ion- -was growing much f a s t e r than the Mexican p o p u l a t i o n (Apendix 1, T a b l e 3 ) . D u r i n g the 1970's , maize p r o d u c t i o n c o n t i n u e d to grow, but at a more modest r a t e of 1.5 to 2.5 per cent per year (Appendix 1, Tab le 2, C o l . 5 & 6 ) . P o p u l a t i o n , however, grew at a f a s t e r r a t e d u r i n g these y e a r s , and maize shortages began to appear . For the p o l i t i c i a n s , maize imports were an i r r i t a t i n g f ea ture of the 1970 ' s . There were those who p r e d i c t e d tha t Mexico would never aga in be s e l f - s u f f i c i e n t i n t h i s o l d and c h e r i s h e d s t a p l e . However, h a r v e s t s in 1980 and 1981 were r e c o r d ones and i m p o r t s , a l t h o u g h c o n t i n u e d , were much l e s s important i n 1982. D u r i n g the f i r s t three year s of t h i s decade, p r o d u c t i o n has aga in been growing at an i m p r e s s i v e 10 per cent per y e a r , a c c o r d i n g to one s o u r c e , or at a more modest 3.6 per cent per 10 y e a r , a c c o r d i n g to another (see Appendix 1, T a b l e 2, C o l . 5 & 6 ) . However, these l a t e s t maize p r o d u c t i o n f i g u r e s have yet to be c o n f i r m e d . I_I_.j_.2. Area h a r v e s t e d and y i e l d s The area h a r v e s t e d i s an i n d i c a t o r of the t o t a l a rea c u l t i v a t e d . Crop y i e l d s are u s u a l l y r e p o r t e d as the y e a r l y q u o t i e n t between t o t a l p r o d u c t i o n and area h a r v e s t e d , i . e . , i n tonnes per h e c t a r e per year (t h a " 1 y r _ 1 ) . They i n d i c a t e l and p r o d u c t i v i t y . A p l o t of these two components of p r o d u c t i o n c o u l d r e v e a l the source of p r o g r e s s : more l a n d ; b e t t e r y i e l d s ; or b o t h . F i g u r e 2 .1b shows tha t the area c u l t i v a t e d w i t h maize has undergone dramat ic changes d u r i n g t h i s c e n t u r y . When compared w i t h the t r ends i n p r o d u c t i o n shown i n F i g . 2 . 1 a , i t seems that c o n t r a c t i o n and expans ion of the area c u l t i v a t e d c o n t r o l l e d p r o d u c t i o n i n the f i r s t f o r t y year s of the c e n t u r y . Note how the a rea h a r v e s t e d d e c l i n e d d u r i n g the Mexican r e v o l u t i o n ( i n the 1910 ' s ) , - and how t h i s c o i n c i d e d wi th the worst p r o d u c t i o n ever r e c o r d e d i n t h i s c e n t u r y . The area h a r v e s t e d to maize expanded c o n t i n u o u s l y i n the 1940 ' s , the 1950's , and d u r i n g the f i r s t h a l f of the 1960 ' s . As a r e s u l t there was 2.3 t imes more l a n d p r o d u c i n g maize i n the 1960's than i n the 1930 ' s . However, d u r i n g the 1970's and e a r l y 1980's the area remained p r a c t i c a l l y the same, as i f i t had reached a p l a t e a u . F i g u r e 2.1c shows the changes i n maize y i e l d s i n t h i s 11 c e n t u r y . ' U n t i l 1940 there was a p p a r e n t l y no s i g n i f i c a n t change. T h e r e a f t e r , an upward-swing began. Average y i e l d s grew from l i t t l e more than 0.5 t h a " 1 y r " 1 i n the 1930's to more than 1.1 t h a - 1 y r - 1 i n the 1970's . In these f o r t y y e a r s , the annual r a t e of i n c r e a s e of y i e l d s f l u c t u a t e d between 1.0 per cent and 3.0 per cent (see Appendix 1, T a b l e 2, C o l . 3&4) For M e x i c o , t h i s was a good achievement i n improving l a n d p r o d u c t i v i t y . P r e l i m i n a r y data from the 1980 ' s , however, suggest even g r e a t e r i n c r e a s e s . A c c o r d i n g to one s o u r c e , y i e l d s were growing at an a s t o n i s h i n g 11 per cent a y e a r ; a c c o r d i n g to another a t 5 per cent a year (Appendix 1, Tab le 2, C o l . 3 & 4 ) . Tab le I_I.J_. Rates of growth fo r p r o d u c t i o n , a r e a , and y i e l d s by s e l e c t e d decades of the 20th Century Decades % Product ion Annual Changes Y i e l d s Areas 1930-1960 4.8 2.0 2.7 1960-1970 2.0 1 .6 0.5 1970-1980 2 7.0 8.3 - 0 . 7 2 P r e l i m i n a r y c a l c u l a t i o n for the 1980's (based on 1980-1982 data o n l y ) Source:DGEA-USDA s e r i e s combined, see Appendix 1, T a b l e s 1-2 Tab le I I .1 summarizes the r a t e s of growth of the three v a r i a b l e s d i s c u s s e d so f a r . The f o l l o w i n g c o n c l u s i o n s can be 12 drawn: a) d u r i n g the f i r s t f o r t y year s of the c e n t u r y , Mexican maize p r o d u c t i o n was b a s i c a l l y c o n t r o l l e d by the amount of l a n d h a r v e s t e d ; b) from the 1940' u n t i l the 1970's a combina t ion of more l a n d and b e t t e r y i e l d s r e s u l t e d i n s i g n i f i c a n t p r o d u c t i o n g a i n s ; c) d u r i n g the 1970 ' s , the area h a r v e s t e d d i d not change; i n s t e a d , i n c r e a s i n g maize y i e l d s s u s t a i n e d the growth of p r o d u c t i o n , but at a s lower pace ; d) p r e l i m i n a r y data from the 1980's suggest a net decrease i n the area h a r v e s t e d but a tremendous i n c r e a s e i n y i e l d , which drove p r o d u c t i o n to unprecedented l e v e l s . Trends in consumption F i g u r e 2.2 shows a s t r o n g h i s t o r i c a l c o r r e l a t i o n between apparent consumption of maize and p o p u l a t i o n i n M e x i c o . As p o p u l a t i o n grew so d i d consumpt ion , a p p a r e n t l y at a cons t an t r a t e . For each new M e x i c a n , about 168 kg of maize were s u p p l i e d every y e a r . But the l i n e a r r e l a t i o n s h i p suggested by these data i s d e c e p t i v e . A c l o s e r look at the extremes of t h i s curve i n d i c a t e s n o n - l i n e a r i t y ; i n f a c t , the s lope changes as p o p u l a t i o n i n c r e a s e s : f l a t f i r s t , s t eep at the m i d d l e , d e f l a t e d at the end . F i g u r e 2.3 p l o t s maize per c a p i t a consumption ver sus t i m e , which o f f e r s a new p e r s p e c t i v e to the p r e v i o u s prob lem. Per c a p i t a consumption of maize decrea sed d r a m a t i c a l l y and then i n c r e a s e d i n t h i s c e n t u r y i n M e x i c o . In 1940, Mexicans were e a t i n g h a l f the amount of maize they a te f o r t y year s be fore and f o r t y year s l a t e r ! 13 F i g u r e 2 . 2 . Consumption of maize and p o p u l a t i o n i n Mexico (1895-1981) . Regre s s ion e q u a t i o n s (1) T o t . C o n s . ( 1 0 6 T) = -2 .026 (time i n y e a r s ) + 0.215 Pop. (people 1 0 6 ) ; r 2 = 0 .96 , SE = 0.74 1 0 6 ; (2) T o t . C o n s . ( 1 0 6 T) = 0.168 (people 1 0 s ) ; r 2 = 0 .96 , SE = 1.098 1 0 6 . 14 M A I Z E CONSUMPTION (t 10 ) 15 F i g u r e 2 . 3 . Per c a p i t a consumption of maize (1895-1981). 10 year s averages u n t i l 1925; y e a r l y data l a t e r . S o u r c e : DGEA, USDA s e r i e s ( t a b l e 2, Appendix 1) . Y E A R 1 7 To the best of t h i s a u t h o r ' s knowledge, no such phenomenon has been r e p o r t e d i n the Mexican l i t e r a t u r e . T h i s l i t e r a t u r e s i m p l y mentions damage to p r o d u c t i o n caused by the r e v o l u t i o n (1910-1917) and the subsequent year s of r e o r g a n i z a t i o n , and p r o d u c t i o n r e h a b i l i t a t i o n a f t e r P r e s i d e n t Cardenas ' government (1934-1940). T h i s r e d u c t i o n i n maize consumption was not a mere s h i f t from one type of food to a n o t h e r . Wheat, for example, was not a s u b s t i t u t e fo r maize d u r i n g the f i r s t f o r t y year s of t h i s c e n t u r y (Hewitt de A l c a n t a r a , 1980). One would e x p e c t , however, tha t a r e d u c t i o n by h a l f of the per c a p i t a consumption of the most important f o o d s t u f f of a c o u n t r y would p r e c i p i t a t e tremendous s o c i a l and p o l i t i c a l consequences . E i t h e r the people went hungry i n the 1930's or the a v a i l a b l e data are i n c o r r e c t . Two data s e r i e s of per c a p i t a consumption of maize i n Mexico were a v a i l a b l e for the p e r i o d 1960-1981 (see F i g . 2 . 3 ) . V a r i a b i l i t y in these data i s h i g h , but i t would appear that per c a p i t a maize consumption l e v e l l e d o f f a f t e r 1965. One e s t imate fo r 1981 i s far o f f the mark; t h i s c o i n c i d e s w i t h the h i g h p r o d u c t i o n r e p o r t e d by DGEA i n 1980'. The combined average of both s e r i e s for the p e r i o d 1965-1980 i s 175 kg p e r s o n - 1 y e a r " 1 . T h i s seems a rea sonab le f i g u r e and i t i s commonly r e p o r t e d i n the l i t e r a t u r e (SPP, 1981, Tab le I V ) . When t r ans formed i n t o a f i g u r e for d a i l y i n t a k e , 480 g d a y " 1 p e r s o n " 1 , t h i s matches the s tatement of the i n t r o d u c t i o n to the e f f e c t t h a t Mexicans eat about h a l f a k i l o of maize every day . T h i s i s , by i n t e r n a t i o n a l s t a n d a r d s , a very h i g h i n t a k e i n d e e d . 18 I I . 1 . 3 . 1 . E f f e c t i v e demand I t has been e s t imated tha t some 38 per cent of the maize produced i n Mexico never e n t e r s the market ( A b u r t o , 1979, Tab le XXXIX; CDIA, 1980, Tab le 28 ) . T h i s maize i s consumed by the producer and h i s f a m i l y . Another 34 per cent of the h a r v e s t , and a l l i m p o r t s , are marketed through the N a t i o n a l Company of Popular S u b s i s t e n c e (CONASUPO). T h i s Board s e t s maximum p r i c e s and c o n t r o l s d i s t r i b u t i o n through a c h a i n of n a t i o n a l s t o r e h o u s e s . Only 28 per cent of the Mexican maize i s a p p a r e n t l y exchanged on the f ree market . O b v i o u s l y , such a percentage of the t r a n s a c t i o n s cannot have a s i g n i f i c a n t e f f e c t on the p r i c e of maize i n M e x i c o . I t i s , however, s t i l l i n t e r e s t i n g to see the p a t t e r n s of e x p e n d i t u r e by d i f f e r e n t s t r a t a of the p o p u l a t i o n . In the 1960's two surveys were conducted to determine how Mexicans spent t h e i r income (Banco de M e x i c o , 1963, 1974). The r e s u l t s for food e x p e n d i t u r e showed that d i f f e r e n t income groups spent d i f f e r e n t p r o p o r t i o n s of t h e i r income on ma ize : the w e a l t h i e r the p e r s o n , the s m a l l e r the p r o p o r t i o n . Moreover , w e a l t h i e r per sons purchased l e s s maize than poorer ones . These o b s e r v a t i o n s suggest a n e g a t i v e income e l a s t i c i t y i n demand for ma ize . The surveys compri sed both urban and r u r a l segments of the p o p u l a t i o n and separa te e s t i m a t e s of these income e l a s t i c i t i e s were p r o d u c e d . For i n s t a n c e , i n 1968 the r u r a l e l a s t i c i t y was -0.096 and the urban was - 0 . 1 4 8 . The same c a l c u l a t i o n s for e x p e n d i t u r e s on wheat show p o s i t i v e e l a s t i c i t i e s : 0.621 and 19 0.278 for the r u r a l and urban s e c t i o n s r e s p e c t i v e l y . These r e s u l t s for the urban and r u r a l s e c t i o n s would support the p r e v i o u s sugge s t ion tha t people i n the c o u n t r y s i d e r e l y on t h e i r own maize for consumpt ion . As wheat i s not a popular c r o p i n M e x i c o , the r u r a l consumers must purchase i t . Abur to (1979) and Lamart ine Yates (1981) q u e s t i o n e d the r e l i a b i l i t y of these d a t a , but they agreed tha t the d i r e c t i o n , i f not the a b s o l u t e v a l u e , of the c o e f f i c i e n t s of income e l a s t i c i t y i s p r o b a b l y c o r r e c t . The s i g n i f i c a n c e of these e s t imate s i s tha t fo r any i n c r e a s e i n r e a l s a l a r y there i s a p r o p o r t i o n a l decrease i n the purchase of maize and a s i m i l a r i n c r e a s e i n the purchase of wheat. If wages were to improve i n M e x i c o , i t c o u l d be a n t i c i p a t e d tha t maize consumption would s h i f t to wheat consumpt ion . What about the p r i c e e l a s t i c i t y of demand for maize? In o ther words, how do p e o p l e ' s e x p e n d i t u r e s change w i t h changes i n the p r i c e of maize? There are no s p e c i f i c s t u d i e s on t h i s s u b j e c t in Mexico to date (Lamart ine Y a t e s , 1981). Perhaps the l a ck of t h i s type of i n f o r m a t i o n i s s i g n i f i c a n t i n i t s e l f . I t i s known, however, tha t the whole sa le consumer p r i c e of maize has lagged behind the g e n e r a l food p r i c e index from 1955 u n t i l 1981. T h i s would imply that the r e l a t i v e p r i c e of ma ize , i n r e a l terms, has e i t h e r d e c l i n e d or remained c o n s t a n t . But d u r i n g those y e a r s , per c a p i t a consumption of maize i n c r e a s e d o n l y s l i g h t l y ( F i g . 2 . 3 ) . I t i s r ea sonab le to suggest t h a t r e l a t i v e l y low p r i c e s of maize w i l l prompt i n c r e a s e s i n per c a p i t a consumpt ion , but scanty data do not a l l o w v e r i f i c a t i o n of 20 t h i s h y p o t h e s i s . G iven the imper fec t knowledge of the economic de te rminant s of maize demand i n M e x i c o , e x i s t i n g p r o j e c t i o n s for maize consumption are main ly based on p o p u l a t i o n t r e n d s . In 1977, for i n s t a n c e , i t was e s t i m a t e d tha t n a t i o n a l consumption of maize would i n c r e a s e by 2.4 per cent per y e a r ; p r o j e c t e d 1982 t o t a l Mexican consumption was 11.5 m i l l i o n tonnes ( G a r c i a Mata et a l , 1977, p . 14) . But the two p u b l i s h e d e s t i m a t e s of t o t a l consumption for 1982 (Table 3, Appendix 1) i n d i c a t e that the p r o j e c t i o n s for 1982, made i n 1977, were between 1 and 3.8 m i l l i o n tonnes shor t of the observed va lue ( i . e . , the l a t t e r was 10 to 35 per cent g r e a t e r than e x p e c t e d ) . Had the p r e d i c t e d r a t e of i n c r e a s e i n consumption b e t t e r approximated the r a te of i n c r e a s e of p o p u l a t i o n fo r those year s ( i . e . , around 3.2 per cent per y e a r ) , the p r e d i c t e d and observed v a l u e s would have been much c l o s e r . I_I._1_.4. F o r e i g n t rade The p r e c e e d i n g pages have shown great changes i n the t o t a l p r o d u c t i o n and consumption of maize i n M e x i c o . These changes have not always compensated for each o t h e r , and f o r e i g n t rade has been r e l i e d on to r e l i e v e the p r e s s u r e s a r i s i n g from shor tages and s u r p l u s e s . The l o n g e s t s e r i e s of data on imports and expor t s goes back to 1925. T a b l e 3, Appendix 1, shows a m a j o r i t y of d e f i c i e n t y e a r s , i . e . , w i t h net i m p o r t s , and o n l y b r i e f p e r i o d s of s u r p l u s , i . e . , year s w i t h net e x p o r t s . Expor t s were consp icuous 21 o n l y i n the mid and l a t e 1960's , when they r e p r e s e n t e d some 1 1 .'5 per cent of the n a t i o n a l h a r v e s t ; t h i s o c c u r r e d r i g h t a f t e r the b i g upswing i n n a t i o n a l p r o d u c t i o n that began i n the 1940's , and c o n t i n u e d u n t i l the mid 1960 ' s . P r o d u c t i o n went up so much that i t c o n s i s t e n t l y exceeded t o t a l consumption from 1965 to 1972. For most of t h i s c e n t u r y , however, Mexico has r e l i e d on f o r e i g n source s of maize to ba lance n a t i o n a l needs . G e n e r a l l y s p e a k i n g , imports were 3 per cent of the n a t i o n a l h a r v e s t u n t i l 1965. S t a r t i n g i n 1972, g r e a t e r and more c o n s i s t e n t imports were needed. From 1973 u n t i l 1982, maize imports were, on the average , 19 per cent of Mexican maize p r o d u c t i o n . The year 1982 was o n l y an e x c e p t i o n to t h i s l a t e s t t r e n d , due to the a l s o e x c e p t i o n a l l y l a r g e 1981 h a r v e s t ; y e t , Mexican government o f f i c i a l s c l a i m e d that s e l f - s u f f i c i e n c y i n maize was f i n a l l y a c h i e v e d i n 1981 ( A n o n . , 1982b). As noted i n s e c t i o n I I . 1 . 3 , per c a p i t a consumption of maize s t a r t e d to l e v e l o f f i n 1965. U n t i l economic f o r c e s are b e t t e r u n d e r s t o o d , n a t i o n a l consumption may be assumed to be a l i n e a r p o s i t i v e f u n c t i o n of p o p u l a t i o n . T h i s means t h a t , for Mexico to be s e l f - s u f f i c i e n t i n maize , a c o n s i s t e n t 3 per cent annual r a te of p r o d u c t i o n i n c r e a s e must be s u s t a i n e d . 22 2. P r o d u c t i o n i n the Maize F i e l d A l l of the t h i r t y - t w o Mexican s t a t e s produce maize , some more than o t h e r s . J a l i s c o , for i n s t a n c e , c o n t r i b u t e s 13 per cent of the n a t i o n a l h a r v e s t ; Baja C a l i f o r n i a Sur a mere 0.01 per c e n t . V e r a c r u z , the s t a t e i n which t h i s s tudy w i l l f ocus , i s the second r a n k i n g s t a t e w i t h 9 per cent of the t o t a l . D u r i n g the l a s t decade a group of twelve s t a t e s , c o m p r i s i n g some 78 per cent of M e x i c o ' s maize l a n d , have been p r o d u c i n g more than 80 per cent of the c r o p (CDIA, 1980, T a b l e 19) . Most s t u d i e s and surveys of maize p r o d u c t i o n i n Mexico focus on these l e a d i n g s t a t e s . Galvan Lopez and Delgado Hernandez (1977) d e s c r i b e d the main a g r o e c o l o g i c c o n d i t i o n s i n these main maize p r o d u c i n g r e g i o n s i n M e x i c o . T a b l e I I . 2 summarizes t h e i r f i n d i n g s . The mean a l t i t u d e of these maize r e g i o n s r e f l e c t s the s p l i t between h i g h l a n d and lowland p r o d u c t i o n . The o b s e r v a t i o n t h a t , on the average , these maize p r o d u c i n g areas l i e on r o l l i n g t e r r a i n i s important to t h i s t h e s i s . As water e r o s i o n i s p a r t l y a f u n c t i o n of the s lope of the l a n d , the c u l t i v a t i o n of maize i n a p redominant ly r o l l i n g landscape suggests h i g h r i s k s of s o i l loss . . T a b l e I I . 2 a l s o i n d i c a t e s t empera ture s , r a i n f a l l , and o ther c l i m a t i c events d u r i n g the growing season for ma ize . That these vary i s w e l l i l l u s t r a t e d by the g rea t range of v a l u e s observed i n a l l the parameters s t u d i e d . Maize i s grown from the t r o p i c a l ( t i e r r a c a l i e n t e ) lowlands hav ing shor t growing seasons , 23 Tab le I_I_.,2' Some A g r o e c o l o g i c a l c h a r a c t e r i s t i c s of the Mexican maize f i e l d s Parameters Range Mean 1 A l t i t u d e ( m ) 25-2440 1113 Topography 2 1 . 3 - 4 . 5 3 Growing Season : a) l e n g t h ( d a y s ) 140-224 171 b)mean Temp(C) 16 .2-28 .7 21.9 c)mean Precip . (mm) 251-1811 870 d)Thermal U n i t s 1846-4496 3470 e ) P r o b a b i l i t y La te frost(%) 0-7.9 1.5 e a r l y frost(%) 0-16 3.5 ha i 1 (%) 0-3.5 0.5 1 Weights c o r r e s p o n d to the area of each r e g i o n . 2 C l a s s e s a r e : 1= f l a t ; 2=gent le ; 3 = r o l l i n g ; 4 = h i l l y ; 5=mountainous. S o u r c e : adapted from Galvan Lopez and Delgado Hernandez(1977) abundant water and h e a t , to the temperate ( t i e r r a f r i a ) h i g h l a n d s w i t h longer growing seasons , scanty r a i n f a l l , l e s s h e a t , and r i s k of f r o s t s . G i v e n these c l i m a t i c v a r i a t i o n s , one would expect the s o i l s of Mexico to be d i v e r s e as w e l l . There are X e r o s o l s i n the N o r t h , Andoso l s i n the c e n t r a l V o l c a n i c Range, Rendzinas i n the Yucatan P e n i n s u l a , L u v i s o l s and F e r r a l s o l s i n ' t h e l o w l a n d s , and a p l e t h o r a of o ther s o i l s throughout the remainder of the c o u n t r y . , Maize can be found growing i n a l l these s o i l s . U n f o r t u n a t e l y , there are no e s t i m a t e s of the p r o p o r t i o n of maize tha t i s grown in each group of s o i l s . A c t u a l l y the o n l y complete s o i l map of Mexico (SARH, 1 '972) i s on such a s m a l l 24 s c a l e ( i . e . , 1 :2 ,000 ,000) , that the task of c a l c u l a t i n g the areas of d i f f e r e n t s o i l s o c c u p i e d by r e l a t i v e l y sma l l maize f i e l d s would be next to i m p o s s i b l e . Maize c u l t i v a t i o n has been i n t i m a t e l y a s s o c i a t e d w i t h M e x i c o ' s n a t u r a l v e g e t a t i o n . In the t r o p i c a l r e g i o n s , s h i f t i n g c u l t i v a t i o n has been the t r a d i t i o n a l system of a g r i c u l t u r e (Wat ter s , 1971; Sanchez , 1977, C h . 3 ) . T h i s nomadic c u l t i v a t i o n of maize i s known as m i l p a a g r i c u l t u r e . In t h i s system, a f i e l d i s c ropped fo r two to three y e a r s , abandoned to the f a s t growing bush, and cropped aga in some seven t o f i f t e e n year s l a t e r . The n a t u r a l , or more p r o p e r l y , second growth v e g e t a t i o n h e l p s i n many ways to r e p l e n i s h the f i e l d . N u t r i e n t s and moi s ture are r e s t o r e d to the su r f ace h o r i z o n s . S o i l p h y s i c a l p r o p e r t i e s are improved, and e r o s i o n c o n t r o l l e d . A g g r e s s i v e weeds which o therwi se take over c r o p s are a l s o s u p p r e s s e d . T h i s system w i l l be d i s c u s s e d f u r t h e r when s o i l f e r t i l i t y d e p l e t i o n i s c o n s i d e r e d i n chap te r I I I . I n s i d e the maize f i e l d there i s a l s o b i o l o g i c a l d i v e r s i t y . More than f i f t y races of maize are known to e x i s t i n Mexico (Wellhausen et a l , 1952). H y b r i d s and improved seeds are used i n o n l y 20 to 25 per cent of the f i e l d s (Table I I . 4 , and Appendix 1 ,Table 4 ) . I n t e r c r o p p i n g i s common i n 10 to 15 per cent of the f i e l d s (Appendix 1, T a b l e 4 ) . Beans, squashes , and c h i l e s dominate t h i s complex of a s s o c i a t e d c r o p s , but they are by no means the o n l y ones : a n y t h i n g from v e g e t a b l e s to f r u i t t r e e s can be found growing toge ther w i t h maize . 25 I I . 2 . 1 Maize f i e l d s management Management combines a v a i l a b l e r e s o u r c e s to a c h i e v e a p r o d u c t i o n g o a l . The b a s i c r e s o u r c e s of the maize f i e l d are energy , l a n d , water , seed, and a g r o c h e m i c a l s . CECODES (1982) has d e s c r i b e d ten management systems i n Mexico based on the use of the f i r s t t h r e e , i . e . , energy , l a n d , and water . T a b l e I I . 3 summarizes these systems. The sources of energy can be t o t a l l y human, supplemented w i t h an imal t r a c t i o n , or combined w i t h f u e l - d r i v e n t r a c t o r s . The l a n d can be cropped c o n t i n u o u s l y (one, two, or even three c rops per year ) or o n l y two or three t imes every ten year s or so . Water may be e n t i r e l y p r o v i d e d by r a i n f a l l or most ly d e r i v e d from i r r i g a t i o n . What p r o p o r t i o n of these maize f i e l d s have i r r i g a t i o n , employ t r a c t o r s or f e r t i l i z e r s i n c u l t i v a t i o n ? The f o l l o w i n g three paragraphs e x p l o r e t h i s q u e s t i o n . The a g r i c u l t u r a l censuses of 1960-1970 (see Appendix I , Tab le 4) r e p o r t e d that approx imate ly 10 per cent of the area h a r v e s t e d to maize was i r r i g a t e d . Cont inuous r e c o r d s of the DGEA show tha t between 1960 and 1978, o n l y some 4.5 to 7 per cent of t h i s area was i r r i g a t e d (CDIA, 1980, Tab le 14). Yet the DGEA f i e l d surveys of 1976-79 i n d i c a t e tha t some 13 to 14 per cent of the area i s f u l l y i r r i g a t e d , whi le an a d d i t i o n a l 7 per cent r e c e i v e a u x i l i a r y i r r i g a t i o n (see Tab le I I . 4 ) . I t c o u l d w e l l be tha t the p r e v i o u s two sources have o n l y r e p o r t e d i r r i g a t e d maize i n the i r r i g a t i o n d i s t r i c t s . A u x i l i a r y i r r i g a t i o n i s u s u a l l y o b t a i n e d through s m a l l , p r i v a t e l y - o w n e d water r e s e r v o i r s , or 26 Tab le I I . 3 . Land , water , Mexican maize and energy f i e l d s management i n Management System Sca le (Ha) Land Use 1 i n t e n s i t y Water sources E n e r g y 2 sources 2-crops 1-5 2-3/1 Rain-Sum I r r . - W i n An imai F u e l Chinampa <1 1-2/1 I r r . Human I r r . D i s t r i c t 1-30 1/1 I r r . An imai F u e l D r y l a n d 2-1 00 1/1 Rain ( e r r a t i c ) F u e l Anrmal R a i n f e d 1-20 1/1 Rain ( r e l i a b l e ) An imai F u e l 1/2 R a i n f e d 1-20 1/1 Rain + I r r . F u e l Animal Y e a r - a f t e r 1-5 1/2 Rain Animal Tonalmi1 1-5 2 /3-2/8 Rain An imai T l a l c o l 1-3 1 / 3-1 / 5 Rain Human Mi lpa 1-3 l/>5 Ra in Human 1 #c rops / ca l endar y e a r . 2 Dominant forms; @ harves t a l l systems use l a b o r . S o u r c e : adapted from CECODES (1982) . through i n d i v i d u a l water pumps. I t i s a l s o under s tood that a u x i l i a r y i r r i g a t i o n o n l y serves a f i e l d at c r i t i c a l t i m e s . There i s , t h e r e f o r e , some ambigu i ty i n the d a t a . The percentage of i r r i g a t e d maize f i e l d s may be anywhere from f i v e to twenty . However, the censuse s ' f i g u r e of 10 per cent seems- to be the more rea sonab le one. 27 Only a h a n d f u l of combines are known to h a r v e s t maize i n Mexico ( C a r l o s Montanez, CECODES, p e r s . comm.). T r a c t o r s are the o n l y consp icuous p i e c e of machinery found i n the maize f i e l d s . Tab le I I . 5 shows that about 50 per cent of these f i e l d s have used t r a c t o r s for some o p e r a t i o n s i n the p e r i o d 1976-79. Chemica l f e r t i l i z e r s are r e l a t i v e l y l e s s expens ive input s of p r o d u c t i o n . T h e i r use i n the maize f i e l d s i s modest , however. Tab le I I . 4 shows that a l i t t l e more than h a l f the f i e l d s used f e r t i l i z e r s at a l l . O v e r a l l , the average annua l r a t e s of a p p l i c a t i o n of n i t r o g e n and phosporus are 40 and 15 kg h a " 1 , r e s p e c t i v e l y . In the case of n i t r o g e n , t h i s i s about one t h i r d of the r a te employed i n the U . S . corn b e l t i n 1970 (see F i g . 2 . 4 ) . I I . 2 . 2 Maize f i e l d s p r o d u c t i v i t y Land p r o d u c t i v i t y depends on the e c o l o g i c a l and t e c h n o l o g i c a l f a c t o r s d e s c r i b e d above. T h i s s e c t i o n examines the e f f e c t s of s e v e r a l management i n p u t s on maize y i e l d s . I t a l s o d i s c u s s e s the v a r i a b i l i t y of e x i s t i n g e s t imate s of maize y i e l d s . I I . 2 . 2 . 1 . E f f e c t s of management i n p u t s on p r o d u c t i v i t y T a b l e I I . 4 shows the e f f e c t s of f e r t i l i z e r s , i r r i g a t i o n , and improved seeds on the p r o d u c t i v i t y of maize f i e l d s . When a l l t h r e e f a c t o r s are used , maize y i e l d s are three t imes h i g h e r than when none i s u sed . When o n l y two are used , y i e l d s d o u b l e . The use of f e r t i l i z e r s a lone i s a s s o c i a t e d w i t h a f i f t y per cent 28 T a b l e I_I_.4 E f f e c t of i r r i g a t i o n , f e r t i l i z e r s , and improved seeds on maize f i e l d s p r o d u c t i v i t y Combinat ions #F ie ld s Mean S i z e Y i e l d of seeds , (%) (ha) (t h a " 1 y r " 1 ) f e r t i l i z e r s , i r r i g a t i o n . 1) C r e o l e N o - F e r t i 1 i z e r N o - I r r i g a t i o n 32 3.2 0.8 2) C r e o l e F e r t i l i z e r N o - I r r i g a t i o n 32 3.8 1.2 3) Improved F e r t i l i z e r No I r r i g a t i o n 12 7.7 1 .7 4) C r e o l e F e r t i l i z e r I r r i g a t i o n 6 2.9 1 .7 5) Improved F e r t i l i z e r I r r i ga t ion 6 6.0 2.4 Sum 88 % t o t a l sample w i t h i r r i g a t i o n = 19 % t o t a l sample w i t h f e r t i l i z e r s = 57 % t o t a l sample w i t h improved seeds= 24 S o u r c e : D G E A - s u r v e y s : 1976-79, see Appendix I . i n c r e a s e i n maize y i e l d s . T h i s i s p a r t i c u l a r l y important because f e r t i l i z e r s are the most w i d e l y used of a l l three i n p u t s (see percentages at the bottom of Tab le I I . 4 ) . These f i g u r e s i n d i c a t e average responses of maize y i e l d s to average t rea tments w i t h f e r t i l i z e r s , i r r i g a t i o n and improved seeds i n the Mexican maize f i e l d . Under a p p a r e n t l y i d e a l c o n d i t i o n s of s o i l , water , and p l a n t management, Goldsworthy e_t 29 a l (1974) , and Goldsworthy and Co legrove (1974) have shown that improved maize v a r i e t i e s can y i e l d 6.3 and 7.2 t h a " 1 y r " 1 in the t r o p i c a l and temperate r e g i o n s of M e x i c o , r e s p e c t i v e l y . These y i e l d s are almost three t imes g r e a t e r than the best y i e l d s p r e s e n t e d i n Tab le I I . 4 . ; and c o u l d w e l l i n d i c a t e fu ture t r ends i n the p r o d u c t i v i t y of the maize f i e l d s . For the moment, however, the p i c t u r e i s l e s s o p t i m i s t i c : o n l y s i x per cent of the maize f i e l d s produce more than 2 t h a " 1 y r " 1 of ma ize . T a b l e I I . 5 shows how the use of t r a c t o r s , farm c r e d i t , i n s u r a n c e , and t e c h n i c a l a s s i s t a n c e a f f e c t maize y i e l d s . Y i e l d s are a lmost s i x t y per cent h i g h e r when t r a c t o r s are used than when they are n o t . The a d d i t i o n of c r e d i t , i n s u r a n c e , and t e c h n i c a l a s s i s t a n c e does not seem to make much d i f f e r e n c e . I I . 2 . 2 . 2 . V a r i a b i l i t y on l a n d p r o d u c t i v i t y e s t i m a t e s Maize y i e l d s can be c a l c u l a t e d on the b a s i s of the h a r v e s t e d l ands a l o n e , or on the b a s i s of a l l the c u l t i v a t e d l a n d s . E q u a t i o n s 2.1 and 2.2 below c l e a r l y d e f i n e these two e s t i m a t e s . Y i e l d ( C ) = PROD/CL (2.1) Y i e l d ( H ) = PROD/HL (2.2) where: Y i e l d ( C ) = y i e l d based on c u l t i v a t e d a rea (t h a " 1 Y i e l d ( H ) = y i e l d based on h a r v e s t e d area (t h a " 1 yr y r r " 1 ) " 1 ) 30 Tab le E f f e c t of t r a c t o r , c r e d i t , i n s u r a n c e , and t e c h n i c a l a s s i s t a n c e on maize f i e l d s p r o d u c t i v i t y i n p u t s used # F i e l d s (%) Mean s i z e (ha) Y i e l d (t h a " 1 y r " 1 ) None 39 2 .9 0.95 T r a c t o r 29 4.4 1 .50 T r a c t o r + C r e d i t + Insurance+ T e c h . A s s i s . 1 2 7.9 1 .55 Sum 80 % of t o t a l tha t % of t o t a l tha t use t r a c t o r use c r e d i t = = 49 25 Source : DGEA Surveys 1976-79, see Appendix I . PROD = tonnes of maize h a r v e s t e d CL = area sown w i t h maize (ha) HL = area h a r v e s t e d (ha) E q u a t i o n 2.1 p r o v i d e s the best e s t imate of the p r o d u c t i v i t y of the l a n d because i t takes i n t o account the hazards of p r o d u c t i o n ( e . g . , l o s s e s of c r o p to d r o u g h t s , p e s t s , e t c . ) tha t are c h a r a c t e r i s t i c of any l a n d t y p e . E q u a t i o n 2 .2 , i n t u r n , o v e r e s t i m a t e s l a n d p r o d u c t i v i t y because i t removes from the c a l c u l a t i o n the areas which were sown and l o s t . The g r e a t e r the r i s k s of p r o d u c t i o n , the more these y i e l d e s t i m a t e s w i l l d i f f e r . 31 T a b l e I_I_.6 Harves ted and c u l t i v a t e d maize y i e l d s i n Mexico (1 ) (2) (3) Year Y i e l d ( C ) Y i e l d ( H ) R a t i o (t h a " 1 y r - 1 ) (t h a " 1 y r " 1 ) 1/2 1976 1 . 1 08 1 .282 0.86 1 977 1 .021 1 .247 0.82 1 978 1.144 1 .320 0.87 1 979 0.723 1 .207 0.60 Averages 0.999 1 .264 0.79 S o u r c e : DGEA S u r v e y s , see Appendix I . T a b l e I I . 6 p r e s e n t s e s t imate s c a l c u l a t e d w i t h both equa t ions for the year s 1976-1979, when the DGEA surveys of maize f i e l d s were c o n d u c t e d . As ex pec ted , maize y i e l d s a c c o r d i n g to E q u a t i o n 2.1 are always s m a l l e r than those c o r r e s p o n d i n g to E q u a t i o n 2 . 2 . The r a t i o between these two y i e l d e s t imate s v a r i e s from year to y e a r . In 1979, a year of d r o u g h t , t h i s r a t i o was on ly 0 . 6 . In normal year s ( i . e . , 1976-1978), the average maize f i e l d y i e l d e d o n l y 85 per cent of what was r e p o r t e d for the h a r v e s t e d l a n d s . On average t h i s r a t i o i s 0 . 7 9 . Governments r e p o r t y i e l d s at harves t o n l y because i t i s s i m p l e r to e s t i m a t e . Noth ing i s wrong w i t h u s i n g y i e l d s based on h a r v e s t e d l ands i f i t i s known: a) how d i f f e r e n t these y i e l d s may be from the ones based on a l l c u l t i v a t e d a r e a s , and b) i f these d i f f e r e n c e s are s m a l l or c o n s i s t e n t throughout the y e a r s . The second q u a l i f i c a t i o n does not seem to app ly to the Mexican maize f i e l d s ; the d i f f e r e n c e s between the two methods of y i e l d 32 e s t i m a t i o n are s i g n i f i c a n t and v a r y i n g . T h i s i s an important i n d i c a t i o n of the r i s k s of p r o d u c i n g maize i n M e x i c o . I f , on the average , twenty per cent of the area sown to maize i s l o s t every y e a r , t h i s must have a s i g n i f i c a n t impact on p r o d u c e r s . A farmer that faces such odds c o u l d w e l l d i s r e g a r d the p r o s p e c t s of b e t t e r y i e l d through a new seed or f e r t i l i z e r . T h i s i s p r o b a b l y why i r r i g a t i o n i n Mexico has been so important i n r a i s i n g c r o p p r o d u c t i v i t y , for once drought a reas were i r r i g a t e d , f e r t i l i z e r s and improved seeds c o u l d be p r o f i t a b l y used (see W e l l h a u s e n , 1976; F l o r e s c a n o Mayet et a l , 1980). Another p o i n t worth e x p l o r i n g i s the correspondence between the maize y i e l d s e s t imated through the 1976-79 DGEA surveys and those c o n t i n u o u s l y r e p o r t e d by the same DGEA and by the USDA. DGEA survey data c o r r e s p o n d to the spring-summer maize c r o p o n l y . The o ther two sources i n c l u d e w i n t e r c rops as w e l l . However, t h i s d i s c r e p a n c y can be c o r r e c t e d w i t h e x i s t i n g r e p o r t s on the y i e l d s and areas of the spring-summer and f a l l - w i n t e r c rops between 1972 and 1977 (CDIA, 1980; T a b l e 18). D u r i n g these year s maize y i e l d e d 1.591 t h a " 1 y r - 1 and 1.128 t h a - 1 y r - 1 i n the winter and summer c y c l e s , r e s p e c t i v e l y . S ince the maize w i n t e r c r o p r e p r e s e n t s 7.4 per cent of the t o t a l annual area h a r v e s t e d , the weighted average of the y i e l d s i n the two c y c l e s i s 1.162 t h a - 1 y r - 1 . T h i s i s o n l y three per cent h i g h e r than the average y i e l d of the summer c y c l e a l o n e , which p r o v i d e s an s t andard fo r compar i son . An e x c e p t i o n to t h i s s imple r u l e would occur i f the a rea of the w i n t e r c r o p were to i n c r e a s e 33 s i g n i f i c a n t l y , something that a p p a r e n t l y d i d not happen between 1976 and 1979 (SARH-DGEA,1980, p . 2 5 ) . T a b l e IJ_*Z D i f f e r e n t maize y i e l d e s t imate s fo r M e x i c o . Year Y i e l d s (t h a " 1 y r - 1 ) Rat i o s (1 ) DGEA-Sur (2) DGEA-C (3) USDA-C 2/1 3/1 3/2 1976 • 1 977 1978 1979 1 .282 1 .247 1 .320 1 .207 1 . 