UBC Theses and Dissertations

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UBC Theses and Dissertations

Hydrologic behaviour of some structurally organic soils Bonsu, Mensah 1984

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HYDROLOGIC BEHAVIOUR OF SOME STRUCTURALLY DEGRADED ORGANIC SOILS B.Sc. Hons. ( A g r i c . Mech.) U n i v e r s i t y o f Ghana, 1972 M.Sc. ( S o i l S c i e n c e ) U n i v e r s i t y o f Ghana, 1978 A T h e s i s S u b m i t t e d i n P a r t i a l F u l f i l l m e n t Of the Requirements f o r t h e Degree o f The F a c u l t y o f Graduate S t u d i e s (Department of S o i l S c i e n c e ) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA Ja n u a r y 1984 w M e n s a h Bonsu, 1984 by M a s t e r of S c i e n c e i n 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 s c h o l a r l y 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 t h e s i s 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 DE-6 (3/81) ABSTRACT I I T h i s t h e s i s d e s c r i b e s the h y d r o l o g i c b e h a v i o u r of some o r g a n i c s o i l s o f the S e r p e n t i n e - N i c o m e k l a r e a i n the Lower F r a s e r V a l l e y near C l o v e r d a l e i n B r i t i s h Columbia, i n r e l a t i o n t o t h e i r s t r u c t u r a l d e g r a d a t i o n . An i n d e x was developed t o c h a r a c t e r i z e the s t r u c t u r a l s t a b i l i t y of the o r g a n i c s o i l s based on r e l a t i v e h y d r a u l i c c o n d u c t i v i t y approach f o l l o w i n g d i s p e r s i o n . The study showed t h a t the s t r u c t u r a l s t a b i l i t y of the o r g a n i c s o i l s d ecreased w i t h i n c r e a s i n g ash c o n t e n t . I t was suggested t h a t s t r u c t u r a l s t a b i l i t y r a t i n g s i n o r g a n i c s o i l s c o u l d be e s t a b l i s h e d from a knowledge o f t h e i r ash c o n t e n t . High b u l k d e n s i t y and p e n e t r a t i o n r e s i s t a n c e , low s a t i a t e d h y d r a u l i c c o n d u c t i v i t y and f i e l d p r o f i l e i n v e s t i g a t i o n s c o n f i r m e d the presence of a c u l t i v a t i o n pan w i t h i n the depth of c u l t i v a t i o n . Ponding of water on these o r g a n i c s o i l s was a t t r i b u t e d t o the c u l t i v a t i o n pan. On f l o o d i n g ^ eroded o r g a n i c and m i n e r a l s o i l p a r t i c l e s s e t t l e d t o form a s u r f a c e s e a l of low i n f i l t r a b i l i t y . The presence of t r a s h d i s r u p t e d a co n t i n u o u s f o r m a t i o n of the s u r f a c e s e a l . S t r u c t u r a l d e g r a d a t i o n o f the o r g a n i c top s o i l due t o c u l t i v a t i n g the s o i l when i t was not a d e q u a t e l y t r a f f i c a b l e o r workable was r e s p o n s i b l e f o r the l i m i t e d c a p a c i t y o f the o r g a n i c top s o i l t o r e l e a s e water. T h i s e f f e c t was shown I l l by r e l a t i v e l y f l a t p a r t i a l water r e t e n t i o n c u r v e s . Recommendations are g i v e n as t o how o r g a n i c s o i l s found i n c o l d , wet c l i m a t e s c o u l d be managed i n o r d e r t o m a i n t a i n good s t r u c t u r a l c h a r a c t e r i s t i c s . IV TABLE OF CONTENTS Pa£e ABSTRACT 1 1 LIST OF TABLES V I 1 LIST OF FIGURES I x LIST OF PLATES X I ACKNOWLEDGEMENTS X I 1 CHAPTER 1 - INTRODUCTION 1 1.1 SOILS AND ENVIRONMENTAL CONDITIONS 5 1.2 REVIEW OF LITERATURE 11 1.2.0 Nature and f o r m a t i o n of o r g a n i c s o i l s 11 1.2.1 Subsidence i n o r g a n i c s o i l s 12 1.2.2 C o n t r o l of sub s i d e n c e i n o r g a n i c s o i l s 13 1.2.3 P h y s i c a l p r o p e r t i e s of o r g a n i c s o i l s 23 1.2.4 S o i l water management i n o r g a n i c s o i l s 26 1.2.5 S o i l s t r u c t u r a l d e t e r i o r a t i o n 31 CHAPTER 2 - MATERIALS AND METHODS 36 2.1 DETERMINATION OF FIBER CONTENT 36 2.2 DETERMINATION OF SHRINKAGE VOLUME 37 2.3 ORGANIC MATTER AND ASH CONTENTS DETERMIN-ATION 37 2.4 DETERMINATION OF WATER RETENTION CHARACTER-ISTICS 38 2.4.1 The hanging water column t e c h n i q u e .. 38 2.4.2 The p r e s s u r e p l a t e e x t r a c t i o n method 40 V TABLE OF CONTENTS  Page 2 Page 2.5 DETERMINATION OF "SATIATED" HYDRAULIC CONDUCTIVITY 40 2.6 METHODS FOR CHARACTERIZING STRUCTURAL STABILITY 45 2.6.1 R e l a t i v e C o n d u c t i v i t y Technique 45 2.6.2 The Waterdrop Technique 46 2.7 FIELD INVESTIGATION OF PENETRATION RESISTANCE 50 2.8 ESTABLISHMENT OF EVIDENCE OF PONDING IN THE FIELD 51 2.9 SIMULATION OF THE PROCESS OF FORMATION OF A SURFACE LAYER OF LOW INFILTRABILITY DUE TO SETTLED ORGANIC AND MINERAL PARTICLES IN PONDED WATER 51 2.10 BULK DENSITY AND PARTICLE DENSITY 54 2.11 SAMPLING PROCEDURE 54 CHAPTER 3 - RESULTS AND DISCUSSION 56 3.1 SOME PERTINENT PROPERTIES OF THE SOILS 56 3.1.1 B u l k D e n s i t y and P a r t i c l e D e n s i t y ... 56 3.1.2 Ash and Organic M a t t e r Contents 58 3.1.3 The F i b e r Content 60 3.1.4 S o i l pH 60 3.2 SHRINKAGE 61 3.3 PENETRATION RESISTANCE 63 3.4 EVIDENCE OF PONDING AND RECESSION RATE OF PONDED WATER IN THE FIELD 68 VI TABLE OF CONTENTS  Page 3 Page 3.5 HYDROLOGIC CHARACTERISTICS 7 4 3.5.1 Water R e t e n t i o n 7 4 3.5.2 S a t i a t e d H y d r a u l i c C o n d u c t i v i t y ..... 7 B 3.6 SIMULATION OF THE PROCESS OF FORMATION OF A LAYER OF LOW INFILTRABILITY DUE TO SETTLING OF ORGANIC AND MINERAL PARTICLES ON PONDING. 89 3.7 CHARACTERIZATION OF STRUCTURAL STABILITY OF ORGANIC SOILS 91 3.7.1 The R e l a t i v e H y d r a u l i c C o n d u c t i v i t y Approach 91 3.7.2 Waterdrop Technique 96 SUMMARY 99 CONCLUSIONS 101 MANAGEMENT RECOMMENDATIONS 102 LIST OF REFERENCES 106 APPENDIX I - S t a t i s t i c s t o Compare the Means o f H y d r a u l i c C o n d u c t i v i t y ("Ks") V a l u e s . H 5 APPENDIX I I - S a t i a t e d H y d r a u l i c C o n d u c t i v i t y ("Ks") Va l u e s i n Vinod S o i l s 1 1 8 APPENDIX I l i a - S a t i a t e d H y d r a u l i c C o n d u c t i v i t y ("Ks") Va l u e s i n Richmond S o i l ( S t a t e r Farm). 119 APPENDIX I l l b - S a t i a t e d H y d r a u l i c C o n d u c t i v i t y ("Ks") Valu e s i n Richmond S o i l ( C l o v e r d a l e Produce Farm) 1 2 0 v n LIST OF TABLES Table Page 1.1 Ash content of moss-dominated peat and sedge-dominated peat a t d i f f e r e n t l e v e l s o f humi-f i c a t i o n ( Z e l a s n y and C a r l i s l e , 1974) 25 1.2 Bulk d e n s i t y , water r e t e n t i o n and s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y of o r g a n i c s o i l s r e p o r t e d i n the l i t e r a t u r e 27 1.3 Muck s o i l h y d r a u l i c c o n d u c t i v i t y v a l u e s by a u g e r - h o l e method i n c o n t r o l l e d d r a i n a g e p l o t s , N o r t h I n d i a n a Muck E x p e r i m e n t a l Farm (Jongedyk et a l , 1954) 2 9 1.4 Degree of d e c o m p o s i t i o n of peat (Van Post S c a l e ) i n r e l a t i o n t o f i b e r c o n t e n t (Stanek and S i l c , 1977) 3 4 3.1 Some p e r t i n e n t P r o p e r t i e s of Vinod S o i l (Van H a l s t Farm) 5 7 3.2 Ash content of Richmond s o i l s i n S t a t e r Farm and g e n e t i c pan i n C l o v e r d a l e Produce Farm .. 59 3.3 Penetrometer r e s i s t a n c e i n h a r v e s t e d o n i o n seedbed and wheel t r a c k i n Vinod S o i l (Van H a l s t Farm) 6 5 3.4 I n c i d e n c e of ponding a t d i f f e r e n t depths t o w a t e r t a b l e i n Vinod S o i l (Van H a l s t Farm) ... b 9 3.5 L e v e l s of s i g n i f i c a n c e between means of s a t i a t e d h y d r a u l i c c o n d u c t i v i t y ("Ks") u s i n g t - d i s t r i b u t i o n 80 V I I I LIST OF TABLES Table Page 3.6 L e v e l s of s i g n i f i c a n c e between means of "Ks" u s i n g Mann-Whitney U t e s t 81 3.7 W a t e r t a b l e r e c e s s i o n i n h a r v e s t e d p o t a t o f i e l d i n s p r i n g , 1983 i n Vinod S o i l (Van H a l s t Farm) 84 3.8 V a r i a t i o n o f s t r u c t u r a l s t a b i l i t y i n d e x ( S I ^ ^ ) w i t h ash c o n t e n t 92 3.9 K i n e t i c energy (K.E.) of f a l l i n g waterdrop as an i n d e x of s t r u c t u r a l s t a b i l i t y 97 IX LIST OF FIGURES  F i g u r e s Page 1.1 A map showing the l o c a t i o n of the e x p e r i m e n t a l s i t e s 2 1.2 Vinod s o i l p r o f i l e ( L u t t m e r d i n g , 19S1) 7 1.3 Seasonal changes i n s o i l r e s p i r a t i o n r a t e (Wildung et a l , 1975) 16 1.4 E f f e c t of water c o n t e n t on s o i l r e s p i r a t i o n r a t e a t d i f f e r e n t s o i l temperature ranges (Wildung et a l , 1975) 17 1.5 Changes i n v e r t i c a l s o i l motion w i t h time ( I r w i n , 1977) 19 1.6 E f f e c t of CO2 c o n c e n t r a t i o n on the p o p u l a t i o n l e v e l of Fusarium oxysporum f . s . p . m e l o n i s i n s o i l ( L o u v e t , 1970) 22 1.7 E f f e c t of CO2 c o n c e n t r a t i o n on the p o p u l a t i o n l e v e l of Fusarium s o l a n i i n s o i l ( L o uvet, 1970) 22 2.1 A carborundum t e n s i o n p l a t e f o r d e t e r m i n i n g water r e t e n t i o n c h a r a c t e r i s t i c s a t low t e n s i o n 39 2.2 A s k e t c h of f a l l i n g head d e v i c e f o r d e t e r -m i n i n g s a t i a t e d h y d r a u l i c c o n d u c t i v i t y o f l a r g e u n d i s t u r b e d o r g a n i c s o i l samples 42 2.3 A d e v i c e f o r s t u d y i n g s t r u c t u r a l s t a b i l i t y of o r g a n i c s o i l s by d i s p e r s i o n 46 2.4 A s k e t c h of a s i m p l e r a i n s i m u l a t o r 47 X LIST OF FIGURES  F i g u r e s Page 2.5 A d e v i c e f o r s i m u l a t i n g s e t t l i n g o f o r g a n i c - m i n e r a l p a r t i c l e s f o r ming a s u r f a c e l a y e r o f low i n f i l t r a b i l i t y i n the l a b o r a t o r y 53 3.1 The s h r i n k a g e of Vinod s o i l as a f u n c t i o n of water c o n t e n t 62 3.2 V a r i a t i o n of penetrometer r e s i s t a n c e w i t h depth a t 60 cm w a t e r t a b l e depth 64 3.3 R a i n f a l l d i s t r i b u t i o n i n A p r i l , 1982 67 3.4 R e c e s s i o n of ponded water on h a r v e s t e d o n i o n f i e l d (Vinod S o i l ) i n S p r i n g , 1982 ... 71 3.5 R e c e s s i o n o f ponded water i n S p r i n g , 1983 on h a r v e s t e d o n i o n f i e l d (Vinod S o i l ) seeded t o W i n t e r Rye 72 3.6 Water r e t e n t i o n c h a r a c t e r i s t i c s on Vinod S o i l 75 3.7 The p a r t i a l water r e t e n t i o n c u r v e s f o r the Vinod S o i l 76 3.8 S a t i a t e d h y d r a u l i c c o n d u c t i v i t y ("Ks") of Vinod S o i l p r e s e n t e d d i a g r a m a t i c a l l y on l o g - s c a l e 79 3.9 S a t i a t e d h y d r a u l i c c o n d u c t i v i t y ("Ks") of Richmond s o i l p r e s e n t e d d i a g r a m a t i c a l l y on l o g - s c a l e 86 3.10 R e l a t i o n s h i p between s t r u c t u r a l s t a b i l i t y i n d e x o f Humisols and t h e i r ash c o n t e n t s ... 93 XI LIST OF PLATES P l a t e Page 1 An a e r i a l photograph o f the study a r e a t h a t i n c l u d e s Vinod s e r i e s (Van H a l s t Farm) 9 2 An a e r i a l photograph of the study a r e a t h a t i n c l u d e s Richmond s e r i e s ( C l o v e r d a l e Produce Farm) 10 X I I ACKNOWLEDGEMENTS I l i k e t o e x p r e s s my s i n c e r e g r a t i t u d e t o the B. C. M i n i s t r y o f Food and A g r i c u l t u r e and the N a t i o n a l S c i e n c e and E n g i n e e r i n g R esearch C o u n c i l f o r p r o v i d i n g funds t o support t h i s r e s e a r c h . I am v e r y g r a t e f u l t o Dr. Jan de V r i e s f o r s u p e r v i s i n g t h i s r e s e a r c h . H i s mental and p h y s i c a l c o n t r i b u t i o n s a r e w e l l acknowledged. I w i s h t o thank Drs. L. M. L a v k u l i c h , L. E. Lowe, and T. A. B l a c k f o r t h e i r c o n s t r u c t i v e s u g g e s t i o n s and as members of my r e s e a r c h committee who p i l o t e d t h i s work t o a f r u i t f u l comple t i on. The h e l p r e c e i v e d from Dr. H. E. S c h r e i e r i n i n t e r p r e t i n g the a e r i a l photograph of the study area and i n c h o o s i n g the r i g h t s t a t i s t i c a l t o o l s t o a n a l y z e the r e s u l t s i s f u l l y acknowledged. I am t h a n k f u l t o Mr. C r a i g Wood of B. C. M i n i s t r y of Food and A g r i c u l t u r e f o r t a k i n g some of h i s time o f f t o accompany us d u r i n g some of the f i e l d t r i p s t o the study a r e a s . I am g r e a t l y i n d e b t e d t o Mr. Van H a l s t who made a v a i l a b l e a p o r t i o n of h i s farm f o r t h i s r e s e a r c h t o be c a r r i e d o ut. The h e l p r e c e i v e d from the managers o f C l o v e r d a l e Produce Farm and S t a t e r Farm i s a l s o acknowledged. X I I I The l a s t but not l e a s t , I am t h a n k f u l t o my w i f e , B e a t r i c e and my sons, H a r r y , C h r i s and Jeremy, who encouraged me t o complete t h i s work s u c c e s s f u l l y . 11 CHAPTER 1 1.0 INTRODUCTION Inadequate water management i s a s e r i o u s c o n s t r a i n t t o a r a b l e f a r m i n g on the o r g a n i c s o i l s o f the S e r p e n t i n e - N i c o m e k l a r e a i n the Lower F r a s e r V a l l e y . S o i l Water management i s concerned w i t h h a n d l i n g the s o i l water so t h a t t h e r e i s n e i t h e r excess nor d e f i c i t of water i n the r o o t zone. A r t i f i c i a l d r a i n a g e i s r e q u i r e d when t h e r e i s a s u r p l u s of water, and i r r i g a t i o n r e q u i r e d when t h e r e i s a d e f i c i t o f water i n the r o o t zone. Even though s u b s u r f a c e d r a i n s i n s t a l l e d i n these s o i l s are o f t e n observed t o be f u n c t i o n i n g , ponding o f water on these s o i l s i s a common o c c u r r e n c e i n s p r i n g due t o the presence o f r e s t r i c t i n g l a y e r s such as t i l l a g e pans. Ponding may be d e f i n e d as the c o n d i t i o n where f r e e water i s p r e s e n t on the s o i l s u r f a c e w i t h the water t a b l e l o c a t e d below the s u r f a c e , and w i t h u n s a t u r a t e d s o i l b e i n g p r e s e n t between the s u r f a c e and the water t a b l e because the i n f i l t r a t i o n requirement i s not met (de V r i e s , 1983). The farmlands i n t h i s a r ea a re l o c a t e d on lowlands ( F i g . 1.1). S p r i n g r u n o f f from the upland urban c e n t r e s ( F i g . 1.1) causes the d r a i n a g e d i t c h e s t o be f u l l o f water r e s u l t i n g i n the r i s e of the w a t e r t a b l e and consequent f l o o d i n g o f the f a r m l a n d s . F l o o d i n g i s the c o n d i t i o n where the water t a b l e r i s e s above the s o i l s u r f a c e because of inadequate water t a b l e and d r a i n a g e d i t c h water l e v e l 2 Fig. 1.1. Note: A map showing the location of the experimental s i t e s . Bold margins demarcate uplands from lowlands. CHAPTER 1 1.0 INTRODUCTION Inadequate water management i s a s e r i o u s c o n s t r a i n t t o a r a b l e f a r m i n g on the o r g a n i c s o i l s o f the S e r p e n t i n e - N i c o m e k l area i n the Lower F r a s e r V a l l e y . S o i l Water management i s concerned w i t h h a n d l i n g the s o i l water so t h a t t h e r e i s n e i t h e r excess nor d e f i c i t of water i n the r o o t zone. A r t i f i c i a l d r a i n a g e i s r e q u i r e d when t h e r e i s a s u r p l u s of water, and i r r i g a t i o n r e q u i r e d when t h e r e i s a d e f i c i t o f water i n the r o o t zone. Even though s u b s u r f a c e d r a i n s i n s t a l l e d i n these s o i l s are o f t e n observed t o be f u n c t i o n i n g , ponding of water on these s o i l s i s a common o c c u r r e n c e i n s p r i n g due t o the presence of r e s t r i c t i n g l a y e r s such as t i l l a g e pans. Ponding may be d e f i n e d as the c o n d i t i o n where f r e e water i s p r e s e n t on the s o i l s u r f a c e w i t h the water t a b l e l o c a t e d below the s u r f a c e , and w i t h u n s a t u r a t e d s o i l b e i n g p r e s e n t between the s u r f a c e and the water t a b l e because the i n f i l t r a t i o n requirement i s not met (de V r i e s , 1983). The farmlands i n t h i s area a re l o c a t e d on lowla n d s ( F i g . 1.1). S p r i n g r u n o f f from the upland urban c e n t r e s ( F i g . 1.1) causes the d r a i n a g e d i t c h e s t o be f u l l of water r e s u l t i n g i n the r i s e of the w a t e r t a b l e and consequent f l o o d i n g of the fa r m l a n d s . F l o o d i n g i s the c o n d i t i o n where the water t a b l e r i s e s above the s o i l s u r f a c e because of inadequate water t a b l e and d r a i n a g e d i t c h water l e v e l 3 e l e v a t i o n c o n t r o l (de V r i e s , 1983). The a n t i c i p a t i o n of f l o o d on the farmlands due t o inadequate c o n t r o l of urban o v e r l a n d f l o w makes e a r l y l o w e r i n g of the water t a b l e i n s p r i n g an u n c e r t a i n o p e r a t i o n . B e s i d e s , seedbed p r e p a r a t i o n i s o f t e n d e l a y e d i n s p r i n g . The f l o o d water r e s u l t i n g from o v e r l a n d f l o w from.the urban c e n t r e s has a s t r o n g b e a r i n g on s t r u c t u r a l d e g r a d a t i o n of these o r g a n i c s o i l s . Free water t h a t c o l l e c t s i n d e p r e s s i o n a l areas as a r e s u l t of f l o o d i n g and ponding causes a s u r f a c e l a y e r of low i n f i l t r a b i l i t y t o form. F i n e o r g a n i c and m i n e r a l s o i l p a r t i c l e s a r e eroded from the immediate v i c i n i t y of the "pond" and s e t t l e t o the bottom of the pond to form a s u r f a c e l a y e r of low i n f i l t r a b i l i t y . A second p r o c e s s r e s u l t i n g i n the f o r m a t i o n o f the low i n f i l t r a b i l i t y s u r f a c e l a y e r i s the e r o s i o n o f the bottom of the pond induced by w i n d - d r i v e n wave a c t i o n . When the wind calms down, the eroded s o i l p a r t i c l e s s e t t l e t o the bottom to form a t h i n l a y e r c o n s i s t i n g of r y t h m i t e s (de V r i e s , 1983). T h i s p r o c e s s produces a number o f r y t h m i t e s , each c o r r e s p o n d i n g to a r a i n f a l l - w i n d event. T h i s s u r f a c e l a y e r of low i n f i l t r a b i l i t y c o n t r i b u t e s s i g n i f i c a n t l y t o the impairment of d r a i n a g e i n the d e p r e s s i o n a l a r e a s , r e s u l t i n g i n non-u n i f o r m d r y i n g of the f i e l d when the water t a b l e i s b e i n g lowered i n s p r i n g ( p e r s o n a l o b s e r v a t i o n ) . With the advent of 4-w h e e l - d r i v e t r a c t o r s , some farmers i m p a t i e n t l y perform f i e l d o p e r a t i o n s when the s o i l i s not t r a f f i c a b l e , and the s t r u c t u r e of the s o i l s u f f e r s f u r t h e r d e t e r i o r a t i o n . In a d d i t i o n , r a i n l e s s p e r i o d s d u r i n g the growing season 4 r e q u i r e t h a t the crops be i r r i g a t e d because the r o o t s a re prevented from r e a c h i n g t h e i r p o t e n t i a l r o o t i n g depth due to the presence of compacted l a y e r s . T h e r e f o r e , supplemental i r r i g a t i o n s are o f t e n r e q u i r e d f o r s u c c e s s f u l p r o d u c t i o n o f crops on the s e s o i l s . These supplemental i r r i g a t i o n s are an a d d i t i o n a l c o s t to the farmer, r e d u c i n g h i s net income and p r o f i t a b i l i t y . Ponding on d r i v e n - o n i n t e r - r o w p o s i t i o n s of seedbeds d u r i n g the growing season i s a common oc c u r r e n c e on these s o i l s . T h e r e f o r e , s u r f a c e compaction due t o the passage of heavy farm machinery a l s o c o n t r i b u t e s s i g n i f i c a n t l y t o the s t r u c t u r a l d e t e r i o r a t i o n of these s o i l s . These s o i l s a r e i n t e n s i v e l y used f o r the p r o d u c t i o n of v e g e t a b l e s and t h e r e f o r e v e r y v a l u a b l e a g r i c u l t u r a l l y . I n o r d e r to make sound f u t u r e management d e c i s i o n s , a knowledge of the e x t e n t to which s o i l s t r u c t u r a l d e t e r i o r a t i o n has i m p a i r e d the movement of water i n these s o i l s i s r e q u i r e d . T h i s work was t h e r e f o r e c a r r i e d out w i t h the f o l l o w i n g o b j e c t i v e s : • to i n v e s t i g a t e the l e g a c y o f water management and t i l l a g e o p e r a t i o n s i n r e l a t i o n t o s o i l s t r u c t u r a l d e t e r i o r a t i o n and r e d u c t i o n o f h y d r a u l i c c o n d u c t i v i t y i n t h e s e s o i l s . • t o f o r m u l a t e an i n d e x t o c h a r a c t e r i z e the s t r u c t u r a l s t a b i l i t y of hu m i s o l s based on t h e i r d i s p e r s i n g and c l o g g i n g c h a r a c t e r i s t i c s i n r e l a t i o n t o h y d r a u l i c 5 c o n d u c t i v i t y r e d u c t i o n . • t o suggest s o i l s t r u c t u r e and water management recommendations f o r these s o i l s . 