UBC Theses and Dissertations

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UBC Theses and Dissertations

Cowpea yield response under alternative irrigation scheduling techniques using line-source sprinklers Tyem, Mamkur Ndam 1984

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COWPEA YIELD RESPONSE UNDER ALTERNATIVE IRRIGATION SCHEDULING TECHNIQUES USING LINE-SOURCE SPRINKLERS by MAMKUR NDAM TYEM B . S c , U n i v e r s i t y of I b a d a n , N i g e r i a , 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Dep a r t m e n t o f A g r i c u l t u r a l M e c h a n i c s We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i i / f d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA O c t o b e r 1984 © MAMKUR NDAM TYEM, 1984 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y of B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g of t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head of my D e p a r t m e n t o r by h i s or h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r 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 g a i n s h a l l n ot be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t of AGRICULTURAL MECHANICS The U n i v e r s i t y of B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 D a t e : OCTOBER 1, 1984 i i A b s t r a c t A p o r t a b l e h a n d - m o v e s p r i n k l e r i r r i g a t i o n s y s t e m was d e s i g n e d t o p r o v i d e w a t e r d i s t r i b u t i o n and o p e r a t i n g c h a r a c t e r i s t i c s s u i t a b l e f o r c o n t i n u o u s v a r i a b l e i r r i g a t i o n -f e r t i l i z e r cowpea ( V i g n a u n g u i c u l a t a [ L . ] W a l p . ) p r o d u c t i o n s t u d i e s . B o t h t h e s i n g l e a n d i n t e r a c t i v e e f f e c t s o f i r r i g a t i o n w a t e r , p h o s p h a t e f e r t i l i z e r a n d i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s on cowpea d r y m a t t e r p r o d u c t i o n were i n v e s t i g a t e d i n a f i e l d -c o n d u c t e d r a n d o m i s e d c o m p l e t e b l o c k e x p e r i m e n t . The r e l a t i v e m e r i t s o f i r r i g a t i n g f r e q u e n t l y b u t w i t h s m a l l e r amount s o f w a t e r t h a n t h e d e s i g n w a t e r a p p l i c a t i o n d e p t h a s c o m p a r e d t o s t a g e - o f - g r o w t h a n d n o r m a l i n t e r v a l i r r i g a t i o n were s t u d i e d w i t h f i v e l e v e l s o f f e r t i l i z e r P a n d t h r e e w a t e r l e v e l s . S o i l w a t e r d e p l e t i o n by c r o p was m e a s u r e d by g r a v i m e t r i c m e t h o d s a n d w a t e r u se a s w e l l a s w a t e r u se e f f i c i e n c y were s u b s e q u e n t l y e v a l u a t e d . P l a n t g r o w t h r a t e i n d i c e s m o n i t o r e d w e r e : p l a n t h e i g h t , number o f t r i f o l i a t e s a n d number o f n o d e s on a w e e k l y b a s i s f o r s e v e n weeks f o l l o w i n g e m e r g e n c e . S t a t i s t i c a l a n a l y s e s o f y i e l d a n d P u p t a k e i n d i c a t e d p o s i t i v e r e s p o n s e o f t h e c r o p t o a d d e d f e r t i l i z e r a n d i r r i g a t i o n w a t e r , most n o t i c e a b l y u n d e r t h e h i g h - f r e q u e n c y s c h e d u l e s . W a t e r use e f f i c i e n c y was o b s e r v e d t o be h i g h e s t u n d e r t h e most f r e q u e n t l y - i r r i g a t e d p l o t ( S 3 ) b u t t h i s h a p p e n e d t o be o b t a i n e d a t t h e e x p e n s e o f d e p r e s s e d y i e l d s . I t was t h e r e f o r e c o n c l u d e d t h a t i r r i g a t i n g t w i c e w i t h i n t h e d e s i g n e d i n t e r v a l was o p t i m u m f o r b o t h y i e l d and w a t e r use e f f i c i e n c y c o n s i d e r a t i o n s . The l i n e - s o u r c e i r r i g a t i o n s y s t e m and e x p e r i m e n t a l d e s i g n u s e d i n t h i s p r o j e c t were f o u n d t o be s a t i s f a c t o r y f o r f i e l d s t u d y of t h e i n t e r a c t i v e e f f e c t s of t h e s e t e s t f a c t o r s on t h e c r o p . One major d i f f i c u l t y e n c o u n t e r e d w i t h r u n n i n g t h e e x p e r i m e n t was t h e o p e r a t i o n o f t h e s y s t e m under c a l m wind c o n d i t i o n s t o e n s u r e h i g h u n i f o r m i t y o f w a t e r d i s t r i b u t i i o n . S p e c i a l c a r e was n e e d e d a l s o , t o a c h i e v e s e d i m e n t e x c l u s i o n f r o m t h e l a t e r a l t h a t was f r e q u e n t l y u n c o u p l e d and moved from p l o t t o p l o t , but t h e s e were t h e s p e c i a l f e a t u r e s o f t h e i r r i g a t i o n s y s t e m t h a t made w o r k i n g w i t h i t i n t e r e s t i n g . i v T a b l e o f C o n t e n t s A b s t r a c t i i L i s t o f T a b l e s '. v i L i s t o f F i g u r e s v i i A c k n o w l e d g e m e n t v i i i C h a p t e r I INTRODUCTION 1 1 . 1 B a c k g r o u n d and S t a t e m e n t o f t h e P r o b l e m 1 1.2 S t u d y O b j e c t i v e s 7 C h a p t e r I I L I T E R A T U R E REVIEW 9 2.1 E v a p o t r a n s p i r a t i o n , C r o p G r o w t h a n d Y i e l d 9 2 . 2 I r r i g a t i o n S c h e d u l i n g 13 2 . 3 W a t e r S u p p l y a n d N u t r i e n t A v a i l a b i l i t y t o C r o p s . . . 1 9 2 . 4 S t a t e o f K n o w l e d g e i n I r r i g a t e d Cowpea R e s e a r c h . . . 2 6 C h a p t e r I I I M A T E R I A L S AND METHODS 30 A . E X P E R I M E N T A L METHODS 30 3.1 I r r i g a t i o n S y s t e m D e s i g n 30 3 . 1 . 1 Maximum C r o p E T E s t i m a t i o n By Pan E v a p o r a t i o n . . . 3 1 3 . 1 . 2 I r r i g a t i o n S y s t e m D e s i g n C o m p u t a t i o n s 33 3 . 1 . 3 S p r i n k l e r S e l e c t i o n a n d C o n f i g u r a t i o n 34 3 . 1 . 4 L a t e r a l D e s i g n 34 3 . 1 . 5 M a i n l i n e D e s i g n 36 3 .2 E x p e r i m e n t a l D e s i g n a n d S y s t e m L a y o u t 36 3 . 2 . 1 I r r i g a t i o n S y s t e m C h a r a c t e r i s t i c s 36 3 . 2 . 2 E x p e r i m e n t a l D e s i g n a n d F i e l d L a y o u t . . 3 7 3 . 3 A p p l i c a t i o n o f T r e a t m e n t s 38 3 . 3 . 1 C r o p E s t a b l i s h m e n t and M a i n t e n a n c e . . . .• 38 3 . 3 . 2 I r r i g a t i o n W a t e r T r e a t m e n t s 42 3 . 3 . 3 P h o s p h o r u s F e r t i l i z e r T r e a t m e n t s 46 3 . 3 . 4 I r r i g a t i o n S c h e d u l i n g T e c h n i q u e s 47 B . A N A L Y T I C A L PROCEDURES 51 3 . 4 T h e S o i l 51 3 . 4 . 1 S o i l A n a l y s e s . . . . 51 3 . 4 . 2 S o i l M o i s t u r e D e t e r m i n a t i o n s D u r i n g E x p e r i m e n t . . 5 2 3 . 5 The C r o p 53 3 . 5 . 1 C r o p G r o w t h I n d i c a t o r s 53 3 . 5 . 2 C r o p Y i e l d 54 3 . 5 . 3 N u t r i e n t U p t a k e 54 3 . 5 . 4 S t a t i s t i c a l C o m p a r i s o n o f R e s u l t s 55 C h a p t e r I V RESULTS AND D I S C U S S I O N 56 4.1 S o i l P r o p e r t i e s 56 4 . 2 C r o p D e v e l o p m e n t 59 .4 .3 C r o p Y i e l d R e s p o n s e 60 V 4.3.1 E f f e c t of I r r i g a t i o n on Y i e l d s 66 4.3.2 E f f e c t of Phosphorus F e r t i l i z e r on Y i e l d 76 4.3.3 E f f e c t s of F a c t o r I n t e r a c t i o n s on Cowpea Y i e l d ..77 4.4 E f f e c t of I r r i g a t i o n on F e r t i l i z e r U t i l i z a t i o n ....82 4.5 Water Use E f f i c i e n c y 91 4.6 Crop P r o d u c t i o n F u n c t i o n s 95 4.6.1 Y i e l d - W a t e r Model 96 4.6.2 Y i e l d - F e r t i l i z e r F u n c t i o n a l R e l a t i o n s h i p 99 Chapter V SUMMARY AND CONCLUSIONS 109 5.1 Summary 109 5 . 2 C o n c l u s i o n 114 5.3 Recommendations f o r F u r t h e r S t u d i e s 115 LITERATURE CITED 118 APPENDIX A - METEOROLOGICAL DATA USED IN IRRIGATION DESIGN 1 26 APPENDIX B - CATCH CAN LAYOUT FOR WATER DISTRIBUTION TEST 1 27 APPENDIX C - PLANT ANALYSIS: AUTOANALYSER PLOT 129 APPENDIX D - AUTO ANALYSER N AND P DATA 130 APPENDIX E - SOME GRAVIMETRIC MOISTURE DETERMINATION RESULTS ' 131 v i L i s t of T a b l e s I . S o i l R eport of the E x p e r i m e n t a l Area 31 I I . Computed E v a p o t r a n s p i r a t i o n I n f o r m a t i o n 32 I I I . F e r t i l i z e r d a t a 47 IV. S o i l A n a l y s e s R e s u l t s Based on Morgan S o i l T e s t i n g System 56 V. Mean E x t r a c t a b l e ( A v a i l a b l e ) P i n s o i l i n ppm 59 V I . R e s u l t s of S o i l C h e m i c a l A n a l y s e s a t end of Experiment 62 V I I . Dry M a t t e r Y i e l d , m e t r i c t /ha 64 V I I I . A n a l y s i s of V a r i a n c e of Dry M a t t e r Y i e l d 65 IX. S x P I n t e r a c t i o n on Cowpea Y i e l d , t/ha 72 X. P x W I n t e r a c t i o n on Cowpea Y i e l d , t/ha 72 X I . P l a n t A n a l y s e s R e s u l t s 85 X I I . A n a l y s i s of V a r i a n c e f o r P-Uptake 87 X I I I . S x P I n t e r a c t i o n on P-Uptake, ppm 88 XIV. P x W I n t e r a c t i o n on P-uptake, ppm 88 XV. Y i e l d , E v a p o t r a n s p i r a t i o n and Water Use E f f i c i e n c y R e s u l t s 93 XVI. Comparison of A c t u a l and C a l c u l a t e d Y i e l d s of Cowpea 103 v i i L i s t of F i g u r e s 1. The L i n e - S o u r c e S p r i n k l e r S y s t e m 35 2. S c h e m a t i c o f a T e s t P l o t 40 3. E x p e r i m e n t a l D e s i g n and S y s t e m L a y o u t i n t h e F i e l d ...41 4. E x p e r i m e n t a l u n i t s and s o i l s a m p l i n g s i t e s ...45 5. M o i s t u r e R e t e n t i o n C u r v e s of E x p e r i m e n t a l s o i l 58 6. E x t r a c t a b l e P i n S o i l a t end o f E x p e r i m e n t 61 7. I n t e r a c t i o n o f S and P T r e a t m e n t s on Y i e l d 73 8. E x a m i n a t i o n o f S x P I n t e r a c t i o n on Y i e l d - non-p a r a m e t r i c c o m p a r i s o n 74 9. I n t e r a c t i o n of W and P T r e a t m e n t s on Y i e l d 75 10. I n t e r a c t i o n of S and P T r e a t m e n t s on P-Uptake 89 11. I n t e r a c t i o n o f W and P T r e a t m e n t s on P-Uptake 90 12. Cowpea D r y M a t t e r Y i e l d as R e l a t e d t o S e a s o n a l ET ..... 94 13. A p p l i e d P a s R e l a t e d t o Bray-1 E x t r a c t a b l e P 104 14. M i t s c h e r l i c h E q u a t i o n and Y i e l d C u r v e s 105 Acknowledgement I have gone t h r o u g h e x p e r i e n c e s o f v a r y i n g d e g r e e s of e a s e and h a r s h n e s s but t h e d i f f i c u l t y e n c o u n t e r e d i n t h i s phase of my p u r s u i t was u n p r e c e d e n t e d . My d e e p l y h e a r t - f e l t g r a t i t u d e goes t o D r . S i e t a n C h i e n g whose m o r a l and f i n a n c i a l s u p p o r t d u r i n g t h e major d u r a t i o n o f t h i s e n d e a v o u r went beyond a c a d e m i c a c q u a i n t a n c e . I am a l s o i n d e b t e d t o him f o r h i s t h o r o u g h s u p e r v i s i o n o f t h i s t h e s i s . P a r t i c u l a r m e n t i o n must be made o f P a s t o r J u s t i n L a - N i b e t l e w i t h h i s f a m i l y . H i s t r u e l y C h r i s t i a n D i s c i p l i n e of s i m p l i c i t y w i t h an i n w a r d r e a l i t y , h i s f a m i l y ' s o u t w a r d l i f e - s t y l e of l o v e and, f a i t h i n t h e power and wisdom of God t o p r o v i d e and f u l f i l l human needs and d e s i r e s were most e x e m p l a r y ; t h i s was of d a i l y g u i d a n c e t o me t h r o u g h o u t t h i s s t u d y . T h i s w o n d e r f u l f a m i l y ' s p r e s e n c e i n my l i f e was of i m m e a s u r a b l e s p i r i t u a l and p s y c o l o g i c a l s t r e n g t h s . F i n a l l y , I a c k n o w l e d g e t h e c h a l l e n g e p o s e d t o me by b r o t h e r B i n s h a l ' s t r e mendous d r i v e f o r knowledge, N i m t u r ' s . p e c u l i a r , y e t e x c i t i n g a c a d e m i c a c c o m p l i s h m e n t s , D a n l a d i ' s sudden wake-up f r o m a s l u m b e r and Banyo's g r a d u a l p r o g r e s s ; f a r away i n N i g e r i a , y e t t h e y c o n s t i t u t e d a d r i v i n g f o r c e t h a t c u l m i n a t e d i n t h i s a c h i e v e m e n t . 