182 1 .357 1 .519 1 .517 1 .220 1 .220 1 .280 1.210 0.93 1 .09 1.16 1 .26 0.95 0.98 0.97 1 .00 1 .03 0.90 0.84 0.80 Averages 1 .264 (0.028) 1 .394 1 1 .233 1.10 0.98 0.88 1 95 % conf idence l i m i t s , S o u r c e : see Appendix I . T a b l e I I . 7 i n c l u d e s the three a v a i l a b l e e s t imate s of maize y i e l d s i n Mexico for the year s 1976-1979. In a l l cases y i e l d s c o r r e s p o n d to the h a r v e s t e d areas (Equat ion 2 . 2 ) . A c c o r d i n g to the c o r r e c t i o n made b e f o r e , the two s e r i e s of c o n t i n u o u s y i e l d e s t i m a t e s ( C o l . 2 and 3, Tab le I I . 4 ) s h o u l d t h e o r e t i c a l l y be 1.03 t imes g r e a t e r than the DGEA survey i n a l l y e a r s . As the r a t i o s i n d i c a t e , the survey y i e l d s are o v e r e s t i m a t e d i n a l l but one year by the c o n t i n u o u s r e c o r d s of the DGEA, and c o n s i s t e n t l y u n d e r e s t i m a t e d by the c o n t i n u o u s r e c o r d s of the USDA. From 1977 34 on , the o v e r e s t i m a t i o n by DGEA becomes p r o g r e s s i v e l y worse. The DGEA survey data s h o u l d be c o n s i d e r e d the y a r d s t i c k because they a r e , a f t e r a l l , a sample. The o ther two s e r i e s of maize y i e l d s are based on informed judgement, and as such are s u b j e c t to an unknown degree of e r r o r . The d i f f e r e n c e s in y i e l d e s t i m a t e s shown in Tab le I I . 7 are n o t , as y e t , a l a r m i n g . Perhaps a s imple e r r o r c o u l d e x p l a i n them. However, in the p e r i o d 1980-1982 the average y i e l d s r e p o r t e d by the USDA were o n l y 75 per cent of the average y i e l d s r e p o r t e d by the DGEA ( i . e . , 1.35 t h a - 1 y r " 1 v s . 1.80 t h a " 1 y r " 1 ; see Appendix 1, T a b l e 1) . These are c o n s i d e r a b l e d i f f e r e n c e s , which shou ld worry some o f f i c i a l s i n these a g e n c i e s . If o n l y more f i e l d data were a v a i l a b l e , as i n 1976-79, the matter c o u l d r a p i d l y be s e t t l e d . But the DGEA surveys were d i s c o n t i n u e d i n 1980, and the whole q u e s t i o n of how much maize y i e l d s have been improved i n the l a s t four years remains u n r e s o l v e d . I_I_. 3_. A Tes t of P r o d u c t i v i t y T r a j e c t o r i e s E x t e n s i v e r e s e a r c h by the S o i l C o n s e r v a t i o n S e r v i c e of the U . S . d u r i n g the 1930's and 1940's conc luded tha t t o p s o i l l o s s s i g n i f i c a n t l y reduced c r o p y i e l d s (Murray et a l , 1939; U h l a n d , 1949; O d e l l , 1950; S t a l l i n g s , 1950). On the b a s i s of t h i s d a t a , i t has been e s t imated t h a t for each c e n t i m e t e r of t o p s o i l l o s t , maize y i e l d s were reduced by approx imate ly 100 kg h a " 1 ( L y l e s , 1975; P i m e n t e l et a l , 1976). Assuming that the average annual r a t e of e r o s i o n on the U . S . c r o p l a n d i s 2.5 mm y r " 1 (P imente l et a l , 1976), and that the base maize y i e l d s of the uneroded s o i l s 35 was 4 t y r " 1 , the annual decrease i n maize y i e l d s due to e r o s i o n w i l l r e p r e s e n t l i t t l e more than 0.6 per cent of the base y i e l d . S ince the a f fo rement ioned s t u d i e s were c o n d u c t e d , maize y i e l d s i n the U . S . have c o n t i n u o u s l y i n c r e a s e d to t o d a y ' s average of about 6.5 t y r " 1 (average 1979-1981, FAO, 1981). T h i s c o n t r a d i c t i o n needs e x p l a n a t i o n . That an expected d e c l i n e i n l a n d p r o d u c t i v i t y has been overcome i n excess over the l a s t 30 year s i s not s u r p r i s i n g when due account i s taken of o ther s i g n i f i c a n t changes o c c u r r i n g i n the same p e r i o d . Jugenheimer (1976) has shown how maize y i e l d s i n c r e a s e d i n the U . S . from 1930 to 1970. F i g u r e 2.4 reproduces h i s a n a l y s i s of the maize y i e l d t r ends w h i c h , i n -the U . S . , was accompanied by s i m i l a r i n c r e a s e s i n maize p l a n t i n g d e n s i t i e s and ever i n c r e a s i n g n i t r o g e n f e r t i l i z a t i o n . T h i s graph s t r o n g l y suggests that maize y i e l d improvements i n the U . S . have been a byproduct of denser maize s tands suppor ted by i n c r e a s i n g r a t e s of n i t r o g e n f e r t i l i z a t i o n . A l l t h i s has, happened i n one of the b e s t , i f not the b e s t , maize growing areas of the w o r l d . F i g u r e 2.4 a l s o i n c l u d e s Mexican maize y i e l d s for the same p e r i o d and c u r r e n t n i t r o g e n f e r t i l i z a t i o n r a t e s . Note how the same r a t e of n i t r o g e n f e r t i l i z a t i o n ( i . e . , a p p r o x i m a t e l y 40 kg h a - 1 ) was a s s o c i a t e d w i t h q u i t e d i f f e r e n t responses of maize y i e l d s ( i . e . , 4.5 vs 1.2 t h a " 1 y r - 1 ) i n the e a r l y 1960's i n the U . S . and i n the l a t e 1970's i n M e x i c o . O b v i o u s l y , these two c o u n t r i e s must have c o m p l e t e l y d i f f e r e n t systems of p r o d u c t i o n , but tha t b i g a d i f f e r e n c e i n f e r t i l i z e r ' s e f f i c i e n c y must be a l s o r e l a t e d to d i f f e r e n t q u a l i t i e s of maize 36 F i g u r e 2 . 4 . Maize y i e l d s , maize p l a n t i n g d e n s i t i e s and N f e r t i l i z a t i o n r a t e s i n the U . S . and M e x i c o . S o u r c e s : U . S . d a t a , Jugenheimer (1976) . Mexican maize y i e l d s are f i v e year averages from t a b l e 1, Appendix 1 and average Mexican N r a t e from 1976-1979 DGEA s u r v e y s . Mexican p l a n t i n g d e n s i t i e s not a v a i l a b l e . 37 MAIZE Y I E L D S ( t ha"'yr-1) 5 NITROGEN " * 1 1 1 - 1 -1 PLANT. DENSITY (kg ha yr (plant x103 ha-1) 38 l ands i n the U . S . and i n M e x i c o . I f l a n d d e g r a d a t i o n i s an o l d and c o n t i n u i n g phenomenon i n the Mexican maize f i e l d , t h i s shou ld show i n the l o n g - t e r m s t a t i s t i c s on maize y i e l d s d i s c u s s e d i n t h i s c h a p t e r . S e c t i o n I I . 1 . 2 has shown that these y i e l d s have i n c r e a s e d c o n t i n u o u s l y d u r i n g the l a s t t h i r t y y e a r s . But t h i s has a l s o been a s s o c i a t e d w i t h s imul taneous i n c r e a s e s i n the use of i r r i g a t i o n , f e r t i l i z e r s , improved seeds and farm mach inery . In a v a l i d comparison of l a n d p r o d u c t i v i t y , the data s h o u l d be c o r r e c t e d for concomitant changes i n the i n p u t s of p r o d u c t i o n . The DGEA survey data p re sented i n Tab le I I . 4 p r o v i d e an a p p r o p r i a t e sample for 1976-79. Y i e l d data for the more d i s t a n t past can be o b t a i n e d o n l y i n aggregated form. Hewi t t de A l c a n t a r a (1980, T a b l e s 6 & 12) has shown t h a t the modern management i n p u t s to p r o d u c t i o n became popu la r o n l y i n the 1950 ' s . Maize p r o d u c t i o n before those year s must have been predominant ly u n f e r t i l i z e d , u n i r r i g a t e d , and without improved seed v a r i e t i e s . Maize y i e l d data from before 1930 might not be a p p r o p i a t e because a g r i c u l t u r e i n Mexico was then e x p e r i e n c i n g the a f t e r s h o c k s of the a g r a r i a n r e v o l u t i o n . The comparison of maize y i e l d s thus proposed i s between averages of four year s d u r i n g the l a t e 1940's and the four year s 1976-1979; the more recent data are from f i e l d s tha t d i d not use i r r i g a t i o n , f e r t i l i z e r s , or improved seeds ; data from the f o r t i e s are from a l l Mexican maize f i e l d s , which most p r o b a b l y d i d not use these management i n p u t s . The compari son spans t h i r t y y e a r s , enough t ime for l a n d d e g r a d a t i o n , i f s e r i o u s , to 39 show i t s e f f e c t s . A c c o r d i n g to the DGEA and USDA s e r i e s of data (see Appendix I, Tab le 1) , average maize y i e l d s fo r 1946-1949 were 0.71 t h a " 1 y r " 1 . In 1976-79 the y i e l d s of the u n f e r t i l i z e d , u n i r r i g a t e d , and c r e o l e seeded maize f i e l d s were 0.8 t h a " 1 y r " 1 (see Tab le I I . 4 ) . E v i d e n t l y , maize f i e l d p r o d u c t i v i t y has not d e c l i n e d d u r i n g the l a s t t h i r t y y e a r s . Indeed, i n t h i s compar i son , maize y i e l d s show a s l i g h t i n c r e a s e , even a f t e r the most important t e c h n o l o g i c a l changes of the i n t e r v e n i n g year s are taken i n t o a c c o u n t . Before d i s m i s s i n g the p o s s i b i l i t y of l a n d d e g r a d a t i o n i n the Mexican maize f i e l d , we s h o u l d ponder the data i t s e l f . F i g u r e 2 .1b c l e a r l y shows tha t between 1940 and 1970 the l and c u l t i v a t e d w i t h maize doubled i n M e x i c o . How might such a s u b s t a n t i a l i n c r e a s e in .the l a n d base a f f e c t maize p r o d u c t i v i t y data? E i t h e r the new lands were m a r g i n a l , i n which case average p r o d u c t i v i t y per hec ta re would d e c r e a s e , or the new lands were of good q u a l i t y , i n which case average p r o d u c t i v i t y per hec ta re would i n c r e a s e . Which of these was the case i n M e x i c o , we cannot now say . But i n e i t h e r c a s e , an i n c r e a s e of l a n d c o u l d a l s o i n c r e a s e average l and p r o d u c t i v i t y by c o n t i n u o u s l y b r i n g i n g i n t o p r o d u c t i o n lands that were not c u l t i v a t e d b e f o r e . In o ther words, the s u s t a i n e d a d d i t i o n of l a n d to maize p r o d u c t i o n c o u l d w e l l have had the e f f e c t of a cont inuous r e j u v e n a t i o n of the maize l and base . 40 4_. Summary T h i s c r i t i c a l review of what i s known about maize p r o d u c t i o n i n Mexico has i n c l u d e d h i s t o r i c a l , economic , demographic , g e o g r a p h i c a l , and a g r i c u l t u r a l d a t a . I t was necessary to use such a broad range of data because maize i s paramount among food re source s i n M e x i c o . The h i s t o r y of maize p r o d u c t i o n i n t h i s c e n t u r y shows marked changes i n volume, a r e a , and y i e l d s for t h i s c r o p . D u r i n g the 1940's , maize p r o d u c t i o n s t a r t e d to grow e x p o n e n t i a l l y . T h i s c o i n c i d e d w i t h the w e l l known demographic e x p l o s i o n of M e x i c o . Indeed, maize per c a p i t a consumption had f a l l e n to r e c o r d low l e v e l s j u s t be fore p r o d u c t i o n began to c l i m b i n the 1940 ' s . T h i s i n c r e a s e i n p r o d u c t i o n a p p a r e n t l y brought r e l i e f to m i l l i o n s of Mexicans u n t i l the e a r l y 1970's , by which t i m e , p o p u l a t i o n growth had o u t s t r i p p e d the p r e v i o u s ga ins in p r o d u c t i o n . Maize had to be imported and t h i s has c o n t i n u e d ever s i n c e , c o n s t i t u t i n g a s e r i o u s d r a i n on f o r e i g n c u r r e n c y r e s e r v e s , which has p o l i t i c a l as w e l l as economic i m p l i c a t i o n s . Maize i s produced i n Mexico i n m i l l i o n s of maize f i e l d s . S m a l l , d i v e r s e , w i t h low use of i r r i g a t i o n , of f o s s i l energy , of f e r t i l i z e r s , and of improved seeds , these f i e l d s have low p r o d u c t i v i t y by modern s t a n d a r d s . As Mexico has l i t t l e good q u a l i t y a g r i c u l t u r a l l a n d to spa re , there i s a need to improve the p r o d u c t i v i t y of these f i e l d s . I r r i g a t i o n and farm machinery c o u l d r a i s e p r o d u c t i v i t y , but these are too expens ive fo r many of the s m a l l p r o d u c e r s . The e a s i e s t way to i n c r e a s e maize p r o d u c t i v i t y seems to l i e w i t h f e r t i l i z a t i o n , both because i t i s 41 e f f e c t i v e and cheap . Maize f i e l d p r o d u c t i v i t y data are s c a n t y . There i s no doubt that average y i e l d s are low: 1.2 or 1.8 t h a " 1 y r " 1 makes no d i f f e r e n c e by i n t e r n a t i o n a l s t a n d a r d s . The v a r i a b i l i t y of maize y i e l d r e c o r d s d u r i n g the l a s t few years i s another t h i n g . Some of these maize p r o d u c t i v i t y data show i m p r e s s i v e improvements, o t h e r s show j u s t normal g a i n s , w h i l e the o n l y f i e l d data suggest more modest growth. The q u e s t i o n of maize y i e l d s i s here important because such data r e f l e c t l a n d p r o d u c t i v i t y . In the long r u n , l a n d d e g r a d a t i o n shou ld r e s u l t i n lower l a n d p r o d u c t i v i t y , o ther t h i n g s be ing e q u a l . A p r e l i m i n a r y t e s t of t h i s c o n t e n t i o n was made on the assumption that modern management i n p u t s were not important i n the maize f i e l d s of Mexico u n t i l the l a t e f o r t i e s . Recent y i e l d s for f i e l d s tha t d i d not use those modern i n p u t s are o n l y s l i g h t l y g r e a t e r than y i e l d s i n the l a t e f o r t i e s . Thus , no d e c l i n e i n maize y i e l d s c o u l d be observed between 1946-49 and 1976-79. D u r i n g tha t p e r i o d , however, the maize growing area i n Mexico d o u b l e d . I t was not p o s s i b l e to check whether these l ands were new, p r e v i o u s l y abandoned, or of poor or good q u a l i t y . A l l i n a l l , l a n d p r o d u c t i v i t y i n the Mexican maize f i e l d may not ye t be d e c l i n i n g but i s not growing r a p i d l y e i t h e r . 42 CHAPTER I I I . SOIL EROSION, FERTILITY DEPLETION, AND PRODUCTIVITY: A REVIEW OF THE LITERATURE 111 . 1 . I n t r o d u c t ion In r e v i e w i n g the e f f e c t s that s o i l e r o s i o n and s o i l d e p l e t i o n have on l a n d p r o d u c t i v i t y , the chapter that f o l l o w s d e f i n e s the key e lements i n l and d e g r a d a t i o n c o n s i d e r e d i n t h i s t h e s i s : s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n . The p roce s s of s o i l e r o s i o n , and i t s v a r i o u s components, w i l l be rev iewed f i r s t . E v i d e n c e on the magnitude of s o i l e r o s i o n i n maize f i e l d s i s then p r e s e n t e d . Next , the chapter reviews ev idence on the impact of s o i l e r o s i o n on l a n d p r o d u c t i v i t y . F i n a l l y , s o i l f e r t i l i t y d e p l e t i o n i s examined through a number of s t u d i e s tha t have d e a l t w i t h the l o n g - t e r m e f f e c t s of s h i f t i n g c u l t i v a t i o n on l a n d p r o d u c t i v i t y . 111 .2 . S o i l E r o s i o n S o i l e r o s i o n i s a n a t u r a l l y o c c u r r i n g p roce s s which s l o w l y but p e r s i s t e n t l y t rans forms the su r f ace of the l a n d . I t does so by removing s o i l p a r t i c l e s from one s i t e and t r a n s p o r t i n g them to a n o t h e r . E r o s i o n o c c u r s i n most l andscapes and at a l l t imes , but i t v a r i e s g r e a t l y i n degree and i n t e n s i t y . Water i s the p r i n c i p a l agent of s o i l e r o s i o n i n the humid t r o p i c s (Hudson, 1971, pp . 27-31; E l - S w a i f y et a l , 1982). Ex t remely i n t e n s e r a i n f a l l can r e s u l t i n l a n d s l i d e s and 43 g u l l y i n g , but these c a t a s t r o p h i c events are l o c a l i z e d and are almost beyond human c o n t r o l once they have s t a r t e d . On the c o n t r a r y , sheet and r i l l e r o s i o n i s a p e r v a d i n g and cont inuous form of e r o s i o n that can have s e r i o u s e f f e c t s on s o i l p r o p e r t i e s and f u n c t i o n s , but which can be c o n t r o l l e d by sound management. Young (1969) , and Schum and Harvey (1982) have e s t i m a t e d tha t s o i l s c o v e r e d w i t h n a t u r a l v e g e t a t i o n l o s e from between 0.02 and 1 mm of s o i l per y e a r . Because these s o i l s are a l s o be ing formed at r a t e s which vary from 0.01 to 0.5 mm per year ( H a l l e_t a l , 1982), s o i l e r o s i o n under n a t u r a l v e g e t a t i o n may have no s i g n i f i c a n t e f f e c t on s o i l p r o p e r t i e s and f u n c t i o n s . A problem a r i s e s when people remove the n a t u r a l v e g e t a t i o n for a g r i c u l t u r a l or o ther uses . Under these c o n d i t i o n s , s o i l e r o s i o n r a t e s can a c c e l e r a t e d r a m a t i c a l l y . S o i l l o s s e s as h i g h as 45 mm y e a r " 1 have been r e p o r t e d i n the l i t e r a t u r e (Throeh e_t a l , 1980, p . 8 7 ) . These l o s s e s are c l e a r l y beyond the normal range of s o i l fo rmat ion r a t e s . Good l a n d management can m a i n t a i n a ba lance between s o i l l o s s and s o i l f o r m a t i o n . Indeed, s o i l fo rmat ion r a t e s i n p r o p e r l y c u l t i v a t e d l ands may even be as h i g h as 0.83 mm y r " 1 (Hudson, 1971; but see a l s o P imente l et a l , 1976; and Hudson, 1981). F u r t h e r m o r e , s o i l e r o s i o n can be checked through types of c r o p s , c r o p r o t a t i o n s , p l a n t r e s i d u e management, p lowing t e c h n i q u e s , and e r o s i o n c o n t r o l p r a c t i c e s . The u l t i m a t e goa l of s o i l c o n s e r v a t i o n i s to secure a s teady s t a t e between s o i l l o s s e s and s o i l a d d i t i o n s . Maximum a l l o w a b l e s o i l l o s s e s have been recommended to a c h i e v e t h i s g o a l . For 44 i n s t a n c e , the U . S . S o i l C o n s e r v a t i o n S e r v i c e e s t a b l i s h e d s o i l t o l e r a n c e v a l u e s which range from 0.4 to 1.1 mm y r - 1 depending on s o i l type (Wischmeier and Smi th , 1978). However, e r o s i o n on U . S . c r o p l a n d has been r e c e n t l y found to exceed , even more than b e f o r e , these " t o l e r a b l e " s o i l l o s s e s (McCormack and Young, 1980). The f o l l o w i n g pages d e s c r i b e the p r i n c i p a l f a c t o r s which determine sheet and r i l l e r o s i o n on c u l t i v a t e d l a n d s . 111 . 2 . 1. Determinants of s o i l e r o s i o n R a i n f a l l and l a n d r e l i e f g ive e r o s i o n i t s momentum; v e g e t a t i v e cover and s o i l s o f f e r r e s i s t a n c e . Land management may w e l l change whatever ba lance there i s . These f o r c e s do not ac t s e p a r a t e l y . They a l l i n t e r a c t i n any g iven s i t u a t i o n (Quansah, 1981). For i n s t a n c e , f o r r a i n f a l l to have an independent e f f e c t at a l l , the s o i l s u r f a c e must be ba re , the s o i l i t s e l f must be a p i l e of non-aggregated e a r t h , and the l a n d must have no r e l i e f , c o n d i t i o n s which are a l l t r i v i a l . In r e a l i t y , s o i l e r o s i o n i s a complex p r o c e s s , but one w h i c h , for a n a l y t i c a l purposes , has been separa ted i n t o a number of f a c t o r s . I I I . 2 . 1.1. R a i n f a l l and runof f R a i n f a l l p l a y s two r o l e s i n s o i l e r o s i o n : i t detaches su r f ace s o i l p a r t i c l e s and i t t r a n s p o r t s them downslope. R a i n f a l l h i t t i n g the ground i s a powerfu l f o r c e which sp l a she s p a r t i c l e s 45 a c c o r d i n g to i t s i n t e n s i t y and amount. Sp l a sh e r o s i o n has been d e f i n e d as a f u n c t i o n of mean r a i n d r o p s i z e , storm k i n e t i c energy , and r a i n f a l l i n t e n s i t y (Hudson, 1971, C h . 3 ) . How much i t r a i n s and at what v e l o c i t y are important q u e s t i o n s in d e t e r m i n i n g the e r o s i v i t y of r a i n f a l l . For i n s t a n c e , r a i n f a l l q u a n t i t i e s are a lmost the same, a p p r o x i m a t e l y 1.4 m y r - 1 , i n Vancouver , B r i t i s h Co lumbia , as i n X a l a p a , V e r a c r u z . Autumn r a i n s i n both p l a c e s f a l l as d r i z z l e s or l i g h t r a i n s . In s p r i n g and summer, Vancouver gets a few r a i n s t o r m s w h i l e Xa lapa gets t o r r e n t i a l r a i n s . Because of these d i f f e r e n c e s i n r a i n f a l l d i s t r i b u t i o n , r a i n f a l l e r o s i v i t y i n the two areas i s bound to d i f f e r . Runoff or o v e r l a n d flow can occur o n l y when the s o i l water s torage c a p a c i t y i s f i l l e d , or when the i n t e n s i t y of r a i n exceeds the r a t e of water i n f i l t r a t i o n i n t o the s o i l . A l a y e r of water forms on top of the s o i l and moves w i t h a f o r c e which depends on the mass of the excess water and the s teepness of the t e r r a i n . As runof f c o n t i n u e s , i t detaches new s o i l p a r t i c l e s , f u r t h e r compounding the e r o d i n g e f f e c t s of r a i n f a l l ( s p l a s h e r o s i o n ) . The o ther source of e r o s i v e energy comes from the s lope of the l a n d . ' T h e s teeper or longer a s l ope i s , the f a s t e r runof f p r o c e e d s , and the g r e a t e r the w a t e r ' s power t o abrade the s o i l u n d e r n e a t h . I t has been p o s t u l a t e d tha t s o i l e r o s i o n i s a second order p o l y n o m i a l f u n c t i o n of the s ine of the ang le of the s l o p e , m o d i f i e d by the l e n g t h of the s lope and i t s shape (Wischmeier and Smi th , 1978, pp . 15-16) . Put s i m p l y , the s lope 46 f a c t o r shou ld a f f e c t e r o s i o n e x p o n e n t i a l l y . S e v e r a l indexes of r a i n f a l l e r o s i v i t y have been, proposed to d e a l w i t h both the s p l a s h and runof f components of t h i s p roce s s (Wischmeier and Smi th , 1978; Hudson, 1971). They are a l l e m p i r i c a l and need c a l i b r a t i o n to s p e c i f i c c o n d i t i o n s . I I I . 2 . 1 . 2 . S o i l e r o d i b i l i t y I t has a l r e a d y been mentioned that the i n f i l t r a t i o n of r a i n f a l l i n t o the s o i l p r e c l u d e s runo f f and c o n s e q u e n t l y reduces s o i l e r o s i o n . The p e r m e a b i l i t y of the s o i l a f f e c t s the r a t e at which water i n f i l t r a t e s . O b v i o u s l y there are l i m i t s to the amount of water tha t s o i l s can a b s o r b . I t c o u l d w e l l happen tha t runof f would s t a r t even be fore a s o i l becomes c o m p l e t e l y s a t u r a t e d , as water might not p e n e t r a t e the s o i l f a s t enough. In t h i s r e s p e c t , s o i l s t r u c t u r e i s i m p o r t a n t . A m a s s i v e l y compacted s o i l has few pores i n which to accomodate water . C o n v e r s e l y , a s o f t , w e l l t i l l e d t o p s o i l a l l o w s water to soak i n t o i t r a p i d l y . Moreover , s o i l s t r u c t u r e produces a g g r e g a t i o n of s o i l p a r t i c l e s , which prevent s these p a r t i c l e s from s p l a s h i n g away upon r a i n f a l l impact . S o i l p a r t i c l e d i s t r i b u t i o n i s a l s o r e s p o n s i b l e for the net e f f e c t of s p l a s h e r o s i o n . S i l t y s o i l s are more e a s i l y eroded than e i t h e r sands or c l a y s (Wischmeier and Manner ing , 1969). Because o r g a n i c matter c o n t r i b u t e s to the a g g r e g a t i o n of a s o i l as w e l l as to i t s i n f i l t r a b i l i t y , i t i s too c o n s i d e r e d an important f a c t o r i n c o n t r o l l i n g e r o s i o n ( N e a l , 1939; Wischmeier and Manner ing , 1965). 47 Recent r e s e a r c h on t r o p i c a l s o i l s has brought a t t e n t i o n to a number of o ther s o i l p r o p e r t i e s that might a f f e c t e r o d i b i l i t y . For i n s t a n c e , the amount of s e s q u i o x i d e s (Roth et a l , 1974), and the s t a b i l i t y of s o i l aggregates ( E l - S w a i f y and D a n g l e r , 1976), have been shown to have a s i g n i f i c a n t n e g a t i v e e f f e c t on s o i l e r o d i b i l i t y . G iven the number of s o i l p r o p e r t i e s that might have an e f f e c t on e r o d i b i l i t y , as w e l l as the many p o s s i b l e i n t e r a c t i o n s , i t i s d i f f i c u l t to have a c l e a r p i c t u r e of the f a c t o r s on which e r o d i b i l i t y i s s p e c i f i c a l l y dependent (Rorke, 1968). E a r l y a t tempts at s i m p l i f y i n g t h i s r e l a t i o n s h i p i d e n t i f i e d s o i l p a r t i c l e - s i z e d i s t r i b u t i o n , o r g a n i c m a t t e r , and s o i l p e r m e a b i l i t y and s t r u c t u r e as the key v a r i a b l e s to take i n t o account (Wischmeier et a l , 1971). I I I . 2 . 1 . 3 . V e g e t a t i v e cover V e g e t a t i o n i n t e r c e p t s r a i n f a l l and d i m i n i s h e s i t s v e l o c i t y . V e g e t a t i o n a l s o r e d i r e c t s the e n t r y of water i n t o the s o i l through stemflow and canopy d r i p . F u r t h e r m o r e , a s o i l w i t h v e g e t a t i o n o f f e r s o b s t a c l e s to the o v e r l a n d flow of water . These are the b a s i c r o l e s of v e g e t a t i o n in r e d u c i n g the impact of e r o s i o n . I t a c t u a l l y does not matter what type of v e g e t a t i o n c o v e r s a s o i l , p r o v i d e d that i t i s t h i c k at the ground l e v e l . Annual c r o p s can produce enough p l a n t cover by the end of the growing season to be e f f e c t i v e a g a i n s t e r o s i o n . Maize takes between two 48 and f i v e months to f u l l y deve lop a complete canopy on the f i e l d . The e f f e c t i v e n e s s of t h i s p l a n t cover depends on the sea sona l d i s t r i b u t i o n of e r o s i v e r a i n s . T h e r e f o r e , i n any d i s c u s s i o n of the b e n e f i c i a l e f f e c t of p l a n t c o v e r , the c y c l e of the v e g e t a t i o n shou ld be compared w i t h t h a t of r a i n f a l l ( E l w e l l and S t o c k i n g , 1976). For i n s t a n c e , a t r o p i c a l dry f o r e s t l o s e s most of i t s l e ave s d u r i n g the dry season. When the r a i n season se t s i n , there i s l i t t l e e f f e c t i v e cover and e a r l y r a i n s can produce s i g n i f i c a n t e r o s i o n , i f the s o i l can not soak up excess water . On the o ther hand, mulch spread over the seedbed of a maize c r o p can r e s i s t e r o s i o n as w e l l as a l u x u r i a n t f o r e s t (Roose, 1973; L a i , 1976). P lowing has the double e f f e c t of i n c r e a s i n g s o i l i n f i l t r a t i o n and a l t e r i n g s o i l s t r u c t u r e ( G r e e n l a n d , 1977). Much c o n t r o v e r s y i s r e c e n t l y go ing on i n the U . S . about the advantages and d i s advantage s of z e r o - t i l l a g e and minimum t i l l a g e as a means of p r e v e n t i n g e r o s i o n (see , for i n s t a n c e , the 1983 May-June s p e c i a l i s sue of the J o u r n a l of S o i l and Water C o n s e r v a t i o n ) . P lowing c e r t a i n l y has o ther important f u n c t i o n s i n a g r i c u l t u r e , such as weed c o n t r o l , s o i l a e r a t i o n , and seedbed p r e p a r a t i o n . T h e r e f o r e , the n e g a t i v e e f f e c t s of p lowing on e r o s i o n shou ld be weighed a g a i n s t the known b e n e f i c i a l e f f e c t s of p l o w i n g , i f the worth of t h i s o l d a g r i c u l t u r a l p r a c t i c e i s to be p r o p e r l y a s s e s s e d . 49 I I I . 2 . 1 . 4 . S o i l e r o s i o n c o n t r o l R a i n f a l l e r o s i v i t y , s o i l e r o d i b i l i t y , topography and v e g e t a t i v e cover are the key f a c t o r s to any d i s c u s s i o n of s o i l e r o s i o n c o n t r o l . A l l have an important b e a r i n g on the magnitude and ra te of e r o s i o n and are ext remely v a r i a b l e , which makes i t p r a c t i c a l l y i m p o s s i b l e to fore see them a l l . A g r i c u l t u r e i s an a n c i e n t human a c t i v i t y which has been faced w i t h the l o s s of s o i l many t imes and has s u r v i v e d . T e r r a c i n g has been the most consp icuous form of c o n t r o l l i n g e r o s i o n . There are many k inds and s t y l e s of t e r r a c e s , but t h e i r b a s i c r o l e i s to break the s l ope s and to prevent r u n o f f . Contour p l a n t i n g i s a common modern p r a c t i c e which a l s o c u t s a f i e l d a c r o s s the s l o p e s , a l t h o u g h i t i s c o n f i n e d to g e n t l e s l o p e s . Crop r o t a t i o n s and i n t e r c r o p p i n g w i t h legumes are means of r e s t o r i n g s o i l f e r t i l i t y , but a l s o c o n t r i b u t e to c o n t r o l l i n g e r o s i o n by i n c r e a s i n g v e g e t a t i v e c o v e r . Hudson (1971, pp .195-99) has found tha t h i g h d e n s i t y s tands of ma ize , wi th h i g h l e v e l s of p r o d u c t i o n , reduced e r o s i o n more than ten t i m e s . Crop r e s i d u e management h e l p s to p r o t e c t the s o i l d u r i n g the o f f -season. T h i s can p r o v i d e an adequate c o v e r , p a r t i c u l a r l y when y i e l d s are h i g h and when r e s i d u e s are p r o p e r l y handled i n p o s t -harves t o p e r a t i o n s . Perhaps the more c r u c i a l f a c t o r i n c o n t r o l l i n g e r o s i o n i n c r o p l a n d s i s t i m i n g . Farm o p e r a t i o n s can be e f f e c t i v e l y a d j u s t e d to o f f e r the minimal s o i l exposure to the e r o s i v i t y of r a i n f a l l . Farmers shou ld know t h i s w e l l . R a i n f e d a g r i c u l t u r e i s p a r t i c u l a r l y t i e d to the c y c l e of r a i n f a l l . A good c r o p 50 depends s t r o n g l y on the r i g h t time for p l a n t i n g . In areas w i t h sea sona l r a i n s , farmers have to time p l a n t i n g to c o i n c i d e w i t h the onset of r a i n s . F i e l d s shou ld be plowed i n advance for the f i r s t r a i n s to moisten the s o i l and c r e a t e good seedbed c o n d i t i o n s . But what i s good for the c r o p may not be so good for the s o i l a t t h i s p a r t i c u l a r t i m e ; bare and l o o s e n e d , the s o i l might be hard h i t by heavy r a i n s and e r o d e . T h e r e f o r e , t r a d e - o f f s can be made between the l i k e l i h o o d of a good c r o p and the p r o s p e c t s of s o i l e r o s i o n , which a l s o depend on s o i l t y p e , r a i n f a l l , and c r o p development p a t t e r n s . 111 . 2 . 2 . Ev idence of s o i l e r o s i o n i n maize - f i e l d s  S o i l e r o s i o n r e s e a r c h i s p a i n s t a k i n g . I t r e q u i r e s expens ive and t ime-consuming e x p e r i m e n t a t i o n . The number and v a r i a b i l i t y of f a c t o r s a f f e c t i n g e r o s i o n makes d i f f i c u l t the g e n e r a l i z a t i o n of r e s u l t s . The l e a d i n g a g r i c u l t u r a l n a t i o n i n the w o r l d , the U . S . , i s a l s o the l e a d e r i n s o i l c o n s e r v a t i o n r e s e a r c h . A l t h o u g h that c o u n t r y has e x p e r i e n c e d massive e r o s i o n events ( i . e . , the dust bowl of the 1930 ' s ) , and more r e c e n t l y an o i l c r i s i s t h a t t h r e a t e n e d a model of a g r i c u l t u r a l development based on the s u b s t i t u t i o n of f o s s i l f u e l s for s o i l r e s o u r c e s (P imente l et a l , 1976), i t i s w i d e l y b e l i e v e d tha t the magnitude of s o i l e r o s i o n i n the U . S . i s minimal when compared w i t h e r o s i o n i n the t r o p i c a l , d e v e l o p i n g wor ld (see FAO, 1977; G r e e n l a n d and L a i , 1977; S t o c k i n g , 1980; E l - S w a i f y et a l , 1982). In order to as sess the magnitude of s o i l e r o s i o n in the 51 Mexican maize f i e l d s , three types of sources w i l l be reviewed h e r e : a) g e n e r a l s u r v e y s ; b) d e t a i l e d f i e l d s t u d i e s of maize f i e l d s throughout the w o r l d , p a r t i c u l a r l y i n the t r o p i c s and c) p r e l i m i n a r y s t u d i e s of e r o s i o n i n Mexican maize f i e l d s . I l l . 2 . 2 . 1 . Surveys D u r i n g the e a r l y 1950's , the Food and A g r i c u l t u r e O r g a n i z a t i o n of the U n i t e d N a t i o n s c a r r i e d out a survey of s o i l e r o s i o n t r ends i n L a t i n A m e r i c a , i n c l u d i n g Mexico (FAO, 1954). A paragraph d e s c r i b e d the e r o s i o n problem i n Mexico as f o l l o w s : " In the more humid f o r e s t e d and mountainous l ands of Mexico s o i l e r o s i o n i s c h i e f l y due to the c u l t i v a t i o n of corn (ma ize ) . In many p l a c e s c o r n i s grown under p r i m i t i v e c o n d i t i o n s of s h i f t i n g a g r i c u l t u r e , but r e g a r d l e s s of the method of c u l t i v a t i o n , c o r n grown on s lope s always l eads to a c e r t a i n degree of e r o s i o n . Any d e t a i l e d assessment of such damage must be based on a thorough study of s o i l c h a r a c t e r i s t i c s b u t , as a g e n e r a l r u l e , the deep s o i l s formed from young v o l c a n i c ash are l e a s t s u b j e c t to damage and are most r e a d i l y r e s t o r e d because of t h e i r i n h e r e n t e r o s i o n - r e s i s t a n t p h y s i c a l p r o p e r t i e s and t h e i r h i g h l e v e l of m i n e r a l f e r t i l i t y . Sub jec t to the most severe and permanent damage are the sha l low s l o p i n g s o i l s tha t are u n d e r l a i d by i n f e r t i l e parent rock m a t e r i a l s or by hard imperv ious l a y e r s , such as the t e p e t a t e ( c a l i c h e ) so widespread i n p a r t s of M e x i c o " 1 T h i s s tudy used a e r i a l photographs to map at the s c a l e 1 :10,000,000 areas w i t h d i f f e r e n t degrees of e r o s i o n . On t h i s 1 FAO, 1954, p . 1 6 2 . I t a l i c s and notes i n b r a c k e t s are F A O ' s . 52 b a s i s , Samario Pineda (1965) c a l c u l a t e d the extent of each e r o s i o n c l a s s i n M e x i c o . Pockets of severe e r o s i o n (4 per cent of the c o u n t r y ) were n o t i c e d i n the a r i d lands of the n o r t h and i n the mountainous t r o p i c a l l ands of the s o u t h . Some 33 per cent of the l a n d in the n o r t h and west of the c o u n t r y was moderate ly to s e v e r e l y e r o d e d . C e n t r a l and s o u t h - e a s t e r n Mexico c o n t a i n e d 30 per cent of moderate ly to s l i g h t l y eroded s o i l s . F i n a l l y , the remain ing 32 per cent of the l a n d i n the t r o p i c a l lowlands and the f o r e s t e d mountains was found to be s l i g h t l y eroded or not eroded at a l l . These e s t i m a t e s are t e n t a t i v e at b e s t , because they were based on p h o t o i n t e r p r e t a t i o n of the amount of v e g e t a t i v e cover at a p a r t i c u l a r t i m e , and had l i t t l e f i e l d w o r k to support i t . More r e c e n t l y , the S o i l C o n s e r v a t i o n S e r v i c e of Mexico has been u p d a t i n g these e s t i m a t e s w i t h s a t e l l i t e images and f i e l d work (A. B e n i t e z Omana, D t o . Areas E r o s i o n a d a s , SARH, M e x i c o , 1981, p e r s . comm.). U n f o r t u n a t e l y , these r e s u l t s have not yet been made a v a i l a b l e . I I I . 2 . 2 . 