1.1 SOILS AND ENVIRONMENTAL CONDITIONS T h i s work was c a r r i e d out on the o r g a n i c s o i l s of the Se r p e n t i n e - N i c o m e k l area i n the Lower F r a s e r V a l l e y , C l o v e r d a l e . Most of the work was done on the Van H a l s t Farm. However, some comparative s t u d i e s were a l s o c a r r i e d out on the S t a t e r and the C l o v e r d a l e Produce Farms ( F i g . 1.1).. The maximum depth of the o r g a n i c l a y e r a t the study s i t e s on the Van H a l s t Farm does not exceed 40 cm and the s o i l , t h e r e f o r e , belongs t o Vinod s e r i e s , which i s c l a s s i f i e d as Rego G l e y s o l (peaty phase) i n the Canadian system of s o i l c l a s s i f i c a t i o n ( L u t t m e r d i n g , 1981). The maximum depth of the o r g a n i c l a y e r s a t the study s i t e s on both the S t a t e r and the C l o v e r d a l e Produce Farms does not exceed 60 cm, and t h e r e f o r e the s o i l s b elong to Richmond s e r i e s , which i s c l a s s i f i e d as T e r r i c Humisol i n the Canadian system of s o i l c l a s s i f i c a t i o n ( L u t t m e r d i n g , 1981). Vinod s o i l s c o n s i s t of s h a l l o w (15 t o 40 cm t h i c k ) , well-decomposed o r g a n i c m a t e r i a l t h a t o v e r l i e s a moderately f i n e t e x t u r e d m i n e r a l l a y e r . The m i n e r a l l a y e r c o n s i s t s of d e l t a i c and f l u v i a l d e p o s i t s . V i n o d s o i l s g e n e r a l l y have a c u l t i v a t e d , humic s u r f a c e l a y e r about 20 cm t h i c k , which i s f r i a b l e and s o f t . The c u l t i v a t e d l a y e r 6 i s u n d e r l a i n (where the o r g a n i c d e p o s i t s a r e deep enough) by about 15 cm dark-brown compacted zone ( t i l l a g e pan). Lu t t m e r d i n g (1981) r e p o r t s t h a t the t e x t u r e of the m i n e r a l s u b s o i l i s e i t h e r s i l t y c l a y loam o r s i l t y c l a y . The s u b s o i l s are s a l i n e , and where the o r g a n i c l a y e r i s r e l a t i v e l y s h a l l o w , some o f the u n d e r l y i n g m i n e r a l s o i l has been mixed by c u l t i v a t i o n w i t h the o r g a n i c m a t e r i a l s . The u n d e r l y i n g m i n e r a l m a t e r i a l may c o n t a i n common, y e l l o w m o t t l e s , v e r t i c a l remains of o l d r o o t s , and o c c a s i o n a l v e r t i c a l c r a c k s c o n t a i n i n g o r g a n i c m a t t e r from above ( F i g . 1.2, L u t t m e r d i n g , 1981). Medium o r f i n e sand sometimes o c c u r s at a depth of 1 m and s o i l r e a c t i o n i s e x t r e m e l y a c i d throughout ( L u t t m e r d i n g , 1981). The pH o f the s o i l (>0.95 m) i s around 3.6 (Van V l i e t and Wood, 1982). The Richmond s e r i e s has developed from 40 t o 160 cm o f well-decomposed o r g a n i c m a t e r i a l t h a t o v e r l i e s m o d erately f i n e and medium t e x t u r e d d e l t a i c d e p o s i t s ( L u t t m e r d i n g , 1981). Richmond s o i l s have a c u l t i v a t e d s u r f a c e l a y e r about 20 cm t h i c k , which i s f r i a b l e and w e l l - h u m i f i e d . The c u l t i v a t e d l a y e r i s u n d e r l a i n by between 30 and 100 cm o f b l a c k t o b r o w n i s h - b l a c k , m a s s i v e , well-decomposed hard o r g a n i c m a t e r i a l ( L u t t m e r d i n g , 1981). T h i s hard o r g a n i c l a y e r has been d e s c r i b e d as a g e n e t i c pan by Van V l i e t and Wood, (1982). At the study s i t e s , the g e n e t i c pan was l o c a t e d between 25 and 40 cm of depth. The t e x t u r e of the m i n e r a l s u b s o i l i s e i t h e r s i l t loam o r s i l t y c l a y loam ( L u t t m e r d i n g , 1981). F i g. 1. 2 Vinod soil profile (Rego GleysohsaHne and peaty phase). These poorly drained soils have developed from 6 to 16 In. (IS to 40 cm) of well to moderately decomposed organic material overlying silty, saline deltaic deposits. The dark coloured flecks and streaks in the subsoil are mainly dark brown to reddish-brown, hard lubules around old root channels. Note the wide crack on the left of ine photo—these am oommon In Vinod soils. ( L u t t m e r d i n g , 1981) 8 Also, v e r t i c a l remains of humified old plant stems and roots are found i n the mineral subsoil (Luttermerding, 1981). Cultivated Richmond s o i l s may have a c u l t i v a t i o n pan located at 23 or 25 cm from the surface (personal observation). The top of the pan i s laminated due to the use of r o t o t i l l e r s with f l a t f l a i l s , which shear the s o i l i n s i t u . Plates 1 and 2 show a e r i a l photographs of a part of the Serpentine-Nicomekl area, including the Van Halst Farm and Cloverdale Produce Farm. The Van Halst Farm i s shown on plate 1. The old main tributary of the Serpentine River in the Van Halst Farm runs from east to west, with a network of subtributaries. These old t r i b u t a r i e s are the o r i g i n of the f l u v i a l materials, and are indicated as tortuous and l i g h t e r i n colour. The darker areas indicate the organic deposits. The colour becomes l i g h t e r towards the Serpentine River, indicating the closeness of the mineral layer to the surface. The drainage networks of the old t r i b u t a r i e s are more pronounced on plate 2, where the Cloverdale Produce Farm and the Nicomekl River are shown. It i s clear on plate 2 that the Nicomekl River meandered and changed i t s course pr i o r to the reclamation of the area. The t r i b u t a r i e s to the old Nicomekl River bed are c l e a r l y indicated. X-ray d i f f r a c t i o n analysis (2/rn) carried out i n the pedology laboratory, S o i l Science Department, the University of B r i t i s h Columbia, indicates that the dominant clay P l a t e 1. An a e r i a l photograph o f the study a r e a t h a t i n c l u d e s V i n o d s e r i e s (Van H a l s t Farm). An a e r i a l photograph o f the study a r e a t h a t i n c l u d e s Richmond s e r i e s ( C l o v e r d a l e Produce Farm). 11 mineral of the tra n s i t i o n layer separating the organic layers from the mineral subsoil i s smectite, with f a i r proportions of c h l o r i t e , mica, k a o l i n i t e , and quartz. Further, some traces of feldspar may also be present. The texture of thi s t r a n s i t i o n layer i s s i l t y clay and i t i s composed of diatomaceous earth (Van V l i e t and Wood, 1982). The average annual p r e c i p i t a t i o n of this area i s around 1500 mm. About 78 percent f a l l s from October through A p r i l . The remaining 22 percent f a l l s during the growing season. Precipitation f a l l i n g as snow amounts to about 3 to 6 percent of the t o t a l p r e c i p i t a t i o n . The average evapo-transpiration rate of 350 mm exceeds p r e c i p i t a t i o n rates during the summer months. The climatic moisture d e f i c i t ranges from 100 mm to 150 mm (Van V l i e t and Wood, 1982). 1.2 REVIEW OF LITERATURE 1.2.0 Nature and formation of organic s o i l s Organic s o i l s are formed whenever the rate of accumulation of organic residues i n an area exceeds the rate of decomposition under water-saturated and low temperature conditions (Harris et a l , 1962). In the Canadian System of S o i l C l a s s i f i c a t i o n , a s o i l i s c l a s s i f i e d as organic i f i t contains at least 30 percent organic matter (17 percent organic carbon), and the organic layer i s at least 40 cm deep (Canadian S o i l Survey Committee, 1978). The degree of decomposition of the organic materials forms a strong basis for c l a s s i f y i n g organic s o i l s (Mills et a l , 1977). Based on the degree o f d e c o m p o s i t i o n o r g a n i c s o i l s may be c l a s s i f i e d i n the Canadian System of S o i l C l a s s i f i c a t i o n as (Canada S o i l Survey Committee, 1978): i ) F i b r i s o l - the l e a s t decomposed o r g a n i c s o i l h a v i n g w e l l r e c o g n i z a b l e p l a n t m a t e r i a l s , i i ) M e s i s o l - O r g a n i c s o i l i n an i n t e r m e d i a t e s t a t e of d e c o m p o s i t i o n w i t h r e c o g n i z a b l e p l a n t m a t e r i a l s , i i i ) Humisol - w e l l decomposed o r g a n i c s o i l whose s t a t e of d e c o m p o s i t i o n makes i t d i f f i c u l t t o r e c o g n i z e the n a t u r e of the o r i g i n a l p l a n t m a t e r i a l s . 2 Organic s o i l s c over a p p r o x i m a t e l y 1,215,000 km of Canada or about 12 p e r c e n t of the t o t a l l a n d a r e a (MacFarlane and W i l l i a m s , 1974). However, the h u m i s o l s are the most im p o r t a n t a g r i c u l t u r a l l y (Anon, 1981). 1.2.1 Subsidence i n o r g a n i c s o i l s To r e c l a i m o r g a n i c s o i l s f o r a g r i c u l t u r a l p u r poses, i n i t i a l s u b s u r f a c e d r a i n a g e i s r e q u i r e d . Once the s o i l i s d r a i n e d , i t b e g i n s to l o s e s u r f a c e e l e v a t i o n by a p r o c e s s c a l l e d s u b s i d e n c e . T h i s phenomenon i n o r g a n i c s o i l s has been e x t e n s i v e l y s t u d i e d . F a c t o r s t h a t c o n t r i b u t e t o subsidence i n o r g a n i c s o i l s have been commonly l i s t e d as s h r i n k a g e due t o d r y i n g , c o n s o l i d a t i o n by l o s s o f bouyant f o r c e s o f ground-water, compaction due t o t i l l a g e , b u r n i n g , wind e r o s i o n , and b i o c h e m i c a l o x i d a t i o n (Weir, 1950; Stephens, 1956; S l u s h e r et a l , 1974; S c h o t h o r s t , 1977; I r w i n , 1977). The i n i t i a l s u bsidence a f t e r l o w e r i n g o f the water t a b l e may be accomplished i n about 3 y e a r s , and t h i s may cause about 50 percent r e d u c t i o n i n the t h i c k n e s s o f the o r g a n i c m a t e r i a l s ( S l u s h e r e t a l , 1974). The i n i t i a l s u bsidence i s then f o l l o w e d by c o n t i n u e d subsidence due t o b i o c h e m i c a l o x i d a t i o n , and may p r o g r e s s u n t i l e i t h e r the m i n e r a l m a t e r i a l or the water t a b l e i s re a c h e d , i f no a p p r o p r i a t e management i s e s t a b l i s h e d ( S l u s h e r et a l , 1974; S c h o t h o r s t , 1977). 1.2.2 C o n t r o l of Subsidence i n Organic S o i l s S i n c e subsidence i s a s e r i o u s problem i n the management o f o r g a n i c s o i l s , some s t u d i e s have f o c u s s e d on methods o f m i n i m i z i n g i t . S t o t z y and Mortensen (1957) s t u d i e d the e f f e c t of c r o p r e s i d u e s a d d i t i o n on the d e c o m p o s i t i o n of o r g a n i c s o i l s . They observed t h a t a l t h o u g h the a d d i t i o n of cro p r e s i d u e s i n c r e a s e d the r a t e of d e c o m p o s i t i o n , the net l o s s of carbon was d e c r e a s e d , w h i l e the net g a i n o f carbon showed a l o g a r i t h m i c i n c r e a s e w i t h i n c r e a s e d a d d i t i o n of c r o p r e s i d u e s . Broadbent (1960) a l s o r e p o r t e d a s i m i l a r r e s u l t and emphasized t h a t the a d d i t i o n o f c r o p r e s i d u e s c o u l d c o u n t e r a c t the e f f e c t of s u b s i d e n c e , even though the e f f e c t may not be enough t o m a i n t a i n permanent e l e v a t i o n i n o r g a n i c s o i l s . I n p r a c t i c e , subsidence would cease when the amount of o r g a n i c m a t t e r l o s t each year i s r e p l a c e d through the a d d i t i o n o f c r o p r e s i d u e s ( R i c h a r d s o n and Smith, 1977). Some farmers working on the o r g a n i c s o i l s o f the S e r p e n t i n e - N i c o m e k l area c l a i m t h a t f l o o d i n g the l a n d d u r i n g 14 w i n t e r f a l l o w p e r i o d i s b e n e f i c i a l i n t h a t i t reduces b i o l o g i c a l o x i d a t i o n , c o n t r o l s s o i l - b o r n e a e r o b i c pathogens and e l i m i n a t e s c r o p v o l u n t e e r s ( p e r s o n a l communication w i t h f a r m e r s ) . F i e l d d a t a o b t a i n e d by T e r r y and Tate (1980) on Pahokee muck a t B e l l e G lade, F l o r i d a , support the n o t i o n t h a t f l o o d i n g reduces b i o l o g i c a l o x i d a t i o n i n o r g a n i c s o i l s . Two-year d a t a o b t a i n e d between June and J u l y i n 1977 and 1978 i n d i c a t e d t h a t w i t h f l o o d i n g d e n i t r i f i c a t i o n r a t e i n c r e a s e d by 18 micrograms of n i t r o g e n per c u b i c c e n t i m e t e r per day, w h i l e d e n i t r i f y i n g b a c t e r i a i n c r e a s e d 1 5 - f o l d d u r i n g two days o f f l o o d i n g . They a l s o observed t h a t n i t r a t e c o n c e n t r a t i o n s d e c r e a s e d about 80 p e r c e n t d u r i n g the f i r s t t h r e e - d a y s o f f l o o d i n g and no f u r t h e r change was d e t e c t e d u n t i l the f i e l d was d r a i n e d when the n i t r a t e c o n t e n t i n c r e a s e d t o the p r e - f l o o d e d l e v e l . Raveh and Avnimelech (1973) have examined weekly the n i t r a t e l e v e l s i n a f l o o d e d H u l l a V a l l e y muck, I s r a e l , and noted an approximate 70 pe r c e n t d e c l i n e i n the s o i l n i t r a t e c o n c e n t r a t i o n . I t i s common knowledge t h a t temperature and water content p l a y a key r o l e i n the d e c o m p o s i t i o n of o r g a n i c m a t e r i a l . The i n f l u e n c e o f m o i s t u r e c o n d i t i o n s and temperature on the d e c o m p o s i t i o n of peat was r e p o r t e d l o n g time ago by Waksman and P u r v i s (1932) and Waksman (1938). Waksman and P u r v i s (1932) observed t h a t the optimum m o i s t u r e c o n t e n t f o r the d e c o m p o s i t i o n o f a lowmoor peat was 50 t o 80 per-cent o f the t o t a l l y s a t u r a t e d p e a t . Above and below t h a t optimum the r a t e of peat d e c o m p o s i t i o n r a p i d l y d i m i n i s h e d . Under the optimum m o i s t u r e c o n d i t i o n s about 15 per c e n t o f the t o t a l dry m a t e r i a l o f the lowmoor peat was decomposed i n 18 months. They f u r t h e r noted t h a t d r y i n g the peat and r e m o i s t e n i n g i t g r e a t l y s t i m u l a t e d i t s d e c o m p o s i t i o n . Waksman (1938) r e p o r t e d t h a t the optimum temperatures f o r the d e c o m p o s i t i o n o f f r e s h p l a n t m a t e r i a l s , p a r t l y decomposed p l a n t m a t e r i a l s , and s o i l o r g a n i c matter were r e s p e c t i v e l y 35°C, 45 - 60°C, and 65°C. F i e l d s t u d i e s conducted oh a s e m i - a r i d g r a s s l a n d s o i l s on R a t t l e s n a k e mountains, S o u t h - C e n t r a l Washington S t a t e , by Wildung e t al_ (1975) showed t h a t r e s p i r a t i o n r a t e due t o d e c o m p o s i t i o n by s o i l m i c r o f l o r a and fauna of dead r o o t s and o t h e r o r g a n i c m a t t e r g r e a t l y i n c r e a s e d i n the s p r i n g t o annual maxima i n the summer and decreased d u r i n g the f a l l t o minima i n w i n t e r ( F i g . 1.3). I n the same s t u d i e s Wildung et a l (1975) observed t h a t temperature s t r o n g l y i n f l u e n c e d CC^ e v o l u t i o n from the d e c o m p o s i t i o n p r o c e s s . At temperatures below 6°C, water c o n t e n t had l e s s i n f l u e n c e on CC>2 e v o l u t i o n than when the temperatures o f 18 t o 24°C or when temperatures g r e a t e r than 24°C p r e v a i l e d ( F i g . 1.4). In the s u b t r o p i c a l r e g i o n s of south F l o r i d a , summer daytime a i r temperatures are around 27° to 33°C and the peat temperatures 28° to 35°C, w i t h a decrease of 1° t o 3°C per metre w i t h i n c r e a s i n g depth i n the peat. A i r temperatures i n w i n t e r may range between 18°C and 20°C ( G i v e n and Di c k e n s o n , 1975). T h e r e f o r e i n h i g h temperature areas l i k e 16 1.5 i CO T3 U 00 c o 4J 3 o > 0) <u T J •t-t X o c o u CO (J 4-1 CO OS Dec Time Months F i g . 1.3. Seasonal changes i n s o i l r e s p i r a t i o n r a t e . Adapted from Wildung et a l (1975). Only the observed data were used. The o r i g i n a l showed a l s o p r e d i c t e d v a l u e s . The s c a l e used here was double that of the o r i g i n a l . I >1 I CM u C O •H •P O > w 0) T) •H X o c o X ! l-i <TJ CJ <4-l o 1.2 1.0 -0.8 -0.6 _ 0.4 -0.2 0.0 F i g . 1.4 Legend (1) <6°C (2) 6 - ]2 6 C S o i l Water C o n t e n t , % E f f e c t o f water c o n t e n t on s o i l r e s p i r a t i o n r a t e a t d i f f e r e n t s o i l t e m perature r a n g e s . Adapted from Wildung e t a l ( ] 9 7 5 ) . The s c a l e was doubled and the o r i g i n a l d a t a showed a c o n s i d e r a b l e s c a t t e r not shown here. (3) (4) 12 - 18 18 - 24 18 F l o r i d a and I s r a e l , f l o o d i n g as a means of r e d u c i n g b i o l o g i c a l o x i d a t i o n i n o r g a n i c s o i l s w i l l be b e n e f i c i a l . I n F l o r i d a , o r g a n i c s o i l s a r e f l o o d e d i n summer as a management p r a c t i c e t o c o n t r o l weeds and p e s t s and reduce subsidence (Reddy, 1983). I r w i n (1977) s t u d i e d the v e r t i c a l movement o f the s o i l s u r f a c e of muck a t H o l l a n d Marsh, O n t a r i o , w i t h t i m e . H i s r e s u l t s shown i n F i g . 1.5 i n d i c a t e t h a t the s o i l s u r f a c e o f the muck was not s t a b l e throughout the y e a r , but c o n t i n u a l l y moved up or down depending upon p r e c i p i t a t i o n o r e v a p o t r a n s p i r a t i o n , and hence upon temperature. There was a s u b s t a n t i a l i n c r e a s e i n e l e v a t i o n when the s o i l f r o z e i n the w i n t e r months and when the s o i l thawed the s u r f a c e e l e v a t i o n d e c r e a s e d . I t i s not the purpose of t h i s t h e s i s t o r e p o r t on m i c r o b i o l o g i c a l p r o c e s s e s t h a t o c c u r i n f l o o d e d s o i l s . The r e a d e r who i s i n t e r e s t e d i n t h i s t o p i c i s a d v i s e d t o r e f e r to the r e v i e w r e p o r t by Yo s h i d a (1975). However, i t must be emphasized here t h a t the dominant f a c t o r s t h a t c o n t r o l b i o l o g i c a l o x i d a t i o n i n o r g a n i c s o i l s a r e t e m p e r a t u r e , m o i s t u r e c o n t e n t , and/or a i r c o n t e n t , s o i l pH, C:N r a t i o of the m a t e r i a l , and management f a c t o r s such as t i l l a g e , (Sommers e t a l , 1981). In e f f e c t , c o n t i n u o u s f l o o d i n g may e f f e c t i v e l y reduce b i o l o g i c a l o x i d a t i o n i n o r g a n i c s o i l s , but a t the expense of s t r u c t u r a l d e t e r i o r a t i o n of the s o i l . I n o r d e r t o p r e s e r v e the p o t e n t i a l o f v a l u a b l e o r g a n i c s o i l s f o r food c r o p p r o d u c t i o n i n c o o l temperature a r e a s , water c o n t r o l measures t h a t prevent complete f l o o d i n g i n w i n t e r i s e s s e n t i a l , s i n c e 19 Fi g . 