1 I . INTRODUCTION 1.1 B a c k q r o u n d and S t a t e m e n t of t h e P r o b l e m I r r i g a t i o n i s w i d e l y r e c o g n i z e d a s one of t h e o l d e s t known a g r i c u l t u r a l t e c h n o l o g i e s , but improvement i n i r r i g a t i o n methods and p r a c t i c e s a r e s t i l l b e i n g made. The demand f o r even g r e a t e r i mprovements i n t h e f u t u r e c a n be s e e n i n t h e l i g h t of e v e r -i n c r e a s i n g c o m p e t i t i o n f o r l i m i t e d w a t e r s u p p l i e s b o t h i n a r i d r e g i o n s and i n humid r e g i o n s , where w a t e r i s s u p p l i e d on farms t o o f f s e t more or l e s s t e m p o r a r y d r o u g h t s . Most c o n c e r n i n i r r i g a t i o n w a t e r management i s w i t h d e t e r m i n i n g when t o i r r i g a t e t o p r e v e n t d r o u g h t - i n d u c e d y i e l d r e d u c t i o n s . The t r a d i t i o n a l methods o f w a t e r a p p l i c a t i o n by f u r r o w , b a s i n and s o l i d - s e t s p r i n k l e r s y s t e m s impose, a l m o s t i n e v i t a b l y , an i r r i g a t i o n c y c l e c o n s i s t i n g o f a b r i e f p e r i o d of w a t e r i n f i l t r a t i o n i n t o t h e s o i l f o l l o w e d by a r e l a t i v e l y much e x t e n d e d p e r i o d of water e x t r a c t i o n by t h e c r o p s . T h e s e s y s t e m s have f i x e d c o s t s f o r e a c h a p p l i c a t i o n , and u n d e r s t a n d a b l y , t h e ec o n o m i c c o n s t r a i n t e n c o u r a g e s t h e m i n i m i z a t i o n of i r r i g a t i o n f r e q u e n c y and a m a x i m i z a t i o n of b o t h t h e q u a n t i t y of water s t o r e d i n t h e r o o t zone and t h e p r o p o r t i o n o f t h i s w a t e r u s e d by t h e c r o p b e f o r e t h e n e x t i r r i g a t i o n ( R a w l i n s and R a a t s , 1975). The i n t r o d u c t i o n o f s o p h i s t i c a t e d p r e s s u r i z e d i r r i g a t i o n s y s t e m s ( s u c h as s o l i d - s e t t r i c k l e s y s t e m s and c e n t r e - p i v o t s p r i n k l e r s ) t h a t d i s t r i b u t e w a t e r t o t h e f i e l d i n s m a l l q u a n t i t i e s a s o f t e n as d e s i r e d w i t h no a d d i t i o n a l c o s t r e v e r s e d t h e e c o n o m i c p i c t u r e and c h a n g e d t h e i r r i g a t i o n c y c l e f r o m a s o i l m o i s t u r e e x t r a c t i o n - d o m i n a t e d p r o c e s s t o one t h a t i s 2 p r e d o m i n a n t l y i n f i l t r a t i o n . T h i s brought i n t o p l a y a s e t of laws g o v e r n i n g water f l o w w i t h i n the s o i l which f r e q u e n t l y d i d not a p p l y p r e v i o u s l y . C o n s e q u e n t l y , management c r i t e r i a had t o be updated. Y e t . i t i s c a u t i o n e d t h a t t h e r e i s by t h i s method, the m o d i f i c a t i o n of the a e r i a l and s o i l e n vironments t o which the v a r i o u s c r o p s respond d i f f e r e n t l y depending on the geography of the a r e a , c l i m a t e , c r o p v a r i e t y , endemic d i s e a s e s and p e s t s . S i n c e the development of h i g h - f r e q u e n c y systems does not n e c e s s a r i l y suggest the replacement of s u r f a c e and s o l i d - s e t s p r i n k l e r systems, the c h a l l e n g e t o improve c r o p p r o d u c t i v i t y w i t h these l a t t e r i n h e r e n t l y l o n g - i n t e r v a l ( l o w - f r e q u e n c y ) systems emerges. T h e i r energy demand and water consumption r a t e c o n t i n u e t o f a c e c o m p e t i t i o n not o n l y from modern systems but a l s o from o t h e r u s e r s of t h e s e two(water and energy) e x t r e m e l y i m p o r t a n t r e s o u r c e s . T h i s c a l l s f o r a need t o a d d r e s s c a r e f u l l y , c u r r e n t farm water management p r a c t i c e s w i t h a view t o i m p r o v i n g water c o s t and energy s a v i n g s a l o n g s i d e i n c r e a s e d water use e f f i c i e n c y and b e t t e r f e r t i l i z e r management. The most c r i t i c a l d e c i s i o n f o r an i r r i g a t o r i s d e t e r m i n i n g when t o i r r i g a t e and how much water t o a p p l y . In humid a r e a s l i k e the c o a s t a l a r e a of B r i t i s h Columbia where r a i n f a l l o c c u r s d u r i n g the growing season, s p e c i a l i r r i g a t i o n management p r o c e d u r e s need t o be a p p l i e d t o make e f f e c t i v e use of the s p o r a d i c r a i n f a l l . I r r i g a t i o n s c h e d u l i n g o b j e c t i v e s v a r y : when water s u p p l i e s a r e r e a d i l y a v a i l a b l e and i r r i g a t i o n c o s t s a r e low, the o b j e c t i v e of s c h e d u l i n g f o r m a x i m i z i n g y i e l d per u n i t a r e a i s 3 o b v i o u s and may be e c o n o m i c a l l y j u s t i f i e d ; however, as i r r i g a t i o n water s u p p l i e s become more l i m i t e d or as water c o s t s i n c r e a s e i n an a r e a , the management o b j e c t i v e may s h i f t t o o p t i m i z i n g p r o d u c t i o n per u n i t of a p p l i e d w a t e r . In a d d i t i o n , t h e r e are those methods whose o b j e c t i v e s a r e m a x i m i z i n g net p r o f i t - i n a broad sense, and m i n i m i z i n g energy r e q u i r e m e n t s . These o b j e c t i v e s s e t f o r t h the c o n c e p t s of " c r i t i c a l growth s t a g e i r r i g a t i o n " proposed by Musick and Grimes ( 1 9 6 1 ) ; " e v a p o t r a n s p i r a t i o n ( E T ) d e f i c i t i r r i g a t i o n " d e v e l o p e d by Woodruff et a l ( 1 9 7 2 ) and M i l l e r ( 1 9 7 6 ) ; and " s t r e s s day index method" i n t r o d u c e d by H i l e r and C l a r k ( l 9 7 l ) ; amongst o t h e r s , as the major i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s . They were d e v e l o p e d on the b a s i s of d e f i n e d c r i t e r i a . The c r i t e r i a f o r d e c i d i n g when a c r o p s h o u l d be i r r i g a t e d a r e : ( i ) the d e p l e t i o n of water i n the e f f e c t i v e r o o t i n g zone t o some p r e d e t e r m i n e d l e v e l [ e f f e c t i v e r o o t i n g d e pth i s def-ined (BCMAF, 1 9 8 3 ) as t h a t depth i n the s o i l above which the r o o t s o b t a i n 90% or more of t h e i r water between i r r i g a t i o n s ] , or ( i i ) the d e c r e ase of water p o t e n t i a l a t some g i v e n s o i l depth t o a pr e d e t e r m i n e d l e v e l . The v a l u e one chooses f o r e i t h e r of thes e c r i t e r i a t o i n d i c a t e t h a t i r r i g a t i o n i s needed depends on s o i l p r o p e r t i e s , c r o p r o o t i n g c h a r a c t e r i s t i c s and sta g e of p l a n t growth ( C a r y , l 9 8 l ) . E s s e n t i a l l y , the s c h e d u l i n g p r o c e d u r e s h i g h l i g h t e d i n the p r e c e d i n g p a r a g r a p h and t h e i r v a r i o u s t e c h n i c a l r a m i f i c a t i o n s may be s a i d t o d i f f e r from each o t h e r i n the f o l l o w i n g ways: (a) f i l l i n g the maximum e x p e c t e d r o o t zone t o f i e l d 4 c a p a c i t y on an i n t e r v a l b a s i s t h r o u g h o u t t h e c r o p ' s 1 i f e s p a n , (b) f i l l i n g t h e e f f e c t i v e r o o t i n g zone t o f i e l d c a p a c i t y o n l y a t some ' c r i t i c a l ' s t a g e of g r o w t h of t h e c r o p , ( c ) s e q u e n c i n g of t h e ET d e f i c i t s w i t h i n t h e g r o w t h p e r i o d s u c h t h a t t h e c r o p may n o t be a d v e r s e l y a f f e c t e d , and (d) a p p l y i n g w ater f r e q u e n t l y but i n amounts i n s u f f i c i e n t t o r e f i l l t h e s o i l p r o f i l e ( h i g h - f r e q u e n c y d e f i c i t i r r i g a t i o n ) . T h i s a p p r o a c h a d v o c a t e s a p p l y i n g w a t e r a t a r a t e t o meet t h e d a i l y e v a p o t r a n s p i r a t i o n demand of t h e c r o p t h e r e b y k e e p i n g t h e r o o t zone a t c o n s t a n t l y h i g h m a t r i c p o t e n t i a l but n o t n e c e s s a r i l y a t or above f i e l d c a p a c i t y . T h i s s t u d y i n c o r p o r a t e d , a l m o s t e x p l i c i t l y , t h e f i r s t , s e c o n d and f o u r t h of t h e s e b a s i c p r i n c i p l e s o f i r r i g a t i o n s c h e d u l i n g . A t s o i l w e t n e s s l e v e l s t h a t a r e c o n d u c i v e t o h i g h r a t e s o f s e a s o n a l e v a p o t r a n s p i r a t i o n and h e n c e y i e l d , optimum y i e l d may n o t be a c h i e v e d . F a c t o r s s u c h as r o o t zone a e r a t i o n d e f i c i e n c y , p l a n t l o d g i n g , n u t r i e n t a v a i l a b i l i t y and t h e p o t e n t i a l f o r h i g h e r d i s e a s e i n c i d e n c e may c o n t r i b u t e t o w a r d s t h e f a i l u r e o f t h e w a t e r management p r o c e s s t h a t would o t h e r w i s e a c h i e v e optimum y i e l d . T h u s , as J e n s e n (1975) o b s e r v e d , d e s p i t e t h e i r a v a i l a b i l i t y , i r r i g a t i o n o p e r a t o r s have not been g r e a t l y r e c e p t i v e t o any one p a r t i c l a r s c h e d u l i n g method b e c a u s e o f r e a s o n s t h a t i n c l u d e ( i ) t h e c o s t o f i r r i g a t i o n w a t e r i s o f t e n low r e l a t i v e t o c o s t s of p r a c t i c e s t h a t would improve water 5 management, ( i i ) y i e l d r e d u c t i o n s c a u s e d by d e l a y e d i r r i g a t i o n s , i m p r o p e r f e r t i l i z a t i o n and e x c e s s i v e i r r i g a t i o n s a r e not e a s i l y r e c o g n i z e d o r q u a n t i f i e d and ( i i i ) i r r i g a t i o n management d e c i s i o n s a r e g e n e r a l l y made by busy p e o p l e w i t h l i m i t e d t e c h n i c a l b a c k g r o u n d and t r a i n i n g i n t h e management of a complex c r o p - s o i l - c l i m a t e s y s t e m . T y p i c a l l y , t h e i r r i g a t o r would a d o p t t h a t s c h e d u l i n g p r o c e d u r e t h a t , when i n t e g r a t e d w i t h f e r t i l i z e r management on t h e farm, and weed and p e s t c o n t r o l , p r o d u c e s i m p r o v e d net income from t h e f a r m . F u r t h e r m o r e , a method w i l l be p r e f e r r e d w h i c h , when a p p l i e d t o a p a r t i c u l a r c r o p under a known s o i l n u t r i e n t s t a t u s ( n a t i v e o r a p p l i e d ) w i l l g i v e a s t r o n g p r e d i c t i v e c a p a b i l i t y w i t h r e s p e c t t o y i e l d . Such a method c o u l d o n l y be f o u n d t h r o u g h a d o p t i n g t h e new t e c h n i q u e , t e s t i n g i t a c c u r a t e l y , r e l i a b l y and c o n s i s t e n t l y i n c o m b i n a t i o n w i t h o t h e r f a r m management p r a c t i c e s , and f i n a l l y , e v a l u a t i n g and i n t e r p r e t i n g t h e r e s u l t s so as t o a r r i v e a t t h e b e s t c o m b i n a t i o n o f p r a c t i c e s . S o i l n u t r i e n t s t a t u s c an m a r k e d l y i n f l u e n c e w ater use by c r o p ( B l a c k , 1966). In t u r n , t o t a l n u t r i e n t u p t a k e by t h e c r o p a t h a r v e s t i s a f u n c t i o n of b o t h t h e t i m e of o n s e t o f d r o u g h t s t r e s s where s o i l w a ter i s l i m i t e d and, t h e s t r a t e g y f o r i r r i g a t i o n where w a t e r s u p p l y i s n o t l i m i t e d (Begg and T u r n e r , 1976). In p a r t i c u l a r , t h e s e a u t h o r s s t a t e t h a t n i t r o g e n and p h o s p h o r u s i n f l u e n c e water use i n d i f f e r e n t ways: h i g h e r c o n c e n t r a t i o n of e a c h l e a d s t o i n c r e a s e d l e a f a r e a and p r o l o n g e d d e v e l o p m e n t o r t o i n c r e a s e d r o o t p r o l i f e r a t i o n , b u t b o t h c a n have d e t r i m e n t a l e f f e c t s on y i e l d when s o i l m o i s t u r e i s l i m i t e d . 