2 . E r o s i o n r a t e s i n t r o p i c a l maize f i e l d s As maize i s c u l t i v a t e d today i n most of the t r o p i c a l w o r l d , i t i s worth r e v i e w i n g s o i l e r o s i o n measurements fo r these c o n d i t i o n s . L a i (1976a) found t h a t , on two years of o b s e r v a t i o n s , a N i g e r i a n A l f i s o l on 15 per cent s l ope s l o s t an average of 28 t h a - 1 y r " 1 when c u l t i v a t e d c o n t i n u o u s l y to ma ize . On a 5 per cent s l o p e , the same c r o p p i n g system r e s u l t e d i n 7.1 t h a " 1 53 y r " 1 , or r o u g h l y 1/4 of the l o s s e s of the s teeper s l o p e . The study a l s o showed tha t straw mulch a p p l i e d on these f i e l d s reduced e r o s i o n to i n s i g n i f i c a n t l e v e l s , even on the s teep s l o p e s . Hudson (1971; t a b l e I I . 1) r e p o r t e d two year s of data on h i s e a r l i e r work i n Zimbabwe. S o i l e r o s i o n i n a g e n t l y s l o p i n g maize f i e l d was reduced from an average 12.3 t h a " 1 y r " 1 to an average 0.7 t h a " 1 y r " 1 when p l a n t d e n s i t y , f e r t i l i z a t i o n , and p l a n t r e s i d u e s a d d i t i o n s were i n c r e a s e d . A c c o r d i n g to Temple (1972) these s t u d i e s a l s o showed tha t s o i l e r o s i o n i s .dependent on s o i l t y p e . In s o i l c a p a b i l i t y c l a s s e s for a g r i c u l t u r e II to IV, s o i l l o s s e s ranged from 1.8 to 21.9 t h a " 1 y r " 1 , r e s p e c t i v e l y , w i t h poor management, and from 1.2 to 10.6 t h a " 1 y r " 1 , r e s p e c t i v e l y , w i t h good management. Temple (1972) a l s o r e p o r t e d t h r e e year s of measured s o i l e r o s i o n r a t e s for a v o l c a n i c s o i l i n T a n z a n i a . C o n t i n o u s maize on a 40 per cent s lope was a s s o c i a t e d w i t h average s o i l l o s s e s of 12 t h a " 1 y r " 1 . The same treatment w i t h the a d d i t i o n of s t o v e r and t r a s h bunds r e s u l t e d i n average s o i l l o s s e s of 1 t h a " 1 y r " 1 . Roose (1967) s t u d i e d s o i l e r o s i o n of an a lmost l e v e l s o i l i n Senegal w i t h s e v e r a l c r o p p i n g systems. Cont inuous c r o p p i n g of maize r e s u l t e d i n the g r e a t e s t s o i l l o s s e s , a 10 year average of more than 10 t h a " 1 y r " 1 . Q u i n t i l i a n o et a l (1961) r e p o r t e d ten year s of s o i l e r o s i o n measurements for s e v e r a l s o i l s i n southern B r a z i l . These au thor s found tha t c o n t i n u o u s c r o p p i n g of maize on s lope s from 8 to 12 per cent r e s u l t e d i n s o i l l o s s e s r ang ing from 8 to 30 t 54 h a " 1 y r " 1 . T h i s review suggests that measured s o i l l o s s e s i n t r o p i c a l maize f i e l d s range from 1 to 30 t h a " 1 y r " 1 , w i t h a more common range of 10 to 20 t h a " 1 y r " 1 . The l a t t e r roughly cor re sponds to s o i l depth l o s s e s of between 1 and 2 mm per y e a r . Steep s lopes and good management can d r a m a t i c a l l y change t h i s p i c t u r e , for b e t t e r or worse. By way of compar i son , average annual s o i l l o s s r a t e s i n the U . S . c o r n b e l t were r e c e n t l y e s t i m a t e d at s l i g h t l y more than 10 t h a " 1 y r " 1 ( L a r s o n , 1981; Lar son et a l , 1983). I I I . 2 . 2 . 3 . E r o s i o n r a t e s i n Mexican maize f i e l d s Only a few s t u d i e s have a c t u a l l y measured s o i l e r o s i o n i n Mexican maize f i e l d s . T e r r a z a Gonza lez (1977) t e s t e d for one year the e f f e c t s of d i f f e r e n t c r o p management p r a c t i c e s on s o i l e r o s i o n . The s tudy , near Mexico C i t y , examined three s o i l s on 3.5 per cent s l o p e s . When maize was i n t e r c r o p p e d w i t h beans, s o i l l o s s e s v a r i e d from 2.3 to 6.2 t h a " 1 y r " 1 . When maize was c u l t i v a t e d a l o n e , l o s s e s v a r i e d among s o i l s from between 3.1 and 5.5 t h a " 1 y r " 1 . Z e r o - t i l l a g e and a d d i t i o n s of manure and p l a n t r e s i d u e s reduced s o i l e r o s i o n by between twenty and f o r t y per c e n t . F i g u e r o a Sandoval (1975) a l s o s t u d i e d s o i l e r o s i o n i n the v i c i n i t y of Mexico C i t y . S o i l s and v e g e t a t i o n were the main v a r i a b l e s of the s t u d y . Two maize f i e l d s , l o c a t e d on 2 per cent s l o p e s , e x p e r i e n c e d annual s o i l l o s s e s of 1.6 and 3.1 t h a " 1 y r " 1 d u r i n g the year of the s t u d y . These r a t e s were exceeded 55 o n l y by a p o o r l y gras sed Tepeta te (hardpan s o i l ) which l o s t 16.1 t h a " 1 y r " 1 . U n f o r t u n a t e l y , no d e t a i l s of the c r o p p i n g system were g i v e n i n the r e p o r t of t h i s s tudy , but a p p a r e n t l y one of the maize f i e l d s s t u d i e d had been i n r o t a t i o n w i t h o ther c r o p s . Trueba C . et a l , (1979) s t u d i e d four c o n s e r v a t i o n p r a c t i c e s i n 0.7 ha p l o t s c u l t i v a t e d w i t h maize . L i t t l e i n f o r m a t i o n was p r o v i d e d on the s o i l s , l o c a t i o n , and topography of the study s i t e s , a l t h o u g h i t c o u l d be assumed that these s o i l s are a l s o i n the v i c i n i t y of Mexico C i t y . Average s o i l l o s s e s ( 2 year s ) under t r a d i t i o n a l maize c u l t i v a t i o n were a mere 0.5 t h a " 1 y r " 1 , w h i l e a l l c o n s e r v a t i o n works ( i . e . , contour p l a n t i n g and d i f f e r e n t types of t e r r a c e s ) reduced these l o s s e s by up to 80 per c e n t . F i n a l l y , Wegener (1979) r e p o r t e d p r e l i m i n a r y data from e r o s i o n p l o t s i n the h i g h l a n d s of P u e b l a . Maize was used as the p r i n c i p a l c r o p i n h a l f the p l o t s , w h i l e the o t h e r h a l f was kept b a r e . S lopes ranged from 4-11 per cent and o b s e r v a t i o n s were made d u r i n g o n l y one growing season . Only graphs d e p i c t i n g s o i l l o s s e s from s i n g l e storm events were p r o v i d e d . T h i s r e p o r t c o n c l u d e d , however, ' tha t s o i l l o s s e s ranged from medium to very h i g h , depending on s o i l t y p e . The g e n e r a l impres s ion l e f t a f t e r r e v i e w i n g these s t u d i e s i s one of inadequacy . At b e s t , o n l y a c o u p l e of year s of measurements are r e p o r t e d . They were a l l c o n c e n t r a t e d i n the Mexican h i g h l a n d s - - i m p l y i n g tha t the p r o b a b l y most e r o s i v e l ands i n M e x i c o , the t r o p i c s , have not been s t u d i e d at a l l - - a n d i n c l u d e d o n l y g e n t l e s l o p e s i n t h e i r t r e a t m e n t s . An i n t e r e s t i n g 56 f e a t u r e of a l l these s t u d i e s i s that they r e p o r t e d low r a t e s of s o i l e r o s i o n . The i n f o r m a t i o n provided by the authors about methods and procedures suggests that these were p i l o t s t u d i e s , and that more and b e t t e r planned s t u d i e s are about to come. 111.3. S o i l E r o s i o n and P r o d u c t i v i t y T h i s s e c t i o n f i r s t d e s c r i b e s the e f f e c t s of s o i l e r o s i o n on the s o i l p r o p e r t i e s that c o n t r o l l a n d p r o d u c t i v i t y . A review of the l i t e r a t u r e c o n t a i n i n g evidence of land p r o d u c t i v i t y d e c l i n e a s s o c i a t e d with s o i l e r o s i o n w i l l f o l l o w . III.3.1 S o i l e r o s i o n e f f e c t s on s o i l p r o p e r t i e s S o i l i s the product of long-term s o i l forming processes i n v o l v i n g c l i m a t e , l i v i n g organisms, r e l i e f , rocks and v e g e t a t i o n a c t i n g on s o i l parent m a t e r i a l s over time. S o i l depth i s an important f e a t u r e of s o i l s because i t d e f i n e s the volume and r o o t i n g depth of a s o i l . S o i l depth can be measured from the s u r f a c e to the c o n t a c t with u n d e r l y i n g rocks or the u n a l t e r e d s o i l parent m a t e r i a l s . The depth of a s o i l p r o v i d e s the s t a r t i n g c a p i t a l f o r l a n d use a c t i v i t i e s . S o i l e r o s i o n can s i g n i f i c a n t l y reduce the depth of a s o i l . A net s o i l l o s s of 40 t ha" 1 y r " 1 ( i . e . , approximately 4 mm y r - 1 ) c o u l d reduce the depth of a shallow ( i . e . 40 cm) s o i l by 20 per cent i n 20 y e a r s . On the other hand, a deep ( i . e . 100 cm), moderately e r o d i n g s o i l ( i . e . 5 t ha" 1 y r " 1 ) w i l l only s u f f e r a 1.0 per cent r e d u c t i o n d u r i n g a s i m i l a r p e r i o d of time. These 57 v a l u e s are r e p r e s e n t a t i v e and i l l u s t r a t e the range of impact tha t s o i l e r o s i o n can have on s o i l r e s o u r c e s . S o i l i s a heterogeneous assortment of p a r t i c l e s of v a r i o u s s i z e s . G r a v e l and s tones are l e s s e a s i l y eroded than f i n e r p a r t i c l e s . E r o s i o n of c o a r s e - t e x t u r e d s o i l s w i l l i n c r e a s e the r e l a t i v e p r o p o r t i o n of coar se fragments i n the p r o f i l e , and reduce even f u r t h e r the e f f e c t i v e r o o t i n g volume of the s o i l . The f i n e p a r t i c l e s of a s o i l are o r g a n i z e d i n r e l a t i v e l y s t a b l e p h y s i c a l aggrega te s . To a great e x t e n t , these aggregates c o n t r o l the movement of a i r , water , and n u t r i e n t s i n the s o i l . The s p l a s h i n g of r a i n f a l l and the a b r a s i o n of r u n o f f , which precede- s o i l e r o s i o n , compact s o i l aggregates and s e a l the sur f ace of the s o i l . A compacted s o i l s t r u c t u r e r e s u l t s i n poor t i l t h and i n problems for p l a n t g e r m i n a t i o n and growth. Organ ic matter i s the product of s o i l - p l a n t i n t e r a c t i o n s over t i m e , and n a t u r a l l y accumulates on the topmost p o r t i o n of the s o i l . Organ ic matter i s r e s p o n s i b l e for a number of important f u n c t i o n s of the s o i l : a) i t s u p p l i e s most of the n i t r o g e n and phosphorus ; b) i t enhances the c a t i o n exchange c a p a c i t y , p a r t i c u l a r l y i n a c i d s o i l s ; c) i t r e t a i n s o therwise f i x a b l e or l e a c h a b l e n u t r i e n t s i n a form a v a i l a b l e to p l a n t s ; d) i t improves s o i l a g g r e g a t i o n and s t r u c t u r e ; and e) i t g e n e r a l l y enhances s o i l water r e t e n t i o n c a p a c i t y (Sanchez, 1976). E r o s i o n reduces s o i l volume, a f f e c t s s o i l s t r u c t u r e , and i m p a i r s the l e v e l s of s o i l o r g a n i c m a t t e r , a l l of which are i n d i r e c t de te rminant s of the s o i l water b a l a n c e . F u r t h e r , and foremost , e r o s i o n leads to reduced water i n f i l t r a b i l i t y and 58 i n c r e a s e d runof f which d i r e c t l y reduce the water a v a i l a b l e for c r o p p r o d u c t i o n . T h i s may not be important i n humid c l i m a t e s , but i n a l l those lands where r a i n f a l l i s s c a r c e , any l o s s of water can be c r u c i a l for the success of a c r o p . N u t r i e n t s are a l s o l o s t w i t h the e r o d i n g s o i l . As w i t h o r g a n i c mat te r , these n u t r i e n t s are more r e a d i l y a v a i l a b l e i n the t o p s o i l . D i f f e r e n t n u t r i e n t s have d i f f e r e n t water s o l u b i l i t i e s and t h i s c o u l d l e a d to d i f f e r e n t i a l l o s s e s of n u t r i e n t s wi th water e r o s i o n . The f o r e g o i n g d i s c u s s i o n i n d i c a t e s tha t there i s a wide range of e f f e c t s of s o i l e r o s i o n on s o i l p r o p e r t i e s . The p r o p e r t i e s j u s t d i s c u s s e d are a l l important f a c t o r s d e t e r m i n i n g the p r o d u c t i v i t y of the l a n d . One would expect t h a t s e r i o u s impairment of these s o i l p r o p e r t i e s would r e s u l t i n a l o s s of p r o d u c t i v i t y . Indeed, i f the p r o p e r t i e s of t o p s o i l and s u b s o i l s are markedly d i f f e r e n t , i t c o u l d be a n t i c i p a t e d that the l o s s of p r o d u c t i v i t y would be p r o p o r t i o n a l l y g r e a t e r than the r a te of e r o s i o n . The f o l l o w i n g s e c t i o n checks t h i s p r o p o s i t i o n a g a i n s t l i t e r a t u r e e v i d e n c e . 1 1 1 . 3 . 2 . S o i l e r o s i o n impact on p r o d u c t i v i t y O d e l l (1950) surveyed maize p r o d u c t i o n i n two p r a i r i e s o i l s (Tama s i l t loam and Swygert s i l t loam) for two y e a r s . A p p r o x i m a t e l y 250 data s e t s were o b t a i n e d w i t h i n f o r m a t i o n on s o i l d e p t h , phosphorus and l ime a p p l i c a t i o n s , c r o p r o t a t i o n , and maize y i e l d s . R e g r e s s i o n a n a l y s i s of the data y i e l d e d s e v e r a l i n t e r e s t i n g c o n c l u s i o n s . Tama s o i l produced more maize (4 .5 t 59 h a - 1 y r " 1 ) than Swygert s o i l (3 .3 t h a " 1 y r " 1 ) , because i t had a more f r i a b l e and permeable s u b s o i l . For each c e n t i m e t e r of Tama s o i l m i s s i n g , about 30 kg h a " 1 y r " 1 l e s s maize was h a r v e s t e d . In Swygert each cm of s o i l l o s s r e s u l t e d i n a l o s s of 67 kg h a " 1 of g r a i n . There were no s i g n i f i c a n t l y c o n s i s t e n t e f f e c t s of f e r t i l i z a t i o n upon t h i s g e n e r a l r e l a t i o n s h i p . On the o ther hand, t o p s o i l depth had a r e l a t i v e l y s m a l l e r e f f e c t on y i e l d s d u r i n g the dry y e a r , when y i e l d s were o n l y 71 per cent those of the moist y e a r . However, t h i s e f f e c t was o n l y important i n Swygert s o i l (25 kg h a " 1 y r " 1 l e s s g r a i n per cm of t o p s o i 1 l o s s ) . E n g e l s t a d et a l (1961) l a i d out f e r t i l i z e r t r i a l s on d i f f e r e n t i a l l y eroded s i t e s of two M o l l i s o l s , M a r s h a l and Monona s i l t loams (minimal B r u n i z e n s , permeable , d e r i v e d from l o e s s ) c u l t i v a t e d w i t h ma ize . The au thor s found i t d i f f i c u l t to measure t o p s o i l depth v i s u a l l y and used the conten t of o r g a n i c carbon (C) as a su r roga te of s o i l t h i c k n e s s . Two year s of data were a n a l y s e d through m u l t i p l e r e g r e s s i o n t e c h n i q u e s . A dry s p e l l i n one year produced c o n t r a s t i n g r e s u l t s . D u r i n g the normal y e a r , N a p p l i c a t i o n s of 112 kg h a " 1 y r " 1 c o m p l e t e l y overrode d i f f e r e n c e s i n maize y i e l d s over the f u l l range of C c o n t e n t s of the t o p s o i l . Lack of N f e r t i l i z a t i o n , however, showed t h a t y i e l d s i n s o i l s w i t h low C c o n t e n t s were o n l y 82 per cent of t o p s o i l s w i t h h i g h C c o n t e n t s . D u r i n g the dry y e a r , both f e r t i l i z e d and n o n - f e r t i l i z e d p l o t s w i t h low C c o n t e n t s y i e l d e d a p p r o x i m a t e l y 8.7 per cent of the p l o t s w i t h h i g h C c o n t e n t s . 60 These r e s u l t s d i d show t h e r e s t o r a t i v e p o t e n t i a l of f e r t i l i z e r s on e r o d e d s o i l s , but they a l s o h i g h l i g h t e d the d i f f e r e n t i a l r e s p o n s e s t h a t can be e x p e c t e d d u r i n g c l i m a t i c a l l y a d v e r s e y e a r s . I t s h o u l d a l s o be p o i n t e d out t h a t t h e l e v e l s of ma ize y i e l d s i n t h e s e e x p e r i m e n t s were h i g h , i . e . , between 6 and 8 t h a " 1 y r " 1 . More r e c e n t l y , two s i m i l a r s t u d i e s have been c o n d u c t e d on s o i l s of t h e s o u t h e a s t r e g i o n of the U . S . . L a n g d a l e e t a l (1979) s t u d i e d a w a t e r s h e d c o n t a i n i n g p r i n c i p a l l y C e c i l sandy loam (a . T y p i c H a p l u d u l t ) . S i t e s w i t h t o p s o i l d e p t h s r a n g i n g from 10 t o 60 cm were c u l t i v a t e d f o r t h r e e y e a r s w i t h m a i z e a t c o n s t a n t r a t e s of NPK (1 4 0 /24 /125 kg h a " 1 y r " 1 ) . Average ma ize y i e l d s f o r t h e whole e x p e r i m e n t i n c r e a s e d a s y m p t o t i c a l l y w i t h t o p s o i l d e p t h from 1.0 t h a " 1 y r " 1 a t 10 cm d e p t h t o 6 t h a " 1 y r " 1 a t 60 cm. T h i s g e n e r a l l y n o n - l i n e a r r e l a t i o n s h i p , h o w e v e r , i s a l m o s t l i n e a r f o r s o i l d e p t h between 10 and 40 cm, w h i c h the a u t h o r s c o n s i d e r e d t o be t h e maximum n a t u r a l t o p s o i l d e p t h w i t h o u t d e p o s i t i o n . W i t h i n t h i s r a n g e , t h e a v e r a g e r e d u c t i o n i n maize y i e l d p e r cm of t o p s o i l l o s s was 150 kg h a " 1 y r " 1 / c m . In t h e d r y y e a r , t h e same r a t i o was 172 kg h a " ' y r " 1 / c m . Abundant f e r t i l i z a t i o n d i d not compensate f o r y i e l d l o s s e s due t o e r o s i o n . F r y e e t a l (1982) r e p o r t e d on a n o t h e r s t u d y i n t h e U . S . S o u t h e a s t where two A l f i s o l s ( T y p i c P a l e u d a l f s ) were t e s t e d . These a u t h o r s c l a s s i f i e d p l o t s as e r o d e d , m o d e r a t e l y e r o d e d and uneroded on t h e b a s i s of the c l a y c o n t e n t of t h e topmost 7 .5 cm of t h e s e s o i l s . V a r i o u s r a t e s of N f e r t i l i z e r s and s e v e r a l 61 win te r p l a n t cover t rea tments were t r i e d in one s o i l . D i f f e r e n t f e r t i l i z e r s , to o b t a i n l a b o r a t o r y p r e d i c t e d y i e l d s of 9.4 t h a " 1 y r " 1 , were used in eroded and uneroded p l o t s of the o ther s o i l . Three year s average y i e l d s i n the l a t t e r experiment showed t h a t , r e g a r d l e s s of the s o p h i s t i c a t e d f e r t i l i z a t i o n scheme used , y i e l d s i n eroded p l o t s were 21 per cent lower than i n uneroded p l o t s (7 .9 t h a " 1 y r " 1 ) . Average y i e l d s for eroded and uneroded p l o t s i n the former experiment were s i g n i f i c a n t l y d i f f e r e n t , and o n l y twelve per cent lower i n the eroded s i t e s . The N f e r t i l i z a t i o n never f u l l y compensated moderate past e r o s i o n ; d i f f e r e n c e s between eroded and uneroded p l o t s were reduced from t h i r t e e n per cent to three per cent when 100 kg h a " 1 of N were s u p p l i e d toge ther w i t h win te r s t a l k r e s i d u e , but i n the o ther four win te r management o p t i o n s , d i f f e r e n c e s remained or even i n c r e a s e d a f t e r N f e r t i l i z a t i o n . U n f o r t u n a t e l y , the amount of s o i l l o s s was not p r e c i s e l y measured i n t h i s o therwi se very complete s t u d y . As s a i d b e f o r e , o n l y a few s t u d i e s c o u l d be l o c a t e d i n t h i s s ea rch which i n c l u d e d a c t u a l measurement of s o i l l o s s and c r o p y i e l d s . Lamb et a l (1950) s t u d i e d e r o s i o n p l o t s l o c a t e d on four s o i l s i n the N o r t h e a s t e r n U . S . . Some p l o t s were i n f a l l o w s o i l , some on s o i l wi th sma l l g r a i n c rops for a p e r i o d of 5-10 y e a r s . A f t e r enough d i f f e r e n t i a l e r o s i o n was p r o d u c e d , maize was c u l t i v a t e d for a c o u p l e of y e a r s . Manure, and/or NPK was u n i f o r m i l y p r o v i d e d to a l l p l o t s . Honeoye g r a v e l l y s i l t loam y i e l d e d an average of 4.1 t h a " 1 y r " 1 of g r a i n i n the s l i g h t l y eroded ( i . e . , 5 cm) p l o t s . T h i s i s a decrease of 186 kg h a " 1 62 y r - 1 of g r a i n per cm of s o i l l o s s . A deep Lords t rom laggy s i l t loam l o s t from almost n o t h i n g up to 2.6 cm of s o i l depth and y i e l d e d between 4.9 and 1.6 t h a " 1 y r - 1 , r e s p e c t i v e l y , w i t h a d e c l i n e i n maize y i e l d of 1.27 t h a " 1 y r " 1 per cm of s o i l l o s s ! A p l o t tha t had supported u n f e r t i l i z e d c o n t i n u o u s maize for ten year s l o s t approx imate ly 0.9 cm, w h i l e another i n a ten year s r o t a t i o n of ma ize -oa t -c l o v e r l o s t 0.15 cm of s o i l d e p t h . Average maize y i e l d s for the next 2 year s of c u l t i v a t i o n wi th f e r t i l i z e r s were 1.6 and 2.5 t h a " 1 y r - 1 , r e s p e c t i v e l y , o r , a g a i n , a p p r o x i m a t e l y 1.2 t h a " 1 y r " 1 d e c l i n e i n maize y i e l d per cm of t o p s o i l l o s s . In the same s tudy , O n t a r i o sandy loam under f a l l o w for ten year s l o s t 3.8 cm of t o p s o i l and y i e l d e d 3.2 t h a " 1 y r " 1 of ma ize . When sodded, O n t a r i o sandy loam l o s t almost n o t h i n g i n those 10 year s and y i e l d e d 6.1 t h a " 1 y r " 1 . T h i s i s a maize y i e l d d e c l i n e of 600 kg h a " 1 y r " 1 per cm of t o p s o i l l o s s . F i n a l l y , Durham s i l t y c l a y loam l o s t 0.6 cm of t o p s o i l i n 10 year s of a r o t a t i o n w i t h v e g e t a b l e s and l a t e r y i e l d e d 5.2 t h a " 1 y r " 1 of m a i z e . Under f a l l o w , p r e v i o u s s o i l l o s s e s amounted to 6.6 cm and y i e l d s of maize were 3.4 t h a " 1 y r " 1 , or 300 kg h a " 1 y r " 1 l e s s per cm of s o i l l o s s . Lamb et a l (1950) o f f e r e d l i t t l e a d d i t i o n a l d i s c u s s i o n for such a g i g a n t i c e x p e r i m e n t a l e n t e r p r i s e . I t i s c l e a r , however, . that the r a t i o s of p r o d u c t i v i t y d e c l i n e caused by s o i l l o s s are i m p r e s s i v e and v a r y i n g w i t h s o i l t y p e . A l t h o u g h no data on the i n t e r a c t i o n between s o i l s and f e r t i l i z e r was p r o v i d e d , i t seems a l s o c l e a r tha t rea sonab le amounts of f e r t i l i z e r s d i d not 63 compensate f o r the l o s s e s due to e r o s i o n . A s i m i l a r long range study s t i l l going on i n Canada seems to r e f u t e a l l the p r e v i o u s f i n d i n g s (Ketcheson and Webber, 1978). E r o s i o n p l o t s on Guelph loam (Typic H a p l u d a l f ) have been monitored from 1953 to present, under a v a r i e t y of crop r o t a t i o n and s o i l c o n s e r v a t i o n p r a c t i c e s . Between 1953 and 1962 ( i . e . , Ketcheson and Webber, 1978, t a b l e 1) p l o t s which l o s t a t o t a l of 1.25 cm of t o p s o i l y i e l d e d 4.1 t ha" 1 y r - 1 of maize. P l o t s which l o s t a t o t a l of 0.12 cm of t o p s o i l y i e l d e d 4.5 t ha" 1 y r " 1 of maize. T h i s i s e q u i v a l e n t to a 355 kg ha" 1 y r " 1 d e c l i n e i n maize y i e l d per cm of t o p s o i l l o s s . Between 1971 and 1976 ( i . e., Ketcheson and. Webber1-978 , t a b l e 2), a v a r i e t y of manures, p l a n t r e s i d u e s , and t i l l a g e p r a c t i c e s were t r i e d . A l l p l o t s r e c e i v e d 168 kg ha' 1 y r " 1 of N , and P and K a c c o r d i n g to t h e i r needs. A c o n t r o l p l o t (unplowed, unmanured, with r e s i d u e s removed) l o s t a t o t a l of 1.2 cm of t o p s o i l and y i e l d e d an average of 2.8 t ha" 1 y r " 1 of maize. Manured p l o t s l o s t 1.4 cm and y i e l d e d 3.6 t ha" 1 y r " 1 . Manured and plowed p l o t s l o s t 2.2 cm and y i e l d e d 4.8 t ha" 1 y r " 1 . Plowed p l o t s with manure and p l a n t r e s i d u e s l o s t 1.4 cm and y i e l d e d 4.6 t ha" 1 y r ' 1 . F i n a l l y , unplowed p l o t s with manure and abundant p l a n t residue l o s t only 0.06 cm and y i e l d e d 4.4 t ha" 1 y r " 1 . O b v i o u s l y , the e f f e c t s of e r o s i o n on p r o d u c t i v i t y were overcome through c a r e f u l s o i l management, f o r the most eroded p l o t had the b e t t e r y i e l d . J u s t r e c e n t l y , Ketcheson and Stonehouse (1983) repo r t e d that maize y i e l d s in 1981-1982 stood at a f l a t 6 t ha" 1 y r " 1 over a 64 range of 4.1 cm of t o p s o i l l o s s , which was o b t a i n e d through twenty-seven year s of e x p e r i m e n t a t i o n . Three b r i e f o b s e r v a t i o n s can be made about t h i s s tudy : a) the r a t e s of measured e r o s i o n are p r o b a b l y normal i n G u e l p h , but they are low when compared w i t h o ther s t u d i e s reviewed h e r e ; b) Gue lph loam must be an extremely good s o i l ; and c) s o i l , p l a n t , and f e r t i l i t y management, a l t h o u g h not r e p o r t e d i n d e t a i l , appears to have been extremely s o p h i s t i c a t e d . F a y e t t e s i l t loam i s a p p a r e n t l y a good s o i l a l s o . Hays et a l (1948) t r i e d a s i m i l a r exper iment on t h i s s o i l . S ix year s of e r o s i o n measurements r e v e a l e d a l o s s of 16 cm of t o p s o i l under f a l l o w and c o n t i n o u s maize , and 2.5 cm w i t h a r o t a t i o n of sma l l g r a i n c r o p s . Manure was a p p l i e d u n i f o r m l y to a l l p l o t s , but l i m e , P, and K were s u p p l i e d a c c o r d i n g to p l o t needs ( a l though the au thor s suggested that a p p l i c a t i o n s to the more eroded p l o t s were on the average twelve per cent g r e a t e r ) . C o n s e q u e n t l y , maize was c u l t i v a t e d i n r o t a t i o n and nine year s of maize y i e l d data were o b t a i n e d . The average y i e l d for the severe and modera te ly eroded p l o t s was 4.9 t h a - 1 y r - 1 , which works to 67 kg h a " 1 y r ' 1 of maize y i e l d d e c l i n e per cm of t o p s o i l l o s s . However, the au thor s p o i n t e d out that by the end of the exper iment , the y i e l d s of both p l o t s were much a l i k e , i m p l y i n g tha t the s o i l s c o u l d be r e c l a i m e d through c a r e f u l management. Another method employed to st.udy the e f f e c t s of e r o s i o n on l a n d p r o d u c t i v i t y i s to s i m u l a t e e r o s i o n by c u t s , i . e . , by p h y s i c a l l y removing chunks of t o p s o i l . T h i s method has he lped to speed up r e s e a r c h i n t h i s area because i t i s s i m p l e , and 65 because i t a l l o w s e x p e r i m e n t a l c o n t r o l of the amount of e r o s i o n . R e c e n t l y , Langdale and Shrader (1982) summarized most of these s t u d i e s for the U . S . . Tab le I I I .1 i n c l u d e s these a u t h o r s ' c o m p i l a t i o n of data p l u s some o t h e r s here found for t r o p i c a l s o i l s . A l l exper iments r e p o r t e d i n t a b l e I I I .1 i n c l u d e d f e r t i l i z a t i o n to remove p o t e n t i a l d e f i c i e n c i e s from the e r o s i o n t r e a t m e n t . A rough c l a s s i f i c a t i o n of these data i n t o "good" i . e . , deep, temperate s o i l s and " p o o r " , i . e . , sha l low and/or t r o p i c a l s o i l s shows an i n t e r e s t i n g c o n t r a s t i n the responses of maize y i e l d s to e r o s i o n . T o p s o i l removal i n the "good" s o i l s r e s u l t e d i n maize y i e l d r e d u c t i o n s from between 10 and 30 per c e n t . T o p s o i l removal i n the " p o o r " s o i l s r e s u l t e d i n g r e a t e r maize y i e l d s r e d u c t i o n s , from 30 to 70 per c e n t . The ev idence reviewed i n t h i s s e c t i o n i n d i c a t e s that the impact of s o i l e r o s i o n on maize p r o d u c t i v i t y i s a f u n c t i o n of the q u a l i t y of s o i l and l a n d management p r a c t i c e s : the poorer the s o i l , the g r e a t e r the impact ; the b e t t e r the management, the l e s s maize y i e l d s w i l l s u f f e r from e r o s i o n . 111 .4 . S o i l F e r t i l i t y D e p l e t i o n through Cont inuous C u l t i v a t i o n S e c t i o n I I . 2 s t a t e d that m i l p a a g r i c u l t u r e has been the t r a d i t i o n a l system of maize c u l t i v a t i o n i n the t r o p i c s of M e x i c o . I t has been long r e c o g n i z e d that a c r i t i c a l f a c t o r fo r the s u r v i v a l of t h i s s h i f t i n g c u l t i v a t i o n system i s the a v a i l a b i l i t y of l and to r o t a t e f i e l d s w i t h second growth v e g e t a t i o n (Cook, 1921). F i g u r e 2.1b showed that the a rea 66 T a b l e 111 . 1 E s t i m a t e d maize y i e l d r e d u c t i o n i n t o p s o i l removal exper iments S o i l Type % Y i e l d Or i g i n a l ( C l a s s i f i c a t i o n ) Decrease Reference Deep, Medium, Temperate S o i l s Memphis s i l t loam, (Typ ic H a p l u d a l f ) 9 (Bunt ley & B e l l 1976) Marsha l s i l t loam, (Typ ic H a p l u d o l l ) 13-17 ( E n g e l s t a d et a l . 1961) Beadle s i l t c l a y loam, ( T y p i c A r g i u s t o l l ) 1 7 ( O l s o n , 1977) Gardena sandy loam, (Pachic Ud ic H a p l o b o r o l l ) 19 ( C a r l s o n et a l . 1961) Ida s i l t loam, ( T y p i c Udor thent ) 8-30 (Spomer et a l . 1973) Grenada s i l t loam, ( G l o s s i c F r a g i u d a l f ) 26 (Bunt ley & B e l l 1976) S h a l l o w , Medium to C o a r s e , T r o p i c a l S o i l s D u r i a n M a l a y s i a , ( A l l u v i u m ) 27 (Siew and F a t t , 1976) G r o s e c l o s e c l a y loam, ( T y p i c H a p l u d u l t ) 36 ( B a t c h e l d e r & Jones , 1972) C e c i l sandy c l a y , (Typ ic H a p l u d u l t ) 40 (Langdale et a l . 1979) Egbeda s i l t c l a y loam, ( T y p i c P a l e u s t a l f ) 41 ( L a i , 1976) Brandon s i l t loam, {Typic H a p l u d u l t ) 44 (Bunt ley et a l . 1976) Sedang M a l a y s i a ( C o l l u v i u m ) 70 (Huat, 1974) S o u r c e : adapted from Langdale and Shrader (1982) d e d i c a t e d to maize i n Mexico has not grown d u r i n g the l a s t twenty y e a r s . T h i s suggests that m i l p a a g r i c u l t u r e must be g i v i n g p l a c e to a more c o n t i n u o u s form of maize c u l t i v a t i o n . 67 Wat te r s (1971) d e d i c a t e d a chapter to s h i f t i n g c u l t i v a t i o n i n Mexico and i n c l u d e d a s p e c i a l s e c t i o n on m i l p a a g r i c u l t u r e i n V e r a c r u z . T h i s author r e p o r t e d t h a t the young v o l c a n i c s o i l s near San Andres T u x t l a might not need f a l l o w i n g i f p lowing and /or f e r t i l i z e r s are used from time to t i m e . Maize y i e l d s d e c l i n e d o n l y a f t e r l ong p e r i o d s of c o n t i n u o u s c r o p p i n g . However, nearby L a t o s o l s had more problems i n s u s t a i n i n g c o n t i n u o u s c u l t i v a t i o n w i t h ma ize . The l a t t e r s o i l s were c u l t i v a t e d w i t h maize fo r about four to f i v e yea r s and then abandoned for two to t h r e e y e a r s . D e c l i n i n g s o i l f e r t i l i t y , i . e . , " the s o i l was t i r e d " , was the common e x p l a n a t i o n p r o v i d e d by l o c a l peasants as the reason for abandoning t h e i r f i e l d s . As p r o o f , Watters (1971) c i t e d r e s u l t s from f e r t i l i z e r t r i a l s which i n d i c a t e d t h a t n i t r o g e n and phosphorus had a s i g n i f i c a n t e f f e c t i n r a i s i n g the p r o d u c t i v i t y of the maize f i e l d s . A s i g n i f i c a n t response of maize to n i t r o g e n and phosphorus has been more r e c e n t l y c o n f i r m e d fo r the s t a t e of V e r a c r u z as a whole ( C e b a l l o s P i e d r a , 1980), as w e l l as for the Xa lapa r e g i o n i n p a r t i c u l a r (Marten and S a n c h o l u z , 1981; A g u i l a r Acuna , 1981). D e s c r i b i n g mi lpa a g r i c u l t u r e i n the p e n i n s u l a of Yuca tan , Wat ter s (1971) r e f e r r e d to the Rendzina s o i l s c h a r a c t e r i s t i c of tha t r e g i o n of M e x i c o . He s t a t e d t h a t these c a l c a r e o u s s o i l s might have a h i g h e r f e r t i l i t y l e v e l than p r e v i o u s l y b e l i e v e d , but a l s o tha t y i e l d s of maize d e c l i n e d wi th time of c u l t i v a t i o n u n l e s s the f i e l d s were f a l l o w e d and b u r n t , or a p p l i c a t i o n s of n i t r o g e n and potass ium were made. 68 Sanchez (1977, Ch.3) reviewed a number of s t u d i e s on f o r e s t -s o i l n u t r i e n t c y c l e s i n C e n t r a l America and the C a r i b b e a n . Second growth f o r e s t s accumulated n u t r i e n t s and biomass at a f a s t e r r a t e than crops i n the same s o i l . Most n u t r i e n t s were r e t a i n e d w i t h i n the l i v i n g p a r t s of the f o r e s t , and o n l y s m a l l f r a c t i o n s of these n u t r i e n t s were s t o r e d in the s o i l . A l t h o u g h l i t t e r p r o d u c t i o n and decompos i t ion was h i g h , the n u t r i e n t s r e l e a s e d through t h i s mechanism were r a p i d l y taken up by the r o o t s of the f a s t growing v e g e t a t i o n . The b u r n i n g of these f o r e s t s d u r i n g s h i f t i n g c u l t i v a t i o n p r a c t i c e s produced s e v e r a l e f f e c t s on s o i l p r o p e r t i e s . A h i g h amount of bases was r e l e a s e d from the a shes . These bases r a i s e d the s o i l pH and t h i s c o n t r i b u t e d to n e u t r a l i z i n g the s o i l s ' r e a c t i o n which i s t y p i c a l l y a c i d i c . Whi le o rgan ic matter and n i t r o g e n were e a s i l y v o l a t i l i z e d d u r i n g b u r n i n g , l o s s e s were consp icuous on ly on the topmost s u r f a c e of the s o i l . Phosphorus c o n c e n t r a t i o n s have been shown to i n c r e a s e i n the s o i l a f t e r b u r n i n g . S ta rk (1978) c o n s i d e r e d that the c r u c i a l f a c t o r i n t r o p i c a l s h i f t i n g c u l t i v a t i o n i s the management of f i r e . I f f i r e s are c a r e f u l l y p l a n n e d , an e q u i l i b r i u m between n u t r i e n t l o s s e s and n u t r i e n t a d d i t i o n s can be o b t a i n e d . T h i s wise use of f i r e , the author c o n c l u d e d , c o u l d l e a d to good c r o p p r o d u c t i o n and sound l a n d u se . But l o c a l people shou ld know about t h i s , for they have used f i r e for a very long t i m e . The q u e s t i o n remains : What i f there i s not enough l a n d to keep t h i s o therwi se e f f i c i e n t m i l p a system a l i v e ? Answers to t h i s q u e s t i o n are not s i m p l e , but a number of exper t s have 69 p r o v i d e d some re sponse s . Agboola and Fayemi (1972) and Arnason et a l (1982) proposed c a r e f u l s e l e c t i o n of c r o p r o t a t i o n s and p l a n t s p e c i e s to r e b u i l d s o i l f e r t i l i t y i n c o n t i n u o u s l y c u l t i v a t e d f i e l d s . They recommended the study of legumes and P - c o n c e n t r a t i n g s p e c i e s which c o u l d supply t r o p i c a l s o i l s w i t h bad ly needed n i t r o g e n and phosphorus . In a r a t h e r d i f f e r e n t v e i n , B l e v i n s et a l (1977) and A i n a (1979) recomended minimum t i l l a g e p r a c t i c e s , not o n l y to p r o t e c t the s o i l ' s p h y s i c a l p r o p e r t i e s but a l s o to b u i l d up the o r g a n i c m a t t e r , and thus the n u t r i e n t s u p p l y i n g c a p a c i t y of t r o p i c a l s o i l s . F i n a l l y , Sanchez et a l (1982) proposed the i n t e n s i f i c a t i o n of a g r i c u l t u r e i n the best s o i l s of the t r o p i c s t o r e l e a s e p r e s s u r e from the m a r g i n a l l a n d s . They reached t h i s c o n c l u s i o n from s u c c e s s f u l exper iments tha t s i g n i f i c a n t l y r a i s e d c r o p p r o d u c t i v i t y . These exper iments i n c l u d e d h i g h dosages of f e r t i l i z e r and l i m e , improved c r o p v a r i e t i e s p l a n t e d i n d i f f e r e n t r o t a t i o n s , good r a i n f a l l or i r r i g a t i o n , and f l a t , deep, and w e l l aggregated t r o p i c a l s o i l s . The au thor s p r o p e r l y c a u t i o n e d t h e i r readers that to a c h i e v e s i m i l a r r e s u l t s e l s ewhere , not o n l y must the b i o p h y s i c a l c o n s t r a i n t s be removed, but a proper economic and i n s t i t u t i o n a l environment needs to be c r e a t e d to promote the a d o p t i o n of these a g r i c u l t u r a l i n n o v a t i o n s among sma l l f a rmers . 