1.5. Changes i n elevation of s o i l surface with time (Irwin, 1977) r the f o r e g o i n g l i t e r a t u r e i n d i c a t e s t h a t a t low temperatures b i o l o g i c a l o x i d a t i o n does not occur at a s i g n i f i c a n t r a t e . A l s o , the optimum s o i l water c o n t e n t f o r o r g a n i c m a t t e r d e c o m p o s i t i o n i n F l o r i d a h i s t o s o l s has been r e p o r t e d t o be a t 100 cm s u c t i o n ( T e r r y , 1980). The c l a i m t h a t f l o o d i n g e l i m i n a t e s the i n c i d e n c e of s o i l - b o r n e d i s e a s e due to a e r o b i c pathogens needs f u r t h e r e l u c i d a t i o n . As the s o i l becomes f l o o d e d , the m i c r o b i a l p o p u l a t i o n changes from p r e d o m i n a n t l y a e r o b i c organisms t o a n a e r o b i c organisms. S i n c e c o n d i t i o n s t h a t f a v o u r the growth of h i g h e r p l a n t s a l s o f a v o u r the growth and r e p r o d u c t i o n o f a e r o b i c microbes t h a t cause numerous p l a n t d i s e a s e s i n s o i l , f l o o d i n g c o u l d e s s e n t i a l l y reduce the i n c i d e n c e of d i s e a s e s due t o a e r o b i c s o i l - b o r n e pathogens. S t o l z y e t a l (1963) w o r k i n g w i t h n e m a t o d e - i n f e s t e d sweet orange s e e d l i n g s growing i n a greenhouse s o i l i n R i v e r s i d e , C a l i f o r n i a , observed t h a t low oxygen d i f f u s i o n r a t e s i n the s o i l were d e t r i m e n t a l t o the r e p r o d u c t i o n o f the nematodes. A l s o they observed t h a t c o n d i t i o n s t h a t f a v o u r e d s e e d l i n g growth a l s o were f a v o u r a b l e f o r the r e p r o d u c t i o n of the c i t r u s nematodes. Louvet (1970) has r e p o r t e d on the i n f l u e n c e o f a e r a t i o n and CC^ c o n c e n t r a t i o n on the a c t i v i t y of two s p e c i e s of f u n g i (Fusarium oxysporum and Fusarium s o l a n i ) , i n an u n s t e r i l i z e d greenhouse s o i l i n D i j o n , F r a n c e . H i s r e s u l t s p r e s e n t e d i n F i g s . 1.6 and 1.7 i n d i c a t e d t h a t the number of p r o p a g u l e s of the f u n g i i n c r e a s e d i n the f i r s t t h r e e weeks of the experiment a t a l l c o n c e n t r a t i o n s o f CC^. However, the number of p r o p a g u l e s remained almost c o n s t a n t from the t h i r d to. the seventh week at h i g h CC^ c o n c e n t r a t i o n s , but i n c r e a s e d i n the s o i l exposed t o a i r . He a t t r i b u t e d the marked i n c r e a s e i n the number of p r o p a g u l e s i n the f i r s t t h r e e weeks a t a l l c o n c e n t r a t i o n s of CC^ t o the m o i s t e n i n g o f the s o i l which induced the g e r m i n a t i o n of the chlamydospores formed d u r i n g the 15-month i n c u b a t i o n p e r i o d . B o u r r e t e t a l (1965; 1968), S t o v e r and F r e i b e r g (1958) and Newcombe (1960) r e p o r t e d t h a t C 0 2 f a v o u r e d the g e r m i n a t i o n of chlamydospores and m y c e l i a l development of the f u n g i (Fusarium oxysporum and Fusarium  s o l a n i ) , but i n h i b i t e d the f o r m a t i o n o f new chlamydospores. Louvet's (1970) r e s u l t s showed t h a t a e r a t i o n o f s o i l f a v o u r e d the m u l t i p l i c a t i o n of the two s p e c i e s of f u n g i , whereas CO2, i n p a r t i c u l a r a t h i g h c o n c e n t r a t i o n s , i n h i b i t e d the m u l t i p l i c a t i o n of the two s p e c i e s of f u n g i . S i n c e f l o o d i n g has both u s e f u l and d e t r i m e n t a l e f f e c t s , an i n t e g r a t e d approach of r e s e a r c h w i l l be needed to f i n d out the l e n g t h of time t h a t c u l t i v a t e d o r g a n i c s o i l s may be f l o o d e d i n o r d e r t o reduce the i n c i d e n c e o f s o i l - b o r n e d i s e a s e s , and get r i d o f c r o p v o l u n t e e r s w i t h minimal e f f e c t on s o i l s t r u c t u r e . A new approach t h a t i s now b e i n g suggested t o be e f f e c t i v e i n m i n i m i z i n g subsidence i s the use o f e l e m e n t a l r.o.t%. t i S S r • WMkl Fig. 1.6. Effect of CO-concentration on the population level of Fusarium oxysporum f.s.p. melonis in s o i l (Louvet, 1970). W M k t F i g . 1.7. Effect of C0~ concentration on the population level of Fusarium solani i n s o i l (Louvet, iy/0) copper (Mathur e t a l , 1979; Campbell and M i l l e t t e , 1981). Microorganisms degrade l a r g e m o l e c u l e s of p l a n t m a t e r i a l s such as c e l l u l o s e by u s i n g e x o c e l l u l a r enzymes. I t has been found by Mathur et a l (1979) t h a t copper i n a c t i v a t e s a c i d phosphatase. Because these, enzymes h e l p i n b r e a k i n g down c e l l s , t h e i r a c t i v i t y i s c l o s e l y r e l a t e d t o the degree of d e c o m p o s i t i o n and l o s s of s o i l carbon (Mathur e t a l 1978). F i e l d s t u d i e s c a r r i e d out by Mathur e_t a_l. (1979) i n summer a t the S t e . C l o t h i l d e E x p e r i m e n t a l S u b s t a t i o n of the A g r i c u l t u r a l Canada Research S t a t i o n a t S t . Jean (P.Q.) showed t h a t the r a t e o f CC^ e v o l u t i o n was n e g a t i v e l y and s i g n i f i c a n t l y c o r r e l a t e d w i t h both the t o t a l and e x t r a c t a b l e copper c o n t e n t s . A l s o , a c i d phosphatase a c t i v i t y f e l l s h a r p l y as copper c o n t e n t i n c r e a s e d . They suggested t h a t copper a p p l i c a t i o n s o f l e s s than 100 kg/ha c o u l d be used f o r m i t i g a t i n g s u b s i d e n c e of some o r g a n i c s o i l s . 1.2.3 P h y s i c a l p r o p e r t i e s of o r g a n i c s o i l s The degree of d e c o m p o s i t i o n o f o r g a n i c s o i l s i n p a r t c o n t r o l s the p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s such as f i b r e c o n t e n t , b u l k d e n s i t y , ash c o n t e n t , and h y d r o l o g i c p r o p e r t i e s . The degree o f d e c o m p o s i t i o n i s a r e l a t i v e q u a n t i t y t h a t i s u s u a l l y approximated by measuring one of the c h e m i c a l o r p h y s i c a l c h a r a c t e r i s t i c s such as f i b r e c o n t e n t , b u l k d e n s i t y , and ash co n t e n t ( B b e l t e r , 1974; Sneddon e t a l 1971; F r a z i e r and Lee 1971; Maas - 1972; Stanek and S i l c 1977). I r w i n (1977) has d e f i n e d percentage o f d e c o m p o s i t i o n of a u n i f o r m peat as: 100 [1 - percentage of ash of s u b s o i l / p e r c e n t a g e of ash i n s u r f a c e s o i l ] . Even though the ash i s not a p h y s i c a l p r o p e r t y , i t has a s t r o n g i n f l u e n c e on the p h y s i c a l p r o p e r t i e s and b e h a v i o u r o f o r g a n i c s o i l s . The ash c o n t e n t o f o r g a n i c s o i l s tends t o i n c r e a s e w i t h i n c r e a s e d m i n e r a l i z a t i o n ( N e l l e r , 1944). Pedersen (1979) r e p o r t e d an i n c r e a s e i n ash content of about 0.4 p e r c e n t per year as a r e s u l t of p h y s i c a l compression and m i n e r a l i z a t i o n o f an o r g a n i c s o i l . I t i s c o n s i d e r e d t h a t when the ash c o n t e n t o f an o r g a n i c s o i l exceeds 50 p e r c e n t , the s o i l behaves more l i k e m i n e r a l s o i l (Farnham and F i n n e y , 1965). The ash c o n t e n t s of moss-dominated peat and sedge-dominated peat at d i f f e r e n t s t a g e s of d e c o m p o s i t i o n r e p o r t e d by Z e l a s n y and C a r l i s l e (1974) are shown i n Table 1.1. As d e c o m p o s i t i o n of the o r g a n i c m a t e r i a l s proceeds, the s i z e of the o r g a n i c p a r t i c l e s becomes s m a l l e r and s m a l l e r , r e s u l t i n g i n s m a l l e r pores and h i g h e r b u l k d e n s i t i e s . Thus b u l k d e n s i t y , water r e t e n t i o n as w e l l as pore s i z e d i s t r i b u t i o n a re r e l a t e d t o the degree of h u m i f i c a t i o n ( S t u r g e s , 1968; B o e l t e r , 1969; S i l c and Stanek, 1977). S i l c and Stanek (1977) o b t a i n e d a s i g n i f i c a n t l i n e a r r e l a t i o n between b u l k d e n s i t y and degree of d e c o m p o s i t i o n as determined by the von Post h u m i f i c a t i o n s c a l e . The r e l a t i o n s h i p was r e p r e s e n t e d as: Y = 0.529 + 0.0182 x ( r = 0.94) where Y i s the b u l k d e n s i t y and x i s von Post s c a l e . Dasberg and Neuman (1977) have 25 : Table 1.1 Ash content of moss-dominated peat and sedge-dominated peat at d i f f e r e n t l e v e l s o f hurnif i c a t i o n ( Z e l a s n y and C a r l i s l e , 1974) Ash Content Peat Type C l a s s i f i c a t i o n (%) a. .Moss-dominated F i b r i s o l 3 - 15 M e s i s o l 15 - 26 Humi s o l 24 - > 60 b. Sedge-dominated F i b r i s o l M e s i s o l Humi s o l 5 - 1 2 12 - 23 24 - >60 r e p o r t e d t h a t the b u l k d e n s i t y o f peat i n the Hula B a s i n , I s r a e l i n c r e a s e d towards the s u r f a c e . Table 1.2 shows b u l k d e n s i t y , water r e t e n t i o n and h y d r a u l i c c o n d u c t i v i t y of o r g a n i c s o i l s r e p o r t e d by d i f f e r e n t a u t h o r s . P e r e z - E s c o l a r e t a l (1974) have observed h i g h water r e t e n t i o n a t both " f i e l d c a p a c i t y " and "permanent w i l t i n g p e r c e n t a g e " f o r a muck s o i l , but low a v a i l a b l e water c o n t e n t . G a w l i k and Zawadzki (1981) have r e p o r t e d a s i g n i f i c a n t n e g a t i v e c o r r e l a t i o n between a v a i l a b l e water r e t e n t i o n and b u l k d e n s i t y o f muck s o i l s . The s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y o f o r g a n i c s o i l s has a l s o been r e p o r t e d t o decrease r a p i d l y w i t h i n c r e a s i n g degree of h u m i f i c a t i o n and b u l k d e n s i t y ( B o e l t e r , 1969; K o r p i j a a k o , 1972; S i l c and Stanek, 1977; Burghardt and I l n i c k i , 1978). I t i s deduced from the f o r e g o i n g l i t e r a t u r e t h a t i n c r e a s e d degree of h u m i f i c a t i o n r e s u l t s i n d i m i n i s h e d pore s i z e , i n c r e a s e d b u l k d e n s i t y , and decreased h y d r a u l i c c o n d u c t i v i t y . T i l l a g e f u r t h e r encourages d e c o m p o s i t i o n , and compaction a s s o c i a t e d w i t h t i l l a g e o p e r a t i o n s a d v e r s e l y a f f e c t s d r a i n a g e . T h e r e f o r e , a water t a b l e depth t h a t ensures t h a t e f f e c t i v e s o i l s t r e n g t h and t r a f f i c a b i l i t y as w e l l as good s o i l s t r u c t u r e a re m a i n t a i n e d i s a p r e r e q u i s i t e f o r s u c c e s s f u l f a r m i n g on o r g a n i c s o i l s ( P a u l and de V r i e s , 1979b). 1.2.4 S o i l water management i n o r g a n i c s o i l s E f f e c t i v e water t a b l e depth t h a t m i n i m i z e s subsidence 27 Table 1.2 Bulk density, water retention and saturated hydraulic conductivity of organic s o i l s reported i n the l i t e r a t u r e . Peat Type So i l Property Value Author Sphagnum moss peat Bulk density (Kg m"3) Herbaceous peat Decomposed peat Bulk density (Kg m-3) Bulk density (Kg m-3) 20 140 240 Boelter (1964 a) Decomposed peat Bulk density (Kg m~3) 230 S i l c & Stanek (1977) Sphagnum moss peat Ov at -10.2 m of water (%) Herbaceous peat Decomposed peat Ov at -10.2 m of water (%) Ov at -10.2 m of water (7„) 95 - 100 65 - 75 65 - 75 Boelter (1964 a) Decomposed peat Ks (m s *) 1.86 x 10 8 f t Sturges -2.77 x 10~° (1968) Ohio muck (85% 0. M. ) Ks (m s ) 8.3 x 10 -8 -3.1 x 10 -6 Hundal and Taylor (1979) F i b r i s o l Mesi sol Humi sol Ks (m s" 1) Ks (m s - 1 ) Ks (m s l) 1.4 x 10 -3 -4 2.6 x 10 -4.0 x 10 *-6 -4 8.4 x 10 -4.0 x 10 -4 Walmsley and Lavkulich (1975) 28 and m a i n t a i n s good s o i l s t r u c t u r e i n the r o o t i n g depth as w e l l ns p r o v i d e s i n c r e a s e d s o i l s t r e n g t h and t r a f f i c a b i l i t y and reduced s o i l compaction, i s d e s i r a b l e f o r s u c c e s s f u l farming on o r g a n i c s o i l s . T h i s i d e a l s i t u a t i o n i s o f t e n d i f f i c u l t t o a c h i e v e because r a i s i n g the water t a b l e too h i g h t o c o u n t e r a c t s u b s i d e n c e r a t e s may c r e a t e s o i l a e r a t i o n and t r a f f i c a b i l i t y problems. Jongedyk e t . a l (1954) have r e p o r t e d on the i n f l u e n c e o f d r a i n a g e on the p h y s i c a l p r o p e r t i e s o f a muck i n I n d i a n a . In t h e i r s i x y e a r s ' study (1944 t o 1950), they observed t h a t m a i n t a i n i n g a water t a b l e w e l l below the s u r f a c e p r o v i d e d f a v o u r a b l e s o i l s t r u c t u r a l c h a r a c t e r i s t i c s f o r crop growth. The d r a i n a g e p l o t s were w e l l a e r a t e d t o g r e a t e r d e p t h s , accompanied by p r o l i f i c r o o t growth and earthworm p o p u l a t i o n s . F u r t h e r they observed t h a t the s o i l s above the water t a b l e i n the deep and medium water t a b l e p l o t s were more permeable w i t h adequate pore s i z e d i s t r i b u t i o n , and easy water c o n t r o l , as compared w i t h the s o i l s above the h i g h water t a b l e p l o t s , which had spongy s t r u c t u r e and low p e r m e a b i l i t y . The h y d r a u l i c c o n d u c t i v i t y v a l u e s r e p o r t e d by Jongedyk et a l (1954) f o r I n d i a n a muck above d i f f e r e n t water t a b l e depths a r e p r e s e n t e d i n Ta b l e 1.3. They used the auger h o l e method. H a r r i s e t a l (1962) e s t a b l i s h e d c o n t r o l l e d d r a i n a g e p l o t s experiments on the Purdue U n i v e r s i t y Muck S o i l E x p e r i m e n t a l Farm a t W a l k e r t o n , I n d i a n a , i n 1943. The area had been d r a i n e d but not c o n t i n u o u s l y cropped f o r o v e r 30 ye a r s p r i o r t o the i n i t i a t i o n of the experiment. They observed t h a t y i e l d s of o n i o n s , p o t a t o , c a r r o t and c o r n i n p l o t s h a v i n g 29 Table 1.3 Muck s o i l h y d r a u l i c c o n d u c t i v i t y - v a l u e s by auger h o l e method i n c o n t r o l l e d d r a i n a g e p l o t s , North I n d i a n a Muck Experiment Farm, W a l k e r t o n , I n d i a n a . (Jongedyk et a l , 1954) 0.40 0.40 0.66 0.64 1.01 5 . 6 x l 0 ~ 7 1 . 4 x l 0 ~ 6 S . 5 x l 0 ~ 6 1.5x10 6 5.6x10 1 . 4 x l 0 ~ 7 l . l x l O " 6 5 . 6 x l 0 ~ 6 7.3x10 6 3.9x10 f o r n = 2 or 5 Average W a t e r t a b l e h e i g h t (M) 1944-1950 H y d r a u l i c C o n d u c t i v i t y Ks (m s-1) Standard D e v i a t i o n i n Ks (m s" 1) -'The au t h o r s used the term p e r m e a b i l i t y r a t e s . the water t a b l e l o c a t e d at 40.6 cm below the s u r f a c e were reduced as compared w i t h the y i e l d s o b t a i n e d i n p l o t s w i t h water t a b l e s l o c a t e d a t 61.0-, 81.0-, and 101.6 - cm below the s u r f a c e , which were e s s e n t i a l l y the same. F u r t h e r , crop response t o n i t r o g e n was l i m i t e d t o the 40.6 cm water t a b l e t r e a t m e n t s o n l y i n d i c a t i n g i n s u f f i c i e n t n i t r i f i c a t i o n i n the h i g h water t a b l e p l o t s . S o i l water b e h a v i o u r i n r e l a t i o n t o d i f f e r e n t water t a b l e depths were s t u d i e d by Rahi and S h i h (1980) i n s u b s i d i n g o r g a n i c s o i l s i n F l o r i d a . L y s i m e t e r s were used i n t h e i r study i n a sugar cane f i e l d . Water t a b l e depths were m a i n t a i n e d a t 30-, 60-, and 90-cm below the s u r f a c e i n the l y s i m e t e r s , and samples were c o l l e c t e d a t 5 to 15, and 15 to 30 cm depths from the l y s i m e t e r s f o r a p e r i o d of t h r e e y e a r s . T h e i r r e s u l t s i n d i c a t e d t h a t w a t e r-y i e l d c o e f f i c i e n t v a l u e s ( a i r c o n t e n t ) of the s o i l from the 50- and 90-cm water t a b l e t r e a t m e n t s were 2 t o 3 times those o b t a i n e d f o r s o i l s from the 30-cm water t a b l e t r e a t m e n t . They a l s o noted t h a t s o i l s from the 30-cm water t a b l e treatment r e t a i n e d about 10 t o 15 p e r c e n t l e s s water a t any g i v e n t e n s i o n , and possessed s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s 1/3 t o 1/2 those f o r s o i l s w i t h 60- and 90-cm water t a b l e t r e a t m e n t s . I r w i n (1977) r e p o r t s t h a t a water t a b l e depth between 0.6 m and 0.76 m below the s u r f a c e i s the most s u i t a b l e f o r c r o p p i n g purposes on the H o l l a n d Marsh i n O n t a r i o , and under t h i s c o n d i t i o n a water c o n t e n t of 60-75% by volume and an a i r c o n t e n t of 12-21% a re produced i n the r o o t zone. In a greenhouse s t u d y , Campbell and M i l l e t t e (1981) r e p o r t e d t h a t the optimum water t a b l e depth f o r p r o d u c i n g good q u a l i t y c a r r o t s was 30 cm below the s u r f a c e . I t must be emphasized t h a t i n f i e l d s i t u a t i o n s , t h i s depth may not produce the n e c e s s a r y s o i l s t r e n g t h f o r e f f e c t i v e t r a f f i c a b i l i t y and w o r k a b i l i t y o f o r g a n i c s o i l s . 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 t h a t low water t a b l e s f a v o u r e f f e c t i v e c r o p p r o d u c t i o n i n o r g a n i c s o i l s , w h i l e h i g h water t a b l e s l i m i t p r o f i t a b l e p r o d u c t i o n of many c r o p s ; but many r e s e a r c h f i n d i n g s r e v e a l t h a t s ubsidence i s d i r e c t l y r e l a t e d t o the depth o f the water t a b l e below the s u r f a c e (Jongedyk, et a l , 1950; Stephens and Johnson, 1951; Stephens, 1956; S c h o t h o r s t , 1977; S h i and Gascho, 1980). T h e r e f o r e the c r i t i c a l water t a b l e depth t h a t l i m i t s s u b s i d e n c e as w e l l as p r o v i d e s f a v o u r a b l e atmosphere f o r e f f e c t i v e s o i l a e r a t i o n , i n c r e a s e d s o i l s t r e n g t h a t the s u r f a c e , good r o o t p e n e t r a t i o n , and i n c r e a s e d c r o p y i e l d , i s e s s e n t i a l . P a u l and de V r i e s ( 1 9 7 9 b ) r e p o r t e d t h a t the c r i t i c a l water t a b l e depth f o r the upper l a y e r s o f Lumbum muck i n B r i t i s h Columbia t o be t r a f f i c a b l e was 53 cm below the s u r f a c e . They a l s o r e p o r t e d t h a t t h e r e was a c o n s i s t e n t i n c r e a s e i n s o i l s t r e n g t h w i t h l o w e r i n g of the water t a b l e . 1.2.5 S o i l s t r u c t u r a l d e t e r i o r a t i o n S t r u c t u r a l d e g r a d a t i o n r e s u l t i n g from management o f o r g a n i c s o i l s may be e v a l u a t e d i n terms o f changes i n s o i l p h y s i c a l p r o p e r t i e s such as b u l k d e n s i t y , a e r a t i o n p o r o s i t y and h y d r a u l i c c o n d u c t i v i t y . P a u l and de V r i e s (1979b)observed s i g n i f i c a n t changes i n these s o i l p h y s i c a l parameters o f Lumbum muck due t o compaction a s s o c i a t e d w i t h the passage of farm machinery. They r e p o r t e d t h a t the b u l k d e n s i t y i n c r e a s e d by 24 percent due t o t r a f f i c , when the average s o i l water t e n s i o n was 38 cm. Vorhees et a l (1978) r e p o r t e d t h a t f o r m i n e r a l s o i l s , penetrometer r e s i s t a n c e was a more s e n s i t i v e i n d i c a t o r of compaction t h a n b u l k d e n s i t y . U s i n g 20 p e r c e n t s l i p as a c r i t i c a l v a l u e f o r t r a c t i o n e f f i c i e n c y , P a u l and de V r i e s (1979b)again r e p o r t e d t h a t the c r i t i c a l s o i l s t r e n g t h f o r Lumbum muck as determined by cone penetrometer was about 2.6 kg cm (254.8 kPa ) . The d e t e r i o r a t i o n o f s o i l s t r u c t u r e and t i l t h r e s u l t i n g from the use of h e a v i e r farm machinery and i n t e n s i v e c u l t i v a t i o n i n m i n e r a l s o i l s was a g a i n r e p o r t e d by Vorhees (1979). A r n o l d and So j k a (1980) r e p o r t d t h a t i n m i n e r a l s o i l s compaction r e s u l t e d i n h i g h e r s o i l water r e t e n t i o n and s o i l s t r e n g t h but a s i g n i f i c a n t r e d u c t i o n i n the y i e l d s of p o t a t o c r o p . Fauk (1977) a s c r i b e s the s o i l d e g r a d a t i o n p r o c e s s t o i r r e v e r s i b l e m o d i f i c a t i o n of s o i l s t r u c t u r e l e a d i n g t o lower p r o d u c t i o n p o t e n t i a l of the s o i l . He emphasizes t h a t mechanical p r a c t i c e s such as t i l l a g e o p e r a t i o n s i n f l u e n c e s o i l p h y s i c a l p r o p e r t i e s . In t h e i r r e v i e w r e p o r t L a r s o n and Osborne (1982) remark t h a t t i l l a g e which shears the s o i l a t some depth below the s u r f a c e s e a l s o f f channels developed by p l a n t r o o t s , or s h r i n k a g e c r a c k s which conduct water t o lower l e v e l s f o r s t o r a g e i n o r d r a i n a g e from the p r o f i l e . T h e r e f o r e f o r o r g a n i c s o i l s t o m a i n t a i n t h e i r r o l e i n food crop p r o d u c t i o n , s t u d i e s i n v o l v i n g the i n f l u e n c e o f c u l t u r a l p r a c t i c e s such as s o i l t i l l a g e and seedbed p r e p a r a t i o n , c r o p r e s i d u e s management, weed and p e s t s c o n t r o l , and c r o p r o t a t i o n s , on t h e i r s t r u c t u r e are e s s e n t i a l . S o i l s t r u c t u r a l s t a b i l i t y t e s t i n g i s not an attempt t o measure the s t r u c t u r e i t s e l f , but i t i s an i m p o r t a n t supplement t o s t r u c t u r a l o b s e r v a t i o n s , e s p e c i a l l y f o l l o w i n g management of the s o i l (Smith and Cernuda, 1951). A s e a r c h o f the l i t e r a t u r e r e v e a l s t h a t t h e r e i s no s p e c i f i c i n d e x a t p r e s e n t f o r c h a r a c t e r i z i n g the s t r u c t u r a l s t a b i l i t y o f o r g a n i c s o i l s . S i n c e s o i l s t r u c t u r e i s r e l a t e d t o the pore s i z e d i s t r i b u t i o n , the changes i n h y d r a u l i c c o n d u c t i v i t y f o l l o w i n g d i s t u r b a n c e of the s o i l c o u l d be used as an i n d e x f o r s t r u c t u r a l s t a b i l i t y . S t r u c t u r a l s t a b i l i t y o f m i n e r a l s o i l s has been d e s c r i b e d u s i n g t h i s approach by W i l l i a m s e t a l (1966), Hamid and Mustafa (1975), and Hartage (1978). T h e r e f o r e the r e l a t i v e d ecrease i n h y d r a u l i c c o n d u c t i v i t y f o l l o w i n g m e c h a n i c a l d i s p e r s i o n of o r g a n i c s o i l s may be used to c h a r a c t e r i z e t h e i r s t r u c t u r a l s t a b i l i t y . The d a t a o f Stanek and S i l c (1977) as shown i n Table 1.4 c l o s e l y i n d i c a t e t h a t as the degree of h u m i f i c a t i o n 3 4 Table 1.4 Degree of d e c o m p o s i t i o n of peat (Von Post S c a l e ) i n r e l a t i o n to f i b e r c o n t e n t (Stanek and S i l c , 1977) Unrubbed f i b e r Rubbed f i b e r Van Post S c a l e (% of t o t a l ) (% of t o t a l ) H l 90.4 81.8 72.6 56.0 H 3 75.6 48.2 H 4 61.6 21.7 H 5 43.6 15.8 "6 46.2 18.2 H 7 25.8 4.4 K 8 32. 