6 N u t r i e n t l e v e l s i n t h e f i e l d a r e u s u a l l y h i g h e s t n e a r t h e s u r f a c e and t h i s p o r t i o n of t h e s o i l p r o f i l e i s t h e f i r s t t o d r y out i n a d r y i n g c y c l e . A v a i l a b l e m o i s t u r e p l a y s a s p e c i a l r o l e i n n u t r i e n t u p t a k e n o t o n l y t h r o u g h t h e amount a l o n e but a l s o a c c o r d i n g t o i t s d i s t r i b u t i o n d u r i n g t h e c r o p p i n g s e a s o n . A l t h o u g h many s t u d i e s on t h e e f f e c t o f h i g h - f r e q u e n c y i r r i g a t i o n on i r r i g a t i o n e f f i c i e n c y and c r o p w a t e r use e f f i c i e n c y have been c o n d u c t e d , most a r e l a b o r a t o r y o r g r e e n h o u s e b a s e d and have p l a c e d emphases on c o r n and wheat. Even w i t h t h e s e l i m i t e d s t u d i e s , c o n f l i c t i n g r e s u l t s have been r e p o r t e d w i t h r e g a r d t o t h e r e l a t i v e w a ter use e f f e c i e n c y ( u n i t s o f c r o p p r o d u c t i o n p e r u n i t of w a t e r ) o f i r r i g a t i n g f r e q u e n t l y but l i g h t l y a s compared t o low f r e q u e n c y ( l o n g i n t e r v a l ) d e f i c i t i r r i g a t i o n . K e l l e r ( l 9 6 5 ) r e p o r t e d t h a t e f f i c i e n c y was d i r e c t l y r e l a t e d t o t h e d e p t h o f w a t e r s t o r e d p e r i r r i g a t i o n , b u t M u s i c k and Dusek (1971 and 1980) w o r k i n g on c o r n and sorghum i n t h e S o u t h e r n H i g h P l a i n s o f t h e U n i t e d S t a t e s , o b t a i n e d a water use e f f i c i e n c y i n c r e a s e . Deboer et_ al_ (1977) f o u n d t h a t f o r c o r n , w a ter use e f f i c i e n c i e s were not a f f e c t e d by a p p l i c a t i o n d e p t h . In s p i t e o f t h e c o n f l i c t i n g f i n d i n g s , Hobbs and Krogman(1978) a r e q u i t e o p t i m i s t i c t h a t t h e p r o s p e c t s f o r h i g h - f r e q u e n c y , l i g h t i r r i g a t i o n s c h e d u l i n g t e c h n i q u e a r e b r i g h t and t h a t a d v a n t a g e s s u c h as e f f e c t i v e use o f summer p r e c i p i t a t i o n , c o n t r o l l i n g d r a i n a g e and m a i n t a i n i n g n u t r i e n t s u p p l i e s abound. T h e i r work was on s p r i n g wheat i n o u t d o o r l y s i m e t e r s a s w e l l as i n t h e g r e e n h o u s e a t L e t h b r i d g e R e s e a r c h s t a t i o n , C a nada. 7 1 . 2 S t u d y O b j e c t i v e s T h i s s t u d y was c o n c e r n e d f u n d a m e n t a l l y w i t h two r e s o u r c e s : s o i l water and s o i l n u t r i e n t . I t was a f i e l d e x p e r i m e n t d e s i g n e d t o t e s t t h e p e r f o r m a n c e o f c o w p e a ( b l a c k - e y e d pea, V i g n a  u n g u i c u l a t a [ L . ] W a l p . ) . In t h e t r o p i c s ( p a r t i c u l a r l y West A f r i c a where cowpea i s t h e most i m p o r t a n t legume) cowpea grown f o r i t s g r a i n s i s o f t e n i n t e r c r o p p e d w i t h o t h e r "more i m p o r t a n t " c r o p s under r a i n - f e d c o n d i t i o n s . I t i s r a r e l y c u l t i v a t e d as a s o l e c r o p , n e i t h e r i s i t i r r i g a t e d a p p r e c i a b l y . Under t h e s e p r a c t i c e s , i t becomes d i f f i c u l t t o know how much w a t e r and n u t r i e n t s t h e c r o p r e q u i r e s a t i t s v a r i o u s s t a g e s o f g r o w t h when grown a l o n e . The p r o p o s e d s t u d y i s aimed a t g a i n i n g an i n s i g h t i n t o t h e g r o w t h and y i e l d r e s p o n s e of t h i s c r o p when monocropped and s u p p l i e d w i t h s u p p l e m e n t a r y i r r i g a t i o n t h a t i s s c h e d u l e d i n v a r i o u s ways and f e r t i l i z e d w i t h p h o s p h o r u s f e r t i l i z e r i n a humid c l i m a t e . The o b j e c t i v e s o f t h i s s t u d y were: 1. To i n v e s t i g a t e cowpea g r o w t h r a t e and d r y m a t t e r y i e l d r e s p o n s e t o i r r i g a t i o n w a t e r and p h o s p h o r u s f e r t i l i z e r u n der t h e c l i m a t i c c o n d i t i o n s o f t h e c o a s t a l a r e a o f B r i t i s h C o l u m b i a , 2. To e v a l u a t e water use e f f i c i e n c y of t h i s c r o p under d i f f e r e n t i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s w i t h a v i e w t o i d e n t i f y i n g w h i c h method s a v e s w a t e r and makes t h e most optimum use of t h e u n p r e d i c t a b l e r a i n s , 3. To make ( b a s e d on 1 and 2) p r e l i m i n a r y r e c o m m e n d a t i o n s o f 8 f e r t i l i z e r P and i r r i g a t i o n water l e v e l s as w e l l as an o p t i m a l w a t e r management p r o c e d u r e f o r t h e c u l t i v a t i o n o f t h e c r o p f o r f o r a g e i n t h e e n v i r o n m e n t i n q u e s t i o n , and 4. To d e v e l o p a t o o l (some r e s p o n s e f u n c t i o n ) f o r p r e d i c t i n g t h e p e r f o r m a n c e o f cowpea under a g i v e n s e t o f a g r o n o m i c management s p e c i f i c a t i o n s o f t h e r e s p o n s e s u r f a c e ( s o i l ) . Upon f i r s t c o n s i d e r a t i o n , one becomes c u r i o u s a t an a t t e m p t t o i r r i g a t e b l a c k - e y e d p e a , a legume t h a t i s l a r g e l y d r o u g h t -r e s i s t a n t and n o r m a l l y c u l t i v a t e d u n d e r r a i n - f e d c o n d i t i o n s . In a summary of t h e v a r i a t i o n i n d r o u g h t t o l e r a n c e among c r o p s p e c i e s , L e v i t t ( 1 972) and L u d l o w ( 1 9 7 6 ) , f o u n d t h a t t h e d e g r e e of d r o u g h t t o l e r a n c e was a s s o c i a t e d more wi t h - t h e e n v i r o n m e n t t o w h i c h the p l a n t s had a d a p t e d t h a n t o t a x o n o m i c g r o u p i n g . The r e s u l t s of t h i s s t u d y w i l l throw more l i g h t on t h e v a l i d i t y o f t h i s s t a t e m e n t i n t h i s e x p e r i m e n t a l s i t u a t i o n . M o r e o v e r , i r r i g a t i o n i n humid a r e a s has o f t e n been e c o n o m i c a l even t h o u g h a n n u a l r a i n f a l l e x c e e d s e v a p o t r a n s p i r a t i o n . Lambert e t a_l (1981) p o i n t o u t t h r e e f a c t o r s n e c e s s i t a t i n g i r r i g a t i o n i n humid r e g i o n s : (a) t h e a n n u a l d i s t r i b u t i o n of r a i n f a l l does n o t c o i n c i d e w i t h t h e e v a p o t r a n s p i r a t i o n d i s t r i b u t i o n , (b) t h e w ater h o l d i n g c a p a c i t y of s o i l g e n e r a l l y i s not s u f f i c i e n t t o p r o v i d e a d e q u a t e water f o r c r o p s d u r i n g t h e d e f i c i t r a i n f a l l p e r i o d s and ( c ) r e s t r i c t e d r o o t i n g due t o m e c h a n i c a l impedance f o r example, l i m i t s s o i l w a t e r a v a i l a b i l i t y t o p l a n t s . 9 11 . LITERATURE REVIEW 2.1 E v a p o t r a n s p i r a t i o n , Crop Growth and Y i e l d The r a t e of e n t r y of water i n t o the s o i l ' and i t s r e t e n t i o n , movement and a v a i l a b i l i t y t o p l a n t r o o t s a r e a l l p h y s i c a l phenomena and c o n s t i t u t e the complex s o i l - p l a n t - w a t e r r e l a t i o n s h i p . T h i s c o n t i n u o u s system may be d i v i d e d i n t o f o u r s e q u e n t i a l p r o c e s s e s : the su p p l y of water t o the r o o t s u r f a c e , the e n t r y of water i n t o the r o o t , the passage of water i n the p l a n t ' s c o n d u c t i n g elements and the movement of water vapour t h r o u g h and out of the l e a v e s . The complete p a t h of water may be a n a l y s e d by e v a l u a t i n g the p o t e n t i a l d i f f e r e n c e between s o i l and atmosphere i n c o n t a c t w i t h r o o t and l e a f r e s p e c t i v e l y ( P i e r r e e_t a l , 1965; K o z l o w s k i , 1968; and H s i a o , 1973), and the r a t e of water movement i s everywhere p r o p o r t i o n a l t o the p o t e n t i a l energy g r a d i e n t and i n v e r s e l y p r o p o r t i o n a l t o the r e s i s t a n c e t o f l o w i n the pathways of water and/or vapour f l o w ( M i c h a e l , 1978). The p r i n c i p a l d r i v i n g f o r c e f o r t r a n s p i r a t i o n i s t h u s , the d i f f e r e n c e between the vapour p r e s s u r e a t e v a p o r a t i n g l e a f s u r f a c e s and the b u l k a i r . A l t h o u g h the t o t a l p o t e n t i a l ( p a r t i a l s p e c i f i c f r e e energy of water r e l a t i v e t o t h a t of pure f r e e l i q u i d water a t the same tem p e r a t u r e and h e i g h t and a t a t m o s p h e r i c p r e s s u r e , ISSS,1974) i n d r y a i r i s t y p i c a l l y e q u i v a l e n t t o -1000 b a r s or l e s s , most p l a n t p r o c e s s e s would have been s e v e r e l y i n h i b i t e d by the time the l e a f water p o t e n t i a l drops t o -10 b a r s . T h e r e f o r e , t o m a i n t a i n the water p o t e n t i a l of the l e a f above t h i s c r i t i c a l l e v e l as water moves from the s o i l i n t o the p l a n t through the 1 0 t r a n s p i r a t i o n s t r e a m t o t h e a t m o s p h e r e , t h e f l o w r e s i s t a n c e i m p e d i n g w a t e r l o s s from t h e l e a f must be a t l e a s t 100 t i m e s t h a t i n the pathway s u p p l y i n g w a t e r t o i t ( R a w l i n s and R a a t s , 1975). S o i l w a t e r p o t e n t i a l a f f e c t s p h o t o s y n t h e t i c r a t e and t r a n s p i r a t i o n i n s o f a r as i t c o n t r o l s s t o m a t a l o p e n i n g o r c l o s u r e w h i c h a l l o w s o r _ i n h i b i t s r e s p e c t i v e l y , c a r b o n d i o x i d e a s s i m i l a t i o n f r o m , and water v a p o u r e s c a p e i n t o t h e a t m o s p h e r e . Most p h y s i o l o g i c a l p r o c e s s e s a r e a f f e c t e d by t h e t i m e t h e p l a n t r e a c h e s permanent w i l t i n g p o i n t a t w h i c h s t a g e t h e s o i l w a t er p o t e n t i a l i n t h e range of w a t e r a v a i l a b i l i t y t o p l a n t i s v e r y h i g h , t h a t i s , s m a l l n e g a t i v e v a l u e ( G a r d n e r , 1960). When l e a f w a t e r p o t e n t i a l d r o p s i n t o a c r i t i c a l r a n g e , s t o m a t a l o p e n i n g s b e g i n t o c l o s e , p r e v e n t i n g e x c e s s i v e l o s s o f water even a t t h e expense o f d e c r e a s e d c a r b o n d i o x i d e a s s i m i l a t i o n . " I t i s g e n e r a l l y known and a c c e p t e d ( S l a t y e r , 1969; H s i a o , 1973; and Begg and T u r n e r , 1976) t h a t ( i ) c e l l e x p a n s i o n and d i v i s i o n p r o c e s s e s r e q u i r e h i g h t u r g o r p r e s s u r e ( t h a t p o t e n t i a l component of l e a f w a t e r p o t e n t i a l due t o t u r g o r p r e s s u r e a c t i n g o u t w a r d on c e l l w a l l s - a t e r m a n a l o g o u s t o h y d r o s t a t i c p r e s s u r e i n s o i l s ) l e v e l s and ( i i ) p l a n t g r o w t h ( t h e c o n v e r s i o n o f a s s i m i l a t e s t o l i v i n g t i s s u e ) c e a s e s a t l e a f w a t e r p o t e n t i a l l e v e l s much above t h o s e w h i c h c a u s e s t o m a t a l c l o s u r e and a d e c r e a s e i n t r a n s p i r a t i o n r a t e . In e f f e c t , i n most c r o p s , g r o w t h p r o c e e d s c o m p l e t e l y u n i m p a i r e d i f t h e e v a p o t r a n s p i r a t i o n r a t e i s not l i m i t i n g and y i e l d i s maximal o n l y when water p o t e n t i a l r e m a i n s h i g h t h r o u g h o u t t h e l i f e of t h e c r o p . O f t e n , 11 however, c r o p p r o d u c t i o n c o n s t r a i n t s do not p e r m i t t h i s . F o r most c r o p s , k e e p i n g p l a n t w a t e r p o t e n t i a l h i g h by e n s u r i n g c o n t i n u o u s l y ample s u p p l y from t h e s o i l r e s u l t s i n maximum p r o d u c t i o n p e r u n i t of a r e a b u t a s i m u l t a n e o u s m a x i m i z a t i o n o f p r o d u c t i o n p e r u n i t water consumed i s not g u a r a n t e e d . M a x i m i z i n g water use e f f i c i e n c y (Wue) as p o i n t e d out by V e i t s (1962) may not be d e s i r a b l e s i n c e c r o p s grown on d r y l a n d (no i r r i g a t i o n ) f r e q u e n t l y use w a t e r more e f f i c i e n t l y t h a n ' w e l l -w a t e r e d ' c r o p s , but a t much l o w e r l e v e l s of p r o d u c t i o n . H i l l e l and Guron (1973) s t a t e t h a t i t a p p e a r s more p r o m i s i n g t o i n c r e a s e Wue by i n c r e a s i n g c r o p y i e l d s t h a n by d e c r e a s i n g ET, s i n c e p l a n t s g r o w i n g i n t h e f i e l d a r e s u b j e c t t o an e x t e r n a l l y i m p o s e d e v a p o r a t i v e demand. T h i s s t a t e m e n t i s v a l i d f o r w e l l -w a t e r e d c r o p r e g i m e s but H o w e l l and H i l e r (1975) a r g u e t h a t i t d o e s n o t f u l l y e x p l o r e t h e p o s s i b l e i m p l i c a t i o n s of l i m i t e d i r r i g a t i o n i n r e g i o n s o f s h o r t a n d / o r c o s t l y w a t e r s u p p l i e s . They a g r e e t h a t l i m i t e d i r r i g a t i o n w i l l p r e s u m a b l y d e c r e a s e c r o p y i e l d s but t h i s y i e l d d e c r e a s e may n o t be d i r e c t l y p r o p o r t i o n a l t o the' water d e f i c i t imposed on t h e c r o p . B e s i d e s , m a i n t a i n i n g h i g h l e a f