70 111.5 . Summary Land d e g r a d a t i o n has been d e f i n e d as the r e s u l t of s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n . The mechanics of both these p r o c e s s e s have been r e v i e w e d . C o n t i n u o u s l y c u l t i v a t e d t r o p i c a l s o i l s are s u b j e c t e d to h i g h r i s k s of s o i l e r o s i o n and to s o i l f e r t i l i t y s t r e s s e s . L i t e r a t u r e data suggest t h a t s o i l l o s s e s on maize f i e l d s i n the t r o p i c s c o u l d range from 10 to 20 t h a " 1 y r " 1 . In M e x i c o , a few s t u d i e s conducted on g e n t l y s l o p i n g s i t e s i n the c o o l -temperate c l i m a t i c zone measured much s m a l l e r s o i l l o s s e s . The e f f e c t s of e r o s i o n on maize p r o d u c t i v i t y have been s t u d i e d m a i n l y in the U . S . . T o p s o i l removal exper iments i n deep temperate s o i l s i n d i c a t e t h a t y i e l d s c o u l d d e c l i n e from 9 to 30 per c e n t . T o p s o i l removal i n sha l low t r o p i c a l s o i l s produced maize y i e l d d e c l i n e s from between 30 and 70 per c e n t . A few complete s t u d i e s conducted under e x c e l l e n t s o i l and management c o n d i t i o n s suggest that c a r e f u l s o i l f e r t i l i t y management can compensate for the d e c l i n e i n maize y i e l d brought about by e r o s i o n . S o i l f e r t i l i t y d e p l e t i o n i s a p e r v a s i v e f a c t o r i n the t r o p i c s . Few s t u d i e s have documented the magnitude of the c r o p l o s s e s i n c u r r e d through c o n t i n u o u s c u l t i v a t i o n , but i t i s g e n e r a l l y a c c e p t e d that t r o p i c a l s o i l s cannot s u s t a i n c r o p p r o d u c t i v i t y i n d e f i n i t e l y , u n l e s s i n t e n s i v e l a n d management p r a c t i c e s are adopted , or the l a n d i s r e v e r t e d to f a l l o w from time to t i m e . 71 CHAPTER I V : ENVIRONMENTAL FRAMEWORK FOR A CASE STUDY IN CENTRAL VERACRUZ I V . J_. I n t r o d u c t i o n Chapter II examined the a g r i c u l t u r a l p r o d u c t i o n system under s tudy : the Mexican maize f i e l d . Chapter III surveyed the key q u e s t i o n of t h i s t h e s i s : how s e r i o u s i s the t h r e a t of l a n d d e g r a d a t i o n i n those f i e l d s ? I t i s the task of the p re sen t chapter to show which e x p e r i m e n t a l c o n d i t i o n s were chosen for f i e l d t e s t s and how. The p o s s i b l e combina t ions of e c o l o g i c a l and manager i a l f a c t o r s of maize p r o d u c t i o n i n Mexico are so numerous that o n l y a case s t u d y c o u l d t a c k l e the problem i n a reasonable and p r o d u c t i v e way.. The Xa lapa r e g i o n , i n the c e n t e r of the S t a t e of V e r a c r u z , i s a sma l l e c o l o g i c a l model of M e x i c o . T h i s r e g i o n i n c l u d e s examples of most of the c l i m a t i c , v e g e t a t i o n a l and edaphic c o n d i t i o n s of the c o u n t r y . Maize i s no e x c e p t i o n and i t grows t h e r e from the coas t to the mountains i n a 4000 m a l t i t u d e range tha t s t a r t s i n the t r o p i c a l lowlands and c u l m i n a t e s i n the a r i d h i g h l a n d s . F a m i l i a r i t y w i t h t h i s area and i t s maize f i e l d s made t h i s author t h i n k that there he c o u l d f i n d the range of b i o p h y s i c a l c o n d i t i o n s r e q u i r e d to t e s t the c o n t e n t i o n s of t h i s s t u d y . The next s tep was to f u r t h e r s e l e c t , w i t h i n the Xa lapa r e g i o n , a subset of c o n d i t i o n s on which f i e l d exper iments c o u l d be r u n . Throughout t h i s a r e a , maize i s grown fo r s u b s i s t e n c e 72 purposes u s ing t r a d i t i o n a l t echn iques and t o o l s . T h e r e f o r e , management was c o n s i d e r e d to be u n i f o r m . C l i m a t e , p a r t i c u l a r l y r a i n f a l l , i s c r u c i a l i n d e t e r m i n i n g l a n d p r o d u c t i v i t y . Maize p r o d u c t i o n i n the r e g i o n has been shown to be extremely s e n s i t i v e to water s t r e s s (Marten and S a n c h o l u z , 1981). R a i n f a l l i s c h a r a c t e r i s t i c a l l y u n p r e d i c t a b l e . T h e r e f o r e i t c o u l d not be a n t i c i p a t e d when p l a n n i n g the f i e l d e x p e r i m e n t s . However, s i t e s w i t h d i f f e r e n t average r a i n f a l l and maize growing seasons are c o n s i d e r e d i n the s e l e c t i o n d e s c r i b e d below. Having set a s i d e management and c l i m a t e as e x p e r i m e n t a l pa rameter s , l o g i c a l l y the v a r i a b l e to s tudy was s o i l s . S o i l depth i s a pr imary f a c t o r i n d e t e r m i n i n g the volume of s o i l tha t i s a v a i l a b l e for p l a n t g rowth . Deep s o i l s s t o r e p r o p o r t i o n a l l y g r e a t e r amounts of n u t r i e n t s and water . Moreover , deep s o i l s a l l o w b e t t e r r o o t i n g and e a s i e r c u l t i v a t i o n than sha l low s o i l s . T h e r e f o r e , t h r e e s o i l s , c o v e r i n g a range of deep, medium, and sha l low p r o f i l e s , were chosen to i n c l u d e a range of i n h e r e n t l a n d p r o d u c t i v i t i e s . I V . 2 . G e o g r a p h i c a l L o c a t i o n The S ta te of V e r a c r u z l i e s between the o r i e n t a l mountain range ( S i e r r a Madre O r i e n t a l ) and the G u l f of Mexico (see f i g . 4 . 1 a ) . I t extends from 1 7 ° to 2 2 ° l a t i t u d e N o r t h . Among the mountains to the west of the S t a te i s the h i g h e s t peak i n Mexico ( C i t l a l t e p e t l , 5747 m). On the e a s t e r n s ide of t h i s l o n g i t u d i n a l l y e l o n g a t e d s t a t e l i e , a lmost u n i n t e r r u p t e d , the 73 F i g u r e 4 . 1 . L o c a t i o n of the study a r e a s . 4 .1a , map w i t h the Xa lapa r e g i o n i n c e n t r a l V e r a c r u z ; 4 . 1 b , i d e a l i z e d p r o f i l e c o r r e s p o n d to arrows i n f i g . 4 . 1 a . Curves numbers are s i t e s of Appendix 2. 74 (a) 20 40 60 Distance (km) 75 t r o p i c a l lowlands of M e x i c o . Because of these grea t l a t i t u d i n a l and a l t i t u d i n a l v a r i a t i o n s , the s t a t e of V e r a c r u z i s p r o b a b l y the most e c o l o g i c a l l y d i v e r s e of a l l the Mexican s t a t e s . G a r c i a (1970) has i d e n t i f i e d more than 12 d i f f e r e n t c l i m a t i c types i n the Koppen c l a s s i f i c a t i o n sys tem. Mean annual temperatures range from more than 26 ° C to l e s s than 2 ° C ; r a i n f a l l from l e s s than 800 mm to more than 4000 mm. The s o i l s of the s t a t e of V e r a c r u z , a l t h o u g h p o o r l y s t u d i e d , i n c l u d e the f o l l o w i n g o r d e r s of the FAO-UNESCO sys tem: L a t o s o l s , V e r t i s o l s , A n d o s o l s , P l a n o s o l s , M o l l i s o l s , and L u v i s o l s (SARH,1972). There are twenty-two great types of v e g e t a t i o n , from t r o p i c a l r a i n f o r e s t s , i n the southern l o w l a n d s , to t u n d r a - l i k e g r a s s l a n d s , i n the h i g h e s t mountain peaks (Gomez Pompa, 1973). A g r i c u l t u r e i s a l s o d i v e r s e . More than seventy p l a n t c r o p s , as v a r i e d as wheat and mangoes, compete for the use of the l a n d w i t h c a t t l e and f o r e s t r y o p e r a t i o n s (Marten and S a n c h o l u z , 1 9 7 7 ) . M a i z e , however, i s the most common c r o p i n V e r a c r u z , and , as shown i n Chapter I I , t h i s s t a t e o c c u p i e s second p l a c e i n the t o t a l n a t i o n a l p r o d u c t i o n of ma ize . F i g u r e 4 .1a shows the l o c a t i o n of t h i s s t u d y . A l a n d use survey of c e n t r a l V e r a c r u z found t h e r e most of the a g r i c u l t u r a l , v e g e t a t i o h a l , e d a p h i c , and c l i m a t i c v a r i a n t s of the s t a t e of V e r a c r u z (Marten and S a n c h o l u z , 1982). F i f t y - s i x l a n d use systems and t h i r t y three l a n d types were d e s c r i b e d on the b a s i s of c l i m a t e , s o i l s and l a n d forms . D u r i n g 1977, the same study surveyed more than 300 maize f i e l d s (Marten and S a n c h o l u z , 76 1981). These s t u d i e s , i n c e n t r a l V e r a c r u z , p r o v i d e d a framework from which to s e l e c t more d e t a i l e d s i t e s for the pre sent s t u d y . I V . 3 . S o i l s The s o i l s to be d i s c u s s e d here were chosen as c o n t r a s t i n g examples of the v a r i e t y of s o i l s i n the c e n t e r of V e r a c r u z . They i n c l u d e : a) deep loam Andoso l s i n the h i l l s s u r r o u n d i n g the c i t y of X a l a p a ; b) sha l low sandy loam Tepe ta te s (a common name i n Mexico meaning s o i l s w i t h an u n d e r l y i n g hardpan) i n the r o l l i n g l andscape of E l C h i c o - M i r a d o r e s ; and c) medium depth s toney , c l a y e y C a l i c h e s ( r e n d z i n a s ) on the g e n t l y u n d u l a t i n g o l d marine t e r r a c e s around C a r r i z a l and Chauapan. The s p a t i a l v a r i a t i o n of s o i l s i s a w e l l known p r o c e s s , p a r t i c u l a r l y when r e l i e f and c l i m a t e vary a b r u p t l y . A l l the three s o i l s of t h i s s tudy can be found w i t h i n 40 km, but a lmost 1000 m of a l t i t u d e separa te s the Andoso l s from the C a l i c h e s . W i t h i n t h i s 40 km, the c l i m a t e ranges from humid warm temperate to subhumid t r o p i c a l . G iven the env i ronmenta l v a r i a t i o n of t h i s r a t h e r sma l l a r e a , i t was d e c i d e d to s tudy a t o t a l of e i g h t s o i l p r o f i l e s , to i n c l u d e not o n l y the main s o i l types but the p r i n c i p a l v a r i a n t s as w e l l . F i g u r e 4.1b shows the r e l a t i v e p o s i t i o n of the e i g h t s o i l s i t e s i n the r e g i o n a l l a n d s c a p e . S i t e s 2, 4 and 7 are the modal s i t e s fo r the A n d o s o l , Tepeta te and C a l i c h e r e s p e c t i v e l y . S i t e s 1 ,5 ,6 , and 8 are t o p o g r a p h i c a l v a r i a n t s w i t h i n these s o i l s , w h i l e s i t e 3 r e p r e s e n t s a c l i m a t i c t r a n s i t i o n , between the 77 Ando so l s and the T e p e t a t e s . S o i l p r o f i l e s of a l l e i g h t s i t e s were d e s c r i b e d i n the f i e l d . D e t a i l e d l a b o r a t o r y a n a l y s e s were conducted on samples of the modal s i t e s and fewer a n a l y s e s were performed on samples from the v a r i a n t s ( s i t e s 1 , 3 , 5 , 6 , and 8 ) . The methods of a n a l y s i s and the t a b u l a t e d r e s u l t s are i n c l u d e d i n Appendix 2. The f o l l o w i n g pages d i s c u s s the p r i n c i p a l p h y s i c a l and c h e m i c a l p r o p e r t i e s of the three s o i l t y p e s . The n o n - s p e c i a l i s t reader w i l l f i n d i n these pages s u f f i c i e n t i n f o r m a t i o n about the s o i l s of t h i s s t u d y . IV.3_.J[. Andoso l s The h i l l s around the c i t y of Xa lapa have c h a r a c t e r i s t i c a l l y deep brown s o i l s w i t h r e d - y e l l o w s u b s o i l s . They s i t on huge d e p o s i t s of v o l c a n i c ash and c i n d e r s of c o n t r a s t i n g white c o l o r , which are g e o l o g i c a l l y r ecent ( P l e i s t o c e n e ) . T h i s u n d e r l y i n g white m a t e r i a l i s a c t u a l l y sand w h i c h , as e v i d e n c e d by a number of a c t i v e q u a r r i e s in the a r e a , i s used for c o n s t r u c t i o n p u r p o s e s . L o o k i n g at the exposures of these e x c a v a t i o n s , one can see , from top to bottom, a brown l a y e r 20-80 cm deep, f o l l o w e d by 1.5 to 2 m of y e l l o w - r e d e a r t h , and 30 to 50 m o.f white s and . The topmost 2 m of t h i s m e g a - p r o f i l e are the Andoso l s d i s c u s s e d i n t h i s t h e s i s . Abundant r a i n , warm tempera tures , l u x u r i o u s v e g e t a t i o n , and the s o f t white rock have combined to produce a c o n s i d e r a b l e amount of s o i l i n a r e l a t i v e l y short p e r i o d of t i m e . 78 The l andscape i s made of h i l l s and v o l c a n o e s . The landforms on which the Andoso l s are formed i n c l u d e h i l l s l o p e s , h i l l tops and h i l l bottoms and cover about 20 per cent of the l a n d area i n c e n t r a l V e r a c r u z (Table I V . 1 ) . Topography i s rough: an average s lope measures c l o s e to 30 per cent i n c l i n a t i o n . T a b l e IV.J_. E x t e n s i o n , a l t i t u d e , and s lope s of the s o i l s i n t h i s s t u d y . S o i l Type Area (ha) A l t i tude (m) Weighed Avge. — slope- ..(.%.) - -Andoso l s 88800 1250-1600 29.9 T e p e t a t e s 21 200 800-1200 8.7 C a l i c h e s 25200 300-600 3.8 T o t a l area Xalapa r e g i o n ( c e n t r a l V e r a c r u z ) =475,000ha. S o u r c e : Marten & Sancholuz (1981, t a b l e I V ) . The p r o f i l e s of the Andoso l s c h a r a c t e r i s t i c a l l y i n c l u d e a topmost dark brown o r g a n i c l a y e r . T h i s v a r i e s i n depth a c c o r d i n g to topography and past l a n d use , but ranges from 30 to 80 cm. S i t e s 1 and 2 (see Appendix 2, p r o f i l e d e s c r i p t i o n s ) show these extremes q u i t e w e l l . Below t h i s r i c h humus l a y e r , the s o i l t u r n s to a y e l l o w - r e d c o l o r . T h i s l a y e r i s much more s t r u c t u r e d , compacted, and l e s s i n h a b i t e d by roo t s and fauna a l i k e . There are minute ox ide c o n c r e t i o n s , but no s tones or 79 g r a v e l . T h i s second l a y e r can go as deep as 2.0 m , but as i t goes f u r t h e r down i t becomes much more compacted and t h e r e are no s i g n s of p l a n t r o o t s r e a c h i n g these d e p t h s . There i s a d i s t i n c t boundary between the weathered m a t e r i a l and the parent m a t e r i a l w h i c h , be ing w h i t e , s h a r p l y c o n t r a s t s wi th the s u b s o i l s above i t . Another common c h a r a c t e r i s t i c of these s o i l s i s the almost complete absence of coar se fragments throughout the p r o f i l e . Sur face t e x t u r e s are i n v a r i a b l e c o a r s e r than those of the s u b s o i l s . L o c a l peasants c a l l the eroded s i t e s " b a r r o " (mud), and the uneroded ones " t i e r r a de grano" ( g r a n u l a r e a r t h ) . P a r t i c l e s i z e a n a l y s i s of these s o i l s may be d i f f i c u l t to i n t e r p r e t (Sanchez, 1976). S tandard t e x t u r a l d e t e r m i n a t i o n s wi thout p r e v i o u s removal of Fe203 y i e l d c o n s i d e r a b l y h i g h e r sand c o n t e n t s and much l e s s c l a y than d e t e r m i n a t i o n s made a f t e r the removal of o x i d e s . As noted on the p r o f i l e d e s c r i p t i o n s of the s o i l s , i r o n ox ides c o n c r e t i o n s are a common fea ture and they c o u l d account for the c o a r s e r t e x t u r e s o c c u r r i n g when they are p r e s e n t . As shown in Appendix 2, these s o i l s have very low bulk d e n s i t i e s ( 0 . 7 - 0 . 9 g c m " 3 ) throughout the p r o f i l e . Most p r o b a b l y , t h i s i s due to the presence of a l l o p h a n e , which i s known to g i v e v o l c a n i c s o i l s a c h a r a c t e r i s t i c l i g h t n e s s . The h i g h o r g a n i c matter content i n the t o p s o i l s c e r t a i n l y adds to t h i s l i g h t n e s s , but a lone i t cannot e x p l a i n the c o n s i s t e n t l y low bulk d e n s i t i e s of the s u b s o i l s , where o r g a n i c matter i s much d e c r e a s e d . On a weight b a s i s , water c o n t e n t s at 1/3 bar 80 p r e s s u r e are h i g h , and w h i l e they a r e , as expec ted , lower at 15 b a r , the d i f f e r e n c e i s not g r e a t . The Andoso l s are d e f i n i t e l y a c i d i c s o i l s . The pH ranges from 5 to 5.7 throughout the p r o f i l e and a c r o s s s i t e s . Organ ic matter v a r i e s from 10 per cent i n the sur f ace to 1 per cent at 1m d e p t h . Phosphorus i s i n shor t s u p p l y , and t h i s i s so r e g a r d l e s s of the method of e x t r a c t i o n . A c i d i t y may be r e s p o n s i b l e for the f i x a t i o n of phosphorus i n these s o i l s , but the h i g h c o n c e n t r a t i o n s of i r o n and a luminium ox ide c o u l d a l s o be c l o s e l y l i n k e d to t h i s n o t o r i o u s d e f i c i e n c y . T o t a l n i t r o g e n c o n t e n t i s r e l a t i v e l y h i g h and f o l l o w s a d e c r e a s i n g p a t t e r n w i t h depth as shown by o r g a n i c m a t t e r . Base s a t u r a t i o n i s about 20 per c e n t , which i n d i c a t e s a low c a t i o n exchange c a p a c i t y and p r o b a b l e n u t r i t i o n a l d e f i c i e n c i e s i n these s o i l s . IV.3.2_. Tepe ta te s A few k i l o m e t e r s east of X a l a p a , the h i l l y l andscape becomes g e n t l e r . A l t i t u d e drops s l o w l y but s t e a d i l y , and the mois t and warm weather changes i n t o a d r i e r and h o t t e r one. I t i s a r o l l i n g l andscape , d i s s e c t e d o n l y by deep canyons c a r r y i n g the excess water from the mountains i n t o the sea . The mesas between the canyons have c h a r a c t e r i s t i c s o i l s , the T e p e t a t e s . These s o i l s are much s h a l l o w e r (40-70cm) and p a l e r i n c o l o r than the A n d o s o l s . The u n d e r l y i n g rocks are s t i l l of v o l c a n i c o r i g i n , but i n s t e a d of forming p i l e s of ashes they have hardened i n t o t h i c k l a y e r s of impermeable rocks ( T e p e t a t e s , p r o p e r ) . C u r i o u s l y , another use has been found for t h i s m a t e r i a l , as 81 wi tne s sed by a number of a r t i f i c i a l lagoons r e s u l t i n g from the e x c a v a t i o n s of nearby b r i c k f a c t o r i e s . The n a t u r a l v e g e t a t i o n i s a l s o i n d i c a t i v e of changes : i t i s s c rub dominated by s p e c i e s of oak and i n t e r r u p t e d by patches of g r a s s l a n d . Les s water , warmer tempera tures , and a sparse v e g e t a t i o n are r e s p o n s i b l e for the meagerness of the Tepeta te s o i I s . In t h i s r o l l i n g l andscape , r a v i n e s are s teep (10-25 per cent i n c l i n a t i o n ) , but the s o i l s there b e n e f i t from a l l u v i a l d e p o s i t s which make them deeper and r i c h e r in n u t r i e n t s . I t i s worth n o t i n g that ma in ly co f f ee i s grown i n the r a v i n e s , w h i l e g r a s s l a n d s and maize f i e l d s are found on the c r e s t s . Patches of bare rock on these c r e s t s are an o u t s t a n d i n g f e a t u r e of the l a n d s c a p e . L o c a l people r e c a l l hav ing p a r t s of t h e i r f i e l d s washed away by heavy r a i n s t o r m s . T h i s w i l l be d i s c u s s e d l a t e r , but i t takes l i t t l e i m a g i n a t i o n to see that a sha l low s o i l , s i t t i n g on hard r o c k , can e a s i l y become s a t u r a t e d w i t h water and s l i d e down the s l o p e , even on g e n t l e s l o p e s . The p r o f i l e s of the Tepeta te s o i l s i n c l u d e a t h i n brown o r g a n i c l a y e r . T h i s ranges from 5 to 25 cm i n depth and i t i s w e l l r e p r e s e n t e d by s i t e s 4 and 5 (Appendix 2 ) . The o r g a n i c matter conten t of t h i s h o r i z o n i s 3-4 per c e n t . Below the r e l a t i v e l y dark t o p s o i l there i s e i t h e r a r e d - y e l l o w or a pa le brown h o r i z o n w i t h l e s s o r g a n i c matter and f i n e r t e x t u r e s . I t i s not unusual t o f i n d gray m o t t l e s and i r o n c o n c r e t i o n s . Somewhere between 30 to 40 cm, the s o i l becomes much harder and compacted, and i t g i v e s l i t t l e i n d i c a t i o n of l i f e i n the form of 82 roo t s or f auna . F i n a l l y , at 40-50cm the s o i l has become a so f t rock which hardens f u r t h e r down the p r o f i l e to become the T e p e t a t e . The t o p s o i l s of the Tepe ta te s are s and ie r and l i g h t e r than the s u b s o i l s . Bulk d e n s i t i e s i n c r e a s e w i t h depth and so does the c l a y c o n t e n t . M o i s t u r e r e t e n t i o n t e s t s show very l i t t l e water between 1/3 and 15 bar p r e s s u r e s . I n f i l t r a t i o n i s d e f i n i t e l y impeded at the bottom of the s o i l as shown by the abundant m o t t l i n g ; i t p r a c t i c a l l y s tops at the d u r i p a n and e v e n t u a l l y forms a perched water t a b l e when r a i n s are abundant . Given these p h y s i c a l p r o p e r t i e s , the T e p e t a t e s are not good a g r i c u l t u r a l s o i l s . The Tepe ta te s e x h i b i t c h e m i c a l l i m i t a t i o n s as w e l l . A c i d c o n d i t i o n s are s t i l l p r e v a l e n t . Organ ic carbon i s at the l i m i t of adequacy. N i t r o g e n a l s o seems to be in shor t s u p p l y . Phosphorus p robab ly gets f i x e d by the h i g h c o n t e n t s of i r o n ox ides and the a c i d i c medium. Base s a t u r a t i o n i s not too bad , p o s s i b l y because the bases cannot be l e a c h e d i n t o the hardpan . However, c a t i o n exchange c a p a c i t i e s are the lowest for the t h r e e groups of s o i l s s t u d i e d . A g a i n , the s torage of n u t r i e n t s and the c a p a c i t y to t r a n s f e r them are l i m i t i n g f a c t o r s i n the T e p e t a t e s . I V . 3 . 3 . C a l i c h e s C o n t i n u i n g the descent to the seashore from X a l a p a , one passes from the h i l l s to a l andscape c h a r a c t e r i z e d by a s e r i e s of r e l a t i v e l y f l a t marine d e p o s i t s . The rocks are l i m e s t o n e s 83 d a t i n g back to the P l i o c e n e age. The c l i m a t e shows marked s e a s o n a l changes , wi th r a i n f a l l c o n c e n t r a t e d i n t o 5-6 months of the y e a r . Temperatures are always h i g h . The n a t u r a l v e g e t a t i o n i s a low dec iduous t r o p i c a l f o r e s t . The s o i l s have changed d r a m a t i c a l l y . They are now b l a c k , c l a y e y , and s p o t t e d w i t h white fragments of the l i m e s t o n e u n d e r n e a t h . The landscape i s g e n t l y u n d u l a t i n g . The r e l a t i v e l y h i g h e r p l a c e s i n the toposequence are c l e a r l y d i s t i n g u i s h e d by the s u r f a c e s t o n i n e s s ; these s i t e s are c a l l e d C a l i c h e s p r o p e r . The lower s i t e s have deeper s o i l s , l e s s s t o n e s , and are l o c a l l y d i f f e r e n t i a t e d from the former as " b a r r o s " (mud). S lopes are g e n t l e , w i t h a 3-10 per cent i n c l i n a t i o n . Maize f i e l d s are found everywhere but l a n d i s a l s o used for g r a s s l a n d s and papaya . The p r o f i l e s of these C a l i c h e s o i l s have a c h a r a c t e r i s t i c c o l o r sequence w i t h d e p t h : b l a c k ; g r a y ; w h i t e . The s t rong b l a c k c o l o r of the t o p s o i l s p a r t i a l l y responds to o r g a n i c matter c o n c e n t r a t i o n s , but i t i s a l s o known that c a l c a r e o u s m a t e r i a l s combined w i t h o r g a n i c carbon enhance the darkness of these s o i l s . The gray c o l o r and the m o t t l i n g of the s u b s o i l s are i n d i c a t i v e of poor i n t e r n a l d r a i n a g e . These s o i l s are d e f i n i t e l y f i n e t e x t u r e d and s t i c k when wet. When ca rbona te s are removed for s o i l p a r t i c l e a n a l y s i s (see Appendix 2, S i t e 7 ) , the amount of c l a y i s 60 to 70 per cent of the f i n e e a r t h f r a c t i o n . T h i s a c t u a l l y c o r r e s p o n d s to a heavy c i a y t e x t u r a l c l a s s . However, o r g a n i c matter and c o a r s e 84 p a r t i c l e s h e l p in c r e a t i n g a b e t t e r t i l t h i n the sur f ace h o r i z o n s . G r a v e l and stones are common throughout the p r o f i l e , p a r t i c u l a r l y i n the most eroded s i t e s . T o t a l s o i l depth ranges from 35 to 60 cm, which c o u l d be c l e a r l y d i s t i n g u i s h e d by the sharp c o l o r change on c o n t a c t w i t h the l i m e s t o n e . S o i l s t r u c t u r e i s s t r o n g i n b l o c k s and p r i s m s . A l s o c h a r a c t e r i s t i c are v e r t i c a l c r a c k s which d i s a p p e a r when the s o i l i s m o i s t . Bulk d e n s i t i e s are r e l a t i v e l y low (0.8 to 1.0 g /cm 3 ) g i v e n the f i n e t e x t u r e of these s o i l s . Water r e t e n t i o n i s the h i g h e s t among the 3 groups of s o i l s s t u d i e d h e r e . At f i e l d c a p a c i t y these C a l i c h e s can accumulate between 60 and 70 per cent of t h e i r weight i n water . Water conten t at the permanent w i l t i n g p o i n t i s s t i l l a c o n s i d e r a b l e 40 per cent of f i n e e a r t h w e i g h t . Thus , the amount of water a v a i l a b l e to p l a n t s i s q u i t e h i g h i n these s o i l s , a f a c t tha t i s of g rea t importance i n a subhumid env i ronment . The C a l i c h e s have a l k a l i n e r e a c t i o n ; pH ranges from 7.5 to 8 . 0 . C a l c i u m dominates the o ther c a t i o n s i n the exchange complex. The exchange c a p a c i t y i s h i g h and base s a t u r a t i o n i s c l o s e to 100 per c e n t . There shou ld be no f e r t i l i t y prob lems , o ther than a s l i g h t P f i x a t i o n and N d e f i c i e n c i e s i n the s h a l l o w e r s i t e s . On average , t o t a l N i s i n t e r m e d i a t e between the Andoso l s and the Tepe ta te s p r e v i o u s l y d i s c u s s e d . 85 I V . 4 . C l i m a t e Some c h a r a c t e r i s t i c s of the c l i m a t e of the s tudy area have been mentioned when d i s c u s s i n g s o i l s . There i s an i n t i m a t e c o r r e l a t i o n between s o i l types and c l i m a t e i n t h i s s tudy , but , t h i s s e c t i o n w i l l f u r t h e r d i s c u s s those c l i m a t i c parameters which a f f e c t maize growth: growing seasons , r a i n f a l l p a t t e r n s , and temperature reg imes . I V . 4 . j _ . Growing season The growing season for maize i n Mexico i s e i t h e r c o n t r o l l e d by low temperatures or- by the p a t t e r n of r a i n f a l l throughout the year (Galvan Lopez and Delgado Hernandez , 1977). E a r l y and l a t e f r o s t s , and /or the q u a n t i t y of thermal u n i t s , are determinant f a c t o r s i n c o n t r o l l i n g maize development i n the temperate zones of M e x i c o . C o n v e r s e l y , maize i n the t r o p i c a l and s u b t r o p i c a l l ands of Mexico i s a f f e c t e d by the d i s t r i b u t i o n of r a i n . The study area i n c l u d e s both types of c l i m a t i c c o n t r o l s of maize growth. In the v i c i n i t y of Xa lapa (humid warm t e m p e r a t e ) , temperature r a t h e r than r a i n f a l l i s the c r i t i c a l f a c t o r i n d e t e r m i n i n g the c a l e n d a r fo r ma ize . Tab le IV .2 shows that these humid-warm-temperate zones exper i ence f r o s t s at the r a t e of 20 days a y e a r , mos t ly i n F e b r u a r y . Lower mean annua l temperatures and e v a p o r a t i o n would i n d i c a t e that t h e r e i s l e s s r a d i a n t energy a v a i l a b l e fo r crops to use . A c c o r d i n g l y , l o c a l v a r i e t i e s of maize take 7 to 9 month to f u l l y mature , from February-March to 86 T a b l e I V . 2 . C l i m a t i c parameters of study area C l i m a t e Mean Type Temperature ( ° C ) Mean Annual R a i n f a l l (mm) R a i n f a l l v a r i a b i l i t y (CV%) Tank Evapora t ion (mm) F r o s t (#days / y e a r ) Humid Warm1 Temperate 18.7 1 638 13.8 1 1 46 22.5 Subhumid 2 S u b t r o p i c a l 20.7 1 082 21.4 1313 ? Subhumid 3 T r o p i c a l 24.6 872 17.4 1 736 0 S o u r c e : 15 year averages from nearby m e t e o r o l o g i c a l s t a t i o n s : 1 X a l a p a - C o a t e p e c , 2 Rancho V i e j o - M i r a d o r e s , 3 C a r r i z a l -R i n c o n a d a . September-October . In the next c l i m a t i c zone to be c o n s i d e r e d , the subhumid s u b t r o p i c a l shown i n Tab le I V . 2 , the c l i m a t e i s warmer and d r i e r . A l t h o u g h records of f r o s t s were not a v a i l a b l e for t h i s zone , they are l o c a l l y known to be l e s s i m p o r t a n t - - a n d a l s o i m m a t e r i a l because l i t t l e r a i n f a l l s there u n t i l May, when f r o s t s are r a r e . Higher temperatures and e v a p o r a t i o n r a t e s i n d i c a t e h i g h e r l e v e l s of r a d i a n t energy which are c o r r e l a t e d w i t h a s h o r t e r growing season . T y p i c a l l y , maize i n t h i s c l i m a t e matures w i t h i n 5-6 months, from A p r i l - M a y to September. F i n a l l y , the subhumid t r o p i c a l zone has no f r o s t s on r e c o r d whatsoever (Table I V . 2 ) . High temperatures . w i l l a l l o w year round c u l t i v a t i o n of maize i n t h i s zone, but r a i n f a l l i s much s c a r c e r which s e r i o u s l y l i m i t s maize growth to 4-5 months, from 87 May-June to September. I f i r r i g a t i o n were a v a i l a b l e , t h i s zone c o u l d accomodate two c rops a year and even 3 w i t h f a s t growing h y b r i d s (Marten and S a n c h o l u z , 1982). I V . 4 . 2 . R a i n f a l l p a t t e r n s The three c l i m a t i c zones under d i s c u s s i o n c o r r e s p o n d w i t h the three s o i l types d e s c r i b e d before ( S e c t i o n I V . 3 ) . F i g u r e 4.3 i n c l u d e s t y p i c a l c l imodiagrams for these three s o i l - c l i m a t e zones . The annua l p a t t e r n s of r a i n f a l l and e v a p o r a t i o n f u r t h e r c l a r i f y the d i f f e r e n c e s i n growing season fo r these zones . In a l l cases there i s a c l e a r r a i n y season d u r i n g the summer. However, when r a i n f a l l i s compared w i t h e v a p o r a t i o n r a t e s the d i f f e r e n c e s among the three zones are much c l e a r e r . Roughly s p e a k i n g , the monthly d i f f e r e n c e between r a i n f a l l p r e c i p i t a t i o n and e v a p o r a t i o n i s the amount of water that can be s t o r e d i n the s o i l s for l a t e r use by p l a n t s . The shaded areas of F i g . 4 . 2 a ~ c i n d i c a t e t h i s a tmospher ic s u r p l u s water . C l e a r l y , both the number of months w i t h water s u r p l u s e s and t h e i r magnitude change from zone to zone. On the average , for the temperate , s u b t r o p i c a l , and t r o p i c a l zones , t h e r e are 5 months w i t h a t o t a l of 60 cm of s u r p l u s water , 4 months w i t h 35 cm of s u r p l u s water , and 3 months w i t h 12 cm of s u r p l u s water , r e s p e c t i v e l y . Year to year v a r i a t i o n s , p a r t i c u l a r l y of r a i n f a l l , i s another important c o n s i d e r a t i o n for e s t i m a t i n g water supply i n these zones ; t h i s seems to be i n v e r s e l y c o r r e l a t e d w i t h the amount of r a i n f a l l (see bars i n f i g . 4 . 2 ) . Thus , i t i s not o n l y a q u e s t i o n 88 F i g u r e 4 . 2 . C l imodiagrams for the t h r e e s o i l - c l i m a t e zones . M e t e o r o l o g i c a l S t a t i o n s : 4.1a A n d o s o l : X a l a p a ; 4 .1b T e p e t a t e : Rancho V i e j o ; 4 .1c C a l i c h e : C a r r i z a l . Data are 20 year average s . Bars are S tandard e r r o r s . Shaded areas i n d i c a t e atmospher s u r p l u s of water . 89 mm 300. 200. 100 h u m i d warm t e m p e r a t e (Andosol) 300 200. ioo| subhumid subtropical (Tepetate) 90 of how much i t r a i n s i n these zones , but a l s o of how r e l i a b l e these r a i n s a r e . When both f a c t o r s are combined i t w i l l appear tha t the subhumid s u b t r o p i c a l zone i s a worse environment for maize p r o d u c t i o n than the humid warm temperate zone . However, i t must not be f o r g o t t e n that the v a r i e t i e s of maize change from zone to zone , tha t they d e v e l o p much f a s t e r i n warmer c l i m a t e s , and that r e a l water ba l ances must i n c l u d e not o n l y the a tmospher ic supply but the s o i l ' s c a p a c i t i e s to s t o r e water as w e l l . I V . 5 . Maize F i e l d Management T h i s s e c t i o n d e s c r i b e s the management of maize f i e l d s tha t i s c h a r a c t e r i s t i c of the area of s t u d y . In the p r e v i o u s s e c t i o n , mention was made of the d i f f e r e n c e s i n maize growing seasons of the three s o i l - c l i m a t e zones s e l e c t e d for t h i s s t u d y . These d i f f e r e n c e s do a f f e c t the c h o i c e of seeds and the number of c u l t i v a t i o n s tha t the f i e l d s r e c e i v e i n these zones . O t h e r w i s e , the t o o l s , the methods, and the h a n d l i n g of the c rops are the same. What f o l l o w s i s a s i m p l i f i e d model of maize f i e l d management, common to a l l zones , but w i t h n o t a t i o n s of the s l i g h t d i f f e r e n c e s among them. I V . 5 . j _ . C u l t i v a t i o n Machetes , hoes , p l a n t i n g s t i c k s , E g y p t i a n p lows , and oxen are the b a s i c t o o l s used i n the maize f i e l d s . One r a r e l y sees t r a c t o r s , e l e c t r i c pumps, or t r u c k s . Cropp ing energy i s mos t ly 91 human and a n i m a l . O b v i o u s l y , a peasant u s i n g these methods can o n l y c u l t i v a t e smal l a r e a s . T y p i c a l l y , the f i e l d s range from l e s s than a hec ta re to 15 or 20 h a , w h i l e average f i e l d s i z e i s about 2 h a . Tab le I V . 3 Ca lendar of c r o p p i n g a c t i v i t i e s A c t i v i t y Andoso l (Xalapa) Tepeta te ( E l Ch ico ) C a l i c h e (Chauapan) C l e a n i n g the f i e l d January E a r l y A p r i l A p r i l P lowing February E a r l y A p r i l May Rows and seeding + F e r t i l i z a t i o n La te F e b . / E a r l y March Late A p r i l / May Late May F i r s t C u l t i v a t i o n + F e r t i l i z a t i o n Mid A p r i l June June Second C u l t i v a t i o n ( F e r t i l i z a t i o n ) Mid May J u l y J u l y D o u b l i n g and C lean ing September August August H a r v e s t i n g October September September T a b l e I V . 