3 7.6 H 9 42.2 15.0 H 1 0 29.8 7.0 35 i n c r e a s e s the f i b r e c o ntent d e c r e a s e s . But the ash content o f o r g a n i c s o i l s i n c r e a s e s w i t h i n c r e a s e d degree of h u m i f i c a t i o n (Pedersen, 1979; M o r i t a , 1983). T h e r e f o r e the s t r u c t u r e of o r g a n i c s o i l s might be expected t o be r e l a t e d t o t h e i r ash content. 36 CHAPTER 2 2.0 MATERIALS AND METHODS 2.1 D e t e r m i n a t i o n o f F i b r e c o n t e n t 10 g o f o r g a n i c s o i l a t f i e l d water c o n t e n t were weighed i n t o h a l f - l i t r e p l a s t i c b o t t l e s . 200 ml o f aqueous s o l u t i o n c o n t a i n i n g 2 g of c a l g o n were added t o the s o i l and a l l o w e d to stand o v e r n i g h t (Sneddon et a l , 1971). The sample was shaken by hand f o r a p p r o x i m a t e l y 1 minute, and the o r g a n i c s u s p e n s i o n was c a r e f u l l y poured i n t o a 100 mesh U. S. s t a n d a r d s i e v e (0.15 mm). The s i e v e c o n t a i n i n g the d i s p e r s e d o r g a n i c s o i l was p l a c e d between two 2 mm s i e v e s . The s e t of s i e v e s was clamped i n a s i n k and the o r g a n i c s o i l was washed from above w i t h a r u b b e r hose a t t a c h e d to a tap u n t i l the water p a s s i n g out was c l e a r . Care was taken t o a v o i d a j e t of h i g h p r e s s u r e i n o r d e r t o prevent s p l a s h i n g of the f i b r e . The f i b r e r e t a i n e d on the 0.15 mm s i e v e was c a r e f u l l y washed i n t o a l a r g e p o r c e l a i n d i s h and a l l o w e d t o s e t t l e . The excess water was poured o f f and then was evap o r a t e d t o dryness on a h o t p l a t e . The f i b r e was then d r i e d i n an oven a t 105°C f o r 24 hours and was weighed. An oven d r y weight of a s e p a r a t e 10 g subsample was a l s o determined a t 105°C, and the r e s u l t s were ex p r e s s e d as: o, Wt. o f dry f i b r e ( 0.15 mm) (g) v i n n L F l b r e = Oven dry wt. o i subsample ( g T x 1 0 0 37 2.2 Determination of Shrinkage volume Undisturbed samples of organic s o i l s at f i e l d water content were ca r e f u l l y shaped such that they f i t t e d snugly into small c y l i n d r i c a l cans of 3.7 cm inside diameter and 2.4 cm high. The samples with the containers were weighed. Two subsamples from each layer were dried at 105°C for 2 hours, 4 hours, 8 hours, and 24 hours, respectively. The samples were weighed again after every drying to determine the water content. Shrinkage volume i n each case was determined by noting the mass of fine sand of predetermined bulk density (1,500 kg m ) required to f i l l the unoccupied volume between surface of sample at top of the can aft e r drying (Boelter and Blake, 1964). The volume of s o i l (after shrinkage) was plotted against the volume of water contained i n the s o i l (Baver et a l , 1972). 2.3 Organic matter and ash contents determination The ash and organic matter contents were determined by the i g n i t i o n method (Boggie and Robertson, 1972; Maas 1972). Samples of organic s o i l s at f i e l d water content were oven dried at 105°C for 24 hours. About 5 g subsample of the oven dry s o i l was ignited i n a muffle furnace at 550 C for 16 hours. The loss i n weight aft e r i g n i t i o n was taken to be the organic matter content. The ash l e f t after burning was weighed and expressed as percentage of the oven dry s o i l . The organic matter content was calculated as 100 - % ash. 2.4 D e t e r m i n a t i o n of water r e t e n t i o n c h a r a c t e r i s t i c s Water r e t e n t i o n c h a r a c t e r i s t i c s were determined i n the l a b o r a t o r y by u s i n g the hanging water column method f o r water p o t e n t i a l s r a n g i n g from -0.10 t o -0.9 m o f w a t e r , and the p r e s s u r e p l a t e e x t r a c t o r s f o r water p o t e n t i a l s r a n g i n g from -3 to -150 m of water. 2.4.1 The hanging water column t e c h n i q u e Core samples (7.5 cm h i g h by 7.2 cm di a m e t e r ) were t a k e n from the f i e l d . Each sample was p l a c e d i n a s m a l l p l a s t i c bag, kept i n a s p e c i a l c a rdboard c y l i n d r i c a l c o n t a i n e r w i t h a l i d . The samples were c a r e f u l l y packed i n a l a r g e c a r d b o a r d box and t r a n s p o r t e d t o the l a b o r a t o r y m i n i m i z i n g d i s t u r b a n c e . The samples were kept i n a r e f r i g e r a t o r u n t i l r e q u i r e d . A t e n s i o n p l a t e was made from carborundum. The ends of the sample were c a r e f u l l y trimmed and the sample was p l a c e d on the carborundum t e n s i o n p l a t e ( F i g . 2.1). An upper p l a t e was b o l t e d f i r m l y on top of the c y l i n d e r . The c y l i n d e r s a t on a greased " 0 " - r i n g rubber g a s k e t . In t h i s way no water c o u l d l e a k out when s u b j e c t e d t o w e t t i n g from below. The sample was wetted from below. The o u t f l o w end o f the t e n s i o n p l a t e connected to a p l a s t i c t u b i n g was dipped i n a c o n t a i n e r h a l f - f u l l o f water. The water l e v e l i n the c o n t a i n e r was i n c r e a s e d by adding more water u n t i l the s u r f a c e o f the water was almost l e v e l w i t h the s o i l s u r f a c e . The sample was wetted f o r almost 48 hours u n t i l the whole core was Upper p l a t e Nut S o i l c o r e C y l i n d e r Long screw Carborundum F i n e sand Bottom p e r f o r a t i o n s Nylon c l o t h P l a s t i c t u b i n g Weighing b o t t l e A carborundum t e n s i o n t a b l e f o r d e t e r m i n i n g water r e t e n t i o n c h a r a c t e r i s t i c s a t low t e n s i o n . 40 well saturated. The saturated core sample was then drained successively at water potentials of -0.1m, -0.2m, -0.6m, and -0.9m of water. The amount of water drained was collected i n a p l a s t i c bottle, which was weighed afte r each eq u i l i b r a t i o n . At the l a s t e q u i l i b r a t i o n , the whole sample was weighed and oven dried at 105°C for 24 hours to determine the water content. The weight values were used to calculate the volumetric water contents using bulk densities expressed as dry mass per unit wet bulk volume of s o i l (Boelter and Blake 1964). 2.4.2 The Pressure Plate extraction method The pressure plate extractors were used for water potentials ranging from -3 m of water to -150 m of water. Small blocks of undisturbed organic s o i l (about 5 mm thick) at f i e l d water content were ca r e f u l l y prepared, placed on a slurry made of the same organic s o i l , which covered the porous plate, to insure good contact with i t . Cheesecloth was placed over the slurry before placing the s o i l to prevent contamination of the samples (Sturges, 1968). The samples were soaked for 72 hours before extracting the water with the pressure plate extractors at suctions of 3 m of water, 9 m of water, 30 m of water, and 150 m of water, respectively. The samples were l e f t 4 days at each suction level for e q u i l i b r a t i o n to take place (Boelter 1964b). 2.5 Determination of Satiated hydraulic conductivity The term " s a t i a t i o n " was f i r s t used by M i l l e r and Bresler (1977) to emphasize that s o i l water content obtained through spontaneous w e t t i n g i s l e s s than t h a t at s a t u r a t i o n owing to a i r entrapment. Bouwer (1966) had noted t h a t h y d r a u l i c c o n d u c t i v i t y d u r i n g i n f i l t r a t i o n i n t o f i e l d s o i l s was l e s s than the a c t u a l s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y because of entrapped a i r . The h y d r a u l i c c o n d u c t i v i t y v a l u e s determined through spontaneous w e t t i n g w i t h a i r entrapment may t h e r e f o r e be c a l l e d " s a t i a t e d " h y d r a u l i c c o n d u c t i v i t y (de V r i e s , p e r s o n a l communication). " S a t i a t e d " h y d r a u l i c c o n d u c t i v i t y v a l u e s were determined on b i g u n d i s t u r b e d samples at f i e l d water c o n t e n t u s i n g the f a l l i n g head t e c h n i q u e . Samples o f d i a m e t e r r a n g i n g between 10 cm and 11 cm were c a r e f u l l y " c a r v e d " and f i x e d i n c y l i n d r i c a l cans of 13 cm i n t e r n a l d i a m e t e r and 14 cm l o n g u s i n g wax e i t h e r d i r e c t l y i n the f i e l d o r i n the l a b o r a t o r y . The l e n g t h of the samples ranged between 10 and 11 era, so t h a t t h e r e was at l e a s t 3 cm " f r e e b oard" above the top of the sample. Sampling was done s y s t e m a t i c a l l y a t about 3 m h o r i z o n t a l i n t e r v a l s . In measuring the h y d r a u l i c c o n d u c t i v i t y the samples were p l a c e d on a g r a v e l - f i l l e d r e c t a n g u l a r box made of a c r y l i c sheet ( p l e x i g l a s s ) . The box was open at the base so t h a t water flowed out f r e e l y . The g r a v e l a l l o w e d f r e e d r a i n a g e at the base of the sample so t h a t water flowed out a t z e r o p r e s s u r e p o t e n t i a l from the base. The f l o w r a t e was m onitored by c o n n e c t i n g the samples t o a water manometer w i t h a metre.scale . Water was added to the samples from the top and water was a l l o w e d to f l o w u n t i l the i n f i l t r a b i l i t y 42 F i g . 2.2 A s k e t c h o f f a l l i n g head d e v i c e f o r d e t e r m i n i n g s a t i a t e d h y d r a u l i c c o n d u c t i v i t y o f l a r g e u n d i s t u r b e d o r g a n i c s o i l samples. A. S. b. = A c r y l i c sheet box. 4 had d e c r e a s e d t o i t s constant, minimum v a l u e . The " s a t i a t e d " h y d r a u l i c c o n d u c t i v i t y ("K ") was c a l c u l a t e d by e q u a t i o n 2.1. "Ks" = aL At In HI H7 2.1 where "K " i s s a t i a t e d h y d r a u l i c c o n d u c t i v i t y (m s ~ ^ ) , 2 a i s the i n t e r n a l a r e a of the can, (m ), 2 A i s the s u r f a c e a r e a of the sample (m ), ("a" i s g r e a t e r than "A" by the a r e a of the wax) L i s the l e n g t h o f the sample (m). and t i s the time t a k e n f o r the head t o drop from H^ t o I-I . H-^  i s the head a t time z e r o , and H 2 i s the head a t time t . The procedure a l r e a d y d e s c r i b e d was used to d etermine the s a t i a t e d h y d r a u l i c c o n d u c t i v i t y f o r the f o l l o w i n g s o i l c o n d i t i o n s : (a) A l a y e r of low i n f i l t r a b i l i t y formed from s e t t l e d o r g a n i c and m i n e r a l s o i l p a r t i c l e s as a r e s u l t o f 1982 s p r i n g ponding on the Van H a l s t Farm f o l l o w i n g c u l t i v a t i o n of o n i o n s . ( F i g . 1.1.) (b) Samples from Van H a l s t Farm where ponded water had receded i n the s p r i n g of 1983 f o l l o w i n g c u l t i v a t i o n o f p o t a t o e s - the same s i t e as ( a ) . 44 (c) Samples from Van H a l s t Farm where ponded water had receded i n the s p r i n g of 1983 but the s u r f a c e 10 mm s e a l removed ( i . e . u n s e a l e d s u r f a c e ) . (d) Samples from h a r v e s t e d o n i o n seedbeds under w i n t e r rye cov e r on Van H a l s t Farm where ponded water had receded i n the s p r i n g o f 1983. (e) Management or t i l l a g e pan from Van H a l s t Farm and S t a t e r Farm ( f i g . 1.1). ( f ) Me11 decomposed o r g a n i c s u b s o i l on Van H a l s t Farm and S t a t e r Farm. (g) A s i t e c l o s e to a d r a i n a g e d i t c h on S t a t e r Farm t h a t had not been c u l t i v a t e d f o r a l o n g time. (h) C u l t i v a t e d o r g a n i c g e n e t i c pan on S t a t e r Farm and C l o v e r d a l e Produce Farm, r e s p e c t i v e l y ( f i g . 1.1). ( i ) The t r a n s i t i o n l a y e r between the o r g a n i c l a y e r and the m i n e r a l l a y e r ("diatomaceous e a r t h " ) on Van H a l s t Farm. ( j ) G e n e t i c pan c l o s e to the fence posts of permanent g r a s s p a s t u r e near C l o v e r d a l e Produce Farm where t h e r e had been no t r a m p l i n g due to c a t t l e . (k) C u l t i v a t e d g e n e t i c pan i n C l o v e r d a l e Produce Farm 3m from the fence posts i n ( j ) . 45 2.6 Methods for Characterizing Structural S t a b i l i t y 2.6.1 Relative Conductivity Technique A drawing of the apparatus used for this study Is presented i n Fig. 2.3. The apparatus consisted of a column 30 cm long and 9 cm internal diameter made of plexiglass. A 1.2 cm diameter hole made at the centre of the base of the column was connected to p l a s t i c tubing to serve as the outflow. The open end of the hole inside the column was covered with p l a s t i c screening. The column was h a l f - f i l l e d with gravel of size less than 4 mm but greater than 1 mm i n water. A vent was made i n the gravel compartment to ensure maintenance of atmospheric pressure within the gravel. Thus water flowed out from the s o i l at zero pressure potential. About 8 cm of loose organic s o i l at f i e l d water content was placed on top of the gravel while tapping gently. Water was added to the top using a simple " r a i n former" constructed with hypodermic needles. (Fig. 2.4.) Water was made to flow through the column for a considerable length of time u n t i l the i n f i l t r a b i l i t y had decreased to a constant minimum value. The i n i t i a l hydraulic conductivity was determined by the f a l l i n g head technique. The rate of flow was measured by connecting the water present on top of the column to a water manometer with a metre scale (Fig. 2.3). The satiated hydraulic conductivity was calculated by equation 2.1 with A = a. F i e . 2.3. A d e v i c e f o r s t u d y i n g s t r u c t u r a l s t a b i l i t y of o r g a n i s o i l s by d i s p e r s i o n . 47 s i d e view Hypodermi c Needle C i r c u l a r r u bber diaphragm b. bottom view C o n n e c t i o n t o t a p l=ttJ1H Screw clamp C i r c u l a r a c r y l i c frame C i r c u l a r r u b b e r diaphragm ttypodermic needle F i g 2.4. A s k e t c h of simple r a i n s i m u l a t o r . 48 The i n i t i a l satiated hydraulic conductivity was designated as " K i " . ) After the i n i t i a l hydraulic conductivity determination, the water level i n the column was quickly restored. A p l a s t i c tubing was connected from a source of a i r to the water present on top of the column. The pressure of the a i r , which was not determined, was controlled with a screw clamp on the p l a s t i c tubing such that the water present on top of the column did not s p i l l when i t was agitated with the a i r . This pressure was preselected before the i n i t i a t i o n of the experiment. The water on top of the column was agitated with a i r at the preselected pressure for a period of 10 minutes. More water was added to the column and the suspension was allowed to sett l e while the flow was in progress. When the supernatant l i q u i d was clear, the satiated hydraulic conductivity was again determined and calculated by equation 2.1. This value was designated as "K^". The structural s t a b i l i t y index of the organic s o i l (SI^Q) was defined as SI^Q = , ,K i '7 MK f M. Samples from Van Halst Farm (Vinod s o i l ) and Stater Farm (Richmond s o i l ) were used for thi s study. 2.6.2 The Waterdrop Technique A water-drop former was located at 0.6 m from the laboratory bench under a constant head. The mean volume was determined by catching 50 drops i n a 15 ml graduated c y l i n d e r . T h i s gave an average drop volume o f 1 x 10 m o r 1 x 10 kg assuming the d e n s i t y o f water t o be 1000 kg m . Assuming n e g l i g i b l e a i r r e s i s t a n c e on the f a l l i n g water drop, the v e l o c i t y of the f a l l i n g water drop was c a l c u l a t e d from the f o r m u l a : v 2 = 2gz o r v = (2 g z ) 1 / 2 2.3 In e q u a t i o n 2.3, v i s the v e l o c i t y of the f a l l i n g water drop, g i s the a c c e l e r a t i o n due t o g r a v i t y , and z i s the h e i g h t of f a l l . The assumption t h a t a i r r e s i s t a n c e on the f a l l i n g water drop i s n e g l i g i b l e i s i n c o r r e c t , s i n c e a i r r e s i s t a n c e can be i g n o r e d o n l y i f the f a l l i n g body i s heavy and has a r e l a t i v e l y s m a l l s u r f a c e a rea ( M i l l e r , 1959). For n o n - n e g l i g i b l e a i r r e s i s t a n c e , and assuming t h a t a i r r e s i s t a n c e i s p r o p o r t i o n a l to the i n s t a n t a n e o u s v e l o c i t y of the water drop, the f i n a l v e l o c i t y , v , may be o b t a i n e d from e q u a t i o n 2.4 (Boyce and D i p r i m a , 1965). v = jSS (1 - e " K t / m ) 2.4 Where, k i s a p r o p o r t i o n a l i t y c o n s t a n t , m i s the mass of the f a l l i n g o b j e c t g i s the a c c e l e r a t i o n due to g r a v i t y , and 50 t i s tim e . The use o f e q u a t i o n 2.4 i n t h i s work would be c o m p l i c a t e d by the c o n s t a n t k. T h e r e f o r e , e q u a t i o n 2.3 was used f o r s i m p l i c i t y . The K i n e t i c energy (KE) of a drop was c a l c u l a t e d from the f o r m u l a : KE = j mv 2 2.5 The K i n e t i c energy c a l c u l a t e d by e q u a t i o n 2.5 i s always c o r r e c t r e g a r d l e s s o f how the body was a c c e l e r a t e d to r e a c h t h i s speed. ( M a r s h a l l et a l , 1970) The t o t a l K i n e t i c energy ( i n J o u l e s ) r e q u i r e d t o d e s t r o y a c l o d o f 1 g e q u i l i b r a t e d a t 20 cm s u c t i o n was used as ah i n d e x o f s t r u c t u r a l s t a b i l i t y ( M c C a l l a , 1944). 2.7 F i e l d i n v e s t i g a t i o n o f P e n e t r a t i o n r e s i s t a n c e The p e n e t r a t i o n r e s i s t a n c e i n a h a r v e s t e d o n i o n seedbed i n d r i v e n - o n i n t e r - b e d p o s i t i o n s on the Van H a l s t 2 Farm (Vinod s o i l , F i g . 1.1) was determined w i t h a 0.0005m base a r e a , 60° t i p , cone penetrometer, when the average water t a b l e depth was about 0.60 m below the s u r f a c e on A p r i l 27, 1982. D e t e r m i n a t i o n s were done s y s t e m a t i c a l l y at l o c a t i o n s 5 m a p a r t a t 0.05 m - depth increment t o a maximum depth of 0.60 m. The d e t e r m i n a t i o n was done a t a s i t e where ponded water had j u s t receded. 2.8 E s t a b l i s h m e n t o f evidence o f ponding i n the f i e l d In o r d e r to e s t a b l i s h t h a t ponded c o n d i t i o n s e x i s t e d , two s i t e s w i t h t h i s c o n d i t i o n were s e l e c t e d on Van H a l s t Farm ( F i g . 