w a t e r p o t e n t i a l may e n t a i l e n s u r i n g an e x c e s s i v e l y wet s o i l w h i c h can i m p a i r g a s e o u s e x c h a n g e and r e s t r i c t r o o t d e v e l o p m e n t . R e s t r i c t e d oxygen d i f f u s i o n i n t o p o o r l y d r a i n e d and e x c e s s i v e l y wet s o i l s l i m i t s r o o t r e s p i r a t i o n n e c e s s a r y f o r g r o w t h on t h e one hand (Stegman e t a l , 1980 ), and r e g a r d l e s s of how wet t h e s o i l i s k e p t , c r o p p l a n t s a r e s t i l l s u b j e c t t o w a t e r s t r e s s d u r i n g p e r i o d s o f h i g h t r a n s p i r a t i o n as a c o n s e q u e n c e o f i n c r e a s e d w ater p o t e n t i a l d r o p 1 2 a c r o s s r e s i s t a n c e s w i t h i n ' t h e m on t h e o t h e r ( R a w l i n s and R a a t s , 1975). Thus, i t i s o b v i o u s t h a t m a i n t a i n i n g a c o n s t a n t l y h i g h s o i l m o i s t u r e c o n t e n t i s not a l w a y s d e s i r a b l e f o r optimum p h y s i o l o g i c a l p r o c e s s e s but f r o m t h e s t a n d p o i n t o f d r y m a t t e r p r o d u c t i o n , e i t h e r per u n i t of w a t e r u s e d o r p e r u n i t of l a n d o c c u p i e d , t h e r e seems t o be no a d v a n t a g e i n p e r m i t t i n g c r o p s t o u n d e r g o w a t e r s t r e s s . A v o i d i n g w a t e r s t r e s s i s t h e d e l i c a t e s u b j e c t o f i r r i g a t i o n t i m i n g . F o r t u n a t e l y , r e s u l t s f r o m e x p e r i m e n t s on t h e a f t e r e f f e c t s o f w a t e r s t r e s s on p l a n t g r o w t h s u g g e s t ( H s i a o , 1973) t h a t p e r i o d s o f d e c r e a s e d l e a f water p o t e n t i a l d u r i n g w h i c h p l a n t g r o w t h s l o w s o r s t o p s do n o t d e c r e a s e n e t g r o w t h i f t h e y a r e n o t t o o l o n g . H s i a o o b s e r v e d t h a t d u r i n g e a c h p e r i o d of d e p r e s s e d g r o w t h , a s s i m i l a t e s can be s t o r e d f o r s e v e r a l h o u r s b e f o r e n o n - s t o m a t a l e f f e c t s c a u s e p h o t o s y n t h e s i s t o d e c r e a s e f r om t h e p o t e n t i a l r a t e , and c a n t h e n be u s e d i n a c c e l e r a t e d g r o w t h when s t r e s s i s r e l i e v e d . Hence, r e p e a t e d b r i e f s t r e s s e s , s u c h as i n m i d - a f t e r n o o n maximum e v a p o t r a n s p i r a t i o n p e r i o d s , may o n l y s l i g h t l y a f f e c t y i e l d p o t e n t i a l a n d a r e p r e f e r a b l e t o l e s s f r e q u e n t s t r e s s t h a t l a s t s f o r d a y s s u c h as i s a t t i m e s i n e v i t a b l e i n r a i n - f e d a g r i c u l t u r e and w h i c h may a l s o o c c u r i n s u r f a c e o r l o n g i n t e r v a l i r r i g a t i o n s c h e d u l i n g methods. The l a t t e r s t a t e m e n t i s f u n d a m e n t a l i n i r r i g a t i o n w a ter management and f o r m s t h e b a s i s f o r t h i s s t u d y : an i n v e s t i g a t i o n of d r y m a t t e r p r o d u c t i o n u nder v a r i a b l e i r r i g a t i o n f r e q u e n c i e s ( t h a t i s , programming o f one o r more p e r i o d s o f water s t r e s s of d i f f e r e n t ' d u r a t i o n s i n t h e c r o p ' s 1 3 g r o w t h s p a n ) . L i t e r a t u r e on t h e s u b j e c t of r e l a t i o n s h i p between c r o p y i e l d and ET ( o r c o n s u m p t i v e use of w a t e r ) r e v e a l s t h a t y i e l d r e l a t i o n s h i p s t o w ater c a n r a n g e from l i n e a r t o c u r v i l i n e a r ( b o t h c o n c a v e and c o n v e x ) r e s p o n s e f u n c t i o n s . The v a r i a t i o n s a r e i n f l u e n c e d (Stegman et. a l , 1980) by t h e t y p e of w a t e r p a r a m e t e r t h a t i s c h o s e n , i t s measurement o r e s t i m a t i o n a c c u r a c y and t h e v a r i e d i n f l u e n c e s a s s o c i a t e d w i t h s i t e and p r o d u c t i o n c o n d i t i o n s . The f u r t h e r i n t e r a c t i o n o f t h i s r e s p o n s e f u n c t i o n w i t h s o i l f e r t i l i t y i s r e s e r v e d f o r r e v i e w i n s e c t i o n 2.3. 2 . 2 I r r i g a t i o n S c h e d u l i n g An o p t i m a l i r r i g a t i o n r e g i m e w o u l d be one w h i c h emanates f r o m a good p r o g r a m d e v e l o p e d t o e s t i m a t e s o i l m o i s t u r e d e p l e t i o n , t h e number of d a y s b e f o r e t h e n e x t i r r i g a t i o n , and t h e amount o f w a t e r t h a t s h o u l d be a p p l i e d a t e a c h i r r i g a t i o n . T h i s program c o n s t i t u t e s an i r r i g a t i o n s c h e d u l i n g p r o c e d u r e -t h e a p p l i c a t i o n of t h e r i g h t amount of water a t t h e r i g h t t i m e , many a l t e r n a t i v e s of w h i c h e x i s t . I r r i g a t i o n s c h e d u l i n g c a n be a c c o m p l i s h e d u s i n g d i r e c t r e a d i n g methods s u c h as t e n s i o m e t e r s and s o i l m o i s t u r e b l o c k s as i n d i c a t o r s of s o i l m o i s t u r e c o n t e n t . E s t i m a t e d r a t e s o f c o n s u m p t i v e u s e c o u p l e d w i t h g r a v i m e t r i c d e t e r m i n a t i o n s of e x i s t i n g s o i l m o i s t u r e p r o v i d e an e x c e l l e n t b a s i s f o r p r e d i c t i n g i r r i g a t i o n s ( J e n s e n , 1970) and c o n s t i t u t e p e r h a p s the o l d e s t known method o f s c h e d u l i n g i r r i g a t i o n . E f f i c i e n t i r r i g a t i o n i m p l i e s c o m p l e t e c o n t r o l o f t h e a v a i l a b l e s o i l m o i s t u r e r e s e r v o i r . Such c o n t r o l r e q u i r e s a d e q u a t e knowledge of t h e s o i l m o i s t u r e c o n t e n t a t a l l t i m e s , 1 4 and the a p p l i c a t i o n of j u s t enough water t o r e f i l l t h i s r e s e r v o i r t o the d e s i r e d l e v e l , p l u s the l e a c h i n g r equirement f o r s a l t c o n t r o l where n e c e s s a r y . From l i t e r a t u r e t h e r e appears t o be two major t h e o r i e s r e g a r d i n g p l a n t response t o water s u p p l y and d i f f e r e n t s o i l m o i s t u r e l e v e l s t h a t a r e h e l p f u l t o the i r r i g a t o r : one t h e o r y , put f o r w a r d by Veihmeyer and H e n d r i c k s o n (1950 and 1955) m a i n t a i n s t h a t the u t i l i z a t i o n of s o i l m o i s t u r e by p l a n t s i s u n i f o r m l y e f f e c t i v e t hroughout the whole range between permanent w i l t i n g p o i n t (PWP) and f i e l d c a p a c i t y (FC) c a l l e d the a v a i l a b l e water s t o r a g e c a p a c i t y (AWSC); the o p p o s i n g t h e o r y , a d v o c a t e s of which are F u r r and T a y l o r (1939), Hagan et a l , (1959) and o t h e r s , c l a i m s t h a t p l a n t s do respond d i f f e r e n t i a l l y t o v a r i a t i o n s i n s o i l m o i s t u r e c o n t e n t w i t h i n the a v a i l a b i l i t y range, and t h a t d e p l e t i o n of s o i l m o i s t u r e , even above PWP may a p p r e c i a b l y reduce growth r a t e s and f i n a l y i e l d s . L i t t l e c o n t r o v e r s y e x i s t s over these o p p o s i n g views as the l a t t e r t h e o r y i s of much wider a c c e p t a b i l i t y and a p p l i c a t i o n i n i r r i g a t i o n d e s i g n and s c h e d u l i n g . The reason i s c l e a r : i f the Ve i h m e y e r - H e n d r i c k s o n t h e o r y i s j u s t i f i e d , t h a t p l a n t s indeed u t i l i z e s o i l m o i s t u r e u n i f o r m l y w e l l i n the whole range between PWP and FC, then t h e r e i s no need t o i r r i g a t e as l o n g as the s o i l m o i s t u r e c o n t e n t has not reached PWP and i t f o l l o w s t h a t the o p t i m a l i r r i g a t i o n regime i s one i n which water i s a p p l i e d whenever t h i s p o i n t i s reached. However, s e v e r a l p o i n t s have been put f o r w a r d (Haise and Hagan, 1967; Je n s e n , 1968; Denmead and Shaw, 1962; Stewart 1 5 e t a l , 1975; and E n g l i s h and Nuss, 1982) a g a i n s t a l l o w i n g m o i s t u r e d e p l e t i o n t o a p p r o a c h c l o s e l y , t h e permanent w i l t i n g p e r c e n t a g e b e c a u s e , t h i s may c a u s e f a i l u r e o f t h e c r o p t o o b t a i n enough water from s o i l due i n p a r t t o (a) p o s i t i o n of t h e permanent w i l t i n g p e r c e n t a g e on t h e e n e r g y - s o i l m o i s t u r e ( s o i l m o i s t u r e c h a r a c t e r i s t i c o r r e t e n t i o n ) c u r v e i n t h e r e g i o n where a s l i g h t d e c r e a s e i n m o i s t u r e c o n t e n t r e s u l t s i n a g r e a t i n c r e a s e i n r e s i s t a n c e t o r e m o v a l o f t h e w a t e r , (b) s u p p l y of wa t e r may f a i l b e c a u s e o f t h e s l o w n e s s o f movement o f water i n t o t h e mass of s o i l d r i e d by r o o t s , ( c ) f a i l u r e o f r o o t s t o e l o n g a t e r a p i d l y enough i n t o r e g i o n s where t h e r e i s s t i l l w a t er above t h e w i l t i n g p o i n t , and (d) d e p e n d i n g on a v a i l a b i l i t y and c o s t o f water a n d t h e market v a l u e of t h e c r o p b e i n g c u l t i v a t e d , o n l y a d e f i n e d f r a c t i o n of t h e a v a i l a b l e w a t e r s t o r a g e c a p a c i t y o f t h e s o i l i s a l l o w e d t o be d e p l e t e d by c r o p b e f o r e w ater a p p l i c a t i o n i s n e c e s s a r y i f s t r e s s - i n d u c e d e c o n o m i c y i e l d d e p r e s s i o n i s t o be a v o i d e d . T h i s p e r c e n t a g e o f AWSC o r a v a i l a b i l i t y c o e f f i c i e n t ( a s d e f i n e d i n B.C. I r r i g a t i o n D e s i g n M a n u a l , 1983) m u l t i p l i e d by t h e AWSC g i v e s an e s t i m a t e of t h e maximum s o i l water d e f i c i t . In e f f e c t , t h e a v a i l a b i l i t y c o e f f i c i e n t i s c h a n g e d d e p e n d i n g on t h e m a r k e t ' v a l u e o f t h e c r o p grown on any p a r t i c u l a r s o i l . T h i s c o n c e p t was i m p l i c i t l y u t i l i z e d i n t h r e e of t h e i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s a d o p t e d i n t h i s i n v e s t i g a t i o n . The e x p e r i m e n t was b a s e d on an i r r i g a t i o n s c h e d u l i n g p r o c e d u r e by w h i c h t h e r e was a - f i x e d and e q u a l l e n g t h of time i n t e r v a l between i r r i g a t i o n s f o r any g i v e n s c h e d u l i n g t e c h n i q u e . 1 6 The c o n c e p t of " i r r i g a t i o n i n t e r v a l " i s a t y p i c a l l y low-f r e q u e n c y method (common w i t h g r a v i t y s y s t e m s ) r e q u i r i n g c o m p l e t e l y f i l l i n g t h e e f f e c t i v e r o o t i n g d e p t h of a g i v e n c r o p t o h i g h m a t r i c p o t e n t i a l and o f t e n t o f i e l d c a p a c i t y and t h e n a l l o w i n g g r a d u a l e x t r a c t i o n u n t i l a s t i p u l a t e d w a t e r l e v e l above permanent w i l t i n g i s r e a c h e d when i r r i g a t i o n i s a p p l i e d a g a i n . T h i s a p p l i c a t i o n method may c a u s e a l m o s t i n e v i t a b l y , d e e p p e r c o l a t i o n and p o s s i b l y , s u r f a c e r u n o f f (Hobbs and Krogman, 1978). A c c o r d i n g t o Hobbs and Krogman ( 1 9 7 8 ) , deep p e r c o l a t i o n c a n be m i n i m i z e d o r even e l i m i n a t e d by a p p l y i n g l e s s w a t e r t h a n i s r e q u i r e d t o r e p l e n i s h t h e s o i l p r o f i l e , but a p p l y i n g i t f r e q u e n t l y enough t h a t m o i s t u r e s t r e s s does n o t o c c u r . T h i s s u p p o r t s t h e e a r l i e r s u g g e s t i o n by Phene (1974) t h a t i n s o i l s w i t h d i s t i n c t i v e l y low s t o r a g e c a p a c i t i e s , h i g h - f r e q u e n c y i r r i g a t i o n s y s t e m s make i t p o s s i b l e t o keep t h e s o i l m a t r i c p o t e n t i a l s h i g h and a t t h e same t i m e l e a v e s u f f i c i e n t c a p a c i t y t o s t o r e i n t e r m i t t e n t r a i n t h a t o f t e n o c c u r s i n humid r e g i o n s d u r i n g t h e c r o p p i n g s e a s o n . E a r l i e r , R a w l i n s (1973) had r e c o r d e d t h a t as l o n g as s u f f i c i e n t w a t e r i s a p p l i e d t o meet t h e t r a n s p i r 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 ) demands of t h e c r o p , a d d i n g e x t r a w a t e r s