3 g i v e s the c a l e n d a r of a c t i v i t i e s for each s i t e . The l a n d i s prepared for c u l t i v a t i o n w i t h hoes or p lows , depending on the a v a i l a b i l i t y of oxen as t r a c t i o n for the l a t t e r . T h i s o p e r a t i o n takes p l a c e a f t e r h a r v e s t , or 1 to 3 92 months be fore p l a n t i n g . The s o i l i s then plowed 10 to 20 cm deep, depending on the hardness and s t o n i n e s s of the s o i l . I f plows are used these open rows j u s t be fore p l a n t i n g the f i e l d . P l a n t i n g i s almost always a c h i e v e d w i t h the h e l p of a p l a n t i n g s t i c k which makes h o l e s 5 to 10 cm deep i n the s o i l . Two to four seeds are dropped i n each h o l e , a p p r o x i m a t e l y 90 cm a p a r t , i n each row. Row s p a c i n g i s v a r i a b l e between 70 and 90 cm. F e r t i l i z e r s , i f used , are b u r i e d i n a c i r c l e around the seeds . The f i e l d s are c u l t i v a t e d two or three t imes d u r i n g the growing season, depending on the l e n g t h of the season. F i e l d s of the warm temperate zone u s u a l l y get three weedings , w h i l e the s u b t r o p i c a l and t r o p i c a l f i e l d s get o n l y two. The f i r s t c u l t i v a t i o n can be performed w i t h p lows , but even t h i s i s r a r e . In g e n e r a l , c u l t i v a t i o n w i t h hoes i s the norm. T h i s o p e r a t i o n a f f e c t s the s o i l very s u p e r f i c i a l l y , u s u a l l y the topmost 2 to 4 cm are so f tened to remove weeds. I t i s a l s o s p a t i a l l y c o n c e n t r a t e d to the immediate s u r r o u n d i n g s of the maize h i l l s -machetes chop weeds i n the i n t e r - r o w areas of the f i e l d . I t i s a l s o t y p i c a l to c r e a t e e a r t h mounds around the h i l l s d u r i n g the l a s t c u l t i v a t i o n ( a t i e r r a ) , the purpose be ing to prevent l o d g i n g of the p l a n t s . When the earcones are f u l l y d e v e l o p e d , u s u a l l y a month be fore h a r v e s t i n g , the p l a n t s are f o l d e d over the node j u s t below the earcone (doblada) .. In t h i s way, maize d r i e s out in the f i e l d and i s ready fo r s torage a f t e r h a r v e s t i n g , w h i l e p r e d a t i o n by b i r d s as w e l l as r u s t i n g are p revented by the i n v e r s i o n of the earcone t i p . At the t ime of d o b l a d a , the undergrowing 93 v e g e t a t i o n i s chopped w i t h machetes to f a c i l i t a t e h a r v e s t i n g . Forage from l eave s and p l a n t t i p s i s a l s o c o l l e c t e d at t h i s p o i n t for domest ic an imal s to feed upon. I V . 5 . 2 . P l a n t breeds Maize seeds are p r e d o m i n a n t l y l o c a l v a r i e t i e s known as C r e o l e maize (Zea mays L . v a r . C o n i c o and Tuxpeno; Wel lhausen et a l , 1952). Only the subhumid t r o p i c s are known to have adopted h y b r i d and improved maize v a r i e t i e s ( T u x p e n i t o , H503, H507) . However, these improved seeds are r i s k y g i v e n the u n r e l i a b i l i t y of r a i n s and the p a t c h i n e s s of f e r t i l i t y i n r a i n f e d ' f i e l d s . Only when these seeds are p r o v i d e d almost f ree of charge do the peasants u t i l i z e them i n t h e i r f i e l d s . For i n s t a n c e , i n 1981, T uxpen i to v a r i e t i e s were d i s t r i b u t e d among peasants in Chauapan. C u r i o u s l y , about a t h i r d of the f i e l d s were sown to t h i s l i t t l e known v a r i e t y . That year i t r a i n e d e x t e n s i v e l y i n Chauapan. T uxpen i to d i d w e l l o n l y i n those f i e l d s whose f e r t i l i t y s t a t u s was h i g h ( i . e . , the bottoms of the l a n d s c a p e ) , or tha t were w e l l f e r t i l i z e d . C r e o l e v a r i e t i e s d i d much b e t t e r on the tops of h i l l s (the c r e s t s or s toney s i t e s ) . Seed s e l e c t i o n i s most ly a c r a f t s m a n l i k e a c t i v i t y c a r r i e d on by the peasants every y e a r . They s e l e c t t h e i r best earcones from each h a r v e s t and save the seeds for the next y e a r . Seeds can a l s o be o b t a i n e d i n the l o c a l s torehouses which c a r r y a s u r p l u s of seeds from year t o y e a r . Other p l a n t s p e c i e s are u s u a l l y i n t e r c r o p p e d w i t h ma ize . B lack beans (Phaseolus v u l g a r i s , Phaseo lus spp) are the most 94 common. These v i n e s are sown a l t e r n a t i v e l y w i t h the maize i n the h i l l s at low d e n s i t i e s . They grow under the maize canopy u n t i l the doblada of the maize p l a n t s ; a f t e r tha t the beans f l o u r i s h and mature r a p i d l y . U s u a l l y y i e l d s of beans range from between 200 and '700 kg /ha , depending on seed ing d e n s i t i e s , f e r t i l i t y and c a r e . Squash are a l s o commonly found i n the f l o o r of the maize f i e l d ; these y i e l d o n l y domestic r a t i o n s , but h e l p to ba lance the d i e t of the peasant f a m i l y . I t i s not e i t h e r uncommon to see f r u i t t r e e s i n t e r c r o p p e d i n a maize f i e l d : oranges , bananas, avocadoes , mangoes are the most important of t h e s e . I V . 5 . 3 . Harves t The h a r v e s t of maize i s done by hand. I t i s a h i g h l y s o c i a l task which i n v o l v e s , for a few days , to most members of the community. S t a l k s and p l a n t r e s i d u e s are l e f t behind u n t i l p lowing opens a new c u l t i v a t i n g season in the f o l l o w i n g s p r i n g . But , weeds, tender r e s i d u e s , and gras ses are commonly consumed by domest ic an imal s i n the f a l l . I V . 5 . 4 . Maize y i e l d s Maize y i e l d s range from l e s s than 1 t h a " 1 y r " 1 to more than 4 t h a " 1 y r " 1 , the l a t t e r b e i n g s p e c i a l l y h i g h . O v e r a l l , mean y i e l d s are known to be about 1.5 t h a " 1 y r " 1 (SARH,1979) . Mean maize y i e l d s for the three s i t e s of t h i s s tudy were a s se s sed w i t h two d i f f e r e n t and i n d i r e c t sources of d a t a , as 95 T a b l e I V . 4 Maize y i e l d e s t imate s (t h a " 1 y r " 1 ) for the s o i l s of t h i s study S o i l type Source of data COTECOCA 1 F i e l d s u r v e y 2 Andoso l 2.05 1 .87 Tepe ta te 1 .20 1.15 C a l i c h e 1 .76 1 .55 Mean g r a s s l a n d p r o d u c t i v i t y , COTECOCA (1979) ; P r o j e c t e d average g r a i n y i e l d , Marten and Sancholuz (1981) Tab le I V . 4 i n d i c a t e s . One of these sources (COTECOCA, 1979) p r o v i d e s e s t i m a t e s of g r a s s l a n d p r o d u c t i v i t y for the study a r e a . P h y t o s o c i o l o g i c a l a n a l y s i s and e x t e n s i v e s u r v e y i n g of p roducer s i n the f i e l d have been used to a s ses s forage p r o d u c t i o n and c o n s e q u e n t l y l i v e s t o c k c a r r y i n g c a p a c i t y . Annual grass biomass p r o d u c t i o n data are p r o v i d e d for d i f f e r e n t s i t e q u a l i t i e s and g r a s s l a n d management o p t i o n s . The o ther source i n T a b l e I V . 4 i s the p r e v i o u s l y mentioned survey of maize f i e l d s i n the Xa lapa r e g i o n (Marten and S a n c h o l u z , 1981). Mean maize y i e l d s for the three s o i l types under c o n s i d e r a t i o n are e x t r a p o l a t i o n s from 1977 d a t a . Because maize p r o d u c t i v i t y and p o t e n t i a l e v a p o t r a n s p i r a t ion are s t r o n g l y c o r r e l a t e d , y i e l d p r e d i c t i o n s for an average year are p o s s i b l e . Tab le I V . 4 shows t h a t , on the average , the Andoso l s s h o u l d 96 y i e l d b e t t e r than the C a l i c h e s , and these even b e t t e r than the T e p e t a t e s . The two e s t i m a t e s d i f f e r from one a n o t h e r , but not by much; a l t h o u g h both are c o a r s e , they are the o n l y r e g i o n a l e s t i m a t e s of mean maize p r o d u c t i v i t y for the t h r e e s o i l s under s t u d y . I V . 6 . An E v a l u a t i o n of S o i l E r o s i o n R i s k s for the Tes t S i t e s What are the r i s k s of e r o s i o n for the three s o i l types i n c l u d e d i n t h i s s tudy? The f o l l o w i n g assesment i s based on the U n i v e r s a l S o i l Loss E q u a t i o n (USLE) of the USDA (Wischmeier and S m i t h , 1978). The USLE i s an e m p r i c a l l y d e r i v e d formula which p r e d i c t s average annual s o i l l o s s e s (t h a " 1 y r " 1 ) from a f i e l d . I t i s based on a number of b i o p h y s i c a l f a c t o r s which are known to a f f e c t the r a t e of e r o s i o n . These f a c t o r s a r e : R, r a i n f a l l e r o s i v i t y ; LS , angle and l e n g t h of the s l o p e ; K, s o i l e r o d i b i l i t y ; C , c r o p management; and P, c o n s e r v a t i o n or e r o s i o n c o n t r o l p r a c t i c e s . The f o l l o w i n g four s u b s e c t i o n s d i s c u s s , one at a t i m e , the c a l c u l a t i o n s of these parameters fo r the t h r e e main s o i l s of t h i s s t u d y . S e c t i o n I V . 6 . 5 d i s c u s s e s the r e s u l t i n g s o i l l o s s e s t i m a t e s . I V . 6 . j _ . R a i n f a l l e r o s i v i t y , R In e s t i m a t i n g the e r o s i v i t y of r a i n , t h i s s tudy f o l l o w s the method proposed for maximum 6-hour r a i n f a l l data (Wischmeier , 1974, p . 183). EI30 , the s t andard r a i n f a l l e r o s i v i t y index , and 97 KE>1, a v a r i a n t deve loped for the t r o p i c s (Hudson, 1971) both r e l y on data on i n t e n s i t y and d u r a t i o n of r a i n f a l l , data which are not p u b l i s h e d in Mexico (CP, 1977, p . 33 ) . The m e t e o r o l o g i c a l s t a t i o n s of the area of s tudy have p u b l i s h e d 24-hour r a i n f a l l data for the l a s t twenty y e a r s . In order to use W i s c h m e i e r ' s 6-hour a p p r o x i m a t i o n , an assumption has to be made on the e q u i v a l e n c e of 24 and 6 hour r a i n f a l l e v e n t s . Marten and Sancholuz (1982) have proposed t h a t i n the Xa lapa r e g i o n t h i s i s a r ea sonab le a s sumpt ion , for most of the r a i n s of a year f a l l d u r i n g the summer i n a few c o n c e n t r a t e d storms (see F i g . 4 . 2 ) . On t h i s b a s i s , Marten and Sancholuz (1982, T a b l e I) e s t i m a t e d R for the c l i m a t i c zones c o n s i d e r e d i n t h i s s t u d y . Expres sed i n m e t r i c u n i t s , these R e s t imate s range from 47T t h a - 1 y r " 1 i n the area of the Andoso l s to 850 t h a " 1 y r " 1 i n the area of the C a l i c h e s (see Tab le I V . 9 ) . I V . 6 . 2 . S o i l e r o d i b i l i t i e s , K Wischmeier and Smith (1978, pp . 8-11) proposed that four p r o p e r t i e s determine the i n h e r e n t e r o d i b i l i t i e s of s o i l s . These a r e : s o i l p a r t i c l e - s i z e d i s t r i b u t i o n , o r g a n i c m a t t e r , s o i l s t r u c t u r e , and s o i l p e r m e a b i l i t y . O p e r a t i o n a l l y , these s o i l p r o p e r t i e s are d e f i n e d i n the f o l l o w i n g manner: m, the per cent of very f i n e sand (0.1-.05mm) p l u s s i l t (0.05-.002mm) i n the s o i l p a r t i c l e - s i z e d i s t r i b u t i o n ; a , the s o i l ' s pe rcent o r g a n i c m a t t e r ; b , the s o i l s t r u c t u r e c l a s s ( U S D A ) ; and c , the s o i l p e r m e a b i l i t y c l a s s ( U S D A ) . K i s o b t a i n e d e i t h e r through a non-l i n e a r e q u a t i o n i n which m m u l t i p l i e d by ( l00-%Clay) i s r a i s e d 98 to the 1.14 power, or through a nomograph s o l u t i o n . Tab le IV .5 U S L E - S o i l e r o d i b i l i t y parameters and K (met r i c ) va lue s for the topmost 30 cm of three s o i l s of t h i s s t u d y . S o i l Type Parameters 1 E r o d i b i l i t y m %S a b c K A n d o s o l 2 51 17 1 0 2 2 0.22 T e p e t a t e 2 33 29 2 4 5 0.32 C a l i c h e 2 9 20 3 3 4 0.14 1 m: %Si l t+VFSand; %S: % Sand; a:% OM; b : S o i l s t r u c t u r e c l a s s ; c : S o i l p e r m e a b i l i t y c l a s s . 2 C a l c u l a t e d from s u r f a c e h o r i z o n s of s i t e s #2, 4, and 7, Appendix 1. T a b l e I V . 5 r e p o r t s the i n t e r m e d i a t e c a l c u l a t i o n s and the f i n a l K v a l u e s for the three s o i l s to a depth of 30 cm. The e s t i m a t e s of these parameters are based on the data for s i t e s 2 , 4 , and 7 of Appendix 2. I V . 6 . 3 . S lope e f f e c t , LS The two key components of t h i s t opograph ic parameter are s lope angle and s lope l e n g t h . Another d imens ion to t h i s parameter can be p r o v i d e d by the shape of the s l o p e . To e v a l u a t e these components, a s p e c i a l f i e l d survey was 99 d e s i g n e d . A e r i a l photographs c o v e r i n g the whole area ( s c a l e , 1:50,000) were screened for r e p r e s e n t a t i v e sequences of the l a n d s c a p e . F i v e 500 m l o n g t r a n s e c t s were s e l e c t e d to study i n the f i e l d . At every 25 m on these t r a n s e c t s , the shape and ang le (measured wi th a pocket c l i n o m e t e r ) were r e c o r d e d . S lope l e n g t h s were l a t e r o b t a i n e d as the d i f f e r e n c e between the b e g i n n i n g and end of each s lope segment. T a b l e I V . 6 shows the r e s u l t i n g data for the three s o i l s s t u d i e d . The Tepeta te s o i l s showed s i g n i f i c a n t l y g e n t l e r average s lope s than the o ther two s o i l s . The Andoso l s have the s teeper s l ope s w h i l e the C a l i c h e s have s l i g h t l y more t i l t e d s l ope s than the T e p e t a t e s . There does not seem to be much d i f f e r e n c e i n the shape of the s l ope s of a l l three s o i l s . The dummy averages i n Tab le I V . 6 i n d i c a t e tha t Tepe ta te s and C a l i c h e s tend to have convex s lope s w h i l e the Andoso l s are q u i t e u n i f o r m . The Tepe ta te s showed s i g n i f i c a n t l y s h o r t e r s lope l e n g t h s than the o t h e r two s o i l s . The Andoso l s lopes are s l i g h t l y s h o r t e r than the C a l i c h e ' s , but t h i s d i f f e r e n c e i s i n s i g n i f i c a n t . These e s t imate s can now be used to e v a l u a t e the LS f a c t o r i n the USLE . The c a l c u l a t i o n w i l l assume an i d e a l f i e l d wi th s lope p r o p e r t i e r s as d e p i c t e d i n Tab le I V . 6 . For p r a c t i c a l r ea sons , the s l o p e s of these three s o i l s a re c o n s i d e r e d u n i f o r m . The c a l c u l a t i o n i s now s t r a i g t h f o r w a r d . Us ing the USLE m e t r i c nomograph (Wischmeier and S m i t h , 1978., f i g 11), y i e l d s the LS v a l u e s r e p o r t e d i n t a b l e I V . 6 . As can be r e a d i l y seen t h e r e , LS i s to a grea t extent a f u n c t i o n of s lope angle for these s o i l s . 100 T a b l e I_V.6. S lope c h a r a c t e r i s t i c s of the s o i l s of t h i s study and USLE-LS (met r i c ) v a l u e s . S o i l Type S lope C h a r a c t e r i s t i e s 1 LS length(m) angle(%) shape 2 Andoso l 118.8 (15.0) 22.3 (1 .4) 1 .99 8.8 Tepeta te 72.5 (8.6) 7.5 (0.5) 2.11 1 .4 C a l i c h e 138.6 (16.0) 4.5 (0 .5) 2.31 0.9 1 F i g u r e s in b racke t s are S tandard E r r o r s . 2 V a l u e s coded a s : 1=concave, 2=uniform, 3=convex. The d i f f e r e n c e i n LS between the Andoso l and the o ther two s o i l s i s much g r e a t e r than the d i f f e r e n c e s i n s l ope s because LS i s an e x p o n e n t i a l f u n c t i o n of the angle of the s l o p e . I V . 6 . 4 . Crop management, C S e v e r a l a s p e c t s of the v e g e t a t i v e cover of the l a n d a f f e c t s o i l e r o s i o n . These can be grouped i n t o i n t e r and i n t r a - a n n u a l v a r i a t i o n of p l a n t c o v e r . From year to y e a r , c r o p s and n a t u r a l v e g e t a t i o n can r e p l a c e one another i n a f i e l d . For l o n g - t e r m p r e d i c t i o n s , t h i s s u c c e s s i o n of c r o p s matters because each c r o p or v e g e t a t i o n produces a d i f f e r e n t canopy w h i c h , i n t u r n , d i f f e r e n t l y p r o t e c t s the s o i l from the impac t ing r a i n f a l l and the a b r a d i n g r u n o f f . To a s ses s these e f f e c t s the r o t a t i o n of c r o p s of a f i e l d has to be known, and p r o p e r l y d e f i n e d . The r o t a t i o n s of the maize f i e l d s for t h i s study were 101 Tab le IV . 1_. H i s t o r y of l and use i n the maize f i e l d s of the 3 s o i l s of t h i s study Years w i t h S o i l Maize Agr i c u l t u r e Andoso l 13.6 (1.7) 20.3 (2.2) Tepeta te 8.3 (1 .9) 11.3 (2.1) C a l i c h e 8.0 (2 .1) 38.8 (6.1) 1 F i g u r e s i n b r a c k e t s are S tandard E r r o r s . e s t i m a t e d from i n t e r v i e w s w i t h l o c a l c u l t i v a t o r s conducted i n 1977 (Marten and S a n c h o l u z , 1981). T a b l e I V . 7 c o n t a i n s a summary of t h i s d a t a . These r e v e a l t h a t , on the average , the maize f i e l d s have been c o n t i n u o u s l y c u l t i v a t e d wi th maize fo r between 8 and 13 y e a r s . I t i s i n t e r e s t i n g to note t h a t , on the average , Tepeta te s are the l e s s f r e q u e n t l y used of a l l three s o i l s ( i . e . , for o n l y 11 year s ) which would suggest that these s o i l s may be l e s s s u s t a i n i n g than the o ther two. The number of year s i n any other a g r i c u l t u r a l use v a r i e d from 11 to 39. P r i o r to t h a t , most f i e l d s had been r e s t i n g i n second growth v e g e t a t i o n ( A c a h u a l ) . No i n f o r m a t i o n ' c o u l d be o b t a i n e d on the sequence of c r o p s for the year s p r e c e d i n g the l a s t c y c l e of c o n t i n u o u s maize . However, w i t h the e x c e p t i o n of the C a l i c h e s , which are known to support papayas for two to four year s i n a row, most of these f i e l d s t u r n to another c r o p o n l y fo r one or two y e a r s . In c o n c l u s i o n , most of these f i e l d s are 1 02 c u l t i v a t e d c o n t i n u o u s l y to ma ize , w i t h b r i e f p e r i o d s of a l t e r n a t e crops or A c a h u a l . Another e f f e c t of v e g e t a t i v e cover on e r o s i o n a r i s e s from the sea sona l v a r i a t i o n of p l a n t c o v e r . As the c r o p matures , more and more p l a n t cover d e v e l o p s . The c r o p development schedule must be r e l a t e d to the seasona l d i s t r i b u t i o n of the e r o s i v e r a i n s , for i t i s the c o i n c i d e n c e , or l a ck of i t , between p e r i o d s of e r o s i v e r a i n s and poor p l a n t c o v e r , which matters the most. To e s t imate the sea sona l e f f e c t s of a c r o p on s o i l e r o s i o n , f i r s t the monthly d i s t r i b u t i o n of 24-hour r a i n f a l l was o b t a i n e d for each c l i m a t o l o g i c a l s t a t i o n s d e p i c t e d i n F i g . 4 . 2 . Next the s i x c rops tage p e r i o d s c a l l e d fo r i n the USLE procedure (Wischmeier and Smi th , 1978, pp 17-18) were d e f i n e d as to f i e l d o p e r a t i o n s and p l a n t cover development . Tab le I V . 8 c o n t a i n s the percentage of R i n each p e r i o d for each s o i l t oge ther w i t h the c a l c u l a t e d C v a l u e s . The c r o p management f a c t o r a l s o takes i n t o account the method of c u l t i v a t i n g the l a n d . T i l l a g e and p l a n t r e s i d u e management have a s i g n i f i c a n t e f f e c t on the amount of e r o s i o n tha t a f i e l d would e x p e r i e n c e . The l e v e l of p r o d u c t i o n of a f i e l d i s a l s o important because the more p l a n t cover a f i e l d produces the l e s s e r o s i o n i t shou ld undergo, p r o v i d e d these r e s i d u e s are r e t u r n e d to the s o i l . For the C-va lue c a l c u l a t i o n s , l o c a l c o n d i t i o n s appproached the f o l l o w i n g c o n d i t i o n s d e s c r i b e d by Wischmeier and Smith (1978, t a b l e 5, l i n e 12): moldboard p l o w i n g , p l a n t r e s i d u e removal or b u r r y i n g , and low p r o d u c t i v i t y . C - v a l u e s among s o i l types do not d i f f e r very much, even though l a t e p l a n t i n g i n both 1 03 T a b l e I V . 8 . USLE-crops tage p e r i o d s and Crop-Management F a c t o r (C) for the three s o i l s of t h i s study %R i n Crops tage p e r i o d 1 C S o i l F P1 1 0% 50% 75% H Andoso l 1 1 1 2 7 45 17 7 0 54 Tepeta te 32 1 3 3 27 1 7 8 0 59 C a l i c h e 42 6 3 27 18 4 0 .58 1 F : p e r i o d i n f a l l o w ; P 1 : J u s t p lowed; 10-75: % p l a n t cover development ; H : H a r v e s t . the Tepe ta te s and C a l i c h e s makes the f a l l o w p e r i o d very s u s c e p t i b l e to e r o s i o n . 5. S o i l c o n s e r v a t i o n p r a c t i c e s , P Marten and Sancholuz (1982) found no ev idence of t e r r a c i n g or contour p lowing i n the maize f i e l d s of c e n t r a l V e r a c r u z . S ince s o i l c o n s e r v a t i o n p r a c t i c e s are not be ing used i n t h i s a r e a , the P f a c t o r of the USLE w i l l be assumed to be equa l to 1. I V . 6 . 6 . S o i l l o s s e s t i m a t e s , A T a b l e I V . 9 summarizes the v a l u e s of a l l U S L E - f a c t o r s w i t h the e s t i m a t e d annual s o i l l o s s e s fo r the three s o i l s of t h i s s t u d y ; the l a t t e r vary between 62 and 492 t h a " 1 y r " 1 . These v a l u e s are h i g h in a l l c a s e s , even when compared to the h i g h e s t 1 04 measured s o i l l o s s e s r e p o r t e d i n S e c t i o n I I I . 2 . 2 . Tab le I V . 9 . U S L E - f a c t o r v a l u e s and e s t i m a t e d s o i l l o s s va lue s under cont inuous maize c u l t i v a t i o n for the 3 s o i l s of t h i s s tudy USLE-• F a c t o r s Annua l S o i l Loss S o i l R LS K C (t h a " 1 ) cm 2 Andoso l 471 8.8 0.22 0.54 492 6.3 Tepe ta te 645 1 .4 0.32 0.59 1 70 1 .3 C a l i c h e 850 0.9 0.14 0.58 62 0.7 1 F a c t o r P = 1 i n a l l s o i l s 2 A d j u s t e d wi th bulk d e n s i t i e s , Appendix 2 One problem i n these c a l c u l a t i o n s i s r a i n f a l l e r o s i v i t y . R v a l u e s r e p o r t e d i n Tab le I V . 9 are an a p p r o x i m a t i o n u s i n g maximum 24-hours r a i n f a l l d a t a . However, the R v a l u e s as such are s i m i l a r to the ones r e p o r t e d from o ther t r o p i c a l and s u b t r o p i c a l zones , such as s o u t h e a s t e r n U . S . and Hawai i (Wischmeier and S m i t h , 1978). Another l i m i t a t i o n of these e s t imate s i s the K-va lue s for the t h r e e s o i l s . The Tepe ta te owes i t s r e l a t i v e l y h i g h K - f a c t o r v a l u e to low p e r m e a b i l i t i e s and low o r g a n i c matter c o n t e n t . In c o n t r a s t , the Andoso l has e x c e l l e n t p h y s i c a l p r o p e r t i e s and i s h i g h i n o r g a n i c matter c o n t e n t , but i t s h i g h s i l t and very f i n e 105 sand c o n t e n t s e x p l a i n the i n t e r m e d i a t e K va lue r e p o r t e d i n Tab le I V . 9 . The C a l i c h e s o i l shows the lowest K va lue because t h i s i s a h e a v y - c l a y - t e x t u r e d s o i l wi th medium o r g a n i c matter c o n t e n t s . The v a l u e s of the s l o p e - l e n g t h , LS , and the c r o p management f a c t o r s for the s o i l s of t h i s study seem i n l i n e wi th o ther v a l u e s r e p o r t e d for s i m i l a r c o n d i t i o n s ( E l - S w a i f y et a l , 1982). The range of LS va lues a c r o s s s o i l s i s of the same order of magnitude tha t the range of e s t imated s o i l l o s s e s , i . e . , a lmost one order of magnitude i n both cases (see t a b l e I V . 9 ) . I V . 7 . A F i e l d Tes t on S o i l Losses Is there a s imple way of c h e c k i n g the a c c u r a c y of the p r e c e e d i n g s o i l l o s s e s t imate s ? F i g u r e 4.3 shows s lope and t o p s o i l depth data c o l l e c t e d from more than ten maize f i e l d s in each of the three s o i l s of t h i s s t u d y . The deepest s o i l , the A n d o s o l , showed the g r e a t e s t range of v a l u e s . C o n v e r s e l y , the Tepe ta te v a r i e d the l e a s t in t o p s o i l depth and s lope i n c l i n a t i o n . Depths i n the Tepeta te and the C a l i c h e r e p r e s e n t t o t a l s o i l depth because i t was d i f f i c u l t to r e c o g n i z e s o i l h o r i z o n s from auger samples of these s o i l s . I t i s tempt ing to a t t a c h meaning to these d a t a . As a sample of maize f i e l d s in the area they c o u l d i n d i c a t e past l a n d use management, the n a t u r a l v a r i a t i o n of s o i l depth w i t h i n each s o i l t y p e , or e l s e a s t r o n g i n v e r s e r e l a t i o n s h i p between s o i l depth and s l o p e a n g l e . I f o n l y the l a t t e r p r o p o s i t i o n were t r u e , we would have s t r o n g ev idence of the long term e f f e c t s of s o i l e r o s i o n on s o i l depth i n the maize f i e l d s under s t u d y . S o i l 106 F i g u r e 4.3. T o p s o i l depth and s l o p e s i n the three s o i l s of the s t u d y . T o p s o i l d e f i n e d a s : AB h o r i z o n i n A n d o s o l s ; d u r i p a n i n T e p e t a t e s ; and Limestone i n C a l i c h e s . Each data p o i n t i s an average of 3 measurements of s lope ( c l i n o m e t e r ) and s o i l depth (auger) i n maize f i e l d s . Curves are eye f i t t e d 108 depth f o l l o w s a' dramat ic e x p o n e n t i a l d e c l i n e w i t h i n c r e a s i n g s l o p e s , both i n the Andoso l and the C a l i c h e . In the T e p e t a t e , the change i n s o i l depth w i t h s lope i s m i n o r , o n l y because t h e r e i s not too much of a range i n v a l u e s . Tab le I V . 7 showed the number of year s these s o i l s have been i n a g r i c u l t u r e , i n g e n e r a l , and c u l t i v a t e d to maize , i n p a r t i c u l a r . I t c o u l d be i n f e r r e d from these data that both the Tepeta te and the Andoso l are cropped to maize 7 out of every 10 y e a r s . The C a l i c h e s seem to be l e s s f r e q u e n t l y c ropped to maize , b u t , s i n c e o ther annual c rops are used i n these s o i l s , i t c o u l d be c o n s i d e r e d t h a t , on the average , these s o i l s are c u l t i v a t e d to annual c rops 5 out of every 10 y e a r s . For those year s i n which the f i e l d s are r e s t i n g as Acahua l the C-USLE f a c t o r would approach 0.1 (see Wischmeier and S m i t h , 1978, T a b l e 10) . C a l c u l a t i n g a weighted average C va lue for the t o t a l number of year s i n a g r i c u l t u r e for every s o i l r e s u l t s i n the f o l l o w i n g : A n d o s o l , 0 . 39 ; T e p e t a t e , 0 .46 ; C a l i c h e , 0 .34 . S u b s t i t u t i n g these v a l u e s for each s o i l i n t a b l e I V . 9 and m u l t i p l y i n g fo r the t o t a l number of year s under a g r i c u l t u r e ( i . e . , from Tab le I V . 7 ) g i v e s the expected t o t a l s o i l l o s s e s for the average maize f i e l d s (see t a b l e I V . 1 0 ) . The s l o p e - d e p t h curve s for each s o i l i n F i g . 4 . 3 can be used to c a l c u l a t e observed s o i l depth l o s s e s . The d i f f e r e n c e i n s o i l depth between s i t e s w i t h mean s lope s and s i t e s w i t h the lowest s l ope s i n each s o i l (see arrows i n F i g . 4 . 3 ) i s an a p p r o x i m a t i o n of the observed average l o s s e s n a t u r a l l y exper imented under 1 09 average maize f i e l d management d u r i n g the p e r i o d d e s c r i b e d . T a b l e T V . U L Expected and observed s o i l l o s s e s for the three s o i l s of t h i s s t u d y . S o i l l o s s e s (cm) D i f f e r e n c e 3 S o i l E x p e c t e d 1 O b s e r v e d 2 -6 Andoso l 91 68 + 34 Tepe ta te 1 1 6 + 83 C a l i c h e 1 6 29 -45 1 From t a b l e I V . 9 for a f u l l r o t a t i o n as i n t a b l e I V . 7 . 2 From f i g u r e 4 . 3 , S o i l depth d i f f e r e n c e s are between mean and minimum s lope in each s o i l . 3 C a l c u l a t e d a s : 100X(E-O)/O. Tab le IV.10 compares p r e d i c t e d and observed s o i l l o s s e s for the t h r e e s o i l s of t h i s s t u d y . D i f f e r e n c e s range from 34 to 83%. Expected s o i l l o s s e s for the Andoso l and the Tepeta te are g r e a t e r than the observed s o i l l o s s e s . The c o n t r a r y happens i n the C a l i c h e . Something must be wrong, but where? There are too many b u i l t - i n assumptions i n the e s t i m a t e s p r e s e n t e d i n t a b l e IV..10 to attempt an e x p l a n a t i o n of the d i s c r e p a n c i e s . In r e l a t i v e terms , however, the observed va lue s f a l l w i t h i n the same order of magnitude of the expected v a l u e s a c r o s s s o i l s . The s teep 1 10 s l o p e s of the Andoso l shou ld e x p l a i n the h i g h l o s s e s , both expected and o b s e r v e d . The s h o r t e r r o t a t i o n e x e m p l i f i e d i n the Tepe ta te p r o b a b l y e x p l a i n s the d i f f e r e n c e w i t h the C a l i c h e , which o therwi se shou ld be l e s s e r o d i b l e ( i . e . , see t a b l e I V . 9 ) . F i n a l l y , i t shou ld be remembered that the s o i l s s t u d i e d here have d i f f e r e n t average s o i l d e p t h s . Under average management c o n d i t i o n s , the Tepe ta te s are the most sha l low of a l l , and a l s o the l e a s t e r o d i b l e . The Andoso l s are the deepest of a l l , and a l s o the most e r o d i b l e . Thus , the impact of e r o s i o n on these s o i l s must not o n l y be a f u n c t i o n of the r a te of e r o s i o n , but of the i n i t i a l depth of the s o i l s as w e l l . I_V. 8_. Summary T h i s chap te r d e s c r i b e d the e n v i r o n m e n t a l s e t t i n g for f i e l d t e s t s . S o i l s , c l i m a t e , and management parameters have been d i s c u s s e d . Three s o i l - b a s e d s i t e s of study were c h o s e n . They i n c l u d e a deep v o l c a n i c s o i l , a sha l low hardpan s o i l , and a medium b l a c k c l a y . Maize i n the area i s c u l t i v a t e d almost c o n t i n u o u s l y , and w i t h t r a d i t i o n a l management t e c h n i q u e s . An a p p l i c a t i o n of the U n i v e r s a l S o i l Loss E q u a t i o n p r e d i c t s h i g h r i s k s of e r o s i o n i n these maize f i e l d s . These p r e d i c t i o n s are d i s c u s s e d w i t h regard to l i t e r a t u r e and f i e l d d a t a . Whi le l i t e r a t u r e data would suggest tha t these r a t e s are too h i g h , f i e l d ev idence on t o p s o i l depths a c r o s s the range of s l ope s of these s o i I s . i n d i c a t e s that the magnitude of the p r e d i c t e d s o i l l o s s e s i s c o r r e c t . Under average management c o n d i t i o n s , s o i l l o s s e s w i l l be much g r e a t e r i n the Andoso l than in the o ther two 111 s o i l s , mos t ly because of i t s s teep s l o p e s . 1 1 2 CHAPTER V : AN EXPERIMENTAL APPROACH V . 1 . I n t r o d u c t i o n C r o p s , s o i l s , c l i m a t e and management, these a l l have an e f f e c t on l a n d p r o d u c t i v i t y . To assess the independent e f f e c t of s o i l e r o s i o n on l a n d p r o d u c t i v i t y may not be a s imple t a s k . Research i n so c o m p l i c a t e d a problem must proceed c a u t i o u s l y : o n l y a few f a c t o r s can be v a r i e d at a time i f e x p e r i m e n t a l r e s u l t s are to be p r o p e r l y i n t e r p r e t e d . A b r i e f a n a l y s i s of the key v a r i a b l e s for r e s e a r c h w i l l make that p o i n t c l e a r . I t has a l r e a d y been shown ( S e c t i o n 3.3) tha t s o i l e r o s i o n i s a slow proce s s which takes g rea t ca re and e f f o r t to measure. An i d e a l experiment on s o i l e r o s i o n and l a n d p r o d u c t i v i t y shou ld a l l o w two p l o t s i n the same s o i l to erode d i f f e r e n t i a l l y . A f t e r t h i s i s a c h i e v e d , a c r o p can be t e s t e d to assess p r o d u c t i v i t y i n the two s o i l s . With enough e x p e r i m e n t a l f a c i l i t i e s , perhaps s e v e r a l c r o p s and/or manager i a l p r a c t i c e s can be a l t e r n a t e d to e x p l o r e s i n g l e e f f e c t s and i n t e r a c t i o n terms . Only a handfu l of s t u d i e s have f o l l o w e d t h i s t e d i o u s but sound approach ( i . e . , r e c a l l Lamb et a l , 1950; Ketcheson and Webber, 1978). These s t u d i e s r e q u i r e d between ten and twenty year s to generate r e s u l t s for a few crops on a s i n g l e s o i l . Two a l t e r n a t i v e r e s e a r c h s t r a t e g i e s tha t have been more commonly used a r e : a) to survey s i t e s w i t h a l r e a d y d i f f e r e n t e r o s i o n a l h i s t o r i e s and s e t t i n g exper iments or making o b s e r v a t i o n on them; and b) to s i m u l a t e e r o s i o n by mechan ica l 1 1 3 means. Both these a l t e r n a t i v e s r e s o l v e the problem of w a i t i n g for the s o i l to erode by e i t h e r a) a measurement of s o i l d e p t h , or b) a cut to a s t andard d e p t h . S t i l l a c h o i c e has to be made about the crop-management-so i l type combina t ion to use , but the same treatment of e r o s i o n can be a p p l i e d r e p e a t e d l y . For an i n d i c a t i o n of the p o t e n t i a l number of e x p e r i m e n t a l c e l l s i n one of these exper iments ( l e t a lone r e p l i c a t i o n s ) , c o n s i d e r the f o l l o w i n g : 3 s o i l types x 2 c rops x 3 l e v e l s of N f e r t i l i z a t i o n x 3 l e v e l s of e r o s i o n = 56 t r e a t m e n t s . When s e v e r a l s o i l s have been i n c l u d e d i n one such exper iment , pot exper iments i n the greenhouse have been used because of the o b v i o u s l y g r e a t e r p o s s i b i l i t i e s for i n c r e a s i n g the number of e x p e r i m e n t a l u n i t s . A c c o r d i n g l y , in t h i s Chapter two s e r i e s of exper iments are p r e s e n t e d fo r the s o i l s d e s c r i b e d i n chapter I V . These exper iments e x p l o r e the s imul taneous response of maize y i e l d s to s o i l e r o s i o n and f e r t i l i z a t i o n . One s e r i e s was run i n the greenhouse ; t h e r e , s o i l s , l e v e l s of e r o s i o n and f e r t i l i z a t i o n , and water regimes were s t u d i e d . The o ther s e r i e s was run in the f i e l d ; t h e r e , on ly e r o s i o n and f e r t i l i z a t i o n were s 'tudied on three s o i l s . S e c t i o n V . 2 d e s c r i b e s the greenhouse exper iments and d i s c u s s e s the r e s u l t s . S e c t i o n V . 3 d e s c r i b e s the f i e l d exper iments and d i s c u s s e s the r e s u l t s . S e c t i o n V . 4 compares t h i s e x p e r i m e n t a l ev idence w i t h tha t rev iewed i n Chapter I I I . 1 1 4 V.2. E r o s i o n and P r o d u c t i v i t y i n the Greenhouse The greenhouse exper iments r e p r e s e n t a compromise between a c c u r a c y and p r e c i s i o n , s t a t i s t i c a l l y s p e a k i n g . They attempt to mimic the r e a l wor ld and at the same time to g a i n i n s i g h t i n t o the v a r i o u s proces se s under s t u d y . Pots are seen here as m i n i a t u r e farms i n which e r o s i o n , f e r t i l i z a t i o n , and water management are a l l s i m u l a t e d to match the r e a l wor ld d e s c r i b e d i n Chapter IV . With the s e l e c t i o n of c r o p and c u l t i v a t i o n p r a c t i c e s , a model maize f i e l d i s d e f i n e d for study i n the greenhouse . Greenhouse exper iments a l l o w f i n e t u n i n g of e n v i r o n m e n t a l parameters and r a p i d checks of e x p e r i m e n t a l prob lems . They a l s o permit the s e l e c t i o n of many t rea tments and numerous r e p l i c a t i o n s , a l l of which enhance the scope of r e s e a r c h and the p r e c i s i o n of r e s u l t s . Two pot exper iments were des igned to t e s t four hypothese s : i ) Maize y i e l d s are d i f f e r e n t for the d i f f e r e n t s o i l s i n c l u d e d i n t h i s s tudy . i i ) Maize y i e l d s decrease w i t h e r o s i o n , r e g a r d l e s s of s o i l t y p e . i i i ) F e r t i l i z a t i o n tends to compensate the r e d u c t i o n of y i e l d s produced by e r o s i o n . i v ) Water s t r e s s reduces even more the y i e l d s i n i , i i , and i i i . 