1.1), i n s p r i n g of 1982 and 1983. On A p r i l 19, 1982, f o u r p i e z o m e t e r tubes were i n s t a l l e d c l o s e t o the st a g n a n t water t o mo n i t o r the p o s i t i o n of the groundwater t a b l e i n a h a r v e s t e d o n i o n f i e l d . T h i s f i e l d was cropped t o p o t a t o e s i n s p r i n g 1982. On March 17, 1983, f o u r p i e z o m e t e r tubes were a g a i n i n s t a l l e d a t the same l o c a t i o n , two each i n the o p p o s i t e s i d e s of the stagnant water, a t about 2 m from the water. On the same day one piezometer tube was i n s t a l l e d 3 m from a stagnant water ponding on a h a r v e s t e d onion f i e l d which was under rye c o v e r . On March 24, 1983, one piezometer tube was i n s t a l l e d i n the rye area a f t e r the ponded water had receded. Ponding was e s t a b l i s h e d whenever the water t a b l e was l o c a t e d below the s u r f a c e w i t h water s t a n d i n g on the s o i l s u r f a c e and an u n s a t u r a t e d zone e x i s t i n g between the s o i l s u r f a c e and the water t a b l e . The r a t e of r e c e s s i o n of the ponded water was a l s o determined f o r these l o c a t i o n s . Data on pan e v a p o r a t i o n were o b t a i n e d from Environment Canada, C l i m a t e I n f o r m a t i o n , Vancouver. 2.9 S i m u l a t i o n o f the process o f f o r m a t i o n of a s u r f a c e l a y e r of low i n f i l t r a b i l i t y due to s e t t l e d o r g a n i c and m i n e r a l p a r t i c l e s i n ponded water  A column o f f i n e sand (0.25 - 0.10 mm) o f l e n g t h 0.28 m was prepar e d w i t h v i b r a t i o n i n water i n a 0.05 m i n t e r n a l d i a m e t e r a c r y l i c p l a s t i c tube i n the l a b o r a t o r y . The s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y o f the sand was determined by the c o n s t a n t head method ( F i g . 2.5). An a i r - d r y s u r f a c e c r u s t c o n s i s t i n g o f o r g a n i c and m i n e r a l s o i l p a r t i c l e s t h a t had s e t t l e d on the s o i l s u r f a c e t h r o u g h ponding i n the f i e l d , was ground and put through a 0.5 mm s i e v e . 0.045 kg o f the s i e v e d sample was added t o 100 ml o f water and a l l o w e d t o h y d r a t e f o r about 5 hours and l a t e r s t i r r e d t o make a t h i c k s u s p e n s i o n . The o v e r f l o w u n i t ( t h e c o n s t a n t head d e v i c e ) was r a i s e d above the water l e v e l i n the column. The s u s p e n s i o n was a g a i n v i g o r o u s l y s t i r r e d and about 25 ml o f i t poured on top o f the sand column. The o v e r f l o w u n i t was s l o w l y lowered down a g a i n . I n t h i s way b a c k - f l o w o f s u s p e n s i o n from t h e column t o the o v e r f l o w u n i t was e l i m i n a t e d . The p a r t i c l e s were a l l o w e d t o s e t t l e w h i l e water movement was s t i l l i n p r o g r e s s . When a l l the s o i l p a r t i c l e s and the o r g a n i c t c o l l o i d s had s e t t l e d on top o f the sand column the f l u x t hrough the s e t t l e d p a r t i c l e s was measured and the h y d r a u l i c head r e c o r d e d . The a d d i t i o n o f the s u s p e n s i o n was c u m u l a t i v e l y done f o u r times and s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y determined i n each c a s e . The s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y of t h e l a y e r o f s e t t l e d p a r t i c l e s ( r y t h m i t e s ) was c a l c u l a t e d by the fo r m u l a H i l l e l ( 1 9 8 0 ) : £ = A H / ( L 1 / K 1 + L 2 / K 2 ) 2.6 53 Siphon A device for simulating s e t t l i n g of organic-mineral p a r t i c l e s forming a surface layer of low i n f i l t r a b i l i t y i o the laboratory. C - layer of settled organic - mineral p a r t i c l e s . S - coarse sand G - gravel Where, i s the h y d r a u l i c head drop a c r o s s b o t h l a y e r s (m), £ i s the f l u x d e n s i t y through the s e t t l e d p a r t i c l e s on top the sand column (m s i s the l e n g t h o f the sand column (m), I<2 i s the predetermined s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y o f the sand, (m s ^ " ) , L j i s the t h i c k n e s s of the s e t t l e d p a r t i c l e s (m), and i s the h y d r a u l i c c o n d u c t i v i t y o f the l a y e r o f s e t t l e d p a r t i c l e s to be c a l c u l a t e d (m s ~ ^ ) . 2.10 B ulk D e n s i t y and P a r t i c l e D e n s i t y B u l k d e n s i t y was determined w i t h 7.6 cm by 7.4 cm u n d i s t u r b e d c o r e s ( B o e l t e r , 1964; Dasberg and Neumann, 1977). P a r t i c l e d e n s i t y was determined i n the l a b o r a t o r y by the pycnometer method i n d e - a i r e d d i s t i l l e d water' (Walmsley and L a v k u l i c h , 1975). 2.11 Sampling Procedure F i e l d s ampling f o r " s a t i a t e d " h y d r a u l i c c o n d u c t i v i t y d e t e r m i n a t i o n was done s e q u e n t i a l l y at 3 m a p a r t between rows and 3 m a p a r t w i t h i n rows. Sampling s i t e s were s e l e c t e d from areas where the problem of ponding e x i s t e d . To sample an u n d i s t u r b e d b l o c k of s o i l an area of about 0.5 m by 0.5 m was marked. The s o i l i n i t s n a t u r a l s t a t e was " c a r v e d " i n s i t u and f i x e d i n c y l i n d r i c a l cans of 13 cm i n t e r n a l d i a m e t e r and 14 cm l o n g u s i n g wax. The s o i l was c a r e f u l l y removed w i t h a s h o v e l and the bottom trimmed w i t h a k n i f e . A space o f about 3 cm was always l e f t between the top o f the can and the s o i l s u r f a c e so t h a t water c o u l d be i n t r o d u c e d onto the s u r f a c e of the s o i l . I n c e r t a i n cases l a r g e b l o c k s of samples were o b t a i n e d , and " c a r v i n g " and f i x i n g of the s o i l i n the cans were done i n the l a b o r a t o r y . Core samples, u n d i s t u r b e d b l o c k s of s o i l and b u l k s o i l samples used i n the study were taken l a y e r by l a y e r a l o n g p r o f i l e p i t s dug i n the f i e l d . The t r a n s i t i o n l a y e r c o n s i s t i n g of s o f t , "diatomaceous e a r t h " (Van V l i e t and Wood, 1982) was sampled by pushing the c y l i n d e r c a r e f u l l y i n t o the s o i l and removing the sample w i t h a s h o v e l . 56 CHAPTER 3 3.0 RESULTS AND DISCUSSION 3.1 Some p e r t i n e n t p r o p e r t i e s of the s o i l s 3.1.1 Bulk d e n s i t y and p a r t i c l e d e n s i t y V alues f o r b u l k d e n s i t y and p a r t i c l e d e n s i t y of Vinod s o i l (Rego G l e y s o l ) on Van H a l s t Farm are presented i n T a b l e 3.1. In the o r g a n i c l a y e r s the h i g h e s t average b u l k _3 d e n s i t y of 530 kg m ( c . v . = 1.9%, n = 6) i s observed a t the 0.15-0.25 m s o i l depth, which corresponds t o the t i l l a g e pan. The o r g a n i c s u b s o i l (0.25 - 0.40 m) has the lowest average b u l k d e n s i t y of 220 kg m~3 ( c . v . = 4.5%, n = 6 ) . The bu l k _3 d e n s i t y of the t r a n s i t i o n l a y e r i s 590 kg m ( c . v . = 11.9, n = 6 ) . The average p a r t i c l e d e n s i t y of the o r g a n i c _3 l a y e r s of Vinod s o i l v a r i e s from 1533 kg m ( c . v . = 0.53%, -3 n = 4) i n the top l a y e r to 1295 leg m ( c . v . = 1.2%, n = 4) i n the 0.15 - 0.25 m s o i l depth. The o r g a n i c s u b s o i l (0.25-0.40 m s o i l depth) has an average p a r t i c l e d e n s i t y of 1245 kg m ( c . v . = 0.167,, n = 4 ) . The average p a r t i c l e d e n s i t y of the t r a n s i t i o n l a y e r (0.40 - 0.55 m) i s observed to be 1990 kg m~3, ( c . v . = 4.2%, n = 4 ) . The t r a n s i t i o n l a y e r i s made up o f v e r y l i g h t m a t e r i a l s . The low b u l k and p a r t i c l e d e n s i t i e s o f the t r a n s i t i o n l a y e r a re d i f f i c u l t t o e x p l a i n . Wood ( p e r s o n a l communication) d e s c r i b e s b r i e f l y the s o f t l i g h t 57 Table 3.1 Some p e r t i n e n t p r o p e r t i e s o f s o i l i n Van H a l s t Farm (Vinod s e r i e s ) Depth Bulk P a r t i c l e Ash-" F i b e r pll (m) D e n s i t y D e n s i t y Content Content (0.01 M) (kg m"3) (Kg m-3) (%) (%) ( C a C l 9 ) (n=6) (n=4) (n=9) (n=3) (n=37 -0.15 430+20.0 15SS+10.0 34.7+2.3 31.7+5.5 5.3+0 (c . v . = ( c . v . = ( c . v . = ( c . v . = 4.2%) 0.63%) 6.6%) 17.4%) 0.15-0.25 530+10 1295+15.0 25.1+1.5 40.0+3.3 5.4+0 (c . v . = ( c . v . = ( c . v . = ( c . v . = 1.9%) 1.2%) 6.0%) 8.5%) 0.25-0.40 220+10 1245+2.0 16.4+3.4 54.0+1.6 4.7+0 (c . v . = (c . v . = ( c . v . = ( c . v . = 4.5%) 0.16%) 20.7%) 3.0%) 0.40-0.55 590+70 1990+83.0 89.5+0.5 - 3.9+0 (T. L. ) ( c . v. = ( c . v. = ( c . v. = 11.9%) 4.2%)- 0.6%) T. L. = T r a n s i t i o n Layer * % Or g a n i c m a t t e r = 100 - % Ash + s t a n d a r d d e v i a t i o n , (S) c. v. = c o e f f i c i e n t o f v a r i a t i o n m a t e r i a l s o f the t r a n s i t i o n l a y e r as "sedimentary p e a t . " However, the r e l a t i v e l y low o r g a n i c m a t t e r c o n t e n t o f the t r a n s i t i o n l a y e r makes i t d o m i n a n t l y m i n e r a l . The low b u l k and p a r t i c l e d e n s i t i e s may be due t o the presence o f d i a t o m i t e s . 3.1.2 Ash and o r g a n i c m a t t e r c o n t e n t s The ash and o r g a n i c m a t t e r c o n t e n t s f o r Vinod s o i l (Van H a l s t Farm) a r e p r e s e n t e d i n Table 3.1, w h i l e those f o r Richmond s o i l ( S t a t e r Farm, c u l t i v a t e d g e n e t i c pan a t C l o v e r d a l e Produce Farm, and the g e n e t i c pan under permanent g r a s s p a s t u r e (near C l o v e r d a l e Produce Farm) are p r e s e n t e d i n T a b l e 3.2. In g e n e r a l , the ash c o n t e n t and the o r g a n i c m a t t e r c o n t e n t o f the o r g a n i c l a y e r s i n c r e a s e s and decreases towards the s u r f a c e , r e s p e c t i v e l y . The average ash c o n t e n t o f a 2 cm t h i c k l a y e r c o n s i s t i n g o f o r g a n i c and m i n e r a l p a r t i c l e s s e t t l e d on the s u r f a c e o f V i n o d s o i l (Van H a l s t Farm) from ponded water i s observed t o be 48 p e r c e n t ( c . v . = 8.37o, n = 8) . The average ash c o n t e n t o f the c u l t i v a t e d g e n e t i c o r g a n i c hardpan on C l o v e r d a l e Produce Farm i s s l i g h t l y lower than the pan which i s under permanent g r a s s p a s t u r e . F a v o u r a b l a e r a t i o n due t o abundant dead g r a s s - r o o t channels might be r e s p o n s i b l e f o r the h i g h e r ash c o n t e n t of the p a s t u r e s o i l , even though the d i f f e r e n c e i s not s i g n i f i c a n t . The ash o f t h e s e s h a l l o w o r g a n i c s o i l s c o n t a i n e d c o n s i d e r a b l e amounts o f m i n e r a l s o i l p a r t i c l e s , which were not s e p a r a t e d . 59 Table 3 . 2 Ash co n t e n t of Richmond s e r i e s i n S t a t e r Farm and g e n e t i c pan i n C l o v e r d a l e Produce Farm (a) S t a t e r Farm Depth (m) Ash Content (7.) (b) C l o v e r d a l e Produce Farm Depth Ash Content Remark (m) 0 - 0 . 1 5 2 5 . 5 + 1 . 6 (n = 6 ) ( c . v . = 6 . 3 7 J 0 . 1 5 - 0 . 2 5 2 0 . 9 + 1 . 1 (n = 7 ) ( c . v . = 5 . 3 % ) 0 . 1 5 - 0 . 2 5 1 8 . 3 + 1 . 7 grass (n = 5 ) ( c . v 7 = 9 . 3 7 o ) p a s t u r e 0 . 2 5 - 0 . 4 0 1 0 . 6 + 1 . 3 0 . 2 5 - 0 4 0 (n = 4 ) ( c . v 7 = 1 2 . 3 % ) (n = 4 ) 1 2 . 0 + 0 . 4 ( c . v 7 = 3 . 3 % ) g e n e t i c pan under grass p a s t u r e 0 . 2 5 - 0 . 4 0 (n = 5 ) 1 1 . 1 + 0 . 9 6 c u l t i v a t e d ( c . v . = 8 . 6 7 = ) g e n e t i c p a n 60 The ash co n t e n t of an o r g a n i c s o i l i s an im p o r t a n t p r o p e r t y t h a t d e s c r i b e s the degree o f d e c o m p o s i t i o n of the o r g a n i c m a t e r i a l s (Maas, 1972). The ash c o n t e n t o f an o r g a n i c s o i l i n c r e a s e s w i t h i n c r e a s e d degree o f d e c o m p o s i t i o n ( N e l l e r , 1944), and t h e r e f o r e , has a s t r o n g i n f l u e n c e on the pore s i z e d i s t r i b u t i o n of an o r g a n i c s o i l ( B o e l t e r , 1969). 3.1.3 The f i b r e c o n t e n t The f i b r e c o n t e n t s of the o r g a n i c l a y e r s on Vinod s o i l (Van H a l s t Farm) are a l s o shown i n t a b l e 3.1. The average f i b r e c o n tent (>0.15 mm) i n the top o r g a n i c s o i l i s 31.7 percent ( c . v . = 17.4%, n = 3 ) , w h i l e i n the 0.15 - 0.25m s o i l depth and the 0.25 - 0.40 m s o i l depth the average f i b r e c o n t e n t s a re 40 p e r c e n t ( c . v . = 8.3%,, n = 3 ) , and 54 pe r c e n t ( c . v . = 3.07o, n = 3 ) , r e s p e c t i v e l y . As e x p e c t e d , the f i b r e c o n t e n t d e c r e a s e s towards the s u r f a c e i n accordance w i t h an i n c r e a s e d degree of d e c o m p o s i t i o n towards the s u r f a c e . The f i b r e c o n t e n t ( l i k e the ash c o n t e n t ) of an o r g a n i c s o i l may a l s o be used t o c h a r a c t e r i z e the degree of d e c o m p o s i t i o n of an o r g a n i c s o i l (Stanek and S i l c , 1977; M o r i t a , 1983). 3.1.4 S o i l pH The pH v a l u e s (0.01 M C a C ^ ) f o r the o r g a n i c and the t r a n s i t i o n l a y e r o f Vinod s o i l (Van H a l s t Farm) a r e pres e n t e d i n t a b l e 3.1. The pH v a l u e s o f the o r g a n i c l a y e r s v a r y from 5.3 and 5.4 i n the top l a y e r s t o 4.7 i n the o r g a n i c 61 s u b s o i l . The pH of the t r a n s i t i o n l a y e r (0.40 - 0.55 m) i s 3.9. These s o i l s r e q u i r e l i m i n g a t a r a t e o f 2t - 4t/ha a n n u a l l y to m a i n t a i n s o i l pH at or above 5.0 (Van V l i e t and Wood, 1982). 3.2 Shrinkage Shrinkage c u r v e s o f the o r g a n i c l a y e r s (0.15 - 0.25 m and 0.25 - 0.40 m s o i l depth) and the t r a n s i t i o n l a y e r (0.40-0.55m) f o r f i e l d s a t u r a t e d u n d i s t u r b e d Vinod s o i l (Van H a l s t Farm) are pre s e n t e d i n F i g . 3.1.. The curv e s are almost p a r a l l e l i n the wet r e g i o n and the s l o p e i s a p p r o x i m a t e l y u n i t y . As more water i s removed through d r y i n g the volume of water removed becomes more than the change i n s o i l volume and the approximate l i n e a r i t y i s l o s t i n the lower end of the c u r v e s . T h i s p o r t i o n of the curve has been c a l l e d " r e s i d u a l s h r i n k a g e " ( B a v e r e t a l , 1972), At oven d ry the 0.15 - 0.25 m l a y e r may l o s e as much as 77.4 percent of i t s volume, w h i l e the 0.25 - 0.40 m l a y e r may l o s e about S7.4 p e r c e n t of i t s volume. The t r a n s i t i o n l a y e r a l s o s h r i n k s c o n s i d e r a b l y . At oven d ry the t r a n s i t i o n l a y e r may l o s e as much as 61.1 percent of i t s volume. Shrinkage i n the o r g a n i c l a y e r was l o w e s t i n the c u l t i v a t e d s u r f a c e l a y e r (0.15 - 0.25 m). T h e r e f o r e , s h r i n k a g e g i v e s an i n s i g h t i n t o the p h y s i c a l p r o p e r t i e s of o r g a n i c s o i l s and t h e i r degree of d e c o m p o s i t i o n , (Maas, 1972). For the o r g a n i c s o i l s of Vancouver I s l a n d , Maas (1972) r e p o r t e d t h a t the s u r f a c e l a y e r s of the c u l t i v a t e d s o i l s d i d not s h r i n k S o i l l a y e r s  A 15-25 cm A 25-40 cm • 40-55 cm 5 10 15 Volume o f water cm 20 3 25 F i g . 3.1. The s h r i n k a g e o f Vinod s o i l as a f u n c t i o n o f t h e N t o t a l volume o f water i n a 26 cm 3 s o i l sam 63 e x c e s s i v e l y on d r y i n g because o f t h e i r h i g h b u l k d e n s i t y and p r e v i o u s s h r i n k a g e h i s t o r y . For a s u r f a c e s o i l (0 - 0.15 m) h a v i n g a b u l k d e n s i t y o f 418 kg m , Maas (1972) r e p o r t e d volume s h r i n k a g e of 75 p e r c e n t on d r y i n g a t 30°C. For a s u b s o i l peat he observed 87 p e r c e n t volume s h r i n k a g e . In t h i s study the volume s h r i n k a g e of 77.4 p e r c e n t and 87.4 per c e n t r e p o r t e d on d r y i n g a t 105°C f o r 24 hours f o r _ 3 the c u l t i v a t e d l a y e r h a v i n g a b u l k d e n s i t y o f 530 kg m and _3 the o r g a n i c s u b s o i l h a v i n g a b u l k d e n s i t y of 220 kg m , r e s p e c t i v e l y , c orresponds c l o s e l y w i t h Maas' (1972) f i n d i n g s . The h i g h degree o f s h r i n k a g e o b t a i n e d i n the t r a n s i t i o n l a y e r c o u l d be a s s o c i a t e d w i t h the s i l i c e o u s n a t u r e of the s o i l ( " d i a t o m i t e s " ) and p o s s i b l y the presence of s m e c t i t e , which i s a s h r i n k i n g c l a y m i n e r a l . The r e s u l t s show t h a t i t would be d e t r i m e n t a l t o expose t h i s o r g a n i c s o i l t o e x c e s s i v e d r y i n g s i n c e e x c e s s i v e d r y i n g would induce i r r e v e r s i b l e s h r i n k a g e and consequent l o s s of e l e v a t i o n o f the o r g a n i c s o i l s u r f a c e . 3.3 P e n e t r a t i o n r e s i s t a n c e The v a r i a t i o n of cone penetrometer r e s i s t a n c e w i t h depth i n d r i v e n - o n i n t e r b e d p o s i t i o n s and i n h a r v e s t e d o n i o n seedbeds i n V i n o d s o i l (Van H a l s t Farm) i n s p r i n g 1982 i s shown i n F i g - 3.2. Each p o i n t i s an average o f s i x measure-ments. The da t a used i n p l o t t i n g F i g . 3.2 are shown i n Table 3.3. 64 A Onion seedbed ^ Wheel track • F i e l d moisture content at saturation 0 F i e l d moisture content at time of measurement (60 cm water table). 0 500 1000 1500 Fi g . 3.2 Variation of penetration resistance with depth £,60 cm water table depth on Vinod s o i l (Van a l s t Farm) (April 27, 1982. Table 3.3 Penetrometer r e s i s t a n c e i n h a r v e s t e d o n i o n seedbed and wheel t r a c k on Vinod s o i l (Van H a l s t Farm) Note: A c t u a l depth below o n i o n bed s u r f a c e f o r wheel t r a c k i s i n d i c a t e d depth + .08 m. Depth (m) Penetrometer R e s i s t a n c e (kPa) Wheel t r a c k Onion Seedbed 0 186.2 + 19.6 225.4 + 56.8 0.05 284.2 + 48.0 862.4 + 74.5 0.10 333.2 + 72.5 1352.4 + 56.8 0.15 333.2 + 48.0 1401.4 + 149.9 0.20 509.6 + 260. 7 1156.4 + 311.6 0.25 1029.0 + 320.5 1127.0 + 389.1 0.30 1274.0 + 342.0 764.4 + 172.5 0.40 823.2 + 136.2 539.0 + 98.0 0.50 519.4 + 166.6 294.0 + 0.0 0.60 343.0 + 31.4 264.6 + 28.4 + = s t a n d a r d d e v i a t i o n (n = 6). 66 Measurements were made a f t e r a l l ponded water had d i s a p p e a r e d and the farmer had j u s t s t a r t e d t i l l a g e o p e r a t i o n s . I t r e q u i r e d about 1 0 r a i n l e s s days ( A p r i l 1 7 t o A p r i l 2 7 ) a f t e r ponding b e f o r e the farmer c o u l d c u l t i v a t e the s o i l { F i g . 3.3). (See a l s o the p a r t i a l water r e t e n t i o n c u r v e s of F i g . 3 . 7 . ) . The f l a t n a t u r e of the curve f o r the 0 - 0 . 1 5 m l a y e r d e p i c t s the slow water r e l e a s e c h a r a c t e r i s t i c s of the s u r f a c e s o i l . T h i s b e h a v i o u r w i l l be d i s c u s s e d i n d e t a i l i n s e c t i o n 3 . 5 . The average water t a b l e depth r e c o r d e d from piezometer tubes i n s t a l l e d a t the s i t e a t the time o f d e t e r m i n a t i o n was 0 . 5 9 m ( c . v . = 1 1 . 9 7 o , n = 4 ) . The cone penetrometer r e s i s t a n c e i n the seedbed i n c r e a s e d c o n s i s t e n t l y w i t h depth t o a maximum of 1 2 7 4 kPa a t 0 . 3 0 m of s o i l d e pth, and then d e c r e a s e d c o n s i s t e n t l y t o the m i n e r a l l a y e r . The penetrometer r e s i s t a n c e a t 0 . 1 5 m depth on the seedbed was 3 3 3 . 2 kBa ( 3 . 4 kg cm ), and the average water c o n t e n t of the 0 - 0.15m depth a t the time o f d e t e r m i n a t i o n was 5 8 p e r c e n t by volume. The pore water p r e s s u r e e s t i m a t e d from the water r e t e n t i o n c urve was - 1 . 4 m of water. In the wheel t r a c k , the penetrometer r e s i s t a n c e a l s o i n c r e a s e d c o n s i s t e n t l y w i t h depth to a maximum of 1 4 0 1 . 4 _ 2 kpa ( 1 4 . 3 kg cm ) i n the 0 . 2 3 m of s o i l depth ( t a k i n g i n t o c o n s i d e r a t i o n a f u r r o w depth of 0 . 