h o u l d ' not i n c r e a s e s o i l w a t e r c o n t e n t s i g n i f i c a n t l y ; i t would s i m p l y a l l o w more t o e s c a p e o u t t h e b o t t o m of t h e r o o t z o n e . T h i s means t h a t low s o i l w a t e r c o n t e n t can be e l i m i n a t e d as a f a c t o r a f f e c t i n g p l a n t g r o w t h w i t h o u t w a s t i n g w a t e r t o d eep p e r c o l a t i o n . T h i s l a t t e r r e s e a r c h e r c o n c l u d e d t h a t t h e o n l y r e a s o n f o r a p p l y i n g w ater i n e x c e s s of 1 7 e v a p o t r a n s p i r a t i o n under h i g h - f r e q u e n c y i r r i g a t i o n management i s t o p r o v i d e l e a c h i n g t o c o n t r o l s a l i n i t y and, i n s t e a d of i r r i g a t i n g on t h e b a s i s of s o i l w a t e r p o t e n t i a l o r s o i l water c o n t e n t , the i r r i g a t o r need o n l y a d j u s t t h e r a t e of water a p p l i c a t i o n t o c o n t r o l the f l u x of water out a t t h e b o t t o m o f t h e r o o t z o n e . The water h o l d i n g c a p a c i t y o f t h e s o i l , t h e r e f o r e , becomes u n i m p o r t a n t . I t i s w e l l known t h a t w i t h good d e s i g n and water management, s p r i n k l i n g i n h e r e n t l y e l i m i n a t e s r u n o f f and deep p e r c o l a t i o n but Hobbs and Krogman (1978) f u r t h e r add t h a t a p p l i c a t i o n o f s m a l l amount of w a t e r more f r e q u e n t l y i n c r e a s e s e v a p o r a t i v e l o s s e s f r o m n o z z l e s p r a y s and wet s o i l s u r f a c e s but a d m i t t e d l y , d eep p e r c o l a t i o n and r u n o f f w o u l d be a v o i d e d . U s i n g s p r i n k l e r s , i r r i g a t i o n f r e q u e n c y c a n be h i g h l y m a n i p u l a t e d . V a r i o u s s c h e d u l i n g a l t e r n a t i v e s e x i s t w i t h t h i s t e c h n o l o g y u n l i k e w i t h s u r f a c e i r r i g a t i o n w h i c h i s f a c e d w i t h t h e f u n d a m e n t a l c o n s t a i n t s of f l o w o v e r t h e s o i l s u r f a c e t o d i s t r i b u t e w a t e r f r o m a t u r n o u t t o t h e f i e l d r e q u i r i n g a minimum d e p t h of water s i m p l y t o a c h i e v e c o v e r a g e , and a f i x e d c o s t a s s o c i a t e d w i t h e a c h a p p l i c a t i o n of w a t e r . R a w l i n s and R a a t s (1975) p o i n t e d o u t c e r t a i n e conomic b e n e f i t s t o be g a i n e d by h i g h f r e q u e n c y i r r i g a t i o n w i t h p r e s s u r i z e d s y s t e m s . They o b s e r v e d t h a t t h e c a p i t a l c o s t s of s u c h sytems depend l a r g e l y upon p i p e s i z e ( w i t h i n t h e l i m i t of t h e pumping p r e s s u r e ) , w h i c h i n t u r n d e p e n d s upon w a t e r d e l i v e r y r a t e . D e l i v e r y r a t e , and t h e r e f o r e c a p i t a l c o s t s , c an be m i n i m i z e d by d e s i g n i n g t h e s y s t e m f o r c o n t i n u o u s o p e r a t i o n . 1 8 . They added f u r t h e r m o r e , t h a t f r e q u e n t i r r i g a t i o n s may o p t i m i z e t h e r o o t e n v i r o n m e n t w h i l e r e d u c i n g w a t e r use so t h a t maximum y i e l d s c o u l d be r e a l i z e d w i t h a minimum of w a t e r . T h e s e c o n c l u s i o n s were p r e d i c t e d on s e v e r a l i m p l i c i t a s s u m p t i o n s : t h a t t h e c r o p i s t o be f u l l y i r r i g a t e d , t h e p i p e c o s t i s t h e d o m i n a n t component of s y s t e m c o s t , and t h a t l a b o u r , pumping c o s t and m a i n t e n a n c e c o s t s w i l l not i n c r e a s e s i g n i f i c a n t l y w i t h h i g h -f r e q u e n c y i r r i g a t i o n . W i t h h i g h f r e q u e n c y d e f i c i t i r r i g a t i o n , w a t e r w i l l be a p p l i e d f r e q u e n t l y but i n amounts t o o low t o p r e v e n t t h e d e c l i n e of s o i l m o i s t u r e and c r o p s w i l l e x p e r i e n c e m o d e r a t e s t r e s s more or l e s s c o n t i n u o u s l y . In t h e c a s e of l o w - f r e q u e n c y d e f i c i t i r r i g a t i o n ( l o n g i n t e r v a l method) s o i l m o i s t u r e w i l l f l u c t u a t e w i t h i n a w i d e r r a n g e . A heavy i r r i g a t i o n w i l l be f o l l o w e d by a l o n g p e r i o d o f e x t r a c t i o n d u r i n g w h i c h t h e s t r e s s e x p e r i e n c e d by t h e c r o p w i l l r a n g e from none a t a l l t o s e v e r e . A s u b s e q u e n t f u l l i r r i g a t i o n w i l l t h e n r e f i l l t h e p r o f i l e and t h e p a t t e r n r e p e a t s i t s e l f . Whether y i e l d s w i l l d i f f e r s i g n i f i c a n t l y o r n o t under t h e s e two w a t e r i n g r e g i m e s i s an e s s e n t i a l q u e s t i o n . R e s e a r c h on t h i s q u e s t i o n i s i n c o n c l u s i v e . M i l l e r (1976) r e p o r t e d r e l a t i v e l y good y i e l d s o f s u g a r b e e t s , wheat and beans i n a h i g h - f r e q u e n c y d e f i c i t i r r i g a t i o n . A c c o r d i n g t o M i l l e r , t h e h i g h i r r i g a t i o n f r e q u e n c i e s a p p e a r e d t o m i t i g a t e t h e e f f e c t s o f w a t e r d e f i c i t s . R e s u l t s of Hobbs and Krogman (1978) on wheat i n Canada s u p p o r t e d M i l l e r ' s f i n d i n g s . However, o t h e r w o r k e r s have a r r i v e d a t o p p o s i t e c o n c l u s i o n s i n s i m i l a r s t u d i e s . F e d e r e s and F a c i 1 9 (1980) f o u n d t h a t y i e l d s p r o d u c e d under d e f i c i t h i g h - f r e q u e n c y i r r i g a t i o n were t h e same a s , or l o w e r t h a n , y i e l d s p r o d u c e d u n d e r normal f r e q u e n c i e s w i t h t h e same l e v e l s of d e f i c i t . The l o w e r y i e l d s r e p o r t e d under t h e h i g h - f r e q u e n c y r e g i m e were a t t r i b u t e d t o t h e l o w e r a p p l i c a t i o n e f f i c i e n c y a s s o c i a t e d w i t h t h a t t e c h n i q u e . F o r t h e moment, t h e r e f o r e , t h e e f f e c t of i r r i g a t i o n f r e q u e n c y on c r o p y i e l d s u nder d e f i c i t h i g h - f r e q u e n c y i r r i g a t i o n c a n a t t h e b e s t be r e g a r d e d as u n c e r t a i n . T h i s r e s e a r c h w i l l c e r t a i n l y be a u s e f u l c o n t r i b u t i o n i n t h i s d i r e c t i o n . 2.3 Water S u p p l y and N u t r i e n t A v a i l a b i l i t y t o C r o p s T o d a y ' s e m p h a s i s on e f f i c i e n t c r o p p r o d u c t i o n has s t i m u l a t e d g r e a t e r c o n c e r n f o r t h e methods u s e d t o i n v e s t i g a t e c r o p r e s p o n s e s t o v a r i o u s g r o w t h f a c t o r s i n t h e f i e l d . T h e r e a r e s e v e r a l f a c t o r s but s o i l w a t e r and s o i l f e r t i l i t y have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . Box and H u n t e r (1958) o b s e r v e d t h a t t h e p r o d u c t i o n a p p r o a c h t o d e s c r i b i n g c r o p r e s p o n s e s i s u s e d f o r two r e a s o n s namely: ( i ) t o f i n d the. c o m b i n a t i o n s o f v a r i a b l e s t h a t g i v e t h e b e s t y i e l d a nd ( i i ) t o d e t e r m i n e t h e c h a r a c t e r i s t i c s o f t h e r e s p o n s e s u r f a c e ( s o i l o r b r o a d l y s p e a k i n g , l a n d ) i n t h e n e i g h b o u r h o o d of t h e optimum c o m b i n a t i o n o f g r o w t h f a c t o r s . The r e s p o n s e s u r f a c e can t h e n be u s e d t o d e t e r m i n e whether management c a n be m o d i f i e d i f c o n d i t i o n s c h a n g e . The i n t e r a c t i o n s between w a t e r s u p p l y and s o i l f e r t i l i t y w i t h r e s p e c t t o t h e i r e f f e c t on c r o p y i e l d i s complex. In a c r i t i c a l r e v i e w of t h i s s u b j e c t , B l a c k (1966) i n d i c a t e d t h a t 20 c r o p y i e l d may e v i d e n t l y be i n c r e a s e d , u n a f f e c t e d , or d e c r e a s e d by a g i v e n c h a n g e i n f e r t i l i t y l e v e l , d e p e n d i n g on t h e m a g n i t u d e of t h e c h a n g e , t h e i n i t i a l s o i l f e r t i l i t y l e v e l , and t h e water s u p p l y . F u r t h e r m o r e , V e i t s ( 1 9 6 2 ) , B l a c k ( 1 9 6 6 ) , H s i a o ( 1 9 7 3 ) , and Begg and T u r n e r (1976) have g i v e n good a n a l y s e s and summaries of most o f t h e i n f o r m a t i o n a v a i l a b l e on t h e e f f e c t s of f e r t i l i z e r s on y i e l d and a s s o c i a t e d e v a p o t r a n s p i r a t i o n a s a f f e c t e d by c a n o p y d e n s i t y under c o n d i t i o n s w i t h e i t h e r a d e q u a t e o r d e f i c i e n t s o i l w a t e r . B l a c k (1966) i n h i s c o n t r i b u t i o n , h o l d s t h e view t h a t where t h e r e i s a d e f i c i e n c y of water f o r e v a p o r a t i o n f r o m b a r e s o i l c o n t a i n i n g a v a i l a b l e w a t e r a t some d e p t h below t h e s u r f a c e , t h e h i g h c o n d u c t i v i t y of p l a n t s f o r w a t e r s u b s t i t u t e s i n p a r t f o r t h e l o s s o f c o n d u c t i v i t y f o r w a t e r t h a t h as o c c u r r e d i n t h e s u r f a c e p o r t i o n o f t h e s o i l . Under t h i s c o n d i t i o n , an i n c r e a s e i n d e n s i t y o f v e g e t a t i v e c a n o p y f r o m z e r o up t o t h e maximum t h e s o i l w i l l s u p p o r t , t h e n c a u s e s an i n c r e a s e i n e v a p o t r a n s p i r a t i o n ; t h e c o n t r o l i s i n t h e s o i l . Under m o i s t c o n d i t i o n s , t h e c o n t r o l i s i n t h e a t m o s p h e r e . However, under i n t e r m e d i a t e c o n d i t i o n s , t h e c o n t r o l may be p a r t l y i n t h e a t m o s p h e r e , p a r t l y i n t h e s o i l and p a r t l y i n t h e p l a n t s . The t e r m ' d e n s i t y of v e g e t a t i v e c a n o p y ' as d e f i n e d by B l a c k ( 1 9 6 6 ) , i s t h e m a r k e t a b l e y i e l d i n f o r a g e s and i s u s e d o f t e n b e c a u s e of a s s o c i a t i o n of t r a n s p i r a t i o n w i t h e x p a n s e of t r a n s p i r i n g s u r f a c e . D i f f e r e n c e s i n s o i l , f e r t i l i t y a f f e c t e v a p o t r a n s p i r a t i o n p r i m a r i l y by c h a n g i n g t h e e x p a n s e of t r a n s p i r i n g s u r f a c e and o n l y i n a m i n o r way by c h a n g i n g t h e 21 c h a r a c t e r of t h e s u r f a c e . B l a c k f u r t h e r s u g g e s t s t h a t an i n c r e a s e i n c r o p y i e l d p r o d u c e d by i n c r e a s i n g s o i l f e r t i l i t y d o e s n o t p r o d u c e a c o r r e s p o n d i n g i n c r e a s e i n e v a p o t r a n s p i r a t i o n ( E T ) ; on t h e c o n t r a r y , wide d i f f e r e n c e s i n y i e l d i n d u c e d by d i f f e r e n c e s i n s o i l f e r t i l i t y may r e s u l t i n o n l y r e l a t i v e l y s m a l l d i f f e r e n c e s i n ET. T h i s i s t o s a y , i n e s s e n c e , t h a t t h e m a g n i t u d e of t h e i n c r e a s e i n y i e l d t h a t c an be o b t a i n e d by i n c r e a s i n g t h e s o i l f e r t i l i t y u n der a g i v e n water r e g i m e may be s a i d t o depend on t h e i n i t i a l f e r t i l i t y l e v e l and t h e c a p a b i l i t y o f t h e c r o p t o p r o d u c e a d d i t i o n a l d r y m a t t e r under t h e p r e v a i l i n g c i r c u m s t a n c e s . An i n c r e a s e i n p r o d u c t i o n of d r y m a t t e r by c r o p s w i t h o u t an a p p r e c i a b l e i n c r e a s e i n use o f w a t e r under c o n d i t i o n s o f a d e f i c i e n c y o f w a t e r f o r e v a p o t r a n s p i r a t i o n i s s a i d t o be a c h i e v e d i n many ways i n c l u d i n g : (a) c a p a b i l i t y of p l a n t s a t a h i g h e r s o i l f e r t i l i t y l e v e l t o c a r r y on p h o t o s y n t h e s i s a t g r e a t e r r a t e t h a n t h o s e a t a c o m p a r a t i v e l y l o w e r f e r t i l i t y l e v e l d e s p i t e somewhat g r e a t e r d e f i c i e n c y o f w a t e r ( V e i t s , 1966; Begg and T u r n e r , 1976), ( i i ) d i f f e r e n c e s i n r a t e o f use o f wa t e r a t d i f f e r e n t t i m e s . Two v a r i a t i o n s o f t h e l a t t e r mechanism o f a d d i t i o n a l d r y m a t t e r a c c u m u l a t i o n c o u l d o c c u r : f i r s t , B l a c k (1966) o b s e r v e d i n an e x p e r i m e n t i n w h i c h water was