1 1 5 V . 2 . K M a t e r i a l s and methods S t a t i s t i c a l l y s p e a k i n g , the greenhouse exper iments are compounded f a c t o r i a l s i n a c o m p l e t e l y randomized d e s i g n ( H i c k s , 1973; Ch 8 ) . The exper iments are compounded because—us ing a b a s i c l a y - o u t - - t w o se t s of t rea tments were i n c l u d e d : 1) a set w i t h e r o s i o n and e i g h t s o i l s ; 2) a set w i t h e r o s i o n , f e r t i l i z e r s , water , and 3 s o i l t y p e s . Exper iment 1 can be modeled as f o l l o w s : Y i j k = U + S i +ERj + S E R i j + E k ( i j ) (5 .1) where: Y = biomass y i e l d of maize ( g / p o t ) ; U = p o p u l a t i o n mean; S i = s o i l t y p e , i =1 ,8 ; ERj = e r o s i o n , j=1,3;" Ek = e r r o r , k=1,5. T h i s experiment i s an 8 ( s o i l s ) x 3 ( e r o s i o n l e v e l s ) c o m p l e t e l y randomized f a c t o r i a l w i t h f i v e r e p l i c a t i o n s per c e l l . N e i t h e r f e r t i l i z e r s nor water t rea tments were i n c l u d e d i n experiment 1. Equa t io n 5.1 shows that t h i s experiment c o n t a i n s o n l y one f i r s t order i n t e r a c t i o n , i . e . , s o i l type x e r o s i o n . Exper iment 2 i n c l u d e s s o i l s , e r o s i o n , f e r t i l i z a t i o n , and water reg imes . The s t a t i s t i c a l model for experiment 2 can be w r i t t e n as f o l l o w s : 116 Y i j k l m = U + S i +ERj + S E R i j + Fk + SF ik + ERFj k + SERFi jk + Wl + SWi l + DWJ1+ FWkl + SERWi j l + ERFWjkl + SFWikl + SERFWi jk l + E n ( i j k l ) (5 .2) where: Y = biomass y i e l d of maize ( g / p o t ) ; U = p o p u l a t i o n mean; S i = s o i l t y p e , i =1 ,3 ; ERj = e r o s i o n , j=1 ,3 ; Fk = f e r t i l i z a t i o n , k=1,3; Wl = water reg ime, 1=1,2; En = r e p l i c a t i o n s , n=1,5. T h i s second experiment i s a 3 ( s o i l type) x 3 ( e r o s i o n l e v e l s ) x 3 ( f e r t i l i z a t i o n l e v e l s ) x 2 (water regimes) c o m p l e t e l y randomized f a c t o r i a l w i t h f i v e r e p l i c a t i o n s per c e l l . E q u a t i o n 5.2 shows 6 f i r s t order- i n t e r a c t i o n s , 5 of second o r d e r , and 1 of t h i r d o r d e r . A l l po t s were l a i d out i n a c o m p l e t e l y randomized d e s i g n . Each pot r e c e i v e d a number and was a s s i g n e d a p o s i t i o n i n a g r i d marked on the greenhouse f l o o r u s i n g a t a b l e of random numbers. As r e p l i c a t i o n was the o n l y random f a c t o r i n these exper iment s , the f i x e d e f f e c t s , ANOVA model type I a p p l i e s to the ensu ing d a t a . Data were p r o c e s s e d at the UBC Computer C e n t r e . A l l a n a l y s e s were performed w i t h the v a r i a n c e - c o v a r i a n c e program MFAV (Le , 1980). 1 1 7 V . 2 . 1 . 1 . Greenhouse f a c i l i t i e s A 10 x 10 m temporary greenhouse was b u i l t for these exper iments i n the C l a v i j e r o B o t a n i c a l Garden i n X a l a p a , V e r a c r u z , M e x i c o . T h i s s i t e i s 1380 m above sea l e v e l and has a humid warm temperate c l i m a t e (see S e c t i o n I V . 2 ) . The greenhouse had a t r a n s p a r e n t p l a s t i c roof on a t i l t e d , wooden s t r u c t u r e . The four s i d e s of the greenhouse were kept open to prevent o v e r h e a t i n g d u r i n g hot summer days ( R i t c h e y , 1973). The f l o o r was covered w i t h one i n c h of v o l c a n i c g r a v e l . Pots were p o s i t i o n e d on t h i s g r a v e l i n a g r i d c o n t a i n i n g 345 p o i n t s , each s epara ted by 50 cm i n t e r v a l s . U n l i n e d c l a y pots were used i n the e x p e r i m e n t s . The pots were o b t a i n e d from a manufacturer of garden s u p p l i e s i n X a l a p a , and had t r u n c a t e d cone shapes which measured: h e i g h t , 18.4 cm; d iameter at the t o p , 25.6 cm; d iameter a t the base , 15.5 cm; f u l l volume, 6468 c m 3 . S l i g h t v a r i a t i o n s i n pot weights were observed ( i . e . , 7.3 per cent v a r i a t i o n around a mean weight of 3.08 kg ) , but s i n c e none of the o t h e r d imens ions v a r i e d much, i t was c o n c l u d e d that the d i f f e r e n c e was caused by p o t s ' w a l l t h i c k n e s s e s . As pot weight was an important datum for m o n i t o r i n g water ing and p o t t i n g , a l l pots were weighed be fore the exper iments began. Pot d r a i n a g e was p r o v i d e d by a s i n g l e h o l e (2 .5 cm i n d iameter ) at the c e n t e r bottom of each p o t . 118 V . 2 . 1 . 2 . S o i l s A l l s o i l s r e p o r t e d i n Appendix 2 were r e p r e s e n t e d i n these e x p e r i m e n t s . F i g u r e 4 .1b shows the sampl ing s i t e s for these s o i l s . The Andoso l has two v a r i a n t s , the Tepeta te has one, and the C a l i c h e two. One a d d i t i o n a l v a r i a n t i s t r a n s i t i o n a l between the Andoso l s and the T e p e t a t e s . V a r i a n t s were s e l e c t e d so as to i n c l u d e d e p o s i t i o n a l and e r o s i o n a l phases of the three main s o i l t y p e s . As Tepeta te s were a l r e a d y so s h a l l o w , o n l y one v a r i a n t c o u l d be r e c o g n i z e d . The aim of these exper iments was to study the impact of s o i l e r o s i o n on the pot maize y i e l d s of up to e i g h t d i f f e r e n t s o i l s . I t was thus necessary to s t a n d a r d i z e the l e v e l s of the e r o s i o n t r e a t m e n t . For a l l s o i l s , the topmost 30 cm of the p r o f i l e were d e f i n e d as c o n t r o l s or uneroded, the f o l l o w i n g 15-45 cm were d e f i n e d as e roded , and the lower 30-60 cm of the p r o f i l e were d e f i n e d as s e v e r e l y e r o d e d . Thus , e r o s i o n was produced by s u c c e s s i v e l y removing 15 c m - t h i c k l a y e r s of s o i l . These t h r e e l e v e l s of e r o s i o n were genera ted through a s p e c i a l sampl ing program. In each of the e i g h t s o i l s i t e s , a p i t 1 m deep and 50 cm wide was opened. The topmost s l i c e of s o i l (0-30 cm) was dug out f i r s t . The 15-45 cm l a y e r was removed from the exposed w a l l l e f t by the removal of the 0-30 cm s l i c e , a f t e r the topmost 15 cm had been scraped away. The f i n a l s l i c e , 30-60 cm, was shove led from the p e d e s t a l l e f t by the o ther two samples , i . e . , from the bottom of the 0-30 cm c u t . Care was taken to a v o i d c o n t a m i n a t i n g these bulk samples w i t h one another or wi th the s p o i l s of the e x c a v a t i o n . Approx imate ly 1 19 250 kg of mois t s o i l were needed from each l a y e r i n a s o i l t y p e , and 50 kg i n a v a r i a n t . The m a t e r i a l thus c o l l e c t e d was put i n t o p r o p e r l y l a b e l l e d sacks and t r a n s p o r t e d to the greenhouse . Once t h e r e , the samples were spread over p l a s t i c sheets and a i r d r i e d for 3 to 5 days . F i n a l mo i s ture c o n t e n t s i n a l l sample v a r i e d between 6 and 9 %. Samples were raked for s tones b igger than 2.5 cm. S o i l crumbs were a l s o c rushed when l a r g e r than 2.5 cm. B i g aggregates were e l i m i n a t e d because pot volumes were a l r e a d y too s m a l l , and p l a n t r o o t s would have been f u r t h e r impeded by b i g c l a s t s and crumbs. C o n v e r s e l y , g r i n d i n g beyond 0.5 cm was a v o i d e d because t h i s would have caused too much d i s r u p t i o n of the s o i l s ' a g g r e g a t i o n c h a r a c t e r i s t i c s . When samples were d r y , they were t h o r o u g h l y mixed , and r e a d i e d for p o t t i n g . P r e l i m i n a r y t e s t s showed a range of p o t t i n g d e n s i t i e s among s o i l s . A s o i l volume of 5.5 1 was used as the y a r d s t i c k for p o t t i n g . T h i s volume l e f t enough room i n the pot s fo r the s o i l s to expand when wet and for water to be s u p p l i e d w i t h ease . Dry weight e q u i v a l e n t s of t h i s volume were worked out for every s o i l . T h i s e q u i v a l e n t depends on the bulk d e n s i t y and p a c k i n g d e n s i t y of the s o i l , both of which were a s c e r t a i n e d i n p o t t i n g t r i a l s . S o i l p o t t i n g weights ranged from 4.45 kg i n an Andoso l to 4 . 8 7 . k g i n a T e p e t a t e . Ju s t be fore p o t t i n g , a 2.5 cm t h i c k l a y e r of f i n e v o l c a n i c g r a v e l was l a i d on the bottom of each pot to f a c i l i t a t e d ra inage and a e r a t i o n . The s o i l s were then shaken i n t o the pot s w i t h three s t r o k e s . When f e r t i l i z e r s were to be added, these were f i r s t mixed w i t h the s o i l i n a l i d d e d c a n , and the mix ture was 1 20 p o t t e d a f t e r w a r d s . Two days be fore s e e d i n g , pots were p l a c e d on the greenhouse g r i d and s a t u r a t e d w i t h water . V . 2 . 1 . 3 . F e r t i l i z a t i o n Exper iment 2 t e s t e d three l e v e l s of n i t r o g e n (N) and phosphorus ( P ) : c o n t r o l , medium and h i g h . These were de s igned to match, r e s p e c t i v e l y , the n o n - e x i s t e n t , modest, and h i g h f e r t i l i z a t i o n l e v e l s r e p o r t e d l y used i n the l o c a l maize f i e l d s (SARH, 1980). Lime was a l s o s u p p l i e d to the a c i d i c s o i l s . Pot f e r t i l i z a t i o n r a te s were c a l c u l a t e d on a s u r f a c e b a s i s and c o r r e s p o n d wi th N/P f i e l d r a t e s of 150/100 and 300/200 kg h a " 1 , r e s p e c t i v e l y . . In f a c t , these r a t e s are double those commonly used i n the l o c a l maize f i e l d s , i . e . , a p p r o x i m a t e l y 75/50 for medium and 150/100 fo r h i g h . Pot r a t e s were doubled i n t h i s experiment because i t i s known that f e r t i l i z e r s are a p p r o x i m a t e l y h a l f as e f f i c i e n t in s m a l l pots as i n the f i e l d (Terman and M o r t v e d t , 1978). The Andoso l s and the Tepe ta te s were l imed at a r a t e comparable to 4 t h a - 1 . N i t r o g e n sources were n i t r a t e and ammonium s u l f a t e . The former was s u p p l i e d be fore p l a n t i n g in a formula (18-46-0) c o n t a i n i n g a l l the P as t r i p l e superphosphate . The ammonium s u l f a t e was a p p l i e d when p l a n t s were two weeks o l d . Lime was c o n t a i n e d i n a commercia l mixture ( C a l A g r i c o l a , Guanomex) as h y d r o x i d e of c a l c i u m and sodium and c a l c i u m c a r b o n a t e . In t o t a l , 0.531 g of N , 0.353 g of P, and 14.12 g of l ime were g i v e n to each pot w i t h low ra te of f e r t i l i z a t i o n . The h i g h f e r t i l i z a t i o n ra te c o n t a i n e d double the amount of N and P. 121 V . 2 . 1 . 4 . Water ing procedure The w a t e r i n g treatment r e q u i r e d c a r e f u l p l a n n i n g . A meaningfu l and p r a c t i c a l water s t r e s s t reatment was needed: m e a n i n g f u l , t h a t i s , w i t h regard to the water h o l d i n g p r o p e r t i e s of n ine d i f f e r e n t s o i l s . These , expres sed as water h e l d at f i e l d c a p a c i t y (WFC), ranged from between 50 and 60 per cent i n the c l a y e y C a l i c h e s , to between 20 and 30 per cent i n the sandy loam T e p e t a t e s . The h i g h l y o r g a n i c Andoso l s f e l l i n the middle of t h i s range wi th between 30 and 50 per cent WHC. These water c o n t e n t s , s o i l by s o i l , c o r r e s p o n d to the f i e l d c a p a c i t y t r e a t m e n t . The water s t r e s s t reatment was d e f i n e d as 60 per cent of the water which each s o i l h e l d at f i e l d c a p a c i t y . A p r a c t i c a l water t reatment was needed to a s ses s the water l e v e l s of 345 pots d u r i n g the exper iment . To t h i s end, two pots from each t reatment were s e l e c t e d at random every o ther day, weighed, and t h e i r water c o n t e n t s a s s e s s e d . The procedure worked w e l l throughout most of the exper iment . On very hot days , or when maize was growing very f a s t , the water c o n t e n t s were a s se s sed every day, and e x t r a water was added to those pots showing the g r e a t e s t consumpt ion . Tap water was used to i r r i g a t e the s o i l sur f ace a c c o r d i n g to t reatment needs . Leak ing at the bottom of the pot s was r a r e l y o b s e r v e d . Leak ing at the top d i d occur a f t e r some stormy winds damaged the p l a s t i c r o o f : d r i p p i n g s a t u r a t e d some pot s w i t h water , which a f terwards were c a r e f u l l y watched and a d j u s t e d to s t a n d a r d s . 122 V . 2 . 1 . 5 . Seeds and c u l t i v a t i o n methods Seeds of c r e o l e maize (Zea mays L . v a r . c o n i c o ) were used i n a l l these exper iment s . These seeds were o b t a i n e d from farm s t o r e s i n X a l a p a , V e r a c r u z . There are no sources of c e r t i f i e d seed i n the a r e a ; these would have been p r e f e r r e d to reduce e x p e r i m e n t a l e r r o r . C6nico v a r i e t y i s adapted to s u b t r o p i c a l and warm temperate c l i m a t e s w i t h p a r t i c u l a r l y long growing seasons . A t e s t was conducted to determine the v a r i a b i l i t y i n g e r m i n a t i o n of d i f f e r e n t s e l e c t i o n s of these seeds . G r a i n s from the cen te r of the earcone were s o r t e d i n t o groups of two d i f f e r e n t s i z e s , two c o l o r s , and s c a r i f i c a t i o n or no s c a r i f i c a t i o n of the embryonic apex. R e s u l t s showed no s i g n i f i c a n t d i f f e r e n c e among any of these groups ( X 2 : P<0.05) and , on average , 95 per cent of the seeds g e r m i n a t e d . C o n s e q u e n t l y , the same s e l e c t i o n procedure was f o l l o w e d to o b t a i n enough seeds for the e x p e r i m e n t s . Seeding took p l a c e on 2 8 / 7 / 8 1 . Four seeds were p l a n t e d 5 cm deep in the c e n t e r of each p o t . G e r m i n a t i o n was c l o s e l y watched d u r i n g the f o l l o w i n g week. By then most pot s had produced at l e a s t three s e e d l i n g s . The few pots i n which g e r m i n a t i o n f a i l e d were prompt ly re seeded . Two weeks a f t e r s e e d i n g , a l l pot s were t h i n n e d to two p l a n t s each . The h e i g h t of the t a l l e s t p l a n t i n each pot was measured every week, from base to t i p of u p - s t r e t c h e d l e a v e s . 123 V . 2 . 1 . 6 . Weed and pes t c o n t r o l Every week weeds were removed, a l t h o u g h few were n o t i c e d . Pes t s were more p r o b l e m a t i c . R a b b i t s e n t e r e d the greenhouse soon a f t e r maize had ge rmina ted . S e e d l i n g s i n a p p r o x i m a t e l y 15 pots were damaged and these pot s had to be subsequent ly r e s e e d e d . To prevent f u r t h e r a t t a c k s , a c h i c k e n wire fence was b u i l t around the greenhouse . D u r i n g the exper iment , two s p r a y i n g s w i t h M a l a t h i o n were g i v e n to a l l pot s to prevent i n s e c t a t t a c k s . V . 2 . 1 . 7 . H a r v e s t i n g procedures E i g h t weeks a f t e r sowing date (11 /9 /81 ) , a l l pot s were h a r v e s t e d . The two p l a n t s of each pot were cut at the base , c l i p p e d i n t o s m a l l e r p i e c e s , and put i n t o paper bags. A l l samples were d r i e d on a herbar ium p l a n t d ryer u n t i l they reached cons t an t we ight - -about two days a f t e r h a r v e s t . The samples were then o v e n - d r i e d at 80 ° C , for a n i g h t . Next morning , p l a n t weights were recorded to the neare s t m i l l i g r a m . T h e r e f o r e , i n these exper iment s , y i e l d means the oven dry weights of two p l a n t s of c r e o l e maize grown for s i x weeks i n 5.5 1 p o t s . V.2^.2^. R e s u l t s and d i s c u s s i o n S e c t i o n V . 2 . 3 . 1 p r e s e n t s the r e s u l t s of experiment 1, where e i g h t s o i l s were s t u d i e d at three l e v e l s of e r o s i o n . Two hypotheses w i l l be c o n s i d e r e d : i ) y i e l d s d i f f e r among s o i l s , and i i ) y i e l d s are reduced through e r o s i o n . S e c t i o n V . 2 . 3 . 2 1 24 p r e s e n t s the r e s u l t s of experiment 2, where three s o i l s , three l e v e l s of e r o s i o n , three l e v e l s of f e r t i l i z a t i o n , and two water regimes were combined. The two hypotheses d i s c u s s e d a r e : i i i ) f e r t i l i z a t i o n compensates y i e l d l o s s e s due to e r o s i o n , and i v ) water s t r e s s reduces y i e l d s i n a l l c i r c u m s t a n c e s . V . 2 . 2 . 1 . Experiment 1, e r o s i o n e f f e c t s on maize growth and y i e l d s As F i g u r e 5.1a shows, maize p l a n t biomass i s w e l l c o r r e l a t e d w i t h p l a n t h e i g h t s (see a l s o Marten and Sancho luz , 1981). I t i s thus p o s s i b l e to c o n s i d e r the h e i g h t s p l o t t e d i n F i g . 5 .1b-g as s u r r o g a t e s of biomass p r o d u c t i o n . In g e n e r a l , F i g . 5 . 1 shows that maize growth r a t e s decreased as the experiment p r o g r e s s e d . The exper iment was c o n c l u d e d a f t e r s i x weeks, because some p l a n t s i n the p o o r e s t - y i e l d i n g t rea tment s were a l r e a d y d y i n g o f f . Only maize grown on one or two s o i l s showed t y p i c a l growth c u r v e s , i . e . , f i g . 5 . 1 h , and perhaps f i g . 5 . 1b . Given the shor t p e r i o d of t ime i n v o l v e d , t h i s q u a s i - g e n e r a l t r e n d of d e c l i n i n g maize growth r a t e s must be a t t r i b u t e d to the l i m i t i n g e f f e c t s of s m a l l pot volumes . However, uneroded s o i l s d i d b e t t e r than the eroded ones i n a l l c a s e s . D i f f e r e n c e s i n maize growth were g r e a t e r by the end of the exper iment . T a b l e V.1 shows the a n a l y s i s of v a r i a n c e for t h i s exper iment . Maize y i e l d s were s i g n i f i c a n t l y d i f f e r e n t among s o i l s and a c r o s s e r o s i o n l e v e l s . Note a l s o that the i n t e r a c t i o n between these two e x p e r i m e n t a l f a c t o r s was not s i g n i f i c a n t , which means tha t s o i l s ' y i e l d s decreased i n a s i m i l a r f a s h i o n w i t h e r o s i o n . 125 F i g u r e 5 . 1 . Maize growth p a t t e r n s i n exper iment 1. 5 .1a , c o r r e l a t i o n between maize p l a n t s h e i g h t s and biomass . 5 . 1 b - i , p l a n t Growth p a t t e r n s , p l a n t h e i g h t s are averages of 10 p l a n t s . 126 1 27 Tab le V . K ANOVA fo r y i e l d s of experiment 1 Source DF MS F 1 S o i l ( S ) 7 1 .745 15.3 ** E r o s i o n ( E ) 2 3.331 29.2 ** S x E 1 4 0. 109 1 .0 ns E r r o r 96 0.114 1 * * s i g n i f i c a n t (P<0.01) ; n s : non s i g n i f i c a n t F i g u r e 5.2 p l o t s maize y i e l d s for experiment 1. The p o o r e s t -y i e l d i n g s o i l s , as e x p e c t e d , were the T e p e t a t e s . Whi le the C a l i c h e s d i d b e t t e r than a n t i c i p a t e d , the Andoso l s d i d r e l a t i v e l y worse . For a l l s o i l s , the e r o s i o n a l phase was l e s s p r o d u c t i v e than e i t h e r the normal or d e p o s i t i o n a l phase . Not a l l i n d i v i d u a l s o i l s showed s i g n i f i c a n t d i f f e r e n c e s i n y i e l d s ; r a t h e r , t h e r e were four d i s t i n c t maize y i e l d i n g groups of s o i l s . T h i s o n l y p a r t i a l l y suppor t s hypothes i s - i ) , namely that a l l s o i l s s h o u l d y i e l d d i f f e r e n t l y . A l t e r n a t i v e l y , had a g r e a t e r number of r e p l i c a t e s been p r o v i d e d i n t h i s exper iment , the chances of p i c k i n g up s i g n i f i c a n t d i f f e r e n c e s w i t h t e s t s on means would have i n c r e a s e d . O v e r a l l , means for the three l e v e l s of e r o s i o n ( i . e . , 0-30cm, 15-45cm, and 30-60 cm depth i n t e r v a l s ) were 1.21, 0 .803, and 0.653 g / p o t , r e s p e c t i v e l y . These are a l l s i g n i f i c a n t l y d i f f e r e n t (Duncan's m u l t i p l e range t e s t , P<0.01) . Y i e l d s wi thout 15 cm of t o p s o i l are o n l y 66 per cent of the y i e l d s of the t o p s o i l s . I f 15 cm more are s c raped these s o i l s respond w i t h a f u r t h e r d e c l i n e of 12 per c e n t , to end up y i e l d i n g o n l y 128 F i g u r e 5 . 2 . Y i e l d s i n pot exper iment 1. a ) ' A n d o s o l , b) T e p e t a t e , c) C a l i c h e . Numbers c o r r e p o n d are for d i f f e r e n t s i t e s , d e s c r i b e d i n Append ix2 . 130 54 per cent of what the t o p s o i l s y i e l d . A p p a r e n t l y , the topmost 15 cm l a y e r of these s o i l s i s the most p r o d u c t i v e of them a l l . These f i n d i n g s o b v i o u s l y support h y p o t h e s i s i i ) which proposed tha t e r o s i o n should reduce y i e l d s , r e g a r d l e s s of s o i l t y p e . V . 2 . 2 . 2 . Experiment 2, e r o s i o n , f e r t i l i z a t i o n , and water e f f e c t s on y i e l d s T a b l e V . 2 shows the a n a l y s i s of v a r i a n c e of the r e s u l t s . The p a t t e r n of f a c t o r s ' e f f e c t s i s c o m p l i c a t e d by the number of s i g n i f i c a n t i n t e r a c t i o n s . Almost a l l terms i n the model ( i . e , E q u a t i o n 5.2) were s t a t i s t i c a l l y s i g n i f i c a n t . T h i s i n d i c a t e s tha t the e f f e c t s of e r o s i o n , water and f e r t i l i z e r on y i e l d s are i n t e r d e p e n d e n t . A change i n one of these f a c t o r s a l t e r s the e f f e c t of another on y i e l d s . F i g u r e 5.3 d e p i c t s the i n t e r a c t i o n between s o i l , f e r t i l i z e r and e r o s i o n . These y i e l d - i s o c l i n e diagrams show the s imul taneous response of y i e l d s to e r o s i o n , a b s c i s s a , and f e r t i l i z a t i o n , o r d i n a t e , for the three s o i l s under study (graphs a , b and c ) . E x t r a v a l u e s are computer i n t e r p o l a t i o n s which show a c l e a r e r p i c t u r e of the i n t e r a c t i o n s . The f l a t y i e l d response to e r o s i o n i n the Andoso l ( F i g . 5 . 3 a ) c o n t r a s t s wi th the more, i n c l i n e d response i n the Tepe ta te ( F i g . 5 . 3 b ) , and w i t h the n o t i c e a b l y t i l t e d response i n the C a l i c h e ( F i g . 5 . 3 c ) . An upward t i l t of these curves i n d i c a t e s that more f e r t i l i z e r s are needed to keep y i e l d s cons t an t as e r o s i o n p r o g r e s s e s . Hence, the e f f e c t s of e r o s i o n can be more e a s i l y compensated wi th 131 F i g u r e 5.3. I n t e r a c t i o n between s o i l type, f e r t i l i z e r and e r o s i o n , experiment 2. Curves are computer i n t e r p o l a t i o n s between data p o i n t s with equal y i e l d s ( g / p o t ) . 5.3a, Andosol, 5.3b, Tepetate, 5.3c, C a l i c h e . highf med nonet Andosol .47 (g/pot) (a) LU > high < UJ med none (b) high med: none (c) E R OS ION (cm) 133 T a b l e V . 2 . ANOVA of y i e l d s for experiment 2 Source DF MS F S o i l ( S ) 2 26.67 23.3 ** E r o s i o n ( E ) 2 90.72 79.2 ** F e r t i l i z e r ( F ) 2 340.12 296.9 ** Water(W) 1 109.32 95.4 ** S x E 4 22.30 19 . 4 ** S x F 4 17.57 15.3 ** S x W 2 11.51 9.7 ** E x F 4 17.97 15.7 ** E x W 2 5.93 5.2 ** F x W 2 26.66 23.3 ** S x E x F 8 5.95 5.2 ** S x E x W 4 1 .96 1 .7 ns S X F X W 4 3.39 3.0 * E x F x W 4 1 .29 1 .3 ns S x E x F x W 8 0.89 0.8 ns E r r o r 216 1.15 1 n s : non s i g n i f i c a n t ; * : P<0.05; * * : P<0.01 f e r t i l i z e r s i n the Tepeta te than i n the C a l i c h e ; , a p p a r e n t l y , the p r o d u c t i v i t y of the Andoso l d i d not s u f f e r from e r o s i o n . Average y i e l d s were 4.78 g i n the T e p e t a t e , 3.92 g i n the A n d o s o l , and 2.93 g i n the C a l i c h e . The T e p e t a t e , which when u n f e r t i l i z e d y i e l d e d the l e a s t i n the p r e v i o u s exper iment , when f e r t i l i z e d became the h i g h e s t y i e l d i n g . T h i s i s a somewhat p u z z l i n g r e s u l t , which for the moment can o n l y be i n t e r p r e t e d as a r i s i n g from the c o m p a r a t i v e l y low i n t r i n s i c f e r t i l i t y of the T e p e t a t e . F i g u r e 5.4 shows the s i x , h i g h l y s i g n i f i c a n t , second order i n t e r a c t i o n s . F i g u r e 5.4a f u r t h e r emphasizes the almost complete l a c k of response of y i e l d s to e r o s i o n i n the A n d o s o l . T h i s i s a l s o c o n t r a s t e d w i t h the sharp d e c l i n e i n y i e l d s i n the 1 34 C a l i c h e . F i g u r e 5.4b shows s l i g h t l y d i f f e r e n t y i e l d responses to f e r t i l i z a t i o n among s o i l s . The C a l i c h e ' s poor response to the h i g h l e v e l of f e r t i l i z a t i o n i s worth n o t i c i n g . F i g u r e 5 . 4 c , on the o ther hand, shows tha t f e r t i l i z e r e f f i c i e n c y decreased w i t h i n c r e a s i n g e r o s i o n . For example, i t would take twice as much f e r t i l i z e r t o make the y i e l d s of a s o i l m i s s i n g 15 cm equa l the y i e l d s of an uneroded s o i l . A s i m i l a r t r a d e - o f f a p p l i e s for the l o s s of an a d d i t i o n a l 15 cm of t o p s o i l . However, the f i r s t l e v e l of f e r t i l i z a t i o n (N+P i n f i g 5 .5c) produced a sharp y i e l d re sponse , r e g a r d l e s s of e r o s i o n . For t h i s p a r t i c u l a r c a se , i t must be s a i d tha t f e r t i l i z e r s amply compensated for the l o s s e s to e r o s i o n . But i t shou ld a l s o be remembered tha t the N+P treatment i n c l u d e d q u i t e a h i g h dosage of these n u t r i e n t s , because i t was i n i t i a l l y thought tha t f e r t i l i z e r e f f i c i e n c y i n s m a l l pot s was about h a l f tha t of the f i e l d . Indeed, Terman and Mortvedt (1978) suggest t h i s r a t i o for maximum pot y i e l d s . An informed reader would have by now n o t i c e d tha t t h i s was not the case i n these exper iment s : an average y i e l d of 4g/pot i s l e s s than h a l f the y i e l d s r e p o r t e d by these au thor s as o p t i m a l maize y i e l d s i n s m a l l p o t s . Of c o u r s e , there was no i n t e n t i o n of a t t a i n i n g o p t i m a l v a l u e s i n these e x p e r i m e n t s , which i n c l u d e d e r o s i o n and l a c k of f e r t i l i z a t i o n among o ther t r e a t m e n t s . Hypotheses i i i ) and i v ) have been a l r e a d y d i s c u s s e d . That f e r t i l i z e r s compensate the d e p r e s s i n g e f f e c t s of e r o s i o n on y i e l d s i s i n g e n e r a l suppor ted by the r e s u l t s of Experiment 2. In t h i s r e s p e c t , the Andoso l showed the g r e a t e s t f l e x i b i l i t y . 135 F i g u r e 5 .4 . Second order i n t e r a c t i o n s for exper iment 2. 5 . 4a , S o i l s ( A n d o s o l , T e p e t a t e , C a l i c h e ) vs E r o s i o n (none, 15 cm, 30 cmT. 5 . 4 b , S o i l s vs F e r t i l i z a t i o n (0, NP, 2NP). 5 . 4 c , E r o s i o n vs F e r t i l i z a t i o n . 5 . 4 d , S o i l s vs Water regime ( F i e l d c a p a c i t y , 60% F i e l d C a p a c t i t y ) . 5 . 4e , E r o s i o n vs Water reg ime. 5 . 4 f , F e r t i l i z e r s vs water reg ime. 136 o a none med high Fert i l i za t ion none med high Fert i lization F Capacity Stress Water regime co Q _j LU >-LU N F'Capacity Stress Water regime 5i Ferti medium U none —I F Capacity Water regime Stress 1 37 The Tepe ta te and the C a l i c h e s u f f e r e d r e l a t i v e l y more from e r o s i o n and r e q u i r e d more f e r t i l i z e r s to s u s t a i n y i e l d s . Were i t not for the sharp response of y i e l d s to the f i r s t l e v e l of f e r t i l i z a t i o n , a symmetr i ca l response of y i e l d s to e r o s i o n and f e r t i l i z a t i o n would have been found . The p r o b l e m a t i c symmetry i s as f o l l o w s : for every 15 cm of s o i l l o s s , twice as much f e r t i l i z e r i s r e q u i r e d to keep y i e l d s unchanged (see F i g . 5 . 5 c ) . T h i s symmetry may s t i l l be a p p l i c a b l e . If we concede that p l a n t n u t r i t i o n i n sma l l pots i s the l i m i t i n g f a c t o r , then f e r t i l i z e r s can w e l l o f f s e t t h i s l i m i t a t i o n and open ways for b e t t e r p r o d u c t i o n . A f t e r p l a n t n u t r i t i o n was amended wi th the N/P t r e a t m e n t , e r o s i o n s t i l l h a l v e d p r o d u c t i o n , and 2(N/P) were needed to double y i e l d s a g a i n . That water s t r e s s depres ses y i e l d s even f u r t h e r seems p l a u s i b l e and w e l l supported by the r e s u l t s . Every s i n g l e f a c t o r i n Experiment 2, i . e . , s o i l , e r o s i o n , f e r t i l i z a t i o n , and water , had a s i g n i f i c a n t e f f e c t on maize y i e l d s (see Tab le V . 2 ) . The extent of s i g n i f i c a n c e i s s u s p e c t , however, because the r e l e v a n t i n t e r a c t i o n terms were a l s o s i g n i f i c a n t . -Under these c i r c u m s t a n c e s , s t a t i s t i c i a n s would recommend that the i n t e r a c t i o n s - - n o t the f a c t o r s be i n t e r p r e t e d — w h i c h i s p r e c i s e l y what has been done so f a r . However, from Table V . 2 i t can be c a l c u l a t e d tha t f e r t i l i z e r s , water , and e r o s i o n are s i g n i f i c a n t even when t e s t e d a g a i n s t t h e i r r e s p e c t i v e s i g n i f i c a n t i n t e r a c t i o n terms (F v a l u e s > 3, have P<0.05) . T h i s i s something tha t an s t a t i s t i c i a n shou ld a l s o agree i s proof of the d i f f e r e n c e s among l e v e l s o f each 1 38 f a c t o r . V.2.3. P r e l i m i n a r y c o n c l u s i o n s : greenhouse I t was mentioned in the i n t r o d u c t i o n , tha t these exper iments were a compromise between p r e c i s i o n and a c c u r a c y . P r e c i s i o n has been a c h i e v e d through the r e l a t i v e l l y grea t number of e x p e r i m e n t a l c e l l s and r e p e t i t i o n s c o n t a i n e d i n these exper iment s . For t h i s , the c o m p l e t e l y randomized f a c t o r i a l d e s i g n deserves c r e d i t . A c c u r a c y i s s u s p e c t . The tremendous response of y i e l d s to f e r t i l i z a t i o n seems u n r e a l . F e r t i l i z a t i o n a lone accounts for 40 per cent of the v a r i a n c e r e p o r t e d i n Tab le V . 2 . I t i s a l s o t rue that there was a b e t t e r y i e l d response to f e r t i l i z e r s i n the uneroded s o i l s (see F i g . 5 . 4 c ) , and , on b a l a n c e , there was a net y i e l d l o s s fo r each e r o s i o n l e v e l . T a b l e V . 3 compares the e f f e c t s of v a r y i n g l e v e l s of e r o s i o n on maize y i e l d s in both e x p e r i m e n t s . Columns 2 and 4 of t h i s t a b l e , show a comparable decrease of y i e l d s i n both greenhouse e x p e r i m e n t s . T h i s suggests t h a t , r e g a r d l e s s of the use of f e r t i l i z e r s , e r o s i o n of 15 cm of t o p s o i l reduced y i e l d s by about 68 per c e n t . Another 15 cm of s o i l lowered y i e l d s about 50 per cent more. Column 5 i n t a b l e V . 3 compares the y i e l d s of both e x p e r i m e n t s . Y i e l d s wi thout f e r t i l i z e r s were o n l y 30 per cent those w i t h f e r t i l i z e r s , both i n the noneroded and modera te ly eroded s o i l s . T h i s percentage rose to 55 i n the s e v e r e l y eroded 1 39 Tab le V.3_. y i e l d s and per cent r e d u c t i o n 1 due to e r o s i o n i n pot exper iments 1, and 2 L e v e l of Y i e l d Reduc. Y i e l d Reduc. Y1/Y2 e r o s i o n Exp 1 Exp 1 Exp 2 Exp 2 (cm) (g /pot) % (g /pot ) % % (1) (2) (3) (4) (5) non-eroded (0) 1 .21 3.93 -- 31 moderate (15) 0.80 66 2.71 69 29 severe (30) 0.65 54 1 .93 49 55 1 % based on the y i e l d s of the non-eroded t r e a t m e n t . s o i l s . That i s to say , the e f f i c i e n c y of f e r t i l i z e r decrease s w i t h e r o s i o n . Water s t r e s s reduced y i e l d s p r o p o r t i o n a l y more on f e r t i l i z e d than on u n f e r t i l i z e d pots (see f i g . 5 . 4 f ) . On the average , y i e l d s were depres sed 15 per cent i n u n f e r t i l i z e d pots and almost 40 per cent i n f e r t i l i z e d p o t s . T h i s s imple r e s u l t c o u l d be c o n s t r u e d to e x p l a i n the r e l u c t a n c e of some Mexican farmers to use f e r t i l i z e r s in e r r a t i c r a i n f e d a r e a s . A l t h o u g h the a p p l i c a t i o n of f e r t i l i z e r s can be expected to i n c r e a s e y i e l d s even i n drought y e a r s , the f u l l p o t e n t i a l of f e r t i l i z e r s cannot be r e a l i z e d without proper r a i n f a l l . H i g h e x p e c t a t i o n s d i e h a r d . I f f e r t i l i z e r s account fo r a l a r g e share of p r o d u c t i o n c o s t s , and i t does not r a i n , then f e r t i l i z e r s , r a t h e r than r a i n , w i l l l i k e l y be blamed for the poor r e s u l t s . 140 V . 3_. S o i l E r o s i o n and P r o d u c t i v i t y i n the F i e l d The f o l l o w i n g experiment was conducted under f i e l d c o n d i t i o n s which reproduce the most common maize c u l t i v a t i o n p r a c t i c e s found i n the maize f i e l d s of the study a r e a . However, e r o s i o n had to be s i m u l a t e d to permit both comparable and s i g n i f i c a n t s o i l l o s s e s to be s t u d i e d a c r o s s s o i l t y p e s . E r o s i o n was s i m u l a t e d by removing t o p s o i l i n h a l f the e x p e r i m e n t a l p l o t s . The o ther h a l f of the p l o t s was l e f t untouched . T h i s experiment t e s t s s i m i l a r hypotheses to the ones t e s t e d i n the greenhouse exper iment s : i ) Maize y i e l d s are d i f f e r e n t for the d i f f e r e n t s o i l s i n c l u d e d i n t h i s s t u d y . i i ) Maize y i e l d s decrease w i t h e r o s i o n , r e g a r d l e s s of s o i l t y p e . i i i ) F e r t i l i z a t i o n tends to compensate for the r e d u c t i o n of y i e l d s produced by e r o s i o n . Only the water s t r e s s h y p o t h e s i s i s m i s s i n g from the present exper iment . V.3^J_. M a t e r i a l s and methods Treatments for t h i s experiment were: a) e r o s i o n , as a r t i f i c i a l t o p s o i l r emova l ; b) N/P f e r t i l i z a t i o n ; c) s o i l t y p e . The s t a t i s t i c a l model can be w r i t t e n as f o l l o w s : 141 Y i j k l = U + S i +ERj + S E R i j + Fk + SF ik + ERFj k + SERFi jk + E l ( i j k ) (5 .3) where: Y = maize y i e l d i n t h a - 1 of g r a i n ; U = p o p u l a t i o n mean; S i = s o i l t y p e , i =1 ,3 ; ERj = e r o s i o n , j=1 ,2 ; Fk = f e r t i l i z a t i o n , k=1,2; E l = r e p l i c a t i o n s , 1=1,3. T h i s f i e l d experiment i s thus a 3 ( s o i l type) x 2 ( e r o s i o n l e v e l s ) x 2 ( f e r t i l i z a t i o n l e v e l s ) , c o m p l e t e l y randomized f a c t o r i a l w i t h 3 r e p l i c a t i o n s per c e l l . For a n a l y s i s , the f i x e d e f f e c t s ANOVA model I w i l l be a p p l i e d . Twelve 8 x 5 m p l o t s were d i s t r i b u t e d i n 35 x 20 m r e c t a n g u l a r e x p e r i m e n t a l a r e a s . A f t e r c l e a r i n g the l a n d , p l o t s and a l l e y s were d e l i m i t e d w i t h s takes and s t r i n g s . Dra inage d i t c h e s were dug in the a l l e y s around each p l o t . To prevent up-s lope runo f f from e n t e r i n g the e x p e r i m e n t a l a r e a s , ea r then dykes were b u i l t and p e r i p h e r a l d r a i n s were opened. Each p l o t was a l l o t t e d t reatment and r e p l i c a t i o n at random. V . 