0 8 m of the wheel t r a c k ) , and then decreased c o n s i s t e n t l y t o the m i n e r a l l a y e r . I t was f o r t u i t o u s l y observed i n t h i s study t h a t the 67 farmer c u l t i v a t e d the s o i l when the p e n e t r a t i o n r e s i s t a n c e of the 0.15 m depth o f Vinod s o i l was 333.2 kPa (3.4 kg cm and the average water t a b l e depth was a p p r o x i m a t e l y 0.6 m below the s u r f a c e . P a u l and de V r i e s (1979b) observed t h a t the c r i t i c a l p e n e t r a t i o n r e s i s t a n c e f o r the top 0 - 0.15 in _2 of Lumbum muck t o be t r a f f i c a b l e was 254.8 kPa (2.6 kg era ) w i t h the water t a b l e l o c a t e d at 0.53 m below the s u r f a c e . In the o n i o n seedbed the depth of h i g h e s t p e n e t r a t i o n r e s i s t a n c e and b u l k d e n s i t y corresponded w i t h the depth of c u l t i v a t i o n by the farmer. Hence compaction p e r t i n e n t t o t h i s depth i s due to the s h e a r i n g of the s o i l through r o t o t i l l a g e ( L a r s o n and Osborne, 1982), and t r a f f i c by farm machinery. 3.4 Evidence of ponding and r e c e s s i o n r a t e o f ponded water i n the f i e l d  The e x i s t e n c e of ponded c o n d i t i o n s i n the f i e l d was proven by n o t i n g the presence of u n s a t u r a t e d s o i l between ponded water on the s o i l s u r f a c e and the depth of the water t a b l e as measured w i t h p i e z o m e t e r tubes i n s t a l l e d i n Vinod s o i l (Van H a l s t Farm). Table 3.4 shows the depth of water ponding on top of the s o i l and the d i s t a n c e t o the water t a b l e below the s u r f a c e i n the s p r i n g of 1932 and 19S3. E x a m i n a t i o n of the s o i l p r o f i l e i n d i c a t e d the e x i s t e n c e o f a t i l l a g e pan w i t h i n the 0.15 to 0.25 m s o i l d e p t h , which was always u n s a t u r a t e d . In the o r g a n i c l a y e r the t i l l a g e pan had the h i g h e s t b u l k d e n s i t y (Table 3.1) and p e n e t r a t i o n r e s i s t a n c e ( F i g . 3.2) but the low e s t s a t i a t e d h y d r a u l i c c o n d u c t i v i t y ( F i g . 3.8). 69 Table 3.4 I n c i d e n c e o f ponding a t d i f f e r e n t depths to water t a b l e i n Van H a l s t Farm (Vinod s e r i e s ) Date Ponding Depth" (m) Depth to Water Table (m) Remarks A p r i l 20/32 0.033 0.23 P o s t - h a r v e s t e d o n i o n f i e l d . March 17/83 0.025 0.18 P o s t - h a r v e s t e d p o t a t o f i e l d , March 17/83 0.035 0.44 Piezometer i n s t a l l e d 2 m from pond i n p o s t - h a r v e s t e d o n i o n f i e l d . A p r i l 6/83 0.025 0.30 Pie z o m e t e r i n s t a l l e d 2 m from pond i n p o s t - h a r v e s t e d o n i o n f i e l d . A p r i l 6/83 0.025 0.25 Piezometer i n s t a l l e d i n the ponded a r e a i n p o s t - h a r v e s t e d o n i o n f i e l d . 70 T h i s pan i s b r i t t l e when dry and on the average i t c o n t a i n s about 25 p e r c e n t ash (Table 3.1). The r a t e s of r e c e s s i o n o f ponded water i n the h a r v e s t e d o n i o n f i e l d a t d i f f e r e n t s i t e s on Vinod s o i l (Van H a l s t Farm) i n the s p r i n g of 1982 and 1983 are p r e s e n t e d i n f i g u r e s 3.4 and 3.5, r e s p e c t i v e l y . The s l o p e of the curve a f t e r c o r r e c t i o n f o r e v a p o r a t i o n may be used to e s t i m a t e the f l u x through the s u r f a c e l a y e r . The c o r r e c t e d f l u x r e c o r d e d f o r the 1982 s p r i n g r e c e s s i o n o f ponded water was 1.3 x 10 7 m s . In 1983 the r a t e of r e c e s s i o n of ponded water was r e c o r d e d on a s i t e t h a t had been seeded to w i n t e r r y e , but the rye was k i l l e d as a r e s u l t of ponding. The c o r r e c t e d f l u x r e c o r d e d from the r e c e s s i o n of the ponded water i n the rye a r e a was 1.4 x 10~ 7 m s " 1 . The 1932 and 1983 c o r r e c t e d f l u x e s d i d not d i f f e r s i g n i f i c a n t l y . The s a t i a t e d h y d r a u l i c c o n d u c t i v i t y of s u r f a c e samples taken from the rye area a f t e r the ponded water had receded was about one o r d e r of magnitude h i g h e r than the c o r r e c t e d r e c e s s i o n r a t e , but the s a t i a t e d h y d r a u l i c c o n d u c t i v i t y of the t i l l a g e pan l o c a t e d between 0.15 - 0.25 m depth, corresponded c l o s e l y t o the c o r r e c t e d r e c e s s i o n r a t e o f the ponded water ( F i g . 3.8). T h i s i m p l i e d t h a t i n the rye area the r e c e s s i o n r a t e of the ponded water was b e i n g c o n t r o l l e d by the pan w i t h i n the p r o f i l e . The water was perched on top of the t i l l a g e pan. The rye t h a t was k i l l e d as a r e s u l t of ponding served as a mulch d e c r e a s i n g s u r f a c e 8* 20 c X) I 10 0 10 Time (Hr.) 15 20 F i g . 3.4. Recession of ponded surface on harvested onion f i e l d i n Spring, 1982. Slope = I n f i l t r a t i o n flux ( q ^ + evaporation flux (q ) Slope = 14.6 mm day or 1.7 x 10 7 m s - 1 q e = 3.7 mm or 4.3 x 10 8 m s ~ l day qL = 1.7 x 10~ 7 m s" 1 - 4.3 x 10" 8 m s" 1 = 1.3 x 10" 7 m s" 1 72 e 30T E Q . •S 20 c • r - l I 10 0 10 1 20 30 40 Time (Hours) 50 60 70 Fig. 3.5. Recession of ponded surface i n Spring, 1983 on harvested onion f i e l d seeded to winter rye. Slope Slope = i n f i l t r a t i o n flux ( q ^ + evaporation flux (q g) 12.8 mm or 1.5 x 10 7 m day s or 1.5 x 10"^ m and = 1.3 mm day -7 H 1.5 x 10 ' s ,-7 m - 1.5 x 10 ° s = 1.4 x 10 73 s e a l i n g . The l o w - i n f i l t r a b i l i t y s u r f a c e l a y e r ( s e a l ) was formed i n d e p r e s s i o n a l areas where f r e e water had c o l l e c t e d . The o r i g i n of the f r e e water was a t t r i b u t e d t o f l o o d i n g and ponding. These farmlands are l o w - l y i n g ( f i g . 1.1) and f l o o d i n g i s caused by u n c o n t r o l l e d o v e r l a n d f l o w from the u p l a n d , urban c e n t r e s . Once the l a n d i s f l o o d e d , ponding f o l l o w s a u t o m a t i c a l l y because the t i l l a g e pan r e s t r i c t s movement of water i n the p r o f i l e . With f l o o d i n g , a low i n f i l t r a b i l i t y s u r f a c e l a y e r c o n s i s t i n g of t h i n l a y e r s o f m i n e r a l and o r g a n i c p a r t i c l e s or r y t h m i t e s (de V r i e s , 1983) forms i n the d e p r e s s i o n a l a r e a s . The sediment o r i g i n a t e s from two s o u r c e s : One source i s e r o s i o n induced by o v e r l a n d f l o w on a c o n c e n t r i c area i n the immediate v i c i n i t y of the pond; the o t h e r source of the sediment i s e r o s i o n o f the pond bottom induced by w i n d - d r i v e n wave a c t i o n (de V r i e s , 1983). When the wind s u b s i d e s , the sediment s e t t l e s to the bottom of the pond to form a t h i n l a y e r of low i n f i l t r a b i 1 i t y . Thus d u r i n g the w i n t e r and the e a r l y s p r i n g r a i n s , a low i n f i l t r a b i 1 i t y s u r f a c e l a y e r of t h i c k n e s s r a n g i n g between 1 cm and 2 cm may form. The low i n f i l t r a b i 1 i t y s u r f a c e l a y e r c o n s i s t s of a number of r y t h m i t e s , each r y t h m i t e c o r r e s p o n d i n g to a r a i n f a l l - w i n d event. The ash c o n t e n t of 1982 r y t h m i t e s formed on Vinod s o i l (Van H a l s t Farm) averaged 48 percent ( c . v . - 8. 370, n = 8 ) . Once the low i n f i l t r a b i l i t y s u r f a c e l a y e r has formed, any r a i n f a l l r a t e 74 in excess of the i n f i l t r a t i o n rate of the low i n f i l t r a b i l i t y surface layer (seal) causes water to pond on the s o i l . The ultimate legacy of the flooding-ponding event i s fewer opportunity days to the farmer during the c r i t i c a l spring cultivation-planting period. 3.5 Hydrologic Characteristics 3.5.1 Water retention The water retention c h a r a c t e r i s t i c curves for the water potentials between zero and -150 m of water for Vinod s o i l (Van Halst Farm) are presented i n F i g . 3.6. The p a r t i a l water retention curves for the water potentials between zero and -0.9 m of water are presented i n F i g . 3.7. Fig. 3.6 shows that the top organic layer (0 - 0.15 m) retains 63 percent water by volume at saturation, but about 8 percent i s released at suctions between 0 and 0.9 m of water. The available water storage capacity for this layer i s about 25 percent. The 0.15 - 0.25 m organic layer ( t i l l a g e pan) retains more than 80 percent water by volume at saturation, and about 14 percent i s released i n response to a suction increase to 0.9 m of water. The available water storage capacity for th i s layer i s 29 percent. Of 89 percent water by volume retained by the organic subsoil (0.25 - 0.40 m) at saturation, 10 percent i s released i n response to a suction increase to 0.9 m of water. The available water storage capacity for this layer i s 24 percent. The s o i l of the tr a n s i t i o n layer (0.40 - 0.55 m) retains about 70 percent water by volume at saturation, and only 5 percent i s released i n response to a 75 • S o i l Depth S o i l water p o t e n t i a l (m o f water) F i g . 3.6. Water r e t e n t i o n c h a r a c t e r i s t i c s o f Vinod s o i l . 76 S o i l Depth 0 0 - 15 cm (organic) A 15 - 25 cm (organic) A 25 - 40 cm (organic) • 40 - 55 cm ( t r a n s i t i o n layer) 77 s u c t i o n i n c r e a s e t o 0.9 in of water. The water s t o r a g e c a p a c i t y f o r the t r a n s i t i o n l a y e r i s around 35 p e r c e n t . The shape of a p a r t i a l water r e t e n t i o n curve i n d i c a t e s the water r e l e a s e p o t e n t i a l and the n a t u r e of the s o i l p o r e s . T h e x p a r t i a l water r e t e n t i o n c u r v e s of the top o r g a n i c s o i l (0 - 0.15 m) and the s o i l of the t r a n s i t i o n l a y e r a r e r e l a t i v e l y f l a t up to a s u c t i o n of 0.9 m of water. The f l a t n a t u r e of the p a r t i a l water r e t e n t i o n c u r v e s i n d i c a t e s reduced pore s i z e s and a c o r r e s p o n d i n g tendency t o r e t a i n water. I n t e n s i v e use of r o t o t i l l e r s on the top o r g a n i c l a y e r has caused a breakdown of the s o i l s t r u c t u r e and the f o r m a t i o n of s m a l l e r pores h o l d i n g the water back from b e i n g d r a i n e d . The t i l l a g e pan i s n o r m a l l y u n s a t u r a t e d a t f i e l d c o n d i t i o n (except under f l o o d e d c o n d i t i o n ) . When the pan i s i n t e n t i o n a l l y wetted i n the l a b o r a t o r y i t s w e l l s . The t i l l a g e pan t h e r e f o r e r e l e a s e s more water i n the low s u c t i o n range (0 to 0.9 m of water) than the o t h e r l a y e r s because of s w e l l i n g . The pan i s s u b j e c t t o low t e n s i o n c r a c k i n g . The t r a n s i t i o n l a y e r f a i l s to r e l e a s e water a t lower s u c t i o n s because o f the f i n e n a t u r e of i t s p a r t i c l e s ( s i l t y c l a y ) and c o r r e s p o n d i n g s m a l l pore s i z e s . The s m a l l e r pores of the o r g a n i c top s o i l i s a common f e a t u r e of these i n t e n s i v e l y c u l t i v a t e d h u misols i n f l u e n c i n g the d r a i n a g e e f f i c i e n c y . D u r i n g the growing season i t i s common t o observe a s a t u r a t e d (muddy) top s o i l r e s t i n g on top o f an u n s a t u r a t e d 78 l a y e r ( t i l l a g e pan). 3.5.2 S a t i a t e d h y d r a u l i c c o n d u c t i v i t y The s a t i a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s measured on Vinod s o i l (Van Halst.Farm) are p r e s e n t e d s c h e m a t i c a l l y i n F i g . 3.8. (The data are p r e s e n t e d i n Appendix I I . ) The r e s u l t s r e p r e s e n t o r g a n i c top s o i l w i t h s u r f a c e - s e a l i n g e f f e c t s ; o r g a n i c top s o i l w i t h the s u r f a c e s e a l removed; o r g a n i c t o p s o . i l from an area which had been seeded to w i n t e r r y e , but k i l l e d due to ponding; o r g a n i c t i l l a g e pan; o r g a n i c s u b s o i l ; and the t r a n s i t i o n l a y e r (0.40 - 0.55 m). Comparison of the d i f f e r e n c e s between the means of s a t i a t e d h y d r a u l i c c o n d u c t i v i t y u s i n g t - d i s t r i b u t i o n or the Mann-Whitney U - t e s t are p r e s e n t d i n T a b l e s 3.5 and 3.6, r e s p e c t i v e l y . (See a l s o Appendix I f o r i n f o r m a t i o n r e g a r d i n g the p a r a m e t r i c t - t e s t and the non - p a r a m e t r i c Mann-lJh 1 tney U - t e s t . ) Both tests' r e v e a l e d s i m i l a r t r e n d s of s i g n i f i c a n c e except f o r two cases where U - t e s t gave h i g h e r l e v e l s of s i g n i f i c a n c e than the t - t e s t . The r e s u l t s i n d i c a t e t h a t s a t i a t e d h y d r a u l i c c o n d u c t i v i t y i n the o r g a n i c l a y e r tended t o decrease w i t h s u r f a c e s e a l i n g and the presence o f a t i l l a g e pan. The d i f f e r e n c e between the c o n d u c t i v i t y of s e a l e d s u r f a c e and u n s e a l e d s u r f a c e was h i g h l y s i g n i f i c a n t . The d i f f e r e n c e i n c o n d u c t i v i t y between the t i l l a g e pan and the s e a l e d s u r f a c e was not 79 METHOD BS BS BS BS BS BS CORE TREATMENT SSp SR RPR TP OS SSO TL n 13 13 7 11 9 5 6 c.v. 0.93 0.39 0.70 0.63 0.52 0.49 0.59 -1— Range A Mean n =Number of measurements c.v. C o e f f i c i e n t of var i a t i o n SSp =Sealed surface on potato f i e l d SR =Surface seal removed (0-15 cm) RPR =Receded Ponded area under winter rye cover(0-15 cm) TP =Tillage pan (15-25 cm) OS =0rganic subsoil (25-40 cm) SSO =Sealed surface on onion f i e l d TL =Transition layer (40-55 cm) BS =Block samples Fig: 3.8. "Ks" of Vinod S o i l (Van Halst Farm). 80 Table 3.5 L e v e l s of s i g n i f i c a n c e between means of "K " u s i n g t - d i s t r i b u t i o n . ~ V a r i a b l e s d.f. L e v e l of _t S i g n i f i c a n c e L o c a t i o n S e a l e d s u r f a c e f o l l o w i n g growing o f p o t a t o e s v s . un s e a l e d s u r f a c e 24 Ponded and receded s i t e under w i n t e r rye v s . un s e a l e d s u r f a c e 12 15 3. Ponded and receded s i t e under w i n t e r rye v s . t i l l a g e pan 4. Ponded and receded s i t e under w i n t e r rye v s . s e a l e d s u r f a c e 5. T i l l a g e pan v s . s e a l e d s u r f a c e ( p o t a t o f i e l d ) 6. C u l t i v a t e d v s . u n c u l t i v a t e d s u r f a c e s o i l (0 - 0.15 m) 5 7. C u l t i v a t e d v s . u n c u l t i v a t e d " p l o u g h depth"(0.15 -' 0.25 m) 9 8. C u l t i v a t e d g e n e t i c pan v s . g e n e t i c pan under gras s p a s t u r e 8 9. S e a l e d s u r f a c e f o l l o w i n g growing o f o n i o n s v s . s e a l e d s u r f a c e f o l l o w i n g growing o f p o t a t o e s 12 4.549 0.473 2.628 2.300 0.419 8.902 12.566 3.847 3.466 ns s~ S " ns s~"-' Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm S t a t e r Farm S t a t e r Farm C l o v e r d a l e Produce Farm Van H a l s t Farm d.f . = computed degree of freedom s* = s i g n i f i c a n t a t 5% l e v e l , s** = s i g n i f i c a n t at < 1% l e v e l , ns = not s i g n i f i c a n t 81 Table 3.6 L e v e l o f s i g n i f i c a n c e between means o f "K " usi n -Mann-Whitney U t e s t . V a r i a b l e s S e a l e d s u r f a c e f o l l o w i n g growing o f onions vs., un-s e a l e d s u r f a c e Ponded and receded s i t e under w i n t e r rye v s . u n s e a l e d s u r f a c e . Ponded and receded s i t e under w i n t e r rye v s . t i l l a g e pan Ponded and receded s i t e under w i n t e r r y e v s . s e a l e d s u r f a c e T i l l a g e pan v s . s e a l e d s u r f a c e ( p o t a t o f i e l d ) C u l t i v a t e d v s . u n c u l t i v a t e d s u r f a c e s o i l C u l t i v a t e d v s . u n c u l t i v a t e d plough depth C u l t i v a t e d g e n e t i c pan v s . g e n e t i c pan under gra s s p a s t u r e L e v e l o f U S i g n i f i c a n c e L o c a t i o n S e a l e d s u r f a c e f o l l o w i n g growing o f onions v s . s e a l e d s u r f a c e f o l l o w i n g growing p o t a t o e s . 21 19 4 8 55 0 0 0 0 ns s-c^-ns s" '•' Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm Van H a l s t Farm S t a t e r Farm S t a t e r Farm C l o v e r d a l e Produce Farm Van H a l s t Farm 82 s i g n i f i c a n t , i n d i c a t i n g the d e t r i m e n t a l e f f e c t o f both s u r f a c e s e a l i n g and t i l l a g e pan on water movement i n the s o i l . The s h e a r i n g of the s o i l i n the p r o f i l e due to e x c e s s i v e use of r o t o t i l l e r s w i t h f l a t f l a i l s c o u l d have c o n t r i b u t e d t o the f o r m a t i o n of the t i l l a g e pan ( L a r s o n and Osborne, 1982). The mechanism of f o r m a t i o n of the s u r f a c e s e a l i n d e p r e s s i o n a l areas has a l r e a d y been d i s c u s s e d i n d e t a i l i n s e c t i o n 3.4. The d i f f e r e n c e i n c o n d u c t i v i t y between o r g a n i c t o p s o i l w i t h the s u r f a c e s e a l removed and o r g a n i c t o p s o i l from a receded, ponded area t h a t was seeded to w i n t e r rye but k i l l e d as a r e s u l t of ponding, was not s i g n i f i c a n t , i n d i c a t i n g t h a t growing w i n t e r rye as a c o v e r crop was e f f e c t i v e i n e l i m i n a t i n g the s u r f a c e s e a l i n g e f f e c t s . I t was t h e r e f o r e i n f e r r e d t h a t ponding t h a t o c c u r r e d under the w i n t e r rye c o v e r was due to the t i l l a g e pan, s i n c e s u r f a c e s e a l i n g was not e f f e c t i v e . The dead rye ser v e d as a mulch d i s r u p t i n g a c o n t i n u o u s f o r m a t i o n of a s u r f a c e s e a l . The mean s a t i a t e d h y d r a u l i c c o n d u c t i v i t y o f a " s u r f a c e s e a l " formed i n 1982 f o l l o w i n g c u l t i v a t i o n of on i o n s was s i g n i f i c a n t l y lower than the mean s a t i a t e d h y d r a u l i c c o n d u c t i v i t y o f a " s u r f a c e s e a l " formed i n 1983 ( a t the same s i t e ) when the s o i l was cropped t o p o t a t o e s ( F i g . 3.8). Van H a l s t ( the farmer) has a l s o n o t i c e d t h a t ponding on these s o i l s i s more pronounced f o l l o w i n g grdwing o f '.onions than when pota t o e s a re grown ( p e r s o n a l communication)'. L i k e the dead r y e , the r e s i d u e s of the h a r v e s t e d p o t a t o p l a n t s 83 s e r v e as a mulch d i s r u p t i n g t h e . f o r m a t i o n of a c o n t i n u o u s s u r f a c e s e a l . The t r a n s i t i o n l a y e r has a mean s a t i a t e d -9 -1 h y d r a u l i c c o n d u c t i v i t y v a l u e o f 1.9 x 10 m s ( c . v . =59%, n = 6 ) . The t r a n s i t i o n l a y e r i s f i n e t e x t u r e d and i s composed of diatomaceous e a r t h (Van V l i e t and Wood, 1982). These f e a t u r e s of the t r a n s i t i o n l a y e r are r e s p o n s i b l e i n p a r t f o r the low s a t i a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s . However, sample d i s t u r b a n c e c o u l d not be r u l e d o u t , s i n c e t h i s l a y e r was sampled by j u s t pushing i n the c y l i n d r i c a l cans, because of the s o f t n a t u r e of the s o i l . I n s p i t e of the low s a t i a t e d h y d r a u l i c c o n d u c t i v i t y o f the t r a n s i t i o n l a y e r , the performance of the s u b s u r f a c e d r a i n s i n s t a l l e d i n the Vinod s o i l (Van H a l s t Farm) appeared s a t i s f a c t o r y . Water t a b l e r e c e s s i o n r a t e s measured i n p i e z o m e t e r s i n s t a l l e d i n Vinod s o i l (Van H a l s t Farm) a r e p r e s e n t e d i n T able 3.7. L u t t m e r d i n g (1931) has r e p o r t e d on the abundance of v e r t i c a l remains of o l d r o o t s i n the s u b s o i l , and the o c c a s i o n a l presence of v e r t i c a l c r a c k s i n Vinod s o i l s ( F i g . 