us e d more r a p i d l y from f e r t i l i z e d t h a n f r o m u n f e r t i l i z e d s o i l w h i l e t h e wa t e r s u p p l y was ample and more s l o w l y a f t e r t h e a v a i l a b l e w a t e r was l a r g e l y d e p l e t e d and s e c o n d l y , a c c o r d i n g t o Sneva et a_l ( 1 9 5 8 ) , f e r t i l i z e d w h e a t g r a s s under d e s e r t c o n d i t i o n s i n Oregon made more r a p i d growth e a r l y i n t h e s e a s o n , e x h a u s t e d more 22 r a p i d l y t h e water s u p p l y i n s o i l , and m a t u r e d a t an e a r l i e r d a t e t h a n d i d t h e u n f e r t i l i z e d g r a s s . A l t h o u g h n u t r i e n t and water a b s o r p t i o n a r e i n d e p e n d e n t p r o c e s s e s i n t h e p l a n t r o o t ( V e i t s , 1972), t h e n e c e s s i t y f o r a v a i l a b l e water i n b o t h th e p l a n t and s o i l f o r g r o w t h and n u t r i e n t t r a n s p o r t makes them i n t i m a t e l y r e l a t e d e s p e c i a l l y as t h e q u a n t i t y of w a t e r i n t h e s o i l a f f e c t s not o n l y t h e amount ( c o n c e n t r a t i o n t i m e s volume) of n u t r i e n t i n t h e s o i l s o l u t i o n , b u t a l s o , t h e r a t e o f movement t o t h e r o o t by d i f f u s i o n and f l o w i n t h e water (mass f l o w ) as w ater i s a b s o r b e d by t h e r o o t (Unger e t a l , 1981). V e i t s (1972) s t a t e s t h a t u n d e r a f i e l d s i t u a t i o n , s o i l w a ter c o n t e n t n e a r f i e l d c a p a c i t y a l l o w s f o r t h e b e s t c o m b i n a t i o n of s u f f i c i e n t a i r s p a c e f o r oxygen d i f f u s i o n , t h e g r e a t e s t amount of n u t r i e n t i n s o l u t i o n form, t h e g r e a t e s t c r o s s - s e c t i o n a l a r e a f o r d i f f u s i o n o f i o n s and mass f l o w o f w a t e r and f a v o u r a b l e c o n d i t i o n s f o r r o o t e x t e n s i o n . T h i s l a s t s t a t e m e n t , a s s u m i n g i t i s t r u e f o r a l l e x p e r i m e n t a l s i t u a t i o n s , p o s e s c l e a r l y , two p r a c t i c a l c h a l l e n g e s : • a p l a n t g r o w i n g i n . t h e f i e l d i s u s u a l l y s u b j e c t t o f l u c t u a t i o n s i n water a v a i l a b i l i t y t h a t may r a n g e from s o i l s a t u r a t i o n t o d r o u g h t . The i n v e s t i g a t o r w o u l d t h e r e f o r e be c o n f r o n t e d w i t h t h e p r o b l e m of m a i n t a i n i n g c o n s t a n t s o i l w a t e r a t a s u c t i o n of 1/3 b a r ( w h i c h i s t h e m a t r i c p o t e n t i a l a t FC) a p p r o x i m a t e l y . T h i s r e q u i r e m e n t c o u l d be met by d r a i n a g e p r o v i s i o n s and 23 i r r i g a t i o n s c h e d u l i n g p r e c i s i o n s - the l a t t e r b e i n g p a r t of t h i s s t u d y . • a s i t u a t i o n analogous t o the r e l a t i o n s h i p between the p o t e n t i a l or f r e e energy of - water i n a s o i l ( a v a i l a b i l i t y ) and the amount p r e s e n t ( s u p p l y ) , i s the amount of n u t r i e n t p r e s e n t or added i n the s o l i d phase ( q u a n t i t y f a c t o r ) i n a g i v e n c i r c u m s t a n c e t o g u a r a n t e e c o n t i n u e d r e p l e n i s h m e n t of the a b s o r b a b l e n u t r i e n t form i n s o i l s o l u t i o n ( i n t e n s i t y f a c t o r ) t o ensure o p t i m a l a b s o r p t i o n by a growing c r o p i n view of the p o i n t above. The a b i l i t y of the s o i l system t o r e p l e n i s h the s o i l s o l u t i o n i s measured by the c a p a c i t y f a c t o r d e f i n e d (Sumner and B o s w e l l , 1981) as the r a t i o of t h e change i n the q u a n t i t y f a c t o r t o u n i t change i n i n t e n s i t y f a c t o r . But a c c o r d i n g t o L a r s e n ( 1 9 6 7 ) , h a v i n g a p a r t i c u l a r n u t r i e n t i n the l i q u i d phase i s o n l y one f a c e t of the s o l u t i o n t o the c r o p management problem; i t then has to be s u p p l i e d a t a r a t e ( d i f f u s i o n f a c t o r ) s u f f i c i e n t t o s a t i s f y the needs of the p l a n t f o r h i g h y i e l d . The s i z e and morphology of a r o o t system have a tremendous e f f e c t on the e x t r a c t i o n . Thus, Sumner and B o s w e l l (1981) remark t h a t d i a g n o s e s of a n u t r i e n t problem and i t s c o r r e c t i o n and, s c h e d u l i n g i r r i g a t i o n t o m a i n t a i n t i m e l y and adequate consumptive water use of c r o p s can o n l y be c o n s i d e r e d i n the c o n t e x t of the whole s o i l - p l a n t - a t m o s p h e r e continuum. In o t h e r words, f o r a p a r t i c u l a r e n v i r o n m e n t , recommendations of water r e q u i r e m e n t of a s t a t e d c r o p would be d i c t a t e d by the l e v e l of f e r t i l i z a t i o n t o which economic y i e l d response w i l l be o b t a i n e d . 24 F e r t i l i z e r i s u s u a l l y a p p l i e d e i t h e r o v e r t h e e n t i r e s o i l s u r f a c e by b r o a d c a s t i n g (and a t t i m e s p l o u g h e d i n ) ; i n a l o c a l i s e d a r e a below the s u r f a c e c l o s e t o s e e d o r p l a n t , u s u a l l y t e r med ' b a n d i n g ' or row p l a c e m e n t ; o r by ' f e r t i g a t i o n ' , a r e c e n t i n n o v a t i o n u s e d t o d e s c r i b e t h e i n t r o d u c t i o n and a p p l i c a t i o n o f s o l u b l e f e r t i l i z e r m a t e r i a l t o t h e l a n d t h r o u g h i r r i g a t i o n w a t e r , t y p i c a l l y i n s p r i n k l e r and t r i c k l e s y s t e m s . T h e s e p a t t e r n s of f e r t i l i z e r a p p l i c a t i o n , c o u p l e d w i t h t h e a c c u m u l a t i o n o f o r g a n i c m a t t e r and t h e c y c l i n g o f n u t r i e n t s f r o m t h e s u b s o i l t o t h e s u r f a c e by p l a n t s and e a r t h - d w e l l i n g f a u n a t e n d t o f a v o u r ( V e i t s , 1972) c o n c e n t r a t i o n s o f e x t r a c t a b l e and e x c h a n g e a b l e n u t r i e n t s i n t h e s u r f a c e s o i l , h e l d t h e r e by a d s o r p t i o n on c l a y s and o r g a n i c m a t t e r . U n f o r t u n a t e l y enough, water a v a i l a b i l i t y f l u c t u a t e s most i n t h e s u r f a c e s o i l t h a t u s u a l l y c o n t a i n s h i g h e s t c o n c e n t r a t i o n s of t h e s o i l and f e r t i l i z e r n u t r i e n t s . The p r e p o n d e r a n c e of e v i d e n c e o b t a i n a b l e i n t h e works of V e i t s ( 1 9 6 6 ) , Begg and T u r n e r (1976) and Sumner and B o s w e l l (1981) i n d i c a t e s t h a t d r o u g h t o r w a t e r d e f i c i t d e c r e a s e s n u t r i e n t a v a i l a b i l i t y t o p l a n t s as m e a s u r e d by t o t a l n u t r i e n t u p t a k e and sometimes i n r e d u c e d c o n c e n t r a t i o n . Sumner and B o s w e l l s t a t e i n p r i n c i p l e , t h a t as c r o p d e p l e t e s w ater i n t h e s u r f a c e l a y e r o f s o i l so t h a t n u t r i e n t - l a d e n mass f l o w of w ater i s u n a b l e t o meet i t s demand, i t s g r o w t h r a t e w i l l d e c r e a s e o r i n t h e a l t e r n a t i v e , t h e c r o p w o u l d have t o s p e e d up i t s r a t e of r o o t e x t e n s i o n i n t o new, u n d e p l e t e d s o i l a r e a s i n o r d e r t o i n c r e a s e t h e r a t e s of w ater and n u t r i e n t s u p p l i e s t o s u s t a i n 25 growth. With s p e c i f i c r e f e r e n c e t o phosphate, the t e s t n u t r i e n t i n t h i s s t u d y , most r e s e a r c h e r s i n c l u d i n g L a r s e n (1967), W i l l i a m s (1971), and Hagin and Turner (1982) h o l d the common view t h a t phosphate uptake by c r o p i s i n f l u e n c e d v e r y much by r o o t p r o l i f e r a t i o n s i n c e the u t i l i z a b l e phosphate i o n ( H 2 P O 4 ~ i o n i c s p e c i e s ) i s l i m i t e d i n movement. T h e r e f o r e , m o i s t u r e c o n t e n t of s o i l s , i n f l u e n c i n g b oth r o o t development' and phosphate d i f f u s i o n , i n t e r a c t s t r o n g l y w i t h phosphorus uptake by p l a n t s c u l t i v a t e d and e u t r o p h i c ones a l l a l i k e . In d r y c o n d i t i o n s or not w e l l - i r r i g a t e d f i e l d s , the a b s o r p t i o n of phophorus by c r o p s i s reduced, presumably, by l o n g e r d i f f u s i o n p a t h s which may be compensated f o r by an i n c r e a s e i n phosphate c o n c e n t r a t i o n ( O l s e n e_t a l , 1961). I t was shown f o r soybean t h a t phosphorus uptake was i m p a i r e d by h i g h m o i s t u r e s t r e s s , l a r g e l y t h r o u g h i t s i n f l u e n c e on phosphate d i f f u s i o n i n the s o i l ( M a r a i s and Wiersma, 1976). A c c o r d i n g t o B l a c k (1966), i n humid and i r r i g a t e d r e g i o n s , when the plough l a y e r q u i c k l y goes d r y , the s u b s o i l i s u s u a l l y m o i s t t o depths beyond the maximum e x t e n s i o n of c r o p r o o t s . An i n c r e a s e i n s o i l f e r t i l i t y under t h e s e c o n d i t i o n s may i n c r e a s e the use of s u b s o i l w a t e r , p r o v i d e d t h a t deep p e n e t r a t i o n of r o o t s i s not i n h i b i t e d by u n f a v o u r a b l e p h y s i c a l and c h e m i c a l p r o p e r t i e s of the s o i l . That i s t o say, t h a t , by i n c r e a s i n g the f e r t i l i z e r even i n the f a c e of d e f i c i e n t water s u p p l y i n the upper l a y e r s of a s o i l p r o f i l e , a c r o p w i l l be a b l e t o d e v e l o p deeper and denser r o o t system to e x p l o r e deeper l a y e r s of the 26 s o i l p r o f i l e f o r m o i s t u r e . But t o t h i s r e c o m m e n d a t i o n , V e i t s (1972) c a u t i o n s t h a t t h i s c o u l d be a n o t h e r k i n d of " d r o u g h t " a f f e c t i n g r o o t a c t i v i t y i n w h i c h t h e p l a n t i s g e t t i n g i t s w a t e r f r o m one p a r t of t h e s o i l d e f i c i e n t i n n u t r i e n t s w h i l e t h e s o i l ' s main s u p p l y of n u t r i e n t s i s l o c k e d up i n d r y s o i l l a c k i n g a c t i v e r o o t s . He adds however, t h a t wheat and some s p e c i e s o f w h e a t g r a s s (and p e r h a p s some many o t h e r c r o p s p e c i e s ) g r o w i n g i n wet s o i l have s u f f i c i e n t c a p a b i l i t y t o a b s o r b s u f f i c i e n t P a n d s t o r e i t f o r use d u r i n g p e r i o d s when P a v a i l a b i l i t y i n s u r f a c e s o i l i s low, but t h i s d e p ends on t h e c o n c e n t r a t i o n o f P i n t h e s o i l s o l u t i o n and t h e d u r a t i o n a n d f r e q u e n c y o f t h e wet o r d r y p e r i o d s . 