3 . 1 . 1 . S o i l s S i t e s 2, 4, and 7 from the A n d o s o l , the T e p e t a t e , and the C a l i c h e , r e s p e c t i v e l y , were chosen for these e x p e r i m e n t s . F i g u r e 4 .1b showed the g e o g r a p h i c a l l o c a t i o n of these s i t e s . S e c t i o n I V . 3 d i s c u s s e d the main s o i l p r o p e r t i e s of these s i t e s . Appendix 2 c o n t a i n s d e t a i l e d s o i l d a t a . S i t e s were l o c a t e d on e r o s i o n a l phases of each s o i l t y p e : the s l ope s i n the A n d o s o l ; 1 42 the h i l l - t o p s i n the T e p e t a t e s ; ' the g e n t l e s l o p e s of the C a l i c h e s . Sancholuz et a l (1981) and Marten and Sancholuz (1981) have shown tha t maize i s commonly c u l t i v a t e d i n these p l a c e s . To s i m u l a t e s o i l e r o s i o n , about 17 cm of t o p s o i l was sc raped from h a l f the p l o t s i n a l l s i t e s . S c r a p i n g was done by hand, w i t h spades and hoes , as the random l a y - o u t of the experiment p r e c l u d e d the use of mach inery . Cont inuous measurements of the depth of the e x c a v a t i o n w i t h s t a k e s , l e v e l s , and s t r i n g s were made d u r i n g t h i s o p e r a t i o n . Tab le V . 4 . Average depths of e x c a v a t i o n i n e x p e r i m e n t a l p l o t s P l o t # T o p s o i l Removed (cm) Andoso l Tepe ta te C a l i c h e 1 17.4 17.3 14.8 2 20.4 16.4 12.4 3 21 .3 10.8 12.7 4 20.2 15.3 15.8 5 23.2 16.4 18.6 6 22.7 16.9 18.1 Averages 20.9 15.6 15.4 T a b l e V . 4 shows tha t f i n a l depths v a r i e d w i t h i n and among s i t e s . P l o t s i n the Andoso l were sc raped deeper than p l o t s i n the o ther two s o i l s . C l e a r l y , the e r o s i o n treatment c o u l d not be s t a n d a r d i z e d as would have o therwi se been d e s i r e d . 143 V . 3 . 1 . 2 . F e r t i l i z a t i o n The same c r i t e r i a d i s c u s s e d i n S e c t i o n V . 2 . 1 . 3 were used to s e l e c t f e r t i l i z e r c o n t e n t s , r a t e s , and source of n u t r i e n t s for t h i s exper iment . S i m i l a r f e r t i l i z a t i o n schemes were a p p l i e d to h a l f the p l o t s in a l l s i t e s . They c o n t a i n e d 78 kg h a " 1 of N, and 46 kg h a " 1 of P. Lime at 4t h a " 1 was s u p p l i e d o n l y to the a c i d i c Andoso l s and T e p e t a t e s . One week be fore p l a n t i n g , l ime was banded 5 cm deep to the p repared rows. At p l a n t i n g , a l l the P and 18 kg h a " 1 of N were b u r i e d i n a c i r c l e around each maize h i l l . The r e s t of N was b u r i e d i n each h i l l when maize p l a n t s were i n the p e r i o d of f a s t growth. V . 3 . 1 . 3 . Seeds and c u l t i v a t i o n p r a c t i c e s S i t e s were c l e a r e d of g ra s se s and shrubs w i t h machetes and hoes j u s t be fore t i l l a g e . G iven the s i z e and l a y - o u t of the p l o t s , plows c o u l d not be used to t i l l the l a n d be fore p l a n t i n g . I n s t e a d , s o i l s were t i l l e d w i t h hoes . A l l p l o t s , eroded or n o t , r e c e i v e d the same i n t e n s i t y of t i l l a g e i n t h e i r uppermost 15 cm of s o i l . Seeds were drawn from c r e o l e v a r i e t i e s . L o c a l c u l t i v a t o r s were c o n t a c t e d and asked to p r o v i d e "good s t u f f " f o r each a r e a . Seeds , from the c e n t e r p a r t of the maize ears o n l y , were c o l l e c t e d , d i s c a r d i n g those which looked d e f e c t i v e . For the area of the C a l i c h e s o i l a lone t h e r e are known improved seeds (cf d i s c u s s i o n on T u x p e n i t o , S e c t i o n I V . 5 . 2 ) . These seeds , 144 w h i l e yet be ing t e s t e d for the C a l i c h e s , do not prosper i n e i t h e r the Andoso l s or the T e p e t a t e s , mainly because of c l i m a t i c r ea sons . I t was i m p o s s i b l e then to o b t a i n g e n e t i c homogeneity a c ro s s s i t e s . Seeding r a t e s were the same fo r i n a l l e x p e r i m e n t a l p l o t s . S ix rows w i t h ten h i l l s were each separa ted by 80 cm. H i l l s r e c e i v e d four seeds each . Whenever the f o u r t h p l a n t emerged the s e e d l i n g was removed to approximate 45000 p l a n t s h a " 1 , the recommended p l a n t i n g d e n s i t y i n the area (CDIA, 1977; SARH, 1980). Seeds were b u r i e d 5 cm deep wi th the h e l p of a p l a n t e r s t i c k . F e r t i l i z e r s , i f n e c e s s a r y , were a p p l i e d 10 cm apar t from the s eed ' s h i l l and both covered w i t h s o i l . Sowing dates were d i f f e r e n t i n the three s i t e s . The A n d o s o l , w i t h the l o n g e s t growing season , was seeded f i r s t on 10/4/81 . The T e p e t a t e , hav ing an i n t e r m e d i a t e growing season, was next on 12/6 /81 . The C a l i c h e , w i t h the s h o r t e s t season, was l a s t on the 2 7 / 6 / 8 1 . In a l l cases sowing dates c o i n c i d e w i t h the onset of the r a i n season, which i s c l o s e l y watched by a l l c u l t i v a t o r s i n the r e g i o n . Checks of s o i l moi s ture were made be fore seed ing to guarantee g e r m i n a t i o n . Maize g e r m i n a t i o n was s t u d i e d d u r i n g the f i r s t three weeks. Counts on p l a n t s u r v i v a l were made of a l l p l o t s . As poor g e r m i n a t i o n was n o t i c e d i n both the Tepeta te and C a l i c h e s o i l s , these s i t e s had to be reseeded . Crop growth was moni tored every two weeks. P l a n t h e i g h t s , from base to t i p , were measured i n one randomly chosen row of 145 each p l o t . V . 3 . 1 . 4 . Weed and pest c o n t r o l Three weedings were g i v e n to the Andoso l s i t e s and two to the T e p e t a t e s ' and C a l i c h e s ' . D i f f e r e n t maize growth r a t e s c a l l e d for d i f f e r e n t i n t e n s i t i e s of weeding to m a i n t a i n a p p r o x i m a t e l y the same l e v e l s of p l a n t c o m p e t i t i o n . The t i m i n g of these weedings was s e l e c t e d w i t h the a d v i c e of l o c a l c u l t i v a t o r s and the a g r i c u l t u r a l s e r v i c e ' s o f f i c e r s . Machetes and hoes were used to chop and uproot weeds growing i n between rows and around maize h i l l s . A p e r v a s i v e p e s t , p a r t i c u l a r l y at sowing t i m e , was the g r a c k l e (a b i r d : C a s s i d i x mex icanus ) . S e r i o u s damage was p revented w i t h a l o c a l t a c t i c : c r i s s c r o s s i n g white s t r i n g over the p l o t s which a c t s as a f l i g h t b a r r i e r to the b i r d . The Andoso l s i t e was a t t a c k e d by a b e e t l e (Macrodac ty lus spp) when maize was at the s i l k i n g s t a g e . Hexach lore C i c l e - H e x a n e (3%) was sprayed on p l o t s the day a f t e r the a t t a c k was n o t i c e d . The Tepe ta te s i t e was s a i d to be i n f e s t e d w i t h a c o r n borer and was a l s o t r e a t e d w i t h Hexach lore before sowing. V . 3 . 1 . 5 . Weather D u r i n g the year of these exper iment s , i t r a i n e d more than u s u a l throughout the r e g i o n . P r e l i m i n a r y 1981 r e t u r n s from nearby m e t e o r o l o g i c a l s t a t i o n s show a g r e a t e r than average p r e c i p i t a t i o n (see F i g . 4 . 3 for c o m p a r i s o n ) . A drought s p e l l set 1 46 i n the Xa lapa s i t e soon a f t e r sowing. I t was prompt ly c o r r e c t e d by hand i r r i g a t i o n to each maize h i l l , as f a c i l i t i e s p e r m i t t e d . Dra inage was more of a p rob lem, p a r t i c u l a r l y i n the Tepeta te and C a l i c h e s i t e s . T o r r e n t i a l r a i n s t e m p o r a r i l y c l o g g e d the d i t c h e s of these two s i t e s and some a c c u m u l a t i o n of water took p l a c e , p a r t i c u l a r l y i n the eroded p l o t s . S t rong winds are always a major concern i n the r e g i o n . S ince a l l maize v a r i e t i e s are r e l a t i v e l y t a l l , they a l s o tend to lodge when winds blow h a r d . T h i s nu i sance i s p a r t l y c o r r e c t e d by the support g i v e n to the p l a n t s w i t h the mounds of e a r t h s u r r o u n d i n g each h i l l . However, the exper iments were damaged by the winds , s p e c i a l l y i n the A n d o s o l . Care was taken to r a i s e a l l lodged p l a n t s and to support some w i t h s t i c k s . V . 3 . 1 . 6 . H a r v e s t i n g procedures H a r v e s t i n g was conducted at a l l s i t e s be fore the- g r a i n was c o m p l e t e l y d r i e d . E a r l y h a r v e s t i n g was neces sary because of the d i s p a r i t y of growth between t r e a t m e n t s , as w e l l as to prevent f u r t h e r damage by l a t e - s e a s o n winds . Harves t dates were 26/8/81 for the A n d o s o l , 16/10/81 for the T e p e t a t e , and 17/10/81 for the C a l i c h e . To e l i m i n a t e border e f f e c t s , o n l y the 4 -center rows and the 8 - c e n t e r h i l l s of each p l o t were h a r v e s t e d . F r e s h biomass and g r a i n y i e l d s were r e c o r d e d at the s i t e . Samples of these were oven d r i e d for moi s ture content d e t e r m i n a t i o n s . Y i e l d s r e p o r t e d i n the next s e c t i o n c o r r e s p o n d to 15 per cent m o i s t u r e content for g r a i n , and c o m p l e t e l y dry matter for b iomass . 1 47 V . 3 ^ 2 . R e s u l t s and d i s c u s s i o n G e r m i n a t i o n , growth, and y i e l d da ta w i l l be p r e s e n t e d f i r s t . A more g e n e r a l d i s c u s s i o n of the treatment e f f e c t s c o n c l u d e s t h i s s e c t i o n . V . 3 . 2 . 1 . G e r m i n a t i o n and s u r v i v a l p a t t e r n s T a b l e V . 5 . Maize g e r m i n a t i o n i n f i e l d exper iments % S u r v i v a l a f t e r 10 days Treatment Means Andoso l T e p e t a t e 1 C a l i c h e 1 C o n t r o l 95 39 89 74 E r o s i o n 93 1 6 78 62 E r o s . + F e r t . 92 1 4 78 61 F e r t i l i z e r 95 52 89 79 Means 94 30 84 69 1 A f t e r r e s e e d i n g Tab le V . 5 shows how g e r m i n a t i o n v a r i e d among t r e a t m e n t s . Poor maize g e r m i n a t i o n was observed i n the T e p e t a t e s , s p e c i a l l y when e roded , and even a f t e r r e s e e d i n g . On the the o ther hand, the Andoso l s showed e x c e l l e n t g e r m i n a t i o n i n a l l t r e a t m e n t s . Maize g e r m i n a t i o n i n the C a l i c h e s was h i g h e r than i n the T e p e t a t e s , and s l i g h t l y lower than i n the A n d o s o l s . 1 48 F i n a l p l a n t p o p u l a t i o n s , at h a r v e s t , were approx imate ly 40000 p l a n t s h a " 1 i n the A n d o s o l s , 18000 in the C a l i c h e s , and 13000 i n the T e p e t a t e s . Treatments d i d not make much d i f f e r e n c e i n the f i n a l p l a n t d e n s i t i e s of the Andoso l s and the C a l i c h e s . In the T e p e t a t e s , however, eroded p l o t s ended up w i t h one f o u r t h the p l a n t s of the non-eroded p l o t s . V . 3 . 2 . 2 . Maize growth F i g u r e 5.5 shows maize growth curves for t h i s exper iment . D i f f e r e n c e s i n maize p l a n t s ' h e i g h t s among s i t e s ( i . e . , F i g . 5 . 5 a - c ) mean l i t t l e because maize phenotypes were q u i t e d i f f e r e n t . W i t h i n s i t e s , however, there was much b e t t e r growth i n the uneroded and f e r t i l i z e d p l o t s . Growth i n the uneroded u n f e r t i l i z e d p l o t s was c o m p a r a t i v e l y good i n the Andoso l and the C a l i c h e ( F i g . 5 . 5 a , b ) . Eroded but f e r t i l i z e d p l o t s d i d r e l a t i v e l y w e l l o n l y i n the Andoso l ( F i g . 5 . 5 a ) . There was very l i t t l e d i f f e r e n c e among the uneroded u n f e r t i l i z e d , the eroded f e r t i l i z e d , and the eroded u n f e r t i l i z e d t rea tments i n the Tepe ta te ( F i g . 5 . 5 c ) . V . 3 . 2 . 3 . . G r a i n y i e l d s F i g u r e 5.6 shows average maize y i e l d for a l l t rea tments i n t h i s exper iment . T h i s h i s t o g r a m compares y i e l d s for eroded and uneroded p l o t s for each s o i l type and f e r t i l i z a t i o n l e v e l . In a l l s o i l s e r o s i o n had a tremendous impact on y i e l d s . Only the Andoso l and the C a l i c h e produced some g r a i n when eroded .(the 149 F i q u r e 5.5. G r o w t h p a t t e r n s o f m a i z e p l a n t s i n f i e l d e x p e r i m e n t s . 5.5a, A n d o s o l ; 5.5 . b , T e p e t a t e ; 5.5c, C a l i c h e . 151 F i g u r e 5.6. G r a i n y i e l d s for d i f f e r e n t t rea tment s i n f i e l d e x p e r i m e n t . Note that eroded T e p e t a t e s d i d not y i e l d g r a i n , w i t h or wi thout f e r t i l i z e r s . 152 153 C a l i c h e ' s y i e l d was, however, a meager 0.2 t h a " 1 when, f e r t i l i z e d ) . The Tepeta te d i d not produce g r a i n at a l l when e r o d e d , nor d i d uneroded and u n f e r t i l i z e d p l o t s . F e r t i l i z e r s had a marked e f f e c t on y i e l d s o n l y i n the C a l i c h e . For t h i s s o i l , the average y i e l d of the f e r t i l i z e d p l o t s was twice the average y i e l d of the u n f e r t i l i z e d p l o t s . In the A n d o s o l , y i e l d s wi th f e r t i l i z e r s were almost 30 per cent g r e a t e r than wi thout them. I t shou ld be emphasized tha t the Tepe ta te wi thout f e r t i l i z e r s d i d not y i e l d at a l l ; f e r t i l i z e r s seem mandatory i n t h i s s o i l . The C a l i c h e s o i l y i e l d e d s l i g h t l y more than the A n d o s o l , but d i f f e r e n c e s are not grea t ( i . e . , 1.79 vs 1.65 t h a " 1 ) . The T e p e t a t e , as s a i d b e f o r e , produced g r a i n o n l y when f e r t i l i z e d and uneroded which meant tha t mean y i e l d s for t h i s s o i l were a mere 0.22 t h a " 1 . The grand mean of y i e l d s fo r the experiment i s 1.22 t h a " 1 , a datum tha t w i l l be l a t e r r e c a l l e d . V.3_..3_. P r e l i m i n a r y c o n c l u s i o n s ; f i e l d experiment T a b l e V . 6 p r e s e n t s the ANOVA t a b l e for t h i s exper iment . A g a i n , as i n the greenhouse exper iment , most i n t e r a c t i o n terms were found to be s i g n i f i c a n t . Only the s o i l x f e r t i l i z e r i n t e r a c t i o n was not s i g n i f i c a n t , which means tha t maize y i e l d s i n a l l s o i l s r e a c t e d i n the same ( p o s i t i v e ) way to f e r t i l i z a t i o n . E r o s i o n had a d i f f e r e n t impact (always n e g a t i v e ) on the maize y i e l d s of d i f f e r e n t s o i l s . The most marked decrease of y i e l d s 1 54 T a b l e V . 6 ANOVA for g r a i n y i e l d s i n f i e l d exper iment 1 Source DF MS p 2 S o i l ( S ) 2 0 .687 127 2 ** E r o s i o n ( E ) 1 2 .255 417 8 ** F e r t i l i z e r ( F ) 1 0 .279 51 .7 ** S x E 2 0 .233 43 . 1 ** S x F 2 0 .006 1 2 ns E x F 1 0 .052 9 .7 ** S x E x F 2 0 .063 1 1 .7 ** E r r o r 24 0 .005 1 Data are l o g t rans forms to c o r r e c t fo r he te roge-n e i t y of v a r i a n c e s . 2 n s :non s i g n i f i c a n t ; * : P < 0 . 0 1 ; * * : P<0.05. was produced i n the T e p e t a t e , which y i e l d e d n o t h i n g when e r o d e d . Maize y i e l d s i n the C a l i c h e were a l s o much d e p r e s s e d . The Andoso l y i e l d e d some maize when eroded and f e r t i l i z e d , but i t was not much. F i n a l l y , e r o s i o n and f e r t i l i z e r s i n t e r a c t e d d i f f e r e n t l y i n a l l s o i l s . The s i g n i f i c a n c e of t h i s i n t e r a c t i o n term suggests t h a t maize y i e l d s responded d i f f e r e n t l y to e r o s i o n , wi th and wi thout f e r t i l i z e r s i n each s o i l . Hypotheses I to i i i can now be c o n s i d e r e d . R e s u l t s from t h i s experiment support h y p o t h e s i s i , g r a i n y i e l d s for the A n d o s o l , C a l i c h e , and Tepe ta te were a l l s i g n i f i c a n t l y d i f f e r e n t ( i . e . , P<0.05, l o g - t r a n s f o r m e d d a t a , Duncan m u l t i p l e range t e s t ) . H y p o t h e s i s i i can not be r e j e c t e d e i t h e r : a l l s o i l s s u f f e r e d g rea t r e d u c t i o n s i n y i e l d s when e r o d e d . H y p o t h e s i s i i i , however, has to be r e j e c t e d . A l t h o u g h f e r t i l i z e r s d i d have a s i g n i f i c a n t e f f e c t on maize y i e l d s , t h i s was not enough to compensate the l o s s e s to e r o s i o n . For 1 55 i n s t a n c e , the y i e l d s of eroded f e r t i l i z e d p l o t s never matched the y i e l d s of uneroded and u n f e r t i l i z e d p l o t s (see F i g . 5 . 6 ) . V . 4 . Summary The greenhouse and f i e l d exper iments d i s c u s s e d i n t h i s chap te r produced c o n t r a d i c t o r y r e s u l t s . In the greenhouse , e r o s i o n depres sed y i e l d s of the three main s o i l types but f e r t i l i z e r s r e s t o r e d these y i e l d s . On the c o n t r a r y , e r o s i o n i n the f i e l d had a p e r s i s t e n t and q u i t e important n e g a t i v e e f f e c t on maize y i e l d s ; f e r t i l i z e r s c o u l d not h e l p i t . P h i l l i p s and Kamprath (1973) found that pot maize y i e l d s of cut s o i l s u r f a c e s and s u b s o i l s c o u l d be r a i s e d , through f e r t i l i z a t i o n , to l e v e l s s i m i l a r to those shown by u n a l t e r e d s o i l s u r f a c e s . R i t c h e y (1973) has a l s o shown that c a r e f u l f e r t i l i z a t i o n of s u b s o i l s can r e s t o r e maize y i e l d s to the same l e v e l s of the t o p s o i l s . Both these s t u d i e s have d e a l t w i t h p r o b l e m a t i c s o i l s of the t r o p i c s and s u b t r o p i c s . There i s a problem w i t h the i n t e r p r e t a t i o n of r e s u l t s from greenhouse s t u d i e s . F e r t i l i z a t i o n i n sma l l pot s may not be a good i n d i c a t o r of s i m i l a r responses i n the f i e l d . When s o i l volume i s so l i m i t e d , f e r t i l i z e r s may be the o n l y s i g n i f i c a n t source of n u t r i e n t s for the p l a n t s . T h i s i s p a r t i c u l a r l y t rue for maize which r e q u i r e s r e l a t i v e l y more n u t r i e n t s to grow than o ther c r o p s . The f i e l d exper iments i n c l u d e d r e a l s o i l volumes , r e a l p l a n t p o p u l a t i o n s , r e a l f e r t i l i z e r dosages , and r e a l c u l t i v a t i o n p r a c t i c e s . R e s u l t s compare w e l l w i t h those r e p o r t e d i n the 1 56 l i t e r a t u r e reviewed in chapter I I I . Tab le I I I .1 showed that s i m i l a r l e v e l s of e r o s i o n i n t r o p i c a l s o i l s c o u l d depres s maize y i e l d s between 30 and 70 per c e n t . The exper iments r e p o r t e d here show a range of y i e l d l o s s e s from 27 to 100 per c e n t . 157 CHAPTER V I : SUMMARY AND CONCLUSIONS Mexican maize h a r v e s t s have i n c r e a s e d i n the recent p a s t . In the l a s t t h i r t y year s f e r t i l i z e r s , i r r i g a t i o n , and new lands have been added to i n c r e a s e the p r o d u c t i o n of maize i n M e x i c o . A c c o r d i n g to n a t i o n a l s t a t i s t i c s , even u n i r r i g a t e d and u n f e r t i l i z e d maize f i e l d s today y i e l d s l i g h t l y more than t h i r t y year s ago. T h i s f i n d i n g i s c o n t r a r y to p r e d i c t i o n s c o n t a i n e d i n the l i t e r a t u r e reviewed i n chapter I I I . A c c o r d i n g to these p r e d i c t i o n s , s o i l e r o s i o n and s o i l f e r t i l i t y d e p l e t i o n shou ld have lowered the p r o d u c t i v i t y of the Mexican maize f i e l d s . Whi le a c t u a l r a t e s of e r o s i o n under c u r r e n t management p r a c t i c e s were not measured i n t h i s s t u d y , a l l the assembled ev idence s t r o n g l y suggests that s o i l l o s s e s i n the maize f i e l d s are h i g h . T r a d i t i o n a l and cont inuous c u l t i v a t i o n of maize on s e n s i t i v e s i t e s shou ld f u r t h e r d e p l e t e the f e r t i l i t y of the s o i l s . Maize i s a h i g h l y demanding c r o p . Without f e r t i l i z e r s and/or c r o p r o t a t i o n s , i t can on ly y i e l d s u b s i s t e n c e q u a n t i t i e s of g r a i n . Y e t , the n a t i o n a l s t a t i s t i c s on maize p r o d u c t i o n suggested o t h e r w i s e . W h i l e the amount of l and d e d i c a t e d to maize has a p p a r e n t l y ceased to grow, l a n d p r o d u c t i v i t y i s on the i n c r e a s e . T h i s c o n f l i c t i n g ev idence had to be checked i n the f i e l d . Chapter IV surveyed three c o n t r a s t i n g Mexican s o i l s i n c e n t r a l V e r a c r u z . Assessment of e r o s i o n of these s o i l s under maize c u l t i v a t i o n conf i rmed the expected h i g h s o i l l o s s e s . 1 58 A c c o r d i n g l y , exper iments i n the greenhouse and i n the f i e l d were d e s i g n e d to s imula te these s i g n i f i c a n t s o i l l o s s e s . The r e s u l t s , r e p o r t e d i n Chapter V , are a g a i n c o n f l i c t i n g . In greenhouse t e s t s f e r t i l i z e r s d i d compensate for y i e l d l o s s e s r e s u l t i n g from e r o s i o n . F i e l d exper iment s , on the o ther hand, demonstrated that e r o s i o n has a dramat ic and n e g a t i v e e f f e c t on maize y i e l d s . In these exper iment s , f e r t i l i z e r s d i d not compensate for the l o s s e s caused by e r o s i o n . Having rev iewed the main f i n d i n g s of t h i s t h e s i s , i t i s t ime to put them i n t o p e r s p e c t i v e . Can ev idence coming from f i e l d , greenhouse and n a t i o n a l s t a t i s t i c s be c o n s i d e r e d on an equa l f o o t i n g ? Of what consequence i s an i n d i v i d u a l maize f i e l d i n the n a t i o n a l average? Of what consequence i s a s i x l i t e r pot i n the maize' f i e l d ? Both the n a t i o n a l s t a t i s t i c s and s i x l i t e r pot s are o n l y a b s t r a c t i o n s of a more meaningfu l r e a l i t y : the maize f i e l d i t s e l f . The l ands surveyed i n the n a t i o n a l s t a t i s t i c s today and t h i r t y year s ago are not the same. They have been added t o ; doubled i n f a c t . Is 14 per cent a s i g n i f i c a n t d i f f e r e n c e i n y i e l d 1 between a s e r i e s of samples (1970's ) and a s e r i e s of in formed e s t i m a t e s (1940 ' s ) ? No, i t i s n o t , p a r t i c u l a r l y when we r e c a l l tha t each data set has an i n t e r n a l v a r i a b i l i t y which exceeds the d i f f e r e n c e t e s t e d . In a few more y e a r s , however, i t w i l l be ten year s s i n c e the l a s t comprehensive survey of maize f i e l d s was conducted i n i . e . , between 0.7 and 0.8 t h a - 1 y r ~ 1 1 59 M e x i c o . I f t h i s survey were to be repeated to update a g r i c u l t u r a l s t a t i s t i c s , then a data set would be a v a i l a b l e to check t r e n d s i n maize f i e l d p r o d u c t i v i t y d u r i n g a p e r i o d i n which the extent of Mexican maize l a n d would not have changed s i g n i f i c a n t l y . I b e l i e v e t h i s would be a worthwhi le e n t e r p r i s e . What of the ev idence from the pot t r i a l s ? Pots p r o v i d e a neat and conven ient way of c o l l e c t i n g e x p e r i m e n t a l d a t a . However, pot s are sma l l c o n t a i n e r s of s o i l . In such a l i m i t e d env i ronment , f e r t i l i z e r s are taken up by the p l a n t s immedia te ly . F u r t h e r , p l a n t s cannot be grown to m a t u r i t y i n t h i s env i ronment . T h e r e f o r e , the response of maize to f e r t i l i z e r s i s l i k e l y to be d i s p r o p o r t i o n a t e l y l a r g e . To e v a l u a t e the q u e s t i o n s of t h i s t h e s i s r e a l i s t i c a l l y we must go to the f i e l d . That i s where Mexican maize p r o d u c t i o n takes p l a c e , and that i s where r e s u l t s are d e p r e s s i n g . Maize p r o d u c t i o n w i l l s u f f e r from e r o s i o n , f e r t i l i z e r s n o t w i t h s t a n d i n g . S i m i l a r f i n d i n g s have been r e p o r t e d from other f i e l d s t u d i e s throughout the w o r l d , w i t h a few e x c e p t i o n s which do not o b t a i n i n M e x i c o . C o n c l u s i o n s can be drawn in two a r e a s . "One has to do w i t h s c i e n t i f i c i n v e s t i g a t i o n i t s e l f , w h i l e the second r e l a t e s to Mexican a g r i c u l t u r e . With r e spec t to the f i r s t , q u e s t i o n s posed at one l e v e l of r e s o l u t i o n cannot be answered w i t h e x p e r i m e n t a l br survey data ga thered at a n o t h e r . The p o t e n t i a l impact of s o i l e r o s i o n on maize p r o d u c t i o n can o n l y be answered w i t h data ga thered at the f i e l d l e v e l . In n a t i o n a l s t a t i s t i c s a v a i l a b l e h e r e , i t i s 1 60 suggested that confounding v a r i a b l e s masked the e f f e c t s v i s i b l e i n the f i e l d d a t a . N e i t h e r d i d greenhouse exper iments s i m u l a t e f i e l d c o n d i t i o n s w e l l enough to produce even remotely comparable r e s u l t s . T u r n i n g to the dilemma of Mexican a g r i c u l t u r e , maize c u l t i v a t i o n in Mexico i s a p o p u l a r , e s s e n t i a l and preeminent human a c t i v i t y . Maize p r o d u c t i v i t y i s low, demand for maize i s h i g h and i n c r e a s i n g . Many m i l l i o n s of people have on ly a sma l l p a r c e l of l a n d to c u l t i v a t e for t h e i r l i v e l i h o o d . T h i s dilemma i s compounded by the ev idence on l a n d d e g r a d a t i o n that was p r o v i d e d h e r e . How do we go about f i n d i n g s o l u t i o n s ? One r a d i c a l s o l u t i o n would be to change c u r r e n t l a n d use . At p re sen t the d e l i c a t e maize f i e l d s are c o n f i n e d to s l ope s and sha l low s o i l s . w h i l e c a s h - c r o p and c a t t l e p r o d u c t i o n take p l a c e on the f l a t and deep s o i l s . Were t h i s l a n d use p a t t e r n be a l t e r e d , the combined a m p l i f y i n g e f f e c t s of s lope and s c a r c e p l a n t cover on s o i l e r o s i o n would be r e v e r s e d . T h i s s o l u t i o n , however, i s improbable for economic and p o l i t i c a l r ea sons . At minimum, c u l t i v a t o r s of maize f i e l d s might be g iven b e t t e r i n c e n t i v e s to upgrade p r o d u c t i v i t y and to adopt management p r a c t i c e s tha t would keep the s o i l i n p l a c e . S o i l c o n s e r v a t i o n i s not a new idea to Mex icans , nor i s a g r i c u l t u r a l development , but n e i t h e r i s working i n the maize f i e l d s . Faced w i t h repea ted warnings of the p o t e n t i a l impact of e r o s i o n and the importance of maize , why has there been no attempt to to document the magnitude of the l o s s e s i n c u r r e d ? T h i s study o f f e r s an i n i t i a l e v a l u a t i o n . F u r t h e r s t u d i e s may 161 document the dilemma more c o m p l e t e l y . S t u d i e s of s o i l e r o s i o n i n t r o p i c a l Mexico are bad ly needed. An e x t e n s i v e but s imple survey of s o i l depths a c r o s s t r o p i c a l s o i l s w i l l q u i c k l y document the magnitude of c u r r e n t s o i l l o s s e s . I f such a survey i s c o u p l e d w i t h measurements of l a n d p r o d u c t i v i t y , the impact of these s o i l l o s s e s on c r o p p r o d u c t i o n may a l s o be d e t e r m i n e d . Yet another a l t e r n a t i v e i s p r o v i d e d by market minded e c o n o m i s t s . Mexico c o u l d f o r g e t about p r o d u c i n g maize and c o n c e n t r a t e on p r o d u c i n g c o f f e e , s t r a w b e r r i e s , c h i l e s , c acao , or any o ther cash c r o p at which she e x c e l s . A f t e r a l l , i t s n o r t h e r n , ne ighbor .needs-those- goods and has p l e n t y of c o r n to exchange. T h i s c o u l d work w e l l , and indeed i t has been working for a w h i l e , for those who o b t a i n maize through the market . But t h i s s o l u t i o n does not s o l v e the problem of those r u r a l Mexicans l i v i n g on the edge of the cash economy, s t r i v i n g to s u r v i v e on a s m a l l maize f i e l d i n the h i l l s . 162 BIBLIOGRAPHY A b u r t o , H . , 1979. E l ma iz : p r o d u c c i o n , consume* y p o l i t i c a de p r e c i o s , i n : Montanez, C . & H . 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Comunicac iones P r o j e c t o P u e b l a -T l a x c a l a 16:57-67. W e l l h a u s e n , E . J . , 1976. The a g r i c u l t u r e of M e x i c o . S c i e n t i f i c American 235:129-150. W e l l h a u s e n , E . J . , L . M . R o b e r t s , and E . Hernandez X . , 1952. Races of Maize i n M e x i c o . The Bussey I n s t i t u t i o n of Harvard U n i v e r s i t y . W i l d e , S. A . , G . K. V o i g t , and J . G . I y e r , 1972. S o i l and P l a n t A n a l y s i s for Tree C u l t u r e . Oxford & IBH Pub. Co . W i s c h m e i e r , W. H . , C . B . Johnson , and B . V . C r o s s , 1971. A s o i l e r o d i b i l i t y nomograph fo r farmland and c o n s t r u c t i o n s i t e s . J . S o i l & Water Cons . 26 :189-192. 174 •Wischmeier , W.H. and J . V . Manner ing , 1969. R e l a t i o n of s o i l p r o p e r t i e s to i t s e r o d i b i l i t y . S o i l S c i . Soc . of Amer. 33: 131-137. Wischmeier , W.H. and D . D . Smi th , 1978. P r e d i c t i n g r a i n f a l l e r o s i o n l o s s e s — a guide to c o n s e r v a t i o n p l a n n i n g . USDA, A g r i c . Handbook # 537, Washington DC. Wischmeier , W . H . , 1974. New developments i n e s t i m a t i n g water e r o s i o n , i n : SCSA, 29th annual meet ing of the S o i l C o n s e r v a t i o n S o c i e t y of A m e r i c a , P r o c e e d i n g s , pp. 179-186. Wischmeier , W.W. and J . V . Manner ing , 1965. E f f f e c t s of o r g a n i c matter content of the s o i l on i n f i l t r a t i o n . J . S o i l & Water Conser , 20:150-152. Young, A . , 1969. Present r a te of l a n d e r o s i o n . Nature 224:851-52. 175 APPENDIX 1. STATISTICS OF MAIZE PRODUCTION AND CONSUMPTION IN MEXICO _1_._1_. Sources of maize p r o d u c t i o n and consumption data T h i s S e c t i o n p r o v i d e s a d e s c r i p t i o n of the raw data employed i n C h . 2 . Two types of data w i l l be c o n s i d e r e d : c o n t i n u o u s or long term s e r i e s , and a p r o d u c t i o n survey for the year s 1976-79. T a b l e s c o n t a i n i n g s e r i a l data are i n c l u d e d e d at the end of t h i s a p p e n d i x . J_._1_._1_. P r o d u c t i o n s e r i e s of data S i n c e 1925 the A g r i c u l t u r a l Economic Department (DGEA) of the Mexican M i n i s t r y of A g r i c u l t u r e and Water Resources has kept c o n t i n u o u s r e c o r d s of maize p r o d u c t i o n . Data are o b t a i n e d from m a i l r e p o r t s by the mayors of a l l c o u n t i e s i n the c o u n t r y (FAO, 1975). For the p e r i o d 1895-1924, t h i s agency c o l l e c t e d s i m i l a r data from v a r i o u s o ther s o u r c e s . As these were year s of g rea t p o l i t i c a l upwheal in M e x i c o , some data are m i s s i n g and o t h e r s are very d o u b t f u l l i n d e e d . Gomez Cobo (1977) has however r e v i s e d these s e r i e s and suggest a new s e r i e s which ' i s used h e r e . Data i n c l u d e d a r e : a rea h a r v e s t e d to ma ize , volume of p r o d u c t i o n , y i e l d per h e c t a r e ( o b t a i n e d from the p r e v i o u s two) . T a b l e 1 of t h i s appendix reproduces these d a t a . P r o d u c t i o n data are r e p o r t e d on a c a l e n d a r year b a s i s . Even though t h e r e 1 76 are two c rops of maize i n Mexico every y e a r , a p r o d u c t i o n year i n c l u d e s the h a r v e s t of the l a s t year winter c r o p and the h a r v e s t of t h i s y e a r ' s summer c r o p . P r o d u c t i o n r e p o r t s by the Food and A g r i c u l t u r e O r g a n i z a t i o n of the U n i t e d N a t i o n s (FAO), c o i n c i d e w i t h the DGEA r e p o r t s . T h i s i s no s u r p r i s e because FAO p o l i c y i s to t r a n s c r i b e the o f f i c i a l s t a t i s t i c s of a l l member c o u n t r i e s (see FAO, P r o d u c t i o n Y e a r b o o k s ) . The F o r e i g n S e r v i c e D i v i s i o n of the U n i t e d S t a te s Department of A g r i c u l t u r e (USDA h e r e i n t o ) has two s e r i e s of data on Mexican maize p r o d u c t i o n . Both are c o m p i l e d from a l l a v a i l a b l e Mexican s t a t i s t i c s but they a l s o i n c l u d e judgment by the a g r i c u l t u r a l a t t a c h e s at the USA embassy, who t r a v e l i n the f i e l d and c o n s u l t l o c a l e x p e r t s . One s e r i e s s t a r t s i n 1932 and i s p u b l i s h e d i n A g r i c u l t u r a l S t a t i s t i c s . , a y e a r l y p u b l i c a t i o n of USDA. T h i s s e r i e s i n c l u d e s area h a r v e s t e d and p r o d u c t i o n volumes of maize ( y i e l d s can be deduced) . S t a r t i n g i n 1960 , the F o r e i g n A g r i c u l t u r a l C i r c u l a r of the same department a l s o p u b l i s h i m p o r t - e x p o r t volumes and human consumption (see Tab le 3 ) . The p e r i o d of r e p o r t i n g c o r r e s p o n d s to the i n t e r n a t i o n a l c r o p year ( O c t - S e p t ) . The winter maize c r o p in Mexico i s moved forward one y e a r , thus making i t compat ib l e w i t h a l l o ther r e p o r t i n g p e r i o d s ment ioned . Mexico has taken d e c e n n i a l a g r i c u l t u r a l censuses , from 1930 through 1980. These c o n t a i n data on p r o d u c t i o n , and area h a r v e s t e d . There are a l s o t a b u l a t i o n s by c r o p p i n g p a t t e r n s , use of i n p u t s , and l a n d tenure reg imes . The data are v a l i d for 1 77 the year before the census because t h i s i s u s u a l l y taken on the s p r i n g and q u e s t i o n s r e f e r to the l a s t p r o d u c t i o n y e a r . Tab le 4 c o n t a i n s data from the three l a s t censuses which were a v a i l a b l e at t h i s w r i t t i n g . The n a t i o n a l a g r i c u l t u r a l censuses are the r e s p o n s i b i l i t y of the N a t i o n a l D i r e c t i o n of S t a t i s t i c s w h i c h , to c a r r y out the f i e l d work, convocates enumerators from a l l over the c o u n t r y . J_.J_.2. Consumption s e r i e s of data Maize consumption i s r e p o r t e d p e r i o d i c a l l y as the d i f f e r e n c e between n a t i o n a l p r o d u c t i o n and the net ba lance of t rade for the upcoming y e a r . T h i s i s f u r t h e r a d j u s t e d for the i n i t i a l s tocks of the g r a i n be fore the commercia l year b e g i n s . Both the DGEA and the USDA-FS have produced these d a t a : the former s t a r t i n g i n 1940; the l a t t e r i n 1960 (SARH-DGEA, 1981; USDA-FAS, 1982). Both s e r i e s are reproduced i n Tab le 3 of t h i s appendix . J_.J_.3_. Surveys of p r o d u c t i o n From 1975 t o 1979 the DGEA run a survey of b a s i c c rops i n M e x i c o . A p p r o x i m a t e l y 5000 maize f i e l d s were sampled each year on the 17 more' p r o d u c t i v e s t a t e s of the c o u n t r y . The survey was de s igned to e s t imate n a t i o n a l p r o d u c t i o n and thus i t s t r a t i f i e d the count ry i n t o zones known to have d i f f e r e n t p r o d u c t i v e p o t e n t i a l . Samples were l a t t e r drawn at random i n a m u l t i s t a g e sampl ing procedure (Lozano Lube, 1978). In these s u r v e y s , farmers were i n t e r v i e w e d i n t h e i r f i e l d s 178 near h a r v e s t t ime—which for the spring-summer c r o p corre sponds to the months of November and December. A p r e v i o u s l y des igned q u e s t i o n n a i r e was employed to code i n f o r m a t i o n r e g a r d i n g p r o d u c t i o n i n p u t s , l a n d a r e a s , p r o d u c t i o n l e v e l s , f i n a n c i a l and marke t ing c h a n n e l s . The grea t number of samples o b t a i n e d each y e a r , and the v a r i e t y of data r ecorded makes these surveys one of the most complete and r e l i a b l e sources of i n f o r m a t i o n on the p r o d u c t i v e s t r u c t u r e of the Mexican maize f i e l d s . U n f o r t u n a t e l y , the surveys were d i s c o n t i n u e d i n 1980. Only 1975 r e s u l t s have been so far a n a l y s e d and p u b l i s h e d (Gomez Cobo, 1977). A copy of the computer tape c o n t a i n i n g the raw data for the year s 1976-1979 was here o b t a i n e d d i r e c t l y from DGEA. The data were s c r e e n e d , s e l e c t e d , t r a n s f o r m e d , and a n a l y s e d u s i n g MIDAS (Fox and G u t r e , 1976), a s t a t i s t i c a l package a v a i l a b l e at the Amdhl 470 V / 6 , model I I , computing c e n t r e of UBC. R e s u l t s of these a n a l y s i s are p re sen ted i n C h . I I . 