1.2). L u t t m e r d i n g ' s (1981) r e p o r t a l s o i n d i c a t e s an o c c a s i o n a l presence of b l u i s h t o g r e e n i s h - g r a y sand below 1 m. However, i n t h i s study the depth of sampling was never extended below 0.60 m from the s u r f a c e . I t i s r e a s o n a b l e to assume t h a t water moves l a t e r a l l y a c r o s s the l a y e r s below the pan i n t o these v e r t i c a l Table 3.7. Water t a b l e r e c e s s i o n i n h a r v e s t e d p o t a t o f i e l d i n s p r i n g 1983 (March 17 - 27) (Van H a l s t Farm) 84 Piezometer Test S e c t i o n (cm) I n i t i a l Depth of WT from the S u r f a c e (cm) F i n a l Depth of WT from the S u r f a c e (cm) WT Reces s i o n Rate (m s ) 1 2 3 4 48.0 40.0 42.0 47.0 6.5 11.5 30.5 25.0 39.0 d r y 39.5 42.5 5.3 x 10" 1.5 x 10" 2.9 x 10 -7 Mean of 3 Piezometers c o e f f i c i e n t of v a r i a t i o n 0.60 3.2 x 10 -7 WT - Water t a b l e 85 c r a c k s on i t s way t o the d r a i n s . The o c c a s i o n a l presence o f sand i n the deeper l a y e r s c o u l d a l s o h e l p i n e f f i c i e n t sub-s u r f a c e d r a i n a g e of these s o i l s . The s a t i a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s of samples taken from the Richmond s o i l ( S t a t e r Farm) are pr e s e n t e d i n F i g . 3.9. (The d a t a used i n p l o t t i n g F i g . 3.9 are shown i n Appendix I I I . ) Two types of pan were d i s t i n g u i s h e d : a t i l l a g e pan o c c u r r i n g between 0.15 - 0.25 m s o i l depth and g e n e t i c pan (Van V l i e t and Wood, 1982) o c c u r r i n g between 0.25 - 0.45 m s o i l d e pth. These two l a y e r s had the lowest v a l u e s of s a t i a t e d h y d r a u l i c c o n d u c t i v i t y . Mann-Whitney U - t e s t i n d i c a t e d t h a t the d i f f e r e n c e between the c o n d u c t i v i t i e s of the two pans was s i g n i f i c a n t at o n l y 5 p e r c e n t p r o b a b i l i t y l e v e l . Samples were taken from an a r e a c l o s e to the main d r a i n a g e channel t h a t had not been c u l t i v a t e d f o r a t i m e ( p e r s o n a l communication from permanent farm w o r k e r ) . Comparison of the c o n d u c t i v i t y of the c u l t i v a t e d c a r r o t f i e l d and the area c l o s e to the d r a i n a g e channel i n d i c a t e d s i g n i f i c a n t d i f f e r e n c e between t h e i r means (Tables 3.5 and 3.6). The s a t i a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s f o r a c u l t i v a t e d g e n e t i c pan and a s i m i l a r g e n e t i c pan under permanent g r a s s p a s t u r e on Richmond s o i l ( C l o v e r d a l e Produce Farm) are a l s o shown s c h e m a t i c a l l y i n F i g . 3.9. Samples taken from l o c a t i o n s c l o s e t o the fence p o s t s , where t h e r e 86 Method Treatment n c.v. BS BS BS UCSS CSS UCPS 6 7 10 0.27 0.39 0.22 BS BS BS BS BS BS CPS GPCS OSS GRCP GPPC GPCC 15 6 7 6 8 11 0.51 0.56 0.27 0.29 0.54 0.79 LEGEND Range A Mean n =Number of measurements c.v. C o e f f i c i e n t of v a r i a t i o n BS =Block sample UCSS U n c u l t i v a t e d 0-15 cm l a y e r , S t a t e r Farm CSS C u l t i v a t e d 0-15 cm l a y e r S t a t e r Farm UCPS U n c u l t i v a t e d 15-25 cm l a y e r S t a t e r Farm CPS C u l t i v a t e d 15-25 cm l a y e r ( t i l l a g e pan) S t a t e r Far GPCS C u l t i v a t e d g e n e t i c pan (25-45 cm), S t a t e r Farm OSS C r g a n i c s u b s o i l (45-60 cm), S t a t e r Farm GRCP =0rganic s o i l under grass pasture (10-25 cm), Clov e r d a l e Produce Farm GPPC C e n e t i c pan under grass pasture (25-40 cm), Clov e r d a l e Produce Farm GPCC C e n e t i c pan under c u l t i v a t i o n (25-40 cm), Clo v e r d a l e Produce Farm F i g . 3.9. "Ks" of Richmond s o i l s ( S t a t e r Farm and Cl o v e r d a l e Produce Farm). 87 had been no t r a m p l i n g by c a t t l e were about one o r d e r o f magnitude more c o n d u c t i v e t o water than the samples t a k e n from the c u l t i v a t e d f i e l d about 3 m away from the fence p o s t s . The d i f f e r e n c e between the two c o n d u c t i v i t i e s was h i g h l y s i g n i f i c a n t ( T a b l e s 3.5 and 3.6). The g e n e t i c a l l y developed o r g a n i c pan t h a t was observed to e x i s t i n Richmond s o i l ( C l o v e r d a l e Produce Farm) between 0.25 and 0.45 m of s o i l depth was v e r y hard and d i f f i c u l t t o cut w i t h a k n i f e . T h i s pan d i d not c o v e r the e n t i r e f i e l d but was p r e s e n t as p a t c h e s , w i t h d i s c o n t i n u i t i e s . In the o r g a n i c s o i l s of Vancouver I s l a n d Maas (1972) r e p o r t e d the o c c u r r e n c e of r e s t r i c t i v e l a y e r s j u s t below the depth of c u l t i v a t i o n . He a t t r i b u t e d the f o r m a t i o n of the r e s t r i c t i v e l a y e r s e i t h e r to the movement of o r g a n i c c o l l o i d s i n t o t h a t zone from above or to the c r e a t i o n of a t i l l a g e pan due to compaction by t i l l a g e machinery. The p e d o l o g i c a l p r o c e s s l e a d i n g to the f o r m a t i o n of the h a r d , g e n e t i c o r g a n i c pan i s not c l e a r (Wood, p e r s o n a l communication). S i n c e t h i s pan o c c u r s i n both c u l t i v a t e d and u n c u l t i v a t e d a r e a s , i t s f o r m a t i o n may be a s s o c i a t e d w i t h the movement of o r g a n i c c o l l o i d from the l a y e r above (Maas, 1972). T y p i c a l l y , water movement tends t o be r a p i d i n undecomposed peat, but tends t o decrease w i t h i n c r e a s e d 88 d e c o m p o s i t i o n ( B o e l t e r , 1974). The tendency towards low h y d r a u l i c c o n d u c t i v i t y i s aggravated w i t h c u l t i v a t i o n and h a r v e s t i n g when the s o i l i s not t r a f f i c a b l e . Compaction due to the passage of heavy farm machinery and s h e a r i n g of the s o i l w i t h i n the c u l t i v a t i o n l a y e r as a r e s u l t of e x c e s s i v e use of r o t o t i l l e r s w i t h f l a t f l a i l s c o n t r i b u t e i n no s m a l l way to the s t r u c t u r a l d e t e r i o r a t i o n and the f o r m a t i o n of a c u l t i v a t i o n pan i n these o r g a n i c s o i l s . Soehne (1958) has d i s t i n g u i s h e d , i n t h e o r y , t h r e e d i f f e r e n t k i n d s of s o i l d e f o r m a t i o n due to a p p l i e d e x t e r n a l f o r c e s as e l a s t i c s o i l d e f o r m a t i o n , s o i l compaction, and s o i l d e f o r m a t i o n by p l a s t i c f l o w . In p r a c t i c e , these t h r e e k i n d s of s o i l d e f o r m a t i o n may o c c u r c o n c u r r e n t l y . The e f f e c t s o f s o i l compaction are the r e d u c t i o n o f pore s i z e and the i n c r e a s e i n b u l k d e n s i t y . P l a s t i c f l o w o c c u r s i f the shear s t r e s s e s i n the s o i l exceed y i e l d s t r e n g t h . These d e f o r m a t i o n p r o c e s s e s depend, to a g r e a t e r degree, on the water c o n t e n t of the s o i l . At h i g h water c o n t e n t when the s o i l i s not t r a f f i c a b l e , s l i p p a g e of the t i r e of the t i l l a g e implement may produce s o i l smearing and p l a s t i c f l o w . C a l d w e l l and R i c h a r d s o n (1975) have r e p o r t e d t h a t i n o r g a n i c s o i l s which r e q u i r e a r t i f i c i a l d r a i n a g e , s o i l s t r u c t u r a l d e g r a d a t i o n can oc c u r when crops a r e h a r v e s t e d or the l a n d c u l t i v a t e d i n wet c o n d i t i o n s . In the permanent p a s t u r e f i e l d w i t h no t r a m p l i n g by c a t t l e , p r o l i f i c g r a s s r o o t s i n the s o i l c r e a t e 89 channels f o r the passage o f water, r e s u l t i n g i n r e l a t i v e l y h i g h e r h y d r a u l i c c o n d u c t i v i t y o f the g e n e t i c pan. Even though f l o o d i n g c o u l d be b e n e f i c i a l i n o r g a n i c s o i l s i n terms of r e d u c i n g b i o l o g i c a l o x i d a t i o n of o r g a n i c m a t t e r ( T e r r y and TaCe, 1980), temperature has been observed t o be of paramount importance d u r i n g b i o l o g i c a l o x i d a t i o n of o r g a n i c m a t t e r (Wildung e t a l , 1975). However, c e r t a i n enzymes t h a t r e a c t i v a t e d e c o m p o s i t i o n p r o c e s s i n o r g a n i c s o i l s have been noted t o be a c t i v e a t a temperature as low as -20°C (Mathur, p e r s o n a l communication). A l s o a n a e r o b i c c o n d i t i o n s reduce the i n c i d e n c e of d i s e a s e s due t o a e r o b i c s o i l - b o r n e pathogens ( S t o l z y et a l , 1963; Louvet, 1970). However, i n f o r m a t i o n on the e f f e c t o f f l o o d i n g on the dormancy of the spores of the s e d i s e a s e c a u s i n g organisms i n s o i l s i s s c a r c e . The i n c i d e n c e of "white r o t " d i s e a s e of oni o n s (a f u n g a l d i s e a s e ) i s s t i l l p r e v a l e n t i n these o r g a n i c s o i l s i n s p i t e of f r e q u e n t f l o o d i n g ( p e r s o n a l d i s c u s s i o n w i t h Van H a l s t , the f a r m e r ) . The pre s e n t study r e v e a l s t h a t f l o o d i n g d e s t r o y s the s t r u c t u r e o f these o r g a n i c s o i l s i n terms of reduced pore s i z e s and s a t i a t e d h y d r a u l i c c o n d u c t i v i t y . There i s a need f o r i n t e g r a t e d and i n t e r - d i s c i p l i n a r y r e s e a r c h i n t o the c r i t i c a l f l o o d i n g p e r i o d r e q u i r e d t o e l i m i n a t e crop v o l u n t e e r s , m i n i m i z e the i n c i d e n c e o f s o i l - b o r n e d i s e a s e s and s u b s i d e n c e , and at the same time p r o v i d e good s o i l s t r u c t u r a l c h a r a c t e r i s t i c s . 3.6 S i m u l a t i o n o f the process of f o r m a t i o n of a l a y e r o f low i n f i l t r a b i l i t y due to s e t t l i n g of o r g a n i c and m i n e r a l p a r t i c l e s on ponding  The s a t i a t e d h y d r a u l i c c o n d u c t i v i t y v a l u e s o f a 90 " s u r f a c e s e a l " c o n s i s t i n g o f r y t h m i t e s o f o r g a n i c and m i n e r a l s o i l p a r t i c l e s formed on p o s t - h a r v e s t e d o n i o n f i e l d i n the s p r i n g of 1982 are p r e s e n t e d i n F i g . 3.8. The s a t i a t e d h y d r a u l i c —8 —1 —8 c o n d u c t i v i t y v a l u e s range between 2.9 x 10~ m s~ and 9.2 x 10 —1 —8 —1 m s w i t h an average v a l u e o f 5.8 x 10 m s ( c . v . = 497,, n = 5). A l a y e r o f low i n f i l t r a b i l i t y s i m u l a t e d i n the l a b o r a t o r y u s i n g the " s u r f a c e c r u s t " m a t e r i a l from the f i e l d , had s a t u r a t e d —8 —1 h y d r a u l i c c o n d u c t i v i t y v a l u e s r a n g i n g between 9.8 x 10 m s and 3.4 x 10~ 7 m s -^", w i t h an average v a l u e o f 1.9 x 10~ 7 m ( c . v . = 957o, n = 4) . The average v a l u e o f s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y f o r the s i m u l a t e d low i n f i l t r a b i l i t y s u r f a c e l a y e r was about 3 times h i g h e r than t h a t formed n a t u r a l l y i n the f i e l d . P o s s i b l y the h i g h e r s i m u l a t e d v a l u e c o u l d have been caused by leakage o f water through the i n t e r f a c e s e p a r a t i n g the s u r f a c e s e a l and the w a l l o f the a c r y l i c c y l i n d e r . . I t was observed t h a t as more s u s p e n s i o n was added the h y d r a u l i c c o n d u c t i v i t y o f the s i m u l a t e d l a y e r decreased c o n s i s t e n t l y , a f i n d i n g which was not e x p e c t e d . I t was p o s s i b l e t h a t v e r y f i n e o r g a n i c c o l l o i d s might have moved down i n t o the sand column below d u r i n g each a d d i t i o n of the s u s p e n s i o n . A i r entrapment c o u l d a l s o have been an im p o r t a n t c o n t r i b u t i n g f a c t o r l e a d i n g t o the c o n s t a n t l y d e c r e a s i n g h y d r a u l i c c o n d u c t i v i t y w i t h more a d d i t i o n of s u s p e n s i o n (Bouwer, 1966). One i n t e r e s t i n g o b s e r v a t i o n was t h a t the m i n e r a l s o i l f r a c t i o n i n the added s u s p e n s i o n s e t t l e d f i r s t d u r i n g each a d d i t i o n . Thus a s e r i e s o f d i f f e r e n t l a y e r s f o l l o w i n g each a d d i t i o n o f the s u s p e n s i o n c o u l d be i d e n t i f i e d . T h i s o b s e r v a t i o n b r i e f l y shows the mechanism of f o r m a t i o n of low 91 i n f i l t r a b i l i t y l a y e r due to s e t t l i n g o f o r g a n i c and m i n e r a l s o i l p a r t i c l e s on ponded h u m i s o l s as a r e s u l t of e r o s i o n of s o i l p a r t i c l e s i n the immediate v i c i n i t y of the pond and the bottom of the pond due to wind-induced wave a c t i o n . 3.7 C h a r a c t e r i z a t i o n o f S t r u c t u r a l S t a b i l i t y o f Organic S o i l s 3.7.1 The r e l a t i v e h y d r a u l i c c o n d u c t i v i t y approach The r e l a t i v e h y d r a u l i c c o n d u c t i v i t y c a l c u l a t e d as the r a t i o of s a t i a t e d h y d r a u l i c c o n d u c t i v i t y w i t h o u t d i s p e r s i o n to t h a t w i t h t e n minutes' d i s p e r s i o n (SI-^Q) was used as an index to c h a r a c t e r i z e the s t r u c t u r a l s t a b i l i t y o f the o r g a n i c s o i l s s t u d i e d . The r a t i o might be expected to i n c r e a s e w i t h a decrease i n s t r u c t u r a l s t a b i l i t y . Thus t h i s i n d e x i s e s s e n t i a l l y a measure of s o i l s t r u c t u r a l i n s t a b i l i t y . The data f o r SI-^Q and the c o r r e s p o n d i n g ash c o n t e n t s f o r the Vinod s o i l (V^n H a l s t Farm) and Richmond s o i l ( S t a t e r Farm) are presented i n T a b l e 3.8. The r e l a t i o n s h i p between S I ^ and ash content i s p r e s e n t e d i n F i g . 3 . L 0 . The c o r r e l a t i o n c o e f f i c i e n t between SI-^Q and ash content was p o s i t i v e and s i g n i f i c a n t at 1 p e r c e n t p r o b a b i l i t y l e v e l ( r = 0 . 8 6 ) . The r e g r e s s i o n e q u a t i o n des-c r i b i n g SI-^Q and percentage ash c o n t e n t was o b t a i n e d as: S I ^ Q = -33.9 + 4.3A, where A i s the percentage ash c o n t e n t . Using the t - s t a t i s t i c ( t = r t/ n-2/ J 1 - r 2 ) , the r e g r e s s i o n of • S I , n on ash content was h i g h l y s i g n i f i c a n t . Table 3.8 V a r i a t i o n of S t r u c t u r a l S t a b i l i t y Index ( S I 1 Q ) w i t h Ash c o n t e n t Sampling L o c a t i o n S I i n Ash Content Depth ( 7 o of T o t a l (cm) 0 - 1 0 Van H a l s t 150 52 10 - 20 Van H a l s t 190 46 20 - 30 Van H a l s t 36 24 30 - 40 Van H a l s t 18 15 0 - 1 0 S t a t e r 149 31 10 - 20 S t a t e r 71 25 20 - 30 S t a t e r 24 23 30 - 40 S t a t e r 110 21 40 - 50 S t a t e r 13 15 50 - 60 S t a t e r 10 10 '93 o r - l I—I Ash Content (%) i s . 3.10. Relationship between Structural i n s t a b i l i t y ( S I i n ) of argantc s o i l s and their ash content. 94 In terms of mathematical functions, the correlation c o e f f i c i e n t between two variables i s a measure of how close the function i s to l i n e a r i t y . However, i t i s possible to have two variables perfectly related i n a nonlinear fashion. Hence the need for regression analysis. B r i e f l y , regression i s the amount of change i n one variable associated with a unit change i n the other variable ( L i t t l e and H i l l s , 1972). Thus i n regression one i s interested i n predicting the value of one of the random variables, given a value of the other variable. In short, correlation refers to the fact that two variables are related and how close i s this relationship, while regression, on the other hand, refers to the nature of the relationship ( L i t t l e and H i l l s , 1972). These analyses indicate that the reduction i n hydraulic conductivity of organic s o i l s following mechanical dispersion i s closely related to t h e i r ash contents, and that i t i s possible to predict the structural s t a b i l i t y of organic s o i l s from a knowledge of t h e i r ash contents. The ash content of the humisols increases towards the surface, and therefore, the structural s t a b i l i t y of these humisols decreases towards the surface. There i s a paucity of information regarding studies involving structural s t a b i l i t y of organic soils.; Greenland et_ al (1962) found that the wet strength of mineral surface s o i l aggregates as measured by a Na-saturation permeability technique could be reduced by mild oxidation with sodium 95 periodate. The only chemical treatment that semi-specifically attacks polysaccharides i n s o i l i s oxidation by sodium periodate (Emerson, 1977). The r e s u l t s of Greenland et a l (1962) imply tentatively that polysaccharides play a s i g n i f i c a n t role in the s t a b i l i z i n g property of s o i l organic matter i n mineral s o i l material. However, i n organic s o i l s , a considerable proportion of polysaccharides may be ti e d up i n the undecomposed plant materials i n the form of c e l l u l o s e and hemi-cellulose. Therefore, the role that polysaccharides play i n s t a b i l i z i n g organic s o i l s may be d i f f i c u l t to assess. Morita (1983) has recently correlated the r e l a t i v e abundances of arabinose, xylose, mannose, galactose, glucose, t o t a l pentoses, and t o t a l sugars i n 50 s o i l horizon samples of peat p r o f i l e s from the Provinces of Quebec, Ontario, and Manitoba with fibr e content, pyrophosphate index, ash content, C:N r a t i o , and cation exchange capacity. The c o r r e l a t i o n c o e f f i c i e n t s demonstrated that of the monosaccharides or sugars examined, glucose and t o t a l sugar contents correlated best with the f i v e s o i l properties. He, therefore, concluded that sugar analysis could be as r e l i a b l e as the f i v e s o i l properties for assessing the degree of decomposition and d i f f e r e n t i a t i o n of peats. Fibre content and C:N r a t i o decrease with increasing degree of decomposition, while the pyrophosphate index, ash content, and cation exchange capacity increase with increasing degree of decomposition (Morita, 1983). The present study has shown that structural s t a b i l i t y of organic 96 s o i l s i s closely related to their ash contents. Therefore, i t can be inferred that organic s o i l s become less stable with increased degree of decomposition. Excessive c u l t i v a t i o n of the surface s o i l also contributes to the structural degradation of these humisols. Disrupting s o i l aggregates i n a shearing machine may increase microbial a c t i v i t y since the shearing action exposes organic matter previously protected from micro-organisms (Rovira and Greacen, 1957). This explains in part how and why s o i l structure degrades with intensive c u l t i v a t i o n . 