2.4 S t a t e o f Knowledge i n I r r i g a t e d Cowpea R e s e a r c h Cowpeas- ( V i g n a u n g u i c u l a t a [ L . ] Walp.) a r e grown f o r d r y bean p r o d u c t i o n , m a t u r e . g r e e n p o d s , l e a v e s and hay. E f f i c i e n t i r r i g a t i o n p r a c t i c e s a r e needed f o r b o t h s h o o t biomas p r o d u c t i o n and d r y b e a n s . F o r s o y b e a n s , C o n s t a b l e and H e a r n (1980) p r o p o s e d t h a t f e wer i r r i g a t i o n s c o u l d be a p p l i e d d u r i n g t h e v e g e t a t i v e p h a s e t h a n i s t h e p r e s e n t c o m m e r c i a l p r a c t i c e i n A u s t r a l i a , and t h e w a t e r s a v e d c o u l d be u s e d t o grow a l a r g e r a r e a o f c r o p . S i m i l a r s t u d i e s have been u n d e r t a k e n f o r cowpea i n s e v e r a l e c o l o g i c a l z o n e s . T u r k e_t a_l (1980) and Shouse e t a_l (1981) d e m o n s t r a t e d t h a t f o r cowpeas i n a r a i n - f r e e e n v i r o n m e n t , w a t e r use may be r e d u c e d by w i t h h o l d i n g i r r i g a t i o n d u r i n g t h e v e g e t a t i v e s t a g e w i t h o u t a f f e c t i n g s e e d y i e l d . B u t , a d d e d T u r k and h i s g r o u p , t h e i n f l u e n c e of v e g e t a t i v e s t a g e d r o u g h t on s e e d 27 y i e l d may de p e n d upon s u b s e q u e n t e n v i r o n m e n t a l c o n d i t i o n s d u r i n g f l o w e r i n g and pod f i l l i n g . T h i s ' i s p a r t l y i n agr e e m e n t w i t h one • o f t h e o b s e r v a t i o n s of Summer f i e l d e_t a_l (1976) t h a t m o i s t u r e s t r e s s can r e d u c e p r o d u c t i v i t y c o n s i d e r a b l y d u r i n g t h e p e r i o d f r o m emergence t o f i r s t f l o w e r , b u t w i t h d e t e r m i n a t e c u l t i v a r s , t h i s may not s i g n i f i c a n t l y a f f e c t y i e l d s when s t r e s s o c c u r s t h e r e a f t e r . In a r e c e n t e x p e r i m e n t , Z i s k a and H a l l ( 1 9 8 3 ) , w o r k i n g on cowpea a t R i v e r s i d e , C a l i f o r n i a , f o u n d t h a t v e g e t a t i v e s t a g e d r o u g h t c a u s e d s i g n i f i c a n t r e d u c t i o n s i n s e e d y i e l d when t h e c r o p was i r r i g a t e d a t i n t e r v a l s o f 15 d a y s . They f e l t t h a t t h i s i r r i g a t i o n t r e a t m e n t p r o b a b l y d i d n o t p e r m i t s u f f i c i e n t r e c o v e r y of g r o w t h a f t e r r e s u m p t i o n of n o r m a l i r r i g a t i o n f o l l o w i n g t h e v e g e t a t i v e s t a g e d r o u g h t . The h a r v e s t e d c r o p d i d i n f a c t r e c o r d l o w e r s h o o t b i o m a s s p r o d u c t i o n t h a n t h e ones t h a t were w e l l - w a t e r e d . In t h e same e x p e r i m e n t , v e g e t a t i v e s t a g e d r o u g h t d i d n o t i n f l u e n c e b i o m a s s p r o d u c t i o n u n d e r low or h i g h n i t r o g e n f e r t i l i z a t i o n . The a p p l i c a t i o n of f e r t i l i z e r P t o cowpea has been w i d e l y r e p o r t e d t o i m p r o v e n o d u l a t i o n , i n c r e a s e v e g e t a t i v e g r o w t h r a t e and hence d r y m a t t e r p r o d u c t i o n , and r a i s e g r a i n y i e l d ( T e w a r i , 1965; Rhoades, 1980; and Kang and N a n g j u , 1983). On t h e o t h e r hand, cowpea s u b j e c t e d t o w a t e r s t r e s s d u r i n g t h e a c t i v e v e g e t a t i v e s t a g e e x p e r i e n c e d r e d u c e d s e a s o n a l n i t r o g e n f i x a t i o n ; however, t h e r e d u c t i o n i n n i t r o g e n s u p p l y t o t h e p l a n t s was s m a l l , and d i d n o t r e d u c e s e e d y i e l d ( T e w a r i , 1 9 65). C l e a r l y , f r o m t h e o n s e t , t h e i n v e s t i g a t o r must be aware of whether the c r o p i s t o be i r r i g a t e d f o r hay o r i t s beans b e c a u s e 28 of s e v e r a l u n e q u i v o c a l f i n d i n g s i n d i f f e r e n t e n v i r o n m e n t s . These m a j o r h i g h l i g h t s would s e r v e a s a u s e f u l g u i d e t h o u g h s u c h f i n d i n g s i n t h e m s e l v e s a r e n o t u n i v e r s a l l y e x p e c t e d : i . M a j o r i n c r e a s e s i n water use e f f i c i e n c y may be a c h i e v e d by w i t h h o l d i n g i r r i g a t i o n f r o m p l a n t emergence t o t h e f i r s t a p p e a r a n c e of m a c r o s c o p i c f l o r a l buds, p r o v i d i n g a ' r e a s o n a b l e ' s u p p l y o f water i s p r e s e n t i n t h e s o i l p r o f i l e and no p r e c i p i t a t i o n o c c u r s ( Z i s k a and H a l l , 1983). In many c i r c u m s t a n c e s however ( A l l e n and L a m b e r t , 1971), e c o n o m i c a n a l y s i s i n d i c a t e s t h a t p l a n n e d w a t e r d e f i c i t s w i l l o n l y be p r o f i t a b l e i f y i e l d s a r e m a i n t a i n e d c l o s e t o maximum l e v e l s b e c a u s e t h e s a v i n g s f r o m r e d u c t i o n s i n a p p l i e d water may be s m a l l compared t o t h e v a l u e o f t h e c r o p . i i . E x c e s s i v e m o i s t u r e , a b u n d a n t s o i l n i t r o g e n and h i g h p h o s p h o r u s a v a i l a b i l i t y combine t o f a v o u r l u x u r i a n t f o l i a g e p r o d u c t i o n . T h i s i s v e r y d e s i r a b l e i n hay p r o d u c t i o n o r f r e s h f o r a g e . Where cowpea i s c u l t i v a t e d f o r c o m m e r c i a l s e e d p r o d u c t i o n , e x c e s s i v e l e a f and stem p r o d u c t i o n i s c o n s i d e r e d s u p e r f l u o u s and has no r e a l a d v a n t a g e f o r pod p r o d u c t i o n and u l t i m a t e s e e d y i e l d (Kang and N a n g j u , 1983). i i i . V e g e t a t i v e s t a g e w ater s t r e s s h as been o b s e r v e d by Z i s k and H a l l (1983) t o r e s u l t i n i n c r e a s e d damage 29 due t o l e s s e r c o r n s t a l k b o r e r ' (or E l a s m o p a l p u s l i q n o s e l l u s ) and c h a r c o a l r o t ( M a crophomina p h a s e o l i ). F r e q u e n t i r r i g a t i o n a t any s t a g e of c r o p growth i n c r e a s e s c h a n c e s o f a t t a c k by f u n g i , v i r u s and nematodes ( S u m m e r f i e l d , 1974 and S i n h a , 1977). I t a p p e a r s t h a t t h e f a c t o r s r e s p o n s i b l e f o r t h e b r o a d a d a p t a t i o n of cowpeas a r e p o o r l y u n d e r s t o o d and S i n h a (1977) d i d a g r e e t h a t cowpea can y i e l d s a t i s f a c t o r i l y u n d e r a g r e a t e r d i v e r s i t y of c l i m a t i c , s o i l and c u l t u r a l c o n d i t i o n s t h a n most o t h e r l e g u m i n o u s c r o p s . The i n t e r a c t i v e e f f e c t s o f i r r i g a t i o n v e r s u s p h o s p h o r u s f e r t i l i z e r and i r r i g a t i o n v e r s u s n i t r o g e n f e r t i l i z e r were s t u d i e d r e s p e c t i v e l y by M a l i k (1974) i n I n d i a and Z i s k a and H a l l (1983) i n C a l i f o r n i a , U n i t e d S t a t e s and t h e s e e f f e c t s c o u l d be u s e d t o d e v e l o p cowpea p r o d u c t i o n f u n c t i o n s . But t h e s e c r o p p r o d u c t i o n f u n c t i o n s a r e o f l i m i t e d v a l u e b e c a u s e t h e y a r e s i t e s p e c i f i c , i n t e g r a t i n g a r a n g e of r e l a t i v e l y u n q u a n t i f i e d e n v i r o n m e n t a l e f f e c t s . T h e r e f o r e , f o r cowpea, e x p e r i m e n t a l s t u d i e s r e l a t i n g i r r i g a t i o n , f e r t i l i z e r a p p l i c a t i o n , c r o p y i e l d d e v e l o p m e n t and p h o s p h o r u s u p t a k e a r e n e e d e d f o r w ater and s o i l f e r t i l i t y management, e s p e c i a l l y i n a r e a s where p r o d u c t i o n f u n c t i o n s f o r t h e c r o p a r e n o t a v a i l a b l e a t p r e s e n t . T h i s f i e l d s t u d y , c o n d u c t e d t o o b t a i n i n f o r m a t i o n a b o u t cowpea p e r f o r m a n c e i n a humid e n v i r o n m e n t i s t h e r e f o r e , n o t u n r e a s o n a b l e . 30 I I I . MATERIALS AND METHODS A. EXPERIMENTAL METHODS 3. 1 I r r i g a t i o n System D e s i g n The f i e l d e x p e r i m e n t was c o n d u c t e d u s i n g a hand-move s p r i n k l e r l a t e r a l d e s i g n e d s p e c i f i c a l l y f o r t h i s s t u d y . The i r r i g a t i o n d e s i g n p r o c e d u r e i s e s s e n t i a l l y as d e s c r i b e d e l s e w h e r e ( BCMAF, 1983). M e t e o r o l o g i c a l d a t a from t h e w e a t h e r r e c o r d i n g s t a t i o n l o c a t e d on t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a campus were o b t a i n e d f o r 20 y e a r s and o n l y t h o s e o f t h e r e l e v a n t months were u t i l i z e d ( A p p e n d i x A ) . C r o p p o t e n t i a l e v a p o t r a n s p i r a t i o n was computed from C l a s s 'A' pan e v a p o r a t i o n r e c o r d s and c o e f f i c i e n t s a c c o r d i n g t o t h e method of D o o r e n b o s and P r u i t t ( 1 9 7 7 ) . C r o p c o e f f i c i e n t s f o r t h e i r r i g a t i o n months were e v a l u a t e d u s i n g p a r t o f A p p e n d i x A i n c o n j u n c t i o n w i t h t a b l e s g i v e n i n Doorenbos and Kassam ( 1 9 7 9 ) . S o i l t e x t u r e f r o m t h r e e r e p r e s e n t a t i v e p r o f i l e s i t e s o f t h e e x p e r i m e n t a l a r e a was f o u n d t o be sandy loam, 30cm t h i c k , o v e r l y i n g loamy sand a t l e a s t 60cm t h i c k ( T a b l e I ) . 31 T a b l e I - S o i l R e p o r t o f t h e E x p e r i m e n t a l A r e a SAMPLING CROP AND S 0 I L P R O F I L E MAX. SITES OR ROOT DEPTH DEPTH TEXTURE A W S C AWSC APPL. P I T S * (cm) (mm) RATE A Cowpea 0 - 3 0 SANDY 38. 1 68.6mm 11.4 ( V i g n a LOAM ( 1.5 i n ) mm/hr B u n g u i c u l a t a ) 30 - 60 LOAMY 30.5 SAND ( 1.2 i n ) 0.45 C 60 cm 60 - 90 LOAMY i n / h r • SAND * S o i l s amples were t a k e n f r o m t h r e e d e p t h s (0-30, 30-60, and 60-90 cm) from p i t s A, B and C. 3.1.1 Maximum C r o p ET E s t i m a t i o n By Pan E v a p o r a t i o n From D o o r e n b o s and P r u i t t ( 1 9 7 7 ) , t h e f o l l o w i n g e q u a t i o n s were e x t r a c t e d : E T 0 = Kp x ETp [ 3 . 1 ] and ET(max) = K x E T 0 [ 3 . 2 ] where: ET(max) = r e f e r e n c e c r o p maximum e v a p o t r a n s p i r a t i o n , mm/day 32 K = c r o p c o e f f i c i e n t E T 0 = r e f e r e n c e e v a p o r a t i o n ETp = C l a s s 'A' pan e v a p o r a t i o n r a t e , mm/day Kp = pan c o e f f i c i e n t . B a s e d on cowpea's l i f e s p a n o f 90-100 days when i t i s t o be h a r v e s t e d f o r f o r a g e , t h e t a b u l a t i o n i n T a b l e I I was o b t a i n e d u s i n g D o o r e n b o s and P r u i t t (1977) and D o o r e n b o s and Kassam ( 1 9 7 9 ) . A v e r a g e ( o v e r 2 0 y e a r s ) r e l a t i v e h u m i d i t y between May and A u g u s t , i n c l u s i v e , of 70-80% and wind s p e e d o f 112-144 km/day ( v e r y s t r o n g ) were e s t i m a t e d f r o m t h e q u o t e d r e f e r e n c e and u s e d i n s u b s e q u e n t c o m p u t a t i o n s . T a b l e II - Computed E v a p o t r a n s p i r a t i o n I n f o r m a t i o n MONTH PAN CROP REFERENCE MEAN PEAK COEFFICIENT COEFFICIENT EVAPORATION E'PORATION ET ( K p ) (K). ( E T 0 ) , E T ( m a x ) , mm/ mm/day mm/day day May 0.8 0.75 3.6 2.5 3.3 J u n e 0.8 0.75 3.5 2.5 3.3 J u l y 0.8 1 .05 3.9 4 . 1 4.6 Aug. 0.8 0.95 3.3 3.0 4. 1 S e p t . 0.7 0.90 3.3 3.0 3.6 33 3.1.2 I r r i g a t i o n System D e s i g n C o m p u t a t i o n s Cowpea has an e f f e c t i v e r o o t i n g d e p t h o f 60.5cm ( 2 f t ) . F o r th e s o i l a t t h e s i t e of t h e e x p e r i m e n t , t h e a v a i l a b l e w a t e r s t o r a g e c a p a c i t y , AWSC = 68.58mm ( T a b l e I ) . In t h i s d e s i g n , an a v a i l a b i l i t y c o e f f i c i e n t o f 50% was assumed. A v a i l a b i l i t y c o e f f i c i e n t (AV. C) r e p r e s e n t s t h a t p o r t i o n of t h e t o t a l AWSC t h a t can be d e p l e t e d b e f o r e i r r i g a t i o n i s n e c e s s a r y . Then, t h e f o l l o w i n g c a l c u l a t i o n s were p e r f o r m e d : (a) Maximum s o i l w a t e r d e f i c i t , MSWD, i s AWSC x Av. C = 68.58 x 50% = 34.28mm ( l . 3 5 i n ) . (b) G r o s s Water R e q u i r e m e n t , GWR = M S W D / A p p l i c a t i o n E f f i c i e n c y = 34.28/0.80 = 42.85mm(1.7in). H e r e , an a p p l i c a t i o n e f f i c i e n c y o f 80% was assumed b a s e d on wind s p e e d and a v e r a g e t e m p e r a t u r e d u r i n g t h e i r r i g a t i o n s e a s o n (BCMAF, 1983). (c ) I r r i g a t i o n a p p l i c a t i o n r a t e , A.R = GWR/Time s e t =42.85mm/6hrs = 7.14mm/hr ( 0 . 2 8 i n / h r ) . A t i m e s e t o f 6 h o u r s was c h o s e n b e c a u s e i t was i n t e n d e d t o o p e r a t e t h e i r r i g a t i o n s y s t e m d u r i n g t h e c a l m h o u r s o f t h e m o r n i n g and i f need be, e v e n i n g ; a l o n g e r p e r i o d o f t i m e s e t would e n t a i l e n c h r o a c h i n g i n t o t h e a f t e r n o o n h o u r s w h i c h were q u i t e windy, and t h i s was not d e s i r a b l e . S p r i n k l i n g r a t e o f 7.1 mm/hr (0.28 i n / h r ) i s l e s s than t h e maximum i n t a k e r a t e of t h e s o i l ( T a b l e I ) . T h e r e f o r e , t h i s A.R i s a c c e p t a b l e . (d) I r r i g a t i o n i n t e r v a l , I . I = MSWD/Peak ET. From T a b l e I I , t h e i n t e r v a l s a r e as f o l l o w s : May 10 days J u n e 10 " 34 J u l y 8 " A u g u s t 8 d a y s . 