179 1.2. Data Tab le 1: Y i e l d s , h a r v e s t e d areas and t o t a l p r o d u c t i o n of maize i n Mexico from 1895 to 1982. Year Ma i z e Y i e l d s Area Harves ted T o t a l Product ion (kg/h a) ( x1OOOha) ( t ) USDA DGEA USDA DGEA USDA DGEA 1895 585 3249 1900665 1896 570 3399 1937430 1897 577 521 3 3007901 1898 575 4780 2748500 1899 577 41 66 2403782 1900 572 4036 2308592 1 901 580 4042 2344360 1 902 569 3871 2202599 1 903 568 3950 2243600 1 904 570 3881 2212170 1905 584 3653 2133352 1 906 573 4743 2717739 1 907 584 4900 2861600 1 908 590 4541 2679190 1909 570 4386 2500020 1910 584 5413 3161192 1911 588 2926 1720488 1912 560 2466 1380960 1913 560 21 20 1187200 1914 560 1 922 1076320 1915 540. 2000 1080000 1916 560 1917 560 2320 1299200 1918 550 2480 1364000 1919 565 2680 1514200 1 920 580 2928 1698240 1 921 612 2946 1802952 1 922 607 2856 1733592 To Cont inue 180 T a b l e 1. C o n t i n u e d Year Maize Y i e l d s Area Harves ted T o t a l P r o d u c t i o n (kg/h a) (x lOOOha) (t) USDA DGEA USDA DGEA USDA DGEA 1 923 3209 1924 3267 1 925 670 2936 1967120 1 926 680 31 37 2133160 1 927 647 3181 2058107 1928 698 3112 2172176 1 929 513 2865 1469745 1930 448 3075 1377600 1931 633 3378 2138274 1 932 609 609 3243 3243 1974987 1974987 1 933 603 601 3198 3198 1928394 1921998 1934 577 580 2970 2970 1713690 1722600 1 935 565 565 2966 2966 1 675790 1675790 1936 559 560 2852 2852 1594268 1597120 1 937 546 545 3000 3000 1638000 1635000 1 938 546 547 3091 3094 1687686 1692418 1 939 605 605 3267 3267 1976535 1 976535 1 940 497 491 3342 3342 1660974 1640922 1 941 608 608 3492 3492 2123136 2123136 1 942 628 628 3748 3758 2353744 2360024 1 943 562 587 3048 3083 1712976 1809721 1 944 672 690 3501 3355 2352672 2314950 1 945 634 634 3451 3451 2187934 2187934 1 946 722 719 3313 3313 2391986 2382047 1 947 716 717 3512 3512 2514592 2518104 1 948 760 761 3722 3722 2828720 2832442 1 949 555 757 3998 3792 2218890 2870544 1950 603 721 3998 4328 2410794 3120488 1 951 747 773 4427 4427 3306969 3422071 1 952 729 756 4237 4236 3088773 3202416 1 953 737 766 4340 4857 3198580 3720462 1 954 877 854 4399 5253 3857923 4486062 To c o n t i n u e 181 T a b l e 1. C o n t i n u e d Year Ma i ze Y i e l d s Area Harves ted T o t a l P r o d u c t i o n (kg/ha) (x1OOOha) ( t ) USDA DGEA USDA DGEA • USDA DGEA 1 955 858 836 4000 537 1 3432000 4490156 1 956 780 803 4399 5460 3431220 4384380 1 957 747 835 5500 5392 4108500 4502320 1958 898 828 5500 6372 4939000 5276016 1959 879 880 6325 6324 5559675 5565120 1 960 990 975 641 5 5558 6350850 5419050 1 961 870 993 6391 6288 5560170 6243984 1 962 850 995 6400 6372 5440000 6340140 1 963 1000 987 6700 6963 6700000 6872481 1 964 1 040 1 1 33 7200 7461 7488000 8453313 1 965 1 070 1 1 58 7500 7718 8025000 8937444 1 966 1090 1119 7500 8287 8175000 9273153 1967 1060 1 130 7500 761 1 7950000 8600430 1 968 1 1 20 1181 7600 7676 8512000 9065356 1 969 900 1 184 7250 71 04 6525000 8411136 1 970 .1110 1 1 94 8000 7444 8880000 8888136 1 97 1 1 1 40 1 272 8000 7692 9120000 9784224 1972 1 080 1 264 7500 7292 8100000 9217088 1973 1 1 40 1131 7900 7606 9006000 8602386 1 974 1010 1 1 68 7700 671 7 7777000 7845456 1 975 1 1 70 1 264 7900 6694 9243000 8461216 1 976 1220 1 182 7870 6783 9601400 8017506 1 977 1 220 1 357 7920 7470 9662400 10136790 1 978 1 280 1519 8000 7184 10240000 10912496 1 979 1210 1517 7600 5569 9196000 8448173 1 980 1 280 1 770 81 00 6955 10368000 12310350 1981 1 530 1812 81 50 81 50 12469500 14767800 1 982 1 250 1829 6000 5383 7500000 9845507 Data S o u r c e s : DGEA S e r i e s : 1895-1976, Gomez Cobo, 1977, pp. 36-37; 1977-1978, SARH-DGEA, 1980, pp . 112-115; 1979-1981, SARH-DGEA, 1981; 1982, SARH-DGEA, Departamento de E s t i m a c i o n A g r i c o l a N a c i o n a l , P e r s . Com. June 1983. USDA S e r i e s 1932-1960, USDA,(1936-1964) ; Table 2. Decenial maize production in Mexico (1900-1982). DGEA-USDA series: means and percentual change Area Harvested (ha*103) Yield (kg ha - 1 ) Production (tons*103) Period 1. DGEA 2. USDA 3. DGEA 4. USDA 5. DGEA 6. USDA Mean % Mean % Mean % Mean % Mean % Mean % 1900-1909 4200 576 2420 1910-1919 2703 -4.4 562 -0.2 1532 -4.6 1920-1929 3044 1.1 625 1.1 1879 2.0 1930-1939 3104 0.2 3073** 569 -0.9 576** 1771 -0.6 1774** 1940-1949 3482 1.1 3513 1.3 659 1.5 635 1.0 2304 2.6 2235 2.3 1950-1959 5202 4.0 4713 2.9 805 2.0 785 2.1 4217 6.0 3733 5.1 1960-1969 7104 3.1 7057 4.0 1086 3.0 999 2.4 7762 6.1 7085 6.4 1970-1979 7038 -0.1 7839 1.1 1283 1.7 1158 1.5 8999 1.5 9083 2.5 1980-19901 7126 0.4 7417 -1.8 1804 11.4 1353 5.2 12309 10.4 10113 3.6 * = In (Xt+1/Xt)/t+1-t * * Include years 1932-1939. 1 Projection based on 1980-82 data only. (Source: Table 1, this Appendix) 183 Tab le 3. P o p u l a t i o n , consumption of maize , and maize ba lance of t r ade i n M e x i c o . Year Popu la t ion Maize Consumption Balance of Trade (#people) ( t ) ( t ) USDA DGEA USDA DGEA 1895 12632427 1900700 1 900 13607272 2403800 1910 15160369 3161158 1 921 14334780 1803628 1 925 15208225 66235 1 926 15467986 2077970 109238 1 927 15737944 2163263 28421 1 928 16011729 2068872 9938 1 929 16295901 2180742 7897 1 930 16552722 15481 1 9 7931 4 1931 16875977 1395494 18731 1 932 17169696 2138710 33 1 933 17469782 1973586 1 1 7 1934 17776303 1852862 -71003 1 935 18089633 1642481 -80996 1 936 18409591 1670124 -4442 1 937 18736900 1600865 3662 1 938 19071181 1656792 22062 1 939 19413084 1746563 53897 1 940 19653552 1985002 8271 1 941 20195000 1640003 316 1 942 20751000 2125098 1013 1 943 21323000 2363959 736 1 944 21910000 1971748 163656 1 945 22514000 2364772 48586 1 946 23134000 2195025 8831 1 947 23772000 2383221 589 1 948 24427000 2517625 32 1 949 25099000 2817325 -14614 1 950 2579101 7 2871002 363 1951 26585000 3172777 50735 1 952 27585000 3448942 24820 1953 28246000 3578678 376788 1 954 29116000 3868549 146714 1 955 30012000 4430001 -57636 1956 30935000 4608557 118477 1 957 31887000 5194062 812286 1958 32868000 5310434 810436 1959 33880000 5324561 4781 2 To C o n t i n u e 184 Table 3. Continued Year P o p u l a t i o n Maize Consumption Balance of Trade (#people) (t) (t) USDA DGEA USDA DGEA 1960 34923129 5134288 428966 1 961 36253340 5381000 5453764 8000 33982 1962 37583551 5478000 6260259 1 7000 1 4073 1 963 38913762. 6681000 6812781 450000 457422 1 964 40243973 5980000 6633886 61000 -23631 1965 41574184 6478000 7118890 -1150000 - 1334156 1 966 42904394 6986000 8089018 -1091000 -847363 1 967 44234605 7385000 8022602 -1040000 - 1248883 1 968 45564816 7494000 7712172 -896000 -891107 1 969 46895027 7784000 8281122 -920000 -780621 1 970 48225238 7775000 9169819 729000 758925 1 971 50418000 8240000 8619504 -154000 -259880 1 972 52196000 8767000 9557204 -432000 -238638 1 973 54021900 8754000 10337004 1204000 1116166 1 974 55898700 9150000 9884993 1200000 1275861 1 975 57826700 9600000 10474379 21 00000 2626606 1 976 59801200 10000000 9413731 1450000 955127 1 977 61821800 10480000 9709606 1460000 1727426 1 978 63843900 10800000 11491000 1690000 1465180 1 979 65899300 11100000 11653030 630000 894005 1980 67400000 12700000 11839654 387-0000 3713200 1981 68900700 11900000 15224300 3833000 2844400 1 982 70402000 12500000 15245000 700000 500000 Data Sources: P o p u l a t i o n Data Consumption, DGEA: Consumption, USDA: Balance of Trade: 1895-1970, N a t i o n a l P o p u l a t i o n Censuses ( I -IX); SPP, l 9 8 l ( T a b l e 3.1.3); 1980, Xth. N a t i o n a l Census, reported i n Comercio E x t e r i o r , 1971-1979, a l . , 1977, 1980. 1895-1921, 1925-1939, 1972, G a r c i a Cobo, 1977, Table E n g l i s h ed., I n t e r p o l a t ion Table 3; 1981 27 #1, Jan . 1981; from G a r c i a Mata et e x t r a p o l a t e d from T o t a l Consumption=Production; Hewitt de A l c a n t a r a , 1980; 1940-Mata, Barraza Vazquez, and Cruz Table 3; 1973-1978, CDIA, 1980, #27; 1979-1981, Consumption=Production-Net Balance of Trade f o r the Preceding Year. 1961-1975, 1976-1981, USDA-FAS, 1976, 1982. Same Sources as f o r Consumption. 185 Tab le 4. D e c e n n i a l censuses data on Mexican maize p r o d u c t i o n (Source : CDIA, 1980, Tab le #16; A b u r t o , 1979, Tab le V I I I . 1 950 1 960 1 970 Land Under Maize ( h a x l O 6 ) 5.74 6.82 5.87 P r o d u c t i o n ( t x l O 6 ) 4.85 5.71 5.77 Average Y i e l d ( t /ha ) 0.84 0.84 0.98 C r e o l e Maize %Total Area Y i e l d ( t / h a ) n a 1 na 80.2 0.84 79.7 0.93 I n t e r c r o p p e d Maize %Total Area Y i e l d ( t /ha ) na na 15.2 0.64 10.7 0.71 H y b r i d Maize %Total Area Y i e l d ( t /ha ) na na 4.5 1 .47 8.9 1 .83 I r r i g a t i o n %Total Area na 9.1 11.6 1 Data not a v a i l a b l e . 186 APPENDIX 2. SOIL ANALYSIS 2 M e t h o d s of s o i l a n a l y s i s __._1_._1_. F i e l d methods E i g t h s o i l p r o f i l e s were d e s c r i b e d i n the f i e l d . A s o i l d e s c r i p t i o n form (UBC, 1976) was used to c o d i f y f i e l d o b s e r v a t i o n s . S o i l t e x t u r e s were determined by ' f e e l ' i n the f i e l d . M o i s t s o i l c o l o r s were o b t a i n e d from M u n s e l l c o l o r t a b l e s . Taxonomical c l a s s i f i c a t i o n was a t tempted for the FAO (FAO, 1974) and the USDA ( S o i l Survey S t a f f , 1975) s o i l c l a s s i f i c a t i o n systems. S lope ang le s were r e p e a t e d l y measured w i t h a pocket c l i n o m e t e r i n each s i t e . A l t i t u d e s were e s t i m a t e d wi th an a l t i m e t e r , and doub le -checked on t o p o g r a p h i c a l c h a r t s of the area ( s c a l e 1 :250,000) , from which l a t i t u d i n a l and l o n g i t u d i n a l c o o r d i n a t e s were a l s o o b t a i n e d . Geology of the s tudy s i t e s was d e r i v e d from the a l o c a l g e o l o g i c a l map (Mooser and P a l a c i o s , 1 977) . 2_.__.2. L a b o r a t o r y methods U n l e s s s p e c i f i e d o t h e r w i s e , the f o l l o w i n g methods are d e s c r i b e d i n d e t a i l e l sewhere (UBC, 1981). PH = 1:1 s o l u t i o n of s o i l and water , a f t e r shak ing and r e s t i n g 187 for 1 h , was measured wi th g l a s s e l e c t r o d e . Bulk d e n s i t y = samples were d r i l l e d from the p r o f i l e s w i t h a m e t a l l i c c y l i n d e r (255 cm 3 i n volume) , o v e n - d r i e d for 12 h @ 105 C ° , and weighed to c a l c u l a t e dry weight /volume r a t i o s . P a r t i c l e s i z e d i s t r i b u t i o n = Method of the hydrometer . Samples were t r e a t e d wi th Sodium d i t h i o n i t e to remove f ree i r o n o x i d e s . NaO-Acetate was used to remove carbonate s from the C a l i c h e s o i l s ( s i t e s 6 -8 ) . % moi s ture r e t e n t i o n = Determined w i t h porous membrane apparatus at 1/3, 1, 3, and 15 bar p r e s s u r e s . Only 1/3 a n d . 15 bar r ead ing s are p re sen ted h e r e , on a dry weight b a s i s . F i e l d c a p a c i t y = determined w i t h Colman columns ( W i l d e , V o i g t , and I y e r , 1972) . These were r e p l i c a t e d d e t e r m i n a t i o n s on p o l y e t h y l e n e tubes 25 cm i n d iameter and 30 cm l o n g . S o i l samples were s a t u r a t e d wi th water , sampled 24 h l a t e r , and o v e n - d r i e d for 12 h to c a l c u l a t e % water conten t on a dry weight b a s i s . Exchangeable c a t i o n s = C a t i o n s were e x t r a c t e d w i t h NH4-Acetate @ pH 7. C o n c e n t r a t i o n s were read wi th spect rophotometer of atomic a b s o r p t i o n . 188 % N i t r o g e n = D i g e s t e d w i t h Kyedahl method. T o t a l n i t r o g e n was read i n a u t o a n a l y s e r . % Organ ic matter = Carbon was determined w i t h W a l k l e y - B l a c k method. Organ ic matter was c a l c u l a t e d u s i n g 1.724 as the c o n v e r t i o n f a c t o r . A v a i l a b l e phosphorus = Water s o l u b l e , s o l u b l e i n 0.03N NH4F and 0.025N HC1 (Bray method) , and (Olsen) e x t r a c t s were read c o l o r i m e t r i c a l l y . S e s q u i o x i d e s = these f ree ox ides were e x t r a c t e d wi th c i t r a t e -b i c a r b o n a t e - d i t h i o n i t e s o l u t i o n (Mhera and J a c k s o n , 1960) and read w i t h spectrophotometer of atomic a b s o r p t i o n . 189 2 . 2 . S o i l data Andoso l ( s i t e #1) C l a s s i f i c a t i o n : Loca t i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : V e g e t a t i o n : Parent m a t e r i a l Remarks: O c h r i c Andoso l Dys t randept (USDA) (FAO-Unesco) , Ox i c 150 m behind Normal S c h o o l , eas t ent rance to Xa lapa C i t y . 1 9 ° 31' L N , 9 6 ° 58' LW. A l t i t u d e : 1500 m above sea l e v e l . H i l l y , Xa lapa Land System, l a n d u n i t #57 (Sancho luz , M a r t e n , and Z o l a , 1981). H i l l top ( c r e s t ) , i n c l i n a t i o n . W e l l d r a i n e d . Induced g r a s s e s . convex s l o p e , 9% and c i n d e r s of P l i o c e n e M o d e r a t e l y eroded phase . V o l c a n i c ash e r u p t i o n s . 190 H o r i z o n d e s c r i p t i o n ( A n d o s o l , S i t e #1) A 0-15 cm Dark brown (10 YR 4/3) loam. Moderate coar se g r a n u l a r s t r u c t u r e , f r i a b l e . Abundant f i n e o b l i q u e p o r e s , abundant f i n e v e r t i c a l r o o t s . D i f f u s e i r r e g u l a r boundary. AB 15-30 cm Y e l l o w i s h brown (10 YR 5/6) sandy c l a y loam. Moderate to s t r o n g angu la r b l o c k y s t r u c t u r e ( c o n s i s t e n c y 3 ) . P l e n t y medium o b l i q u e pores i n ex-peds . P l e n t y of medium and f i n e r o o t s i n ex-peds . Few f i n e ox ide c o n c r e t i o n s . Wavy h o r i z o n boundary . B2 30-90 cm Red y e l l o w (7 .5 YR 4/8) c l a y . Moderate to s t r o n g coar se columnar s t r u c t u r e ( c o n s i s t e n c y 4 ) . Few f i n e pores i n ex-peds . Few f i n e r o o t s i n ex-peds . Abundant ox ide c o n c r e t i o n s . Abrupt h o r i z o n boundary. B3 90-150 cm Red y e l l o w (7 .5 YR 5/8) c l a y . S t rong very coar se p r i s m a t i c s t r u c t u r e ( c o n s i s t e n c y 6 ) . Very few coar se pores i n ex-peds . No r o o t s . A n a l y t i c a l data ( A n d o s o l , s i t e #1) H o r i z o n A AB B2 B3 Depth (cm) 0-15 15-30 30-90 90-150 PH 5.6 5.2 5.2 5.1 Bulk d e n s i t y (g/cm 3 ) 0.8 0.81 1 .0 1 . 1 P a r t i c l e s > 2mm (%dry s o i l ) 1.0 2.0 2.0 2.0 F i n e texture(%) Coarse sand(2-.1mm) 30.6 {46.2 {9.3 {8.2 F i n e sand(.1-.05mm) 18.2 S i l t ( . 0 5 - . 0 0 2 m m ) 29.8 20.0 10.9 6.0 Clay(<.002mm) 21 .4 33.8 79.8 85.8 F i e l d Capaci ty(%) 32 34 35 n . a . T o t a l e lements (ppm) Ca 1055 650 1 500 950 K 65 33 93 90 Mg 95 176 580 1 28 % N 0.32 0.11 0.06 0.05 % OM 5.7 1 .7 0.7 0.7 C/N 10.3 9.0 6.8 8.1 A v a i l a b l e P (Bray) 1.21 1.61 n . a . 0.32 192 Andoso l ( s i t e #2) C l a s s i f i c a t i o n : Locat i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : Veqeta t i o n : Parent m a t e r i a l Remarks: Humic Andoso l (FAO-Unesco) , T y p i c Dys t randept (USDA) E x p e r i m e n t a l area of C l a v i j e r o B o t a n i c a l Garden, km 5, O l d Road X a l a p a - C o a t e p e c , V e r a c r u z . 1 9 ° 30' LN, 9 6 ° 56* LW. A l t i t u d e : 1300 m above sea l e v e l . H i l l y , Xa lapa Land System, Land U n i t #56 (Sancho luz , Marten and Z o l a , 1981). Lower s l o p e , s l i g h t l y i n c l i n a t i o n . concave , 1 5% W e l l d r a i n e d , s m a l l stream 150 m a p p a r t . Abandoned orange and c o f f e e o r c h a r d (10 year s o l d ) , has abundant gras s and herbs c o v e r . V o l c a n i c ash and c i n d e r s of P l i o c e n e e r u p t i o n s . S l i g h t l y to moderate ly eroded phase . Next to e x p e r i m e n t a l p l o t of t h i s s tudy (see Chapter V , Xa lapa s i t e ) . 1 93 H o r i z o n d e s c r i p t i o n ( A n d o s o l , S i t e '#2) 0 2-0 cm Dark brown (10 YR 3 /3 ) , p a r t i a l l y decomposed o r g a n i c m a t e r i a l , f i b r o u s , abundant l eaves and many r o o t s . ' Abrupt boundary . A1 0-10 cm Dark brown (10 YR 4/3) loam. Weak to moderate coar se g r a n u l a r s t r u c t u r e , f r i a b l e ( m o i s t ) . P l e n t y medium s i z e , randomly o r i e n t e d p o r e s . Few coar se r o o t s but p l e n t y of medium and f i n e r o o t s . D i f f u s e i r r e g u l a r boundary. AB 10-30 cm L i g h t y e l l o w i s h brown (10 YR 6/4) loam. Moderate , very coarse g r a n u l a r s t r u c t u r e , f r i a b l e . P l e n t y medium s i z e d p o r e s . Common f i n e ox ide c o n c r e t i o n s . P l e n t y of coar se and medium r o o t s . C l e a r wavy h o r i z o n boundary . B2 30-85 cm Y e l l o w i s h brown (10 YR 5/6) sandy c l a y loam. Moderate coar se columnar s t r u c t u r e . Roots f o l l o w peds faces u n t i l boundary. Abundant f i n e ox ide c o n c r e t i o n s . Abrupt boundary. B3 85-150 cm Reddi sh y e l l o w (7 .5 YR 6/8) sandy c l a y loam. S t rong coar se p r i s m a t i c s t r u c t u r e . Few roo t s and p o r e s . A n a l y t i c a l da ta ( A n d o s o l , s i t e #2) Hor i z o n A1 AB B2 B3 Depth (cm) 0-10 10-30 30-85 85-150 PH 5.1 5.2 ' 5.0 5.5 Bulk d e n s i t y (g/cm 3 ) 0.75 0.80 0.83 0.90 P a r t i c l e s > 2mm (%dry s o i l ) 1 .7 0.4 0.1 3.0 F i n e texture(%) Coarse sand(2-.1mm) 32.8 30.2 43.2 47.0 F i n e sand(.1-.05mm) 16.7 14.5 9.3 10.1 S i l t ( . 0 5 - . 0 0 2 m m ) 30.7 38.5 24.3 20.7 Clay(<.002mm) 19.8 16.8 23.2 22.2 % M o i s t u r e r e t e n t i o n (dry. b a s i s ) @ .3 bar 52.5 49.8 50.0 49.0 @ 15 bar 35.6 33.8 43.4 42.0 F i e l d Capaci ty(%) 46 50 59 n . a . E x c h . c a t i o n s (meq/1OOg) Ca 5 .9 3.6 2.2 0.3 K 0.6 0.5 0.3 0.2 Mg 2.6 1 .6 1 .6 2.9 Na 0.2 0.2 0.2 1 .0 Mn 0.2 0.2 0.2 1 .0 H 29.6 31 .3 0.1 0.02 C . E . C . (meq/lOOg) 39. 1 37.2 30.0 20.0 % Base s a t u r a t i o n 24 16 1 5 22 % N 0.52 0.42 0.18 0.06 % OM 11.4 9.2 3.5 1 .0 C/N 12.6 12.6 10.9 9.7 A v a i l a b l e P (H20) 0.03 0.03 n . d . n . d . (Bray) 0.79 n . d . 0.42 0.4 CBD E x t r a c t a b l e (%) Fe203 5.6 6.7 6.6 n . a . A1203 3.4 9.0 3.5 n . a . S i02 0.5 0.5 0.6 n . a . 195 T r a n s i t i o n a l A n d o s o l - T e p e t a t e ( s i t e #3) C l a s s i f i c a t i o n : Loca t i o n : P h y s i o g r a p h y ; Topography: D r a i n a g e ; Vegeta t i o n : Parent m a t e r i a l 300 m East of CONAFRUT, 4 km Eas t of Xa lapa on N a t i o n a l Highway. 1 9 ° 3 0 ' 3 2 " LN, 9 6 ° 4 8 ' 3 6 " LW. A l t i t u d e : 1200 m above sea l e v e l . H i l l c r e s t . T r a n s i t i o n between Xa lapa and Tepeta te Land Systems (Sancho luz , Marten & Z o l a , 1981). Convex s l o p e , 10% i n c l i n a t i o n . P o o r l y d r a i n e d . Medium, s u b t r o p i c a l f o r e s t . V o l c a n i c ash and b a s a l t s . 1 96 H o r i z o n d e s c r i p t i o n ( T r a n s i t i o n a l A n d o s o l - T e p e t a t e , S i t e #3) A 0-6 cm Dark brown (7.5 YR 4/4) c l a y loam. Weak to moderate medium g r a n u l a r s t r u c t u r e . P l e n t y of medium s i z e random and c o n t i n u o u s p o r e s . Abundant coar se roo t s i n peds . Earthworm a c t i v i t y common. Common medium Fe c o n c r e t i o n s , r e d d i s h brown (5 YR 4/4) i n c o l o r . Wavy and d i f f u s e boundary. AB 6-23 cm St rong brown (7 .5 YR 5/6) c l a y loam. Moderate medium angular b l o c k y s t r u c t u r e . S t i c k y c o n s i s t e n c y when wet, s l i g h t l y p l a s t i c . P l e n t y medium s i z e d , o b l i q u e pores in peds . Frequent coar se o b l i q u e r o o t s , abundant f i n e r o o t s . Common Fe c o n c r e t i o n s r e d d i s h brown (5 YR 4/4) i n c o l o r . C l e a r wavy boundary. B 23-60 cm Dark y e l l o w i s h brown (10 YR 4/6) g r a v e l y c l a y loam. S t rong medium angu la r b l o c k y s t r u c t u r e . Very few coarse random pores i n expeds . Few r o o t s . Common medium g r e y i s h (7 .5 YR 6/0) m o t t l e s . Abundant Fe c o n c r e t i o n s . Slow to moderate p e r m e a b i l i t y . C l e a r boundary. R 60-96 cm L i g h t brown (7.5 YR 6 / 4 ) . M a s s i v e . Abundant Fe c o n c r e t i o n s . Few p o r e s , r o o t s . Slow p e r m e a b i l i t y . A n a l y t i c a l data ( T r a n s i t i o n a l A n d o s o l - T e p e t a t e , s i t e #3) H o r i z o n A AB B2 Depth (cm) 0-6 6-23 23-60 pH 5.1 5.3 5.3 Bulk d e n s i t y (g/cm 3 ) 0.9 0.95 1 .0 P a r t i c l e s > 2mm (%dry s o i l ) n . d . 2.0 7.0 F i n e texture(%) Coarse sand(2-.1mm) 24.5 {39.3 {31.3 F i n e s.and ( . 1 - . 05mm) 7.5 S i l t ( . 0 5 - . 0 0 2 m m ) 29.0 24.0 32.0 Clay(<.002mm) 39.0 36.7 36.7 F i e l d Capaci ty(%) n . a . 28 27 T o t a l e lements (ppm) Ca n . a . 850 400 K n . a . 48 1 5 Mg n . a . 1 29 1 54 % N 0.13 0.30 0.08 % OM 4.72 3.96 0.98 C/N 21.1 7.7 7.1 A v a i l a b l e P (Bray) n . a . 1 .05 n . d . 198 Tepe ta te ( S i t e #4) C l a s s i f i c a t i o n : Loca t i o n : P h y s i o g r a p h y : Topography: Dra i nage: Vegeta t i o n : Parent m a t e r i a l : D r a i n a g e : Remarks: T y p i c Durandept (USDA) 3 km NE from E l C h i c o , V e r a c r u z . E j i d o p l o t of E m i l i o M a r t i n e z . 1 9 ° 2 8 ' 3 0 " LN, 9 6 ° 4 5 ' 3 0 " LW. A l t i t u d e : 1080 m above sea l eve l - . H i l l c r e s t i n Dos R ios Land System, Tepeta te l andscape . Convex s lope 6.8% i n c l i n a t i o n , moderate ly eroded phase . Poor . Pangola g ra s s for 15 year s (sugar cane b e f o r e ) . V o l c a n i c ash and b a s a l t , slow to very slow Right b e s i d e s E l C h i c o e x p e r i m e n t a l p l o t s (see C h . V ) . 199 H o r i z o n d e s c r i p t i o n ( S i t e #4) A 0-9 cm Brown (10 YR 5/3) sandy c l a y loam. Moderate medium g r a n u l a r s t r u c t u r e . S l i g h t l y hard c o n s i s t e n c y . P l e n t y of f i n e pores i n pedons . P l e n t y f i n e and medium r o o t s . C l e a r boundary. B1 9-30 cm Pa le brown (10 YR 6/3) c l a y loam. Moderate to s t r o n g medium subangular b l o c k y s t r u c t u r e . Common g r a y i s h (10 YR 6/1) m o t t l e s . P l e n t y of f i n e r o o t s . Many y e l l o w i s h red (5 YR 4/8) f i n e Fe ox ide c o n c r e t i o n s . H o r i z o n boundary c l e a r . B2 30-50 cm L i g h t brown gray (10 YR 6/2) c l a y . Mass ive s t r u c t u r e . Abundant f i n e and medium Fe ox ide c o n c r e t i o n s , y e l l o w i h red i n c o l o r (5 YR 4 / 8 ) . Few r o o t s . Many coar se g r a y i s h (10 YR 6/1) m o t t l e s . Poor d r a i n a g e . D i f f u s e boundary. R 50 + cm L i g h t brown (7 .5 YR .6/4) c l a y e y hardpan . S t rong massive s t r u c t u r e . No r o o t s , few p o r e s . A n a l y t i c a l data ( s i t e #4) H o r i z o n A B1 B2 C Depth (cm) 0-13 1 3-30 30-50 50 + pH 4.6 5.1 4.8 5.3 Bulk d e n s i t y (g/cm 3 ) 1.21 1 .43 1 .44 1 .57 P a r t i c l e s > 2mm (%dry s o i l ) 5.2 4.2 3.6 13.1 F i n e texture(%) Coarse sand(2-.1mm) 39.9 33. 1 22.2 28.6 F i n e sand(.1 -.05mm) 10.7 8.9 6.8 7.4 S i l t ( .05-.002mm) 20.4 26. 1 22.9 22. 1 Clay(<.002mm) 29.0 31.9 48. 1 41 .9 % M o i s t u r e r e t e n t i o n (dry b a s i s ) @ .3bar 31.4 25.5 26.5 25.4 Ci 15bar 16.9 16.2 18.6 17.9 F i e l d Capaci ty(%) 26 25 23 n . a . E x c h . c a t i o n s (meq/1OOg) Ca 3.24 3.12 3.22 2.2 K 0.2 0.1 0.1 0.2 Mg 2.2 4.4 5.4 3.6 Na 0.17 0.59 0.85 0.3 Mn 0.15 0.03 0.01 0.01 H 7.6 6.2 4. 1 15.6 C . E . C . (meq/lOOg) 13.5 14.5 13.6 21.9 % Base s a t u r a t i o n 44.0 57.0 70.0 29.0 % N 0.15 0.06 0.02 0.02 % OM 3.2 1 .3 0.22 0.34 C/N 12.7 12.3 8.7 11.1 A v a i l a b l e P (H20) 0.18 n . d 0.03 n . d . (Bray) 0.5 n . d . n . d . n . d . CBD E x t r a c t a b l e (%) Fe203 n . a 10.3 3.7 5.4 A1203 n . a 1 .2 0.5 0.4 S i02 n . a 0.2 0.2 0.3 201 Tepe ta te ( S i t e #5) C l a s s i f i c a t i o n : Loca t i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : Vegeta t i o n : Parent m a t e r i a l : Remarks: T y p i c Durandept (USDA) Same as S i t e #4. Ravine in Dos R i o s Land System, Tepeta te l a n d s c a p e . Complex s lope 25% i n c l i n a t i o n . I m p e r f e c t l y d r a i n e d . Grass l a n d , u s u a l l y cropped w i t h c o f f e e . B a s a l t and V o l c a n i c C i n d e r s . I n f l u e n c e d by a l l u v i a l d e p o s i t s 202 H o r i z o n d e s c r i p t i o n (Tepe ta te , S i t e #5) 0-13 cm AB 13-22cm B2 22-39cm B3 39-50cm 50+ cm Dark brown (7 .5 YR 4/2) loam. Weak to moderate g r a n u l a r s t r u c t u r e . P l e n t y medium s i z e d o b l i q u e pores i n m a t r i x . Earthworm a c t i v i t y . P l e n t y medium f i n e r o o t s . Common f i n e Fe ox ide c o n c r e t i o n s . C l e a r wavy boundary. Brown (7.5 YR angu la r b l o c k y random p o r e s , c o n c r e t i o n s . Few medium o b l i q u e r o o t s 5/2) c l a y loam. Moderate s t r u c t u r e . P l e n t y f i n e Common medium Fe ox ide Few f i n e g r e y i s h m o t t l e s . Abrupt boundary. Redd i sh brown (5 YR 5/4) c l a y loam. Coarse angu la r b lock s t r u c t u r e . Very few f i n e h o r i z o n t a l pores i n ex-peds . Abundant medium Fe ox ide c o n c r e t i o n s . Very few r o o t s , expeds common medium g r e y i s h m o t t l e s . Reddi sh y e l l o w (7 .5 YR 6/6) c l a y . S t rong medium p r i s m a t i c s t r u c t u r e . No pores or r o o t s . Has water t a b l e at bottom. Abundant p ink (7 .5 YR 7/4) m o t t l e s and r e d d i s h Fe ox ide c o n c r e t i o n s . C l e a r boundary. L i g h t brown (7 .5 YR 6/4) c l a y e y Hardpan. S t rong massive s t r u c t u r e . 203 A n a l y t i c a l data (Tepe ta te , s i t e #5) H o r i z o n A AB B2 B3 Depth (cm) 0-13 1 3-22 22-39 39-60 pH 5.7 5.8 5.7 5.8 Bulk d e n s i t y (g/cm 3 ) 0.91 1 .0 1 . 1 1 .3 P a r t i c l e s > 2mm -(%dry s o i l ) 1 .0 2.0 5.0 8.0 F i n e texture(%) Coarse sand(2-.1mm) n . a n .a {45.0 {34.3 F i n e sand(.1-.05mm) n .a n . a S i l t ( .05-.002mm) n .a n .a 22.3 20.0 Clay(<.002mm) n . a n .a 32.7 40.7 % M o i s t u r e r e t e n t i o n (dry b a s i s ) @ .3bar n . a . 31 .4 25.9 26. 1 @ 15bar n . a . 16.9 17.9 17.6 F i e l d Capaci ty(%) 31 30 27 27 E x c h . c a t i o n s (meq/1OOg) Ca 6.01 6.34 4.34 2.93 K 0.19 0.08 0.04 0.05 Mg 1 .48 1 .05 0.83 0.71 % N 0.24 0.25 0.09 0.05 % OM 4.04 3.79 1.10 1 .00 C/N 9.76 8.79 7.09 1 1 .60 A v a i l a b l e P (Bray) 2.73 1 .58 0.61 n . d . 204 C a l i c h e ( S i t e #6) C l a s s i f i c a t i o n : Locat i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : V e g e t a t i o n : Remarks: P e l l i c V e r t i s o l (FAO-Unesco) , T y p i c P e l l u s t e r t s (USDA) Road C a r r i z a l - C h a u a p a n (km 2 . 5 ) . 19 0 2 1 ' 3 0 " LN, 9 6 ° 4 0 ' L W . 550 m above sea l e v e l . C a r r i z a l Land System, C a r r i z a l Landscape , C r e s t Land U n i t ( Sancho luz , Marten and Z o l a , 1981). Convex s l o p e , 7% i n c l i n a t i o n . W e l l d r a i n e d . maize f i e l d , w i t h papaya i n pas t r o t a t i o n . Low t r o p i c a l des iduous f o r e s t . Parent m a t e r i a l : marine c a l c a r e o u s (marl) d e p o s i t s . Eroded phase , l o c a l l y c a l l e d c a l i c h e s o i l because of abundant g r a v e l and s tones i n su r f ace h o r i z o n s . 205 H o r i z o n d e s c r i p t i o n ( C a l i c h e , S i t e #6) 0-10cm Black (7.5 YR 2/0) g r a v e l l y c l a y loam. Moderate medium angu la r b l o c k y s t r u c t u r e . P l e n t y medium s i z e d , cont inuous p o r e s . Abundant f i n e and medium r o o t s . Moderate e f e r v e s c e n c e w i t h HC1. C l e a r wavy boundary . B 10-22cm Dark gray (7 .5 YR 4/0) g r a v e l l y c l a y . Moderate to s t r o n g coar se angu la r b lock s t r u c t u r e . P l e n t y f i n e cont inuous p o r e s . P l e n t y f i n e and medium r o o t s . Moderate e f e r v e s c e n c e w i t h HC1. Abrupt boundary . BC 22-50cm Gray (7 .5 YR 6/0) s toney c l a y . Mass ive s t r u c t u r e . Very few pores and r o o t s . S t rong e f e r v e s c e n c e w i t h HC1. Gradua l boundary i n t o m a r l . 50cm White (10 YR 8/1) m a r l . 206 A n a l y t i c a l data ( C a l i c h e , s i t e ' #6) H o r i z o n A B BC Depth (cm) 0-10 1 0-22 22-50 pH 7.8 7.9 8.0 Bulk d e n s i t y (g/cm 3 ) 0.75 1.0 1 . 1 P a r t i c l e s > 2mm (%dry s o i l ) 20.0 27.0 35.0 F i n e texture(%) Coarse sand(2-.1mm) 21.9 n . a . n . a . F i n e sand(.1-.05mm) 12.1 n . a . n . a . S i l t ( . 0 5 - . 0 0 2 m m ) 30.7 n . a . n . a . Clay(<.002mm) 35.3 n . a . n . a . F i e l d Capaci ty(%) 55 51 40 E x c h . c a t i o n s (meq/1OOg) Ca 64.2 n . a . 72 .9 K 0.7 n . a . 0.6 Mg 3.1 n . a . 1 .8 Na 0.2 n . a . 0.1 Mn 0.2 n . a . 0.1 H 1 .5 n . a . 0.0 C . E . C (meq/lOOg) 69.8 n . a . 75.3 % Base s a t u r a t i o n 98.0 n . a . 99.0 % N 0.25 n . a . 0.23 % OM 5.1 n . a . 4.7 C/N 11.83 n . a . 11.85 A v a i l a b l e P (H20) 0.43 n . a . 0.21 (Bray) 5.6 4.4 n . a . 207 C a l i c h e ( S i t e #7) C l a s s i f i c a t i o n : Locat i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : Vegeta t i o n : Parent m a t e r i a l ; Remarks: P e l l i c V e r t i s o l (FAO-Unesco) , T y p i c P e l l u s t e r s (USDA) Chauapan, E . Zapata County . Schoo l p l o t in the Chauapan E j i d o . l 9 o 2 T 3 0 n LN , 9 6 ° 4 0 ' 3 0 " L W . 1590 m above sea l e v e l . C a r r i z a l Land System, C a r r i z a l l a n d s c a p e , c r e s t l a n d u n i t ( Sancho luz , Marten and Z o l a , 1981). Upper s l o p e , near c r e s t . 4-5% i n c l i n a t i o n . M o d e r a t l y w e l l d r a i n e d . Maize f i e l d s , c u l t i v a t e d for 15 y e a r s . Marine c a l c a r e o u s d e p o s i t , m a r l . Bes ide Chauapan e x p e r i m e n t a l p l o t (see C h . 5 ) . Moderate e r o s i o n phase . 208 H o r i z o n d e s c r i p t i o n ( C a l i c h e , S i t e #7) 0-15cm B1 15-25cm B2 25-40cm BC 40-65cm Black (7 .5 YR 2/0) f i n e c l a y , weak medium subangular b lock s t r u c t u r e . P l e n t y f i n e c o n t i n u o u s p o r e s . Few t h i n c l a y f i l m s . P l e n t y f i n e and medium r o o t s . Weak e f e r v e s c e n c e to HC1. Abrupt boundary. Very dark gray (10 YR 3/1) very f i n e c l a y . Weak to moderate coar se angu la r b l o c k y s t r u c t u r e . Abundant f i n e and medium r o o t s . Common t h i n c l a y f i l m s i n - p e d . Weak e f e r v e s c e n c e to HC1. Abrupt wavy boundary. Gray (10 YR 6/1) g r a v e l l y very f i n e c l a y . S t rong angular b l o c k i n g s t r u c t u r e . Few f i n e p o r e s . Very few f i n e r o o t s , h o r i z o n t a l . Common modera te ly t h i c k c l a y f i l m s i n . Moderate e f e rve scence to H C l . G r a d u a l boundary. White (10 YR 8/2) g r a v e l l y m a r l . Mass ive s t r u c t u r e a l t e r e d by g r a v e l and c o b b l e s . Common p a l e gray (10 YR 7/2) m o t t l i n g . Very few p o r e s , no r o o t s . Cont inues i n t o deep m a r l . A n a l y t i c a l data ( C a l i c h e , s i t e #7) Hor i z o n A B1 B2 BC Depth (cm) 0-15 15-25 25-40 40-65 pH 7.9 7.7 7.7 7.9 Bulk d e n s i t y (g/cm 3 ) 0.75 1 .05 1 . 1 1 .2 P a r t i c l e s > 2mm (%dry s o i l ) 10.0 20.0 35.0 40.0 F i n e texture(%) Coarse sand(2-.1mm) 10.4 15.0 12.3 n . a . F i n e sand(.1-.05mm) 5.8 8.9 4.7 n . a . S i l t ( . 0 5 - . 0 0 2 m m ) 23.8 2.4 14.3 n . a . Clay(<.002mm) 60.0 73.7 68.7 n . a . % M o i s t u r e r e t e n t i o n (dry b a s i s ) @ .3bar 60.2 70.2 63.5 n . a . @ 15bar 38.3 43.4 37.8 n . a . F i e l d Capaci ty(%) 55 52 55 55 % N 0.19 0.16 0.09 0.06 % OM 4.09 2.30 1 .40 1 .20 C/N 12.5 8.3 9.02 11.6 A v a i l a b l e P (Olsen) (ppm) 8.5 5.4 5.2 3.0 CBD E x t r a c t a b l e (%) Fe203 0.26 0.14 0.11 n . a . A1203 0.30 0.23 0.23 n . a . S i02 0.34 0.30 0.30 n . a . 210 C a l i c h e ( S i t e #8) C l a s s i f i c a t i o n : Loca t i o n : P h y s i o g r a p h y : Topography: D r a i n a g e : V e g e t a t i o n : Parent m a t e r i a l P e l l i c V e r t i s o l P e l l u s t e r s (USDA) 250 m west of S i t e #7. (FAO-Unesco) , T y p i c C a r r i z a l Land System, C a r r i z a l Landscape , Bottom Land U n i t ( Sancho luz , Marten and Z o l a , 1981) G e n t l y concave bottom, s lope i n c l i n a t i o n , a c c u m u l a t i o n s i t e . M o d e r a t e l l y w e l l d r a i n e d . Maize f i e l d c u l t i v a t e d for 15 y e a r s . Mar ine c a l c a r e o u s d e p o s i t s ( m a r l ) . 1 -2% Remarks: Known l o c a l l y as " b a r r o de fondo" ( l o c a l mud) . 21 1 H o r i z o n d e s c r i p t i o n ( C a l i c h e , S i t e #8) A1 0-15cm A2 15-30cm B1 35-45cm, B2 45-60cm Black (7 .5 YR 2/0) c l a y loam. Moderate medium angu lar b lock s t r u c t u r e . P l e n t y f i n e c o n t i n u o u s p o r e s . P l e n t y f i n e and medium r o o t s . Very weak HC1 e f e r v e s c e n c e . D i f f u s e boundary. B lack (7 .5 YR 2/0) c l a y loam. Moderate medium angu lar b l o c k y s t r u c t u r e . P l e n t y f i n e c o n t i n u o u s p o r e s . P l e n t y f i n e and medium r o o t s . Weak HC1 e f e r v e s c e n c e . C l e a r wavy boundary Very dark gray (7 .5 YR 3/0) c l a y . Moderate coar se angu la r -blocky s t r u c t u r e . Few f i n e p o r e s , few medium r o o t s . Moderate e f e rve scence w i t h H C l . D i f f u s e boundary. l i g h t brown-gray (10 YR 6/2) c l a y . S t rong coar se angu la r b l o c k s t r u c t u r e . Very few pores and r o o t s . Common t h i c k c l a y f i l m s . S t rong HCl e f e r v e s c e n c e . BC 55cm White (10 YR 8/2) g r a v e l l y c l a y , m a r l . A n a l y t i c a l data ( C a l i c h e , s i t e #8) H o r i z o n A1 A2 B1 B2 Depth (cm) 0-15 1 5-30 30-45 45-60 PH 7.9 7.7 7.7 7.5 . Bulk d e n s i t y (g/cm 3 ) 0.80 0.85 0.90 1.0 P a r t i c l e s > 2mm (%dry s o i l ) 5.0 5.0 10.0 20.0 F i e l d Capaci ty(%) 57 58 58 % N 0. 19 0.21 0.22 0. 12 % OM 4.09 5.09 4.54 2.78 C/N 12.5 14.1 11.9 13.44 A v a i l a b l e P (Olsen) (ppm) 8.2 3.8 2.8 0.8 

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