3.7.2 Water drop technique An attempt was also made to determine the structural s t a b i l i t y of these organic s o i l s using" the "water drop technique. The results of measurement on four samples are presented i n Table 3.9. The correlation between the amount of k i n e t i c energy required to destroy a 1 g clod and ash content was strongly negative but only s i g n i f i c a n t between 5 and 10 percent probability levels (r = -0.93). The non-significant correlation i s related to the small number of samples. The water drop technique was not e f f e c t i v e for s o i l s with organic matter content greater than 80 percent because of excessively high number of drops required to destroy the clod. Table 3.9 K i n e t i c energy (K.E.) of f a l l i n g water drops as an i n d e x of s t r u c t u r a l s t a b i l i t y 97 Sampling Depth (cm) L o c a t i o n K. E. (Mi H i J o u l e s ) Ash Content ( % ) 10 - 20 20 - 30 30 - 40 Van H a l s t Van H a l s t Van H a l s t 82 + 10.1% 164 + 17.3% 543 + 8.7% 46.0 24.0 15.0 0 - 1 0 S t a t e r 204 + 12.5% 25.0 98 .The water drop t e c h n i q u e f o r d e t e r m i n i n g the s t r u c t u r a l s t a b i l i t y of s o i l was f i r s t used by M c C a l l a (1944). M c C a l l a counted the number of drops of 4 mm d i a m e t e r r e q u i r e d to d r i v e a s i n g l e s o i l a ggregate through a 1 mm s c r e e n , and expressed the r e s u l t s i n terms of k i n e t i c energy of the water drop. Smith and Cernuda (1951) a l s o used a s i m i l a r t e c h n i q u e to determine the s t r u c t u r a l s t a b i l i t y of some m i n e r a l s o i l s of Puerto R i c o , but they n o t i c e d t h a t p r e l i m i n a r y w e t t i n g under reduced t e n s i o n was n e c e s s a r y to g i v e r e l i a b l e r e s u l t s . Moldenhauer and Kemper (1969) have r e p o r t e d on the r e l a t i o n s h i p of s t r u c t u r a l s t a b i l i t y of a m i n e r a l s o i l t o i t s i n f i l t r a t i o n r a t e under a g i v e n r a i n f a l l i n t e n s i t y and drop s i z e . I n f i l t r a t i o n r a t e decreases as s u c c e s s i v e i n c r e m e n t s of water drop energy were a p p l i e d . However the f i n a l i n f i l t r a t i o n r a t e s were not c o r r e l a t e d w i t h aggregate s t a b i l i t y . Bruce-Okine and L a i (1975) have used the water drop t e c h n i q u e to determine the s o i l e r o d i b i l i t y i n d e x o f two N i g e r i a n s o i l s . O r ganic m a t t e r has been observed to p l a y an important r o l e i n the s t a b i l i z a t i o n o f s t r u c t u r e o f m i n e r a l s o i l s i n the t r o p i c s (Monnier 1965; Lugo-Lopez and J u a r e z (1959). Even though the water drop t e c h n i q u e i s not an e f f i c i e n t procedure f o r a s s e s s i n g the s t r u c t u r a l s t a b i l i t y of o r g a n i c s o i l s because of the h i g h number of drops r e q u i r e d to break a c l o d , the t e c h n i q u e can be used to demonstrate t h a t , l i k e m i n e r a l s o i l s , the s t r u c t u r a l s t a b i l i t y of o r g a n i c s o i l s d e c reases w i t h a d ecrease i n o r g a n i c matter c o n t e n t . SUMMARY 99 The study showed t h a t a l a y e r of low h y d r a u l i c c o n d u c t i v i t y , h i g h b u l k d e n s i t y and p e n e t r a t i o n r e s i s t a n c e e x i s t e d i n the 0.15 - 0.30 m depth o f the o r g a n i c l a y e r s and t h a t compaction had a s i g n i f i c a n t e f f e c t on the ease o f water movement i n these s o i l s . E x c e s s i v e c u l t i v a t i o n o f these s o i l s has r e s u l t e d i n the d e g r a d a t i o n o f the s t r u c t u r e of the top s o i l . The r e d u c t i o n i n the s i z e of the f i b r e s and aggregates due t o e x c e s s i v e c u l t i v a t i o n has a l s o r e s u l t e d i n reduced pore s i z e s l i m i t i n g the c a p a c i t y of the s o i l t o r e l e a s e water. T h i s b e h a v i o u r i s shown by the f l a t n a t u r e of the p a r t i a l water r e t e n t i o n curve o f the t o p s o i l . I t i s i n f e r r e d from the p r e s e n t s t u d y t h a t "ponding" on these s o i l s i s d o m i n a n t l y caused by a t i l l a g e pan. A l s o , f l o o d i n g can cause the s t r u c t u r e o f the t o p s o i l t o be f u r t h e r degraded. F i n e o r g a n i c and m i n e r a l s o i l p a r t i c l e s eroded from the immediate v i c i n i t y o f the pond and the bottom o f the pond s e t t l e i n d e p r e s s i o n a l a r e a s t o c r e a t e a s u r f a c e l a y e r o f low i n f i l t r a b i l i t y c o n s i s t i n g of r y t h m i t e s . The s u r f a c e s e a l formed as a r e s u l t of f l o o d i n g has a secondary e f f e c t on ponding. B r e a k i n g the pan through sub-s o i l i n g when t h e p r o f i l e i s r e a s o n a b l y d r y may improve d r a i n a g e i n these s o i l s . The presence o f a g e n e t i c a l l y developed o r g a n i c pan w i t h i n 0.25 - 0.45 m of s o i l depth i n Richmond s o i l c o n t r i b u t e s 100 s i g n i f i c a n t l y t o d r a i n a g e impedance i n these s o i l s . P e d o l o g i c a l f a c t o r s l e a d i n g t o the f o r m a t i o n of the g e n e t i c pan a r e not c e r t a i n (Wood, p e r s o n a l communication). Under permanent gra s s p a s t u r e the h y d r o l o g i c b e h a v i o u r of the g e n e t i c pan i s improved, because of d r a i n a g e channels c r e a t e d by g r a s s r o o t s . The study has a l s o shown t h a t the t r a n s i t i o n l a y e r s e p a r a t i n g the m i n e r a l s u b s o i l from the o r g a n i c l a y e r i s h i g h l y impermeable. The t r a n s i t i o n l a y e r i s b e l i e v e d to be composed of diatomaceous e a r t h (Van V l i e t and Wood, 19S2). In s p i t e of the low h y d r a u l i c c o n d u c t i v i t y of the t r a n s i t i o n l a y e r , s u b s u r f a c e d r a i n a g e appears s a t i s f a c t o r y . (Mean water t a b l e r e c e s s i o n r a t e = 3.2 x 1 0 ~ 7 m s - 1 , c. v. = 0.60, n = 3 ) . In Vinod s o i l L u t t m e r d i n g (19S1) r e p o r t s of an o c c a s i o n a l presence of v e r t i c a l c r a c k s and a l a y e r of sand below 1 m s o i l depth. These f e a t u r e s may c o n t r i b u t e t o the e f f i c i e n t s u b s u r f a c e d r a i n a g e i n these s o i l s . The d i s p e r s i n g and c l o g g i n g b e h a v i o u r o f the o r g a n i c s o i l s was r e l a t e d to t h e i r ash c o n t e n t s . Based on t h i s b e h a v i o u r a r e l a t i v e h y d r a u l i c c o n d u c t i v i t y approach was used to d e v e l o p an i n d e x c a p a b l e of c h a r a c t e r i z i n g the s t r u c t u r a l s t a b i l i t y of the h u m i s o l s . The study showed t h a t the s t r u c t u r a l s t a b i l i t y of the o r g a n i c s o i l s d e c r e a s e d towards the s u r f a c e i n p a r a l l e l w i t h i n c r e a s e i n ash c o n t e n t towards the s u r f a c e . 101 CONCLUSION I t i s deduced from t h i s s tudy t h a t the passage of t r a f f i c and s h e a r i n g the s o i l by r o t o t i l l i n g can c r e a t e a t i l l a g e pan which c o n t r i b u t e s to ponding on o r g a n i c s o i l s . The pan may a l s o o c c u r g e n e t i c a l l y ; however i t s d e l e t e r i o u s e f f e c t on s o i l water movement i s aggravated by c u l t i v a t i o n . F l o o d i n g - p o n d i n g events on o r g a n i c s o i l s are d e t r i m e n t a l to t h e i r good s t r u c t u r a l management. In d e p r e s s i o n a l a r e a s , f i n e o r g a n i c and m i n e r a l s o i l p a r t i c l e s may s e t t l e to the bottom of the pond through e r o s i o n o f the area i n the immediate v i c i n i t y of the pond and the pond bottom to c r e a t e a s u r f a c e l a y e r of low i n f i l t r a b i l i t y . E x c e s s i v e c u l t i v a t i o n can reduce both the p a r t i c l e and pore s i z e s the e f f e c t of which i s r e f l e c t e d i n a f l a t p a r t i a l water r e t e n t i o n c u r v e , i n d i c a t i n g the low a b i l i t y of the s o i l to r e l e a s e water. The d i s p e r s i n g and c l o g g i n g c h a r a c t e r i s t i c s of humisols are r e l a t e d t o t h e i r ash c o n t e n t s . Based on t h i s b e h a v i o u r of humisols the r e l a t i v e h y d r a u l i c c o n d u c t i v i t y approach may be used t o c h a r a c t e r i z e t h e i r s t r u c t u r a l s t a b i l i t y . S t r u c t u r a l s t a b i l i t y o f o r g a n i c s o i l s d e c reases w i t h i n c r e a s e d degree of h u m i f i c a t i o n as r e f l e c t e d i n t h e i r ash c o n t e n t s . T h e r e f o r e , s t r u c t u r a l s t a b i l i t y r a t i n g s i n o r g a n i c s o i l s may be e s t a b l i s h e d u s i n g a knowledge o f t h e i r ash c o n t e n t s . 102 MANAGEMENT RECOMMENDATIONS These o r g a n i c s o i l s s h o u l d be managed such Chat good s o i l s t r u c t u r a l c h a r a c t e r i s t i c s a r e always m a i n t a i n e d . The water t a b l e s h o u l d be c o n t r o l l e d such t h a t i t i s always below the p o t e n t i a l r o o t i n g depth of the crops t o be grown. A l s o , ponding of water on the s o i l s u r f a c e s h o u l d be pr e v e n t e d . Good d r a i n a g e i n these s o i l s w i l l improve s o i l s t r e n g t h and c o n s e q u e n t l y t i m e l y t r a f f i c a b i l i t y and w o r k a b i l i t y of the s o i l . With good d r a i n a g e good s o i l a e r a t i o n and improved r o o t development, a g a i n i n o p p o r t u n i t y days a v a i l a b l e f o r f i e l d o p e r a t i o n s , i n c r e a s e d s o i l temperature t o a s s i s t e a r l y s p r i n g g e r m i n a t i o n , and the o v e r a l l i n c r e a s e d crop y i e l d and q u a l i t y a r e i n s u r e d . To a c h i e v e the f o r e g o i n g o b j e c t i v e s and b e n e f i t s the f o l l o w i n g management recommendations a re suggested: 1. The farmers s h o u l d a l l come t o an agreement to pump t h e i r main d r a i n a g e d i t c h e s the whole of w i n t e r through e a r l y s p r i n g i n o r d e r t o keep the water t a b l e low enough. To a c h i e v e t h i s g o a l r e q u i r e s e f f e c t i v e urban water management. Runoff t h a t r e s u l t s through i n a d e q u a t e urban water c o n t r o l w i l l r e - i n u n d a t e these d r a i n a g e d i t c h e s and cause f l o o d i n g . T h e r e f o r e d i v e r s i o n of o v e r l a n d f l o w from the u p l a n d , urban c e n t r e s away from the l o w - l y i n g farmlands ( F i g . 1.1) 103 i s a p r e r e q u i s i t e f o r e f f e c t i v e water c o n t r o l on these h u m i s o l s . 2. The i n c i p i e n t pan i n these s o i l s s h o u l d be s h a t t e r e d through s u b s o i l i n g i n l a t e summer o r e a r l y f a l l , a f t e r h a r v e s t , when the s o i l p r o f i l e i s s u f f i c i e n t l y d r y . The best r e s u l t s of s u b s o i l i n g are o b t a i n e d when t h e r e are both s h a t t e r i n g and upward l i f t i n g of the b l o c k s o f s o i l . To o b t a i n maximum l i f t i n g and dis p l a c e m e n t o f the t i l l a g e pan, the "shoe" o f the s u b s o i l e r s h o u l d be set 7 - 10 cm below the t i l l a g e pan (Swain, 1975). The pan i s n o r m a l l y b r i t t l e when d r y . T h e r e f o r e , s h a t t e r i n g i s expected to be e f f e c t i v e when tine p r o f i l e i s d r y . Where the n a t u r a l d r a i n a g e of the s o i l below the impeding l a y e r i s good, s u b s o i l i n g a t w i d e r s p a c i n g s d i r e c t l y t o t h e d r a i n s i s recommended as t h i s r e duces t h e aiiiount of energy r e q u i r e d t o p e r f o r m t h e o p e r a t i o n . I n a s i t u a t i o n where s u b s o i l i n g i s used i n c o n j u n c t i o n w i t h an a r t i f i c i a l s u b s u r f a c e d r a i n a g e scheme, s u b s o i l i n g a t 90 degree t o the d i r e c t i o n of the d r a i n s i s recommended as t h i s m i n i m i z e s the d i s t a n c e water has t o move t o the d r a i n s (Swain, 1975). The d e s i g n o f the s u b s o i l e r s h o u l d be such t h a t maximum s h a t t e r i n g and l i f t i n g o f the b l o c k s of s o i l are a c h i e v e d w i t h a minimum compaction o f the s o i l i m m e d i a t e l y around the "shoe" o f the s u b s o i l e r . A l s o the power requirement to draw the s u b s o i l e r s hould be taken i n t o c o n s i d e r a t i o n . For the best r e s u l t s the angle o f the "shoe" of the s u b s o i l e r t o the h o r i z o n t a l s h o u l d be 20 - 25 degree. A s m a l l e r a n g l e than t h i s w i l l c r e a t e i n s u f f i c i e n t f r i c t i o n w i t h the s o i l r e s u l t i n g i n l i t t l e l i f t i n g . S t e e p e r a n g l e s c r e a t e compaction of the s o i l i m mediately around the "shoe" r e s u l t i n g i n the c r e a t i o n of a type of "mole d r a i n s " (Swain, 1975). The s t a n d a r d c a r r y i n g the "shoe" s h o u l d be "V" shaped i n c r o s s s e c t i o n to reduce draught; i t must be s u f f i c i e n t l y s t r o n g and equipped w i t h a "breakback" mechanism i n case of meeting immovable l a y e r s . I t must be s u f f i c i e n t l y l o n g to permit the f r e e upheaval of the s o i l s u r f a c e w i t h o u t f o u l i n g the frame. The use o f a s u b s o i l e r f i t t e d w i t h an o s c i l l a t i n g s t a n d a r d can reduce the power requirement (Swain, 1975). A f t e r b r e a k i n g the pan, f l o o d i n g s h o u l d be e n t i r e l y p r e v e n t e d as f l o o d i n g may n u l l i f y the e f f e c t of the s u b s o i l i n g . F l o o d i n g may be prevented by a d o p t i n g the recommendation as suggested i n ( 1 ) . A l s o , the s o i l may be seeded t o a w i n t e r c o v e r c r o p a f t e r b r e a k i n g the pan. L e a v i n g the l a n d f a l l o w may r e s u l t 105 i n s u r f a c e s e a l i n g as a r e s u l t of w i n t e r - s p r i n g r a i n f a l l . Our s t u d i e s have shown t h a t growing w i n t e r rye as a c o v e r c r o p reduces s u r f a c e s e a l i n g due to e r o s i o n of f i n e o r g a n i c and m i n e r a l s o i l p a r t i c l e s . The co v e r crop may a l s o h e l p i n d r y i n g of the s o i l through i n c r e a s e d e v a p o t r a n s p i r a t i o n i n e a r l y s p r i n g t h e r e b y i m p r o v i n g t i m e l i n e s s f o r f i e l d o p e r a t i o n . 4. C u l t i v a t i n g the s o i l s i n wet c o n d i t i o n can be a major cause of s t r u c t u r a l d e g r a d a t i o n of these s o i l s . D r i v i n g on the wet s o i l w i t h s l i p p i n g wheels causes p u d d l i n g and compaction of the s o i l . T h e r e f o r e , the s o i l s s h o u l d not be c u l t i v a t e d w i t h the h e l p o f 4-wheel d r i v e t r a c t o r s when the s o i l i s too wet and u n t r a f f i c a b l e . Whenever p o s s i b l e , c u l t i v a t i o n s h o u l d be reduced to a minimum. E x c e s s i v e c u l t i v a t i o n can l e a d t o a r e d u c t i o n i n the s i z e s o f f i b r e s and aggregates and c o n s e q u e n t l y s m a l l e r pore s i z e s and low water r e l e a s e c a p a c i t y of the o r g a n i c t o p s o i l . 106 LIST OF REFERENCES Anon, 1981. Management and improvement of o r g a n i c wet-lan d s i n the I n t e r i o r of B r i t i s h Columbia. B. C. 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Techniques f o r the d e t e r m i n a t i o n of the s t a b i l i t y of s o i l a g g r e g a t e s . S o i l S c i . 101: 157-163. 94. Y o s h i d a , T., 1975. M i c r o b i a l metabolism o f f l o o d e d s o i l s . I n : E.A. Paul and A. D. McLaren (eds.) " S o i l B i o c h e m i s t r y " V o l . 3. Academic P r e s s , New York, pp. 83-122. 95. Z e l a z n y , L. W. and V. W. C a r l i s l e , 1974. P h y s i c a l , 114 c h e m i c a l , e l e m e n t a l and oxygen c o n t a i n i n g f u n c t i o n a l group a n a l y s i s of s e l e c t e d F l o r i d a H i s t o s o l s . I n : S t e l l y (ed.) " H i s t o s o l s : T h e i r c h a r a c t e r i s t i c s , c l a s s i f i c a t i o n and use." SSSA Spec. P u b l . #6, S o i l S c i . Soc. Am. Madison, Wise, pp 6.3-78. 115 APPENDIX I S t a t i s t i c s t o compare the means of h y d r a u l i c c o n d u c t i v i t y v a l u e s A t t e s t and Mann-Whitney U t e s t ( S i e g e l , 1956) were s e l e c t e d as s t a t i s t i c a l t o o l s f o r a n a l y z i n g the l e v e l of s i g n i f i c a n c e between the means of the h y d r a u l i c c o n d u c t i v i t y v a l u e s . In the t t e s t i t was assumed t h a t the p o p u l a t i o n v a r i a n c e s were not equal and t h a t the samples were of d i f f e r e n t s i z e s . These c o n d i t i o n s r e q u i r e d a c o r r e c t i o n i n the v a l u e f o r the degree of freedom. The c o r r e c t e d degrees of freedom were found from, (Hays and W i n k l e r , 1971): 2 0 2 v = ( s l + s2** } - 2 i l - a ) 5 5 — ( s 1 ) + ( s 2 ) n^ + 1 n 2 + 1 In e q u a t i o n l a , v i s the c o r r e c t e d degrees of freedom, S j i s the e s t i m a t e d s t a n d a r d d e v i a t i o n of sample of s i z e n^, s 2 i s the e s t i m a t e d s t a n d a r d d e v i a t i o n of sample of s i z e n 2 -Pooled e s t i m a t e of the s t a n d a r d e r r o r was not made. I n s t e a d , s e p a r a t e s t a n d a r d e r r o r s were computed f o r each sample. That i s : 116 (2a) where i s the s t a n d a r d e r r o r f o r the d i f f e r e n c e between the two means. t was computed by: s d where m^  and m2 are the means of samples o f s i z e n^ and n 2 , r e s p e c t i v e l y . In e q u a t i o n 3a i t was f u r t h e r assumed t h a t the d i f f e r e n c e between the p o p u l a t i o n means was z e r o . The Mann-Whitney U t e s t i s a u s e f u l n o n p a r a m e t r i c t e s t a l t e r n a t i v e t o the p a r a m e t r i c t t e s t when one wishes to a v o i d the assumptions of t t e s t or when the measurement i n the r e s e a r c h i s weaker than i n t e r v a l s c a l i n g . Suppose n^ i s the number of cases i n the s m a l l e r of two independent groups and n 2 the number of cases i n the l a r g e r . To apply U t e s t , we f i r s t combine the o b s e r v a t i o n s from both groups and rank these i n o r d e r of i n c r e a s i n g s i z e . Now focus on one of the groups, say the group w i t h n.^  c a s e s . The v a l u e of U i s g i v e n by the number of times t h a t a s c o r e i n the group w i t h n 2 cases precedes a s c o r e i n the group w i t h n^ cases i n the r a n k i n g . I t may happen t h a t the observed v a l u e of U i s so l a r g e t h a t i t does not appear i n the s u b t a b l e f o r the observed 117 v a l u e of r ^ . We denote such a t o o - l a r g e v a l u e by U'. We can t r a n s f o r m U'to U by: U = nlr\2 - U' (4a) For f a i r l y l a r g e v a l u e s o f n^ and n 2 , the c o u n t i n g method of d e t e r m i n i n g the v a l u e of U may be t e d i o u s . In such c a s e s , an a l t e r n a t i v e way i s t o a s s i g n the rank of 1 to the lowest s c o r e (or o b s e r v a t i o n ) i n the combined (n-^ + n 2 ) group of s c o r e s , and rank 2 to the next lowest s c o r e , and so on. Then, n, (n, + 1) U = n L n 2 + — 2 R i ' ( 5 a } o r , e q u i v a l e n t l y , n 0 ( n 0 + 1) U = n l n 2 + - i ± - R 9 (6a) 2 1 where R-^  and I<2 are the sum of ranks a s s i g n e d to group whose sample s i z e i s n^ and n 2 , r e s p e c t i v e l y . I t i s always wise to check whether U ' r a t h e r than U has been found by a p p l y i n g the t r a n s f o r m a t i o n : U = n,n 0 - U' 118 APPENDIX I I S a t i a t e d h y d r a u l i c c o n d u c t i v i t y ("K ") v a l u e s i n Vinod S o i l s (Van H a l s t Farm) Depth (cm) 0- 2 0-15 (n=13) 0-15 (n=13) 0-15 (n=7) 15-25 ( n = l l ) 25-40 (n=9) 40 + (n=6) "Ks" -1 m s m s -1 Range of "K " m s ~ l Remarks 5 . 8 x l 0 " 8 2.8x10 8 9.8x10 7 9.1x10 7 2.9x10 -S 1.3x10 -7 3 . 3 x l 0 ~ 6 1.5x10 6 4 . 0 x l 0 ~ 6 2.8x10 6 (c.v.=93%) l " 6 (c.v.=39%) l ~ 6 (c.v.=70%) 1.3x10 6 - 6.2x10 D S u r f a c e s e a l 1.6x10 -6 8.1x10 7 5.1x10 7 ,-7 1.6x10" 5 8.3x10 6 3 . 2 x l 0 ~ 9 1 . 9 x l 0 " 9 (c.v.=63%) l ~ 6 (c.v.=52%) 1 . 9 x l 0 " 9 (c.v.=59%) 9.2x10 8 Sealed s u r f a c e ( o n i o n f i e l d ) 3 . 1 x l 0 ~ 6 Sealed s u r f a c e ( P o t a t o f i e l d ) -6 removed ( p o t a t o f i e l d ) — 6 9.6x10 Receded ponding area under w i n t e r r y e co v e r -6 5.1x10 - 1.1x10 u T i l l a g e pan 9 . 2 x l 0 ~ 6 - 3. 0 x l O ~ 5 Organic sub-s o i l 1 . 2 x l 0 ~ 9 - 5 . 5 x l 0 ~ 9 T r a n s i t i o n l a y e r s = s t a n d a r d d e v i a t i o n n = number of samples 119 APPENDIX I I I a S a t i a t e d h y d r a u l i c c o n d u c t i v i t y "Ks" v a l u e s i n S t a t e r Farm (Richmond s o i l ) Depth (cm) m s "Ks" -1 m s -1 Ran^e of "K;o" m s 1 Rema rk s 0-15 1.2x10 4 3.2x10 5 8.7x10 -5 (n=6) 0-15 (n=7) 15-25 (n=10) 15-25 (n=15) 1 . 6 x l 0 ~ 6 6.2x10 7 1.1x10 5 2.4x10 6 5.7x10 7 2 . 9 x l 0 ~ 7 (c.v.=27%) l " 7 (c.v.=39%) 2 . 4 x l 0 " 6 (c.v.=51%) 7 (c.v.=51%) 25-45 2.5x10 7 1.4x10 7 (n=6 + 5-6' (n = 7) 4 0 2.2x10 6 6 . 0 x l 0 ~ 7 (c.v.=56%) 7 (c.v.=27%) 1.0x10 -6 6.5x10 -6 1.3x10 -7 1.1x10 -7 1.0x10 -6 1.7x10 4 U n c u l t i v a t e d f o r a l o n g time 2 . 8 x l 0 " 6 C u l t i v a t e d c a r r o t f i e l d 1.5x10" 5 U n c u l t i v a t e d f o r a l o n g time 1 . 3 x l 0 ~ 6 T i l l a g e pan, c u l t i v a t e d c a r r o t f i e l d 5 . 1 x lO~ 7 G e n e t i c pan ( C u l t i v a t e d ) 2 . 3 x l 0 - 6 Organic sub-s o i 1 n = number of samples c.v. = c o e f f i c i e n t of v a r i a t i o n s = s t a n d a r d d e v i a t i o n 120 APPENDIX I I I b S a t i a t e d h y d r a u l i c c o n d u c t i v i t y "K'' v a l u e s o f o r g a n i c g e n e t i c pan i n C l o v e r d a l e Produce Farm (Richmond s o i l ) Depth "!<s" S Range of "K s" Remarks (cm) IT. s ~ l m s ~ l m s--*-10-25 3 . 4 x l 0 " 5 l . O x l O - 5 1 . 3 x l 0 ~ 5 - 5 . 9 x l 0 " 5 Grass p a s t u r e (n=6) (c.v.=29%) 25-40 7 . 0 x l 0 ~ 6 3 . 8 x l 0 ~ 5 3 . 4 x l 0 ~ 6 - l . O x l O - 5 G e n e t i c pan (n=8) (c.v.=54%) under g r a s s p a s t u r e 25-40 9 . 6 x l 0 - 7 7 . 6 x l 0 ~ 7 1.2xlO" 7 - 2 . 2 x l 0 - 6 G e n e t i c pan ( n = l l ) (c.v.=79%) under c u l t i v a t i o n 

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