3.1.3 S p r i n k l e r S e l e c t i o n and C o n f i g u r a t i o n The s e l e c t i o n method was a compromise between t h e d e s i g n i n f o r m a t i o n and t h e w e t t e d d i a m e t e r a t optimum o p e r a t i n g p r e s s u r e (206.9 K P a ) . A R a i n b i r d T u r f S p r i n k l e r , model 2800A S t a n d a r d s p r a y n o z z l e was s e l e c t e d w i t h t h e f o l l o w i n g c o n f i g u r a t i o n : A p p l i c a t i o n r a t e 6.1 mm/hr (24 i n / h r ) N o z z l e 2800A-F [ F u l l ] O p e r a t i n g p r e s s u r e 206.9 KPa (30 p s i ) W e t t e d d i a m e t e r 7.3 m (24 f t ) Flow r a t e / n o z z l e 0.16 1/s (2.5 U.S gpm) C o e f f i c i e n t of U n i f o r m i t y ( m a n u f a c t u r e r ' s ) 84% Wind r a n g e 3.2 - 6.5 km/hr ( 2 - 5 mph) 3.1.4 L a t e r a l D e s i g n • 4 s p r i n k l e r s , e a c h s p r i n k l i n g 0.14 1/s (2.23 gpm) a t 206.9 KPa (30 p s i ) were u s e d . T o t a l f l o w i n t o t h e s i n g l e l a t e r a l was 0.56 1/s (8.92 gpm). T h i s f l o w r a t e c o u l d be d e l i v e r e d by 25.4mm i n t e r n a l d i a m e t e r p i p e w i t h o u t f l o w v e l o c i t y e x c e e d i n g 1.5 m/s (5 f t / s ) . • a 38.1mm d i a m e t e r ( i n t e r n a l ) PVC p i p e was c h o s e n f o r e a s e of h a n d l i n g i n t h e f i e l d . Figure 1 - Line-Source Sprinkler, System 12.80 SPRINKLER UNION 3.8 i™ (1 .5 in . ) PVC PIPE J L JX 1.52 3.66 «_ 1.52 « — 2.13 _ ^ LATERAL 3.66 L... END PLUG 3.0 FLEXIBLE HOSE POINT OF CONNECTION TO MAINLINE OUTLET PRESSURE GAUGE 38 mm (I.D) PVC PIPE 3 -HOSE ADAPTER SHUT-OFF FLOW CONTROL VALVE VALVE * * UNLESS STATED, ALL DIMENSIONS ARE IN m 36 • s p r i n k l e r s p a c i n g was 3.66m - an o v e r l a p of 50% a l o n g t h e l a t e r a l , s i n c e t h e i d e n t i c a l s p r i n k l e r s had a n o z z l e throw o f 3.66m when o p e r a t i n g a t 206.9 KPa. • F i g u r e 1 shows t h e l a t e r a l and o t h e r a c c e s s o r i e s . 3.1.5 M a i n l i n e D e s i g n 76 mm d i a m e t e r a l u m i n i u m p i p e s a l r e a d y b e i n g u s e d on t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a R e s e a r c h f a r m were u s e d . I r r i g a t i o n w a t e r o u t l e t was l o c a t e d a b o u t 3 m from t h e e x p e r i m e n t a l a r e a . Water was d e l i v e r e d a t a maximum p r e s s u r e o f 482.7 KPa f r o m t h e u n d e r g r o u n d n e t w o r k o f m a i n l i n e h y d r a n t c o v e r i n g t h e whole U n i v e r s i t y f a r m . 3.2 E x p e r i m e n t a l D e s i g n and Sy s t e m L a y o u t 3.2.1 I r r i g a t i o n S y stem C h a r a c t e r i s t i c s The s i n g l e l a t e r a l l i n e w i t h s u i t a b l y s p a c e d s p r i n k l e r s was d e s i g n e d t o p r o v i d e a w e t t i n g p a t t e r n w i t h f e a t u r e s s i m i l a r t o t h e ' c o n t i n u o u s v a r i a b l e d e s i g n ' f i r s t d e s c r i b e d by Fox (1973) and f u r t h e r d e v e l o p e d f o r i r r i g a t i o n r e s e a r c h by H u n d t o f t and Wu (1974) and-Hanks e t a l ( 1 9 7 6 ) . The s p r i n k l e r n o z z l e s i z e c h o s e n had a w e t t e d d i a m e t e r o f 7.3 m. T h i s was t h e w i d t h of an e x p e r i m e n t a l p l o t . F i g u r e 2 shows a s c h e m a t i c l a y o u t of t h e l i n e - s o u r c e s p r i n k l e r p l o t d e s i g n . The l i n e of s p r i n k l e r s was t h r o u g h t h e c e n t r e of t h e p l o t and a c r o s s t h e row d i r e c t i o n . The w e t t e d l e n g t h o f e a c h p l o t was 14.5m ( 4 8 f t ) . However, o n l y 10.9m ( 3 6 f t ) o f l e n g t h was u s e d as 3.6m ( 1 2 f t ) was l e f t o ut a t e a c h end o f t h e l a t e r a l f r o m 37 t h e f i r s t a n d l a s t s p r i n k l e r s , t o a c c o u n t f o r b o r d e r e f f e c t s a n d l a c k o f s p r i n k l e r o v e r l a p a t t h e s e e n d s . The p o r t a b l e i r r i g a t i o n s y s t e m p r o d u c e s a w a t e r a p p l i c a t i o n p a t t e r n w h i c h i s u n i f o r m a l o n g t h e l e n g t h o f t h e p l o t a n d c o n t i n u o u s l y , b u t u n i f o r m l y v a r i a b l e a c r o s s t h e p l o t . T h i s i s made p o s s i b l e by u s i n g s p r i n k l e r s o f same c o n f i g u r a t i o n . F u r t h e r m o r e , i n d i v i d u a l s p r i n k l e r s i n h e r e n t l y p r o d u c e t r i a n g u l a r - s h a p e d p r o f i l e when o p e r a t e d i n low w i n d s a t t h e d e s i g n p r e s s u r e . T h e s e a r e t h e c h a r a c t e r i s t i c s t h a t make t h e i r r i g a t i o n s y s t e m s u i t a b l e f o r t h e c o n d u c t o f a c o m p a c t e x p e r i m e n t i n t h e f i e l d . 3 . 2 . 2 E x p e r i m e n t a l D e s i g n a n d F i e l d L a y o u t F r o m F i g . 2 , an e x p e r i m e n t a l p l o t was , i n e f f e c t , 7 . 3 m ( 2 4 f t ) w ide by 10.9m ( 3 6 f t ) l o n g (15 r i d g e s o r r o w s ) . The w i d t h o f t h e p l o t was g o v e r n e d by t h e w e t t e d d i a m e t e r o f t h e s p r i n k l e r s b u t t h e l e n g t h c o u l d be i n c r e a s e d by d e s i g n i n g f o r more t h a n t h e 4 s p r i n k l e r s u s e d i n t h i s s t u d y . W i t h i n e a c h p l o t , t h e r e were two r e p l i c a t i o n s o r b l o c k s — one on e i t h e r s i d e o f t h e s p r i n k l e r l i n e . T h e r e were 5 p l o t s c o m p r i s i n g t h e c o n t r o l o r n o n - i r r i g a t e d p l o t a n d d i f f e r e n t i r r i g a t i o n s c h e d u l i n g p r o c e d u r e s i n t h e r e m a i n i n g 4. T h e e x p e r i m e n t a l d e s i g n was a r a n d o m i z e d c o m p l e t e b l o c k a n d t h e f i e l d l a y o u t i s g i v e n i n F i g u r e 3 . T h i s f i e l d e x p e r i m e n t was c o n d u c t e d d u r i n g t h e Summer o f 1983 on t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a P l a n t S c i e n c e F i e l d , S o u t h Campus Road a l o n g S o u t h - W e s t M a r i n e D r i v e , V a n c o u v e r , 38 B r i t i s h C o l u m b i a . The s o i l t y p e i s Bose sandy loam. The s i t e was l a s t p l a n t e d t o c o r n and f e r t i l i z e d (name and dose of f e r t i l i z e r n o t documented) f o u r y e a r s b e f o r e t h e s t a r t of t h i s e x p e r i m e n t . I t was, however, h a b i t u a l t o h a r r o w t h e whole a r e a s e v e r a l t i m e s d u r i n g S p r i n g and Summer even when n o t h i n g was b e i n g grown. 3.3 A p p l i c a t i o n of T r e a t m e n t s The g r o w th and y i e l d r e s p o n s e o f a t e s t c r o p t o t h r e e i r r i g a t i o n w a ter l e v e l s ( h e r e a f t e r d e n o t e d W1, W2 and W3) under f i v e p h o s p h o r u s f e r t i l i z e r l e v e l s ( P I , P2, P3, P4 and P5) when s u b j e c t e d t o f i v e i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s (S1, S2, S3, S4 and S5) were t o be i n v e s t i g a t e d . Thus, t h e r e were 75 t r e a t m e n t c o m b i n a t i o n s e a c h r e p l i c a t e d t w i c e and, i t f o l l o w s t h a t t h e e x p e r i m e n t c o n s i s t e d of 75 e x p e r i m e n t a l u n i t s e a c h a p p e a r i n g i n two b l o c k s . Cowpea ( V i g n a u n g u i c u l a t a [ L . ] Walp. ) was s e l e c t e d f o r t h e s t u d y . Dry m a t t e r p r o d u c t i o n on t h e b a s i s of m e t r i c t o n s p e r h e c t a r e ( t / h a ) o f a r e a l d r y m a t t e r p r o d u c e d was u s e d as t h e y i e l d i n d i c a t o r . The p l a n t s were h a r v e s t e d when the more a d v a n c e d i n d i v i d u a l s r e a c h e d t h e l a t e b o o t s t a g e i m m e d i a t e l y p r e c e d i n g t h e a p p e a r a n c e o f t h e i r i n f l o r e s c e n c e s . 3.3.1 C r o p E s t a b l i s h m e n t and M a i n t e n a n c e W h i t e v a r i e t y o f b l a c k e y e d pea s e e d s were p l a n t e d . The s e e d s were m o i s t e n e d w i t h c l e a n w a t e r and, N i t r a g i n i n n o c u l a n t ( c u l t u r e s of n i t r o g e n - f i x i n g b a c t e r i a , R h i z o b i u m spp, i n a p e a t -b a s e medium from t h e N i t r a g i n Company, C l e a r w a t e r , F l o r i d a 39 33516, USA) was ad d e d and c a r e f u l l y shaken u n t i l s e e d s were t h o r o u g h l y c o a t e d w i t h t h e i n n o c u l a n t . [ P r e l i m i n a r y s o i l a n a l y s i s of samples from t h e s i t e showed e v i d e n t l y h i g h o r g a n i c m a t t e r c o n t e n t and t o t a l n i t r o g e n b ut t h i s s e e d t r e a t m e n t was a f u r t h e r i n s u r a n c e a g a i n s t n i t r o g e n - i n d u c e d p r o b l e m s ] . Seeds were p l a n t e d on June 2 0 t h , 1983. W i t h i n - r o w s p a c i n g was 30cm ( 1 f t ) w h i l e t h e rows were 60cm ( 2 f t ) a p a r t . D e p t h o f s e e d p l a c e m e n t was 1.27-2.54cm on t h e row, and two s e e d s were p l a c e d p e r h o l e r e q u i r i n g no t h i n n i n g . B e f o r e p l a n t i n g , g e r m i n a t i o n t e s t was r a n and a b o u t 95% v i a b i l i t y was o b t a i n e d . S e e d s g e r m i n a t e d f i v e t o se v e n d a y s a f t e r p l a n t i n g . T h e r e were 12 s t a n d s p e r e x p e r i m e n t a l u n i t , 180 s t a n d s p e r r e p l i c a t e , 360 s t a n d s p e r p l o t and a t o t a l o f 1800 s t a n d s i n t h e e n t i r e e x p e r i m e n t a l a r e a . A s t a n d had 2 p l a n t s or one ( i n t h e l a t t e r c a s e , when t h e s e c o n d f a i l e d t o emerge o r d i e d o f f even a f t e r r e p l a n t i n g ) . F e r t i l i z e r was a p p l i e d two d a y s a f t e r 90 p e r c e n t emergence was r e c o r d e d . Weed c o n t r o l was manual and p e r f o r m e d t h r e e t i m e s - on J u l y 2 0 t h , J u l y 3 1 s t and A u g u s t 2 5 t h , 1983. No p e s t o r d i s e a s e symptom was o b s e r v e d hence no c o n t r o l i n t h a t d i r e c t i o n was u n d e r t a k e n . 40 c Figure 2 - Schematic of a Test Plot 7.31 m • > 3.66 m Direct ion of increase in s o i l moistui WI W2 W3 IRRIGATION LINE SOURCE 3.66m nozzle throw ^ at 206.S KPa -e l eve l s W3 W2 WI MAIN SHUT-OFF VALVE — M A I N L I N E H Y D R A N T FARM IRRIGATION WATER OUTLET SOURCE Figure 3 - Experimental Design and System Layout in the F i e l d TO i—i c n O i - i cr o o m s: m ? 3 I—* VD VO J L M A I N L I N E 0 FLEXIBLE HOSE r~ > —1 rn as _ » j L , — 0.60m foot path round each p lot -o >—( m 5 0 R O W DIRECTION O a 11 | 1 r • 7.31 m 38.41 m 42 3.3.2 I r r i g a t i o n Water T r e a t m e n t s Water t r e a t m e n t s were imposed by p l a c i n g t h e s i n g l e l i n e -s o u r c e s p r i n k l e r s y s t e m d e s i g n e d , i n t h e m i d d l e of e a c h p l o t . T h i s s y s t e m (Hanks e_t a l , 1976), a p p l i e d water u n i f o r m l y down t h e l a t e r a l l i n e i n t h e e x p e r i m e n t a l a r e a and c o n t i n u o u s l y , but i n u n i f o r m l y d e c r e a s i n g amounts w i t h d i s t a n c e f r o m t h e l i n e when o p e r a t e d i n e a r l y m o r n i n g s or l a t e e v e n i n g s under c a l m w i n d s . T h i s water a p p l i c a t i o n p a t t e r n p r o v i d e d h i g h f l e x i b i l i t y i n t h e number of i r r i g a t i o n w a ter l e v e l s t o s e l e c t w i t h i n p r a c t i c a l l i m i t s . F i g u r e 4 shows an o u t l a y o f t h e l a t e r a l and t h e s o i l s a m p l i n g l e g e n d f o r g r a v i m e t r i c s o i l m o i s t u r e d e t e r m i n a t i o n s b e f o r e and a f t e r i r r i g a t i o n a nd a f t e r a r a i n f a l l e v e n t . The t h r e e i r r i g a t i o n w a t e r l e v e l s , W1(3l.2mm), W2(106.8mm) and W3(l69.8mm) were e s s e n t i a l l y , s o i l s a m p l i n g s i t e s l o c a t e d a b o u t 1 m ( 3 . 5 f t ) , 2 m ( 6 . 5 f t ) and 3 m ( 1 0 f t ) r e s p e c t i v e l y away from e i t h e r s i d e o f t h e l a t e r a l . B e c a u s e o f u n i f o r m w ater d i s t r i b u t i o n a l o n g t h e p l o t a t a d i s t a n c e p e r f e c t l y p a r a l l e l t o t h e l a t e r a l , and a s s u m i n g s p r i n k l e r o p e r a t i o n d u r i n g low wind s p e e d , s a m p l i n g a l o n g t h e l i n e W1 o r W2 was assumed t o g i v e an e s t i m a t e of t h e same d e p t h of i r r i g a t i o n w a t e r . A major l i m i t a t i o n of t h e c o n t i n u o u s l y v a r i a b l e d e s i g n u s i n g s p r i n k l e r s was a l s o e x h i b i t e d h e r e : t h e w a t e r l e v e l s c o u l d n o t be r a n d o m i z e d . The s t a t i s t i c a l l i a b i l i t y o f t h i s d e s i g n c h a r a c t e r i s t i c w i l l be d i s c u s s e d l a t e r . S i n c e r e a s o n a b l e u n i f o r m i t y a l o n g t h e l i n e c o u l d be a c h i e v e d o n l y under c a l m wind c o n d i t i o n s (Hanks e t a l , 1976), 43 s p r i n k l i n g i n t h i s e x p e r i m e n t was done i n t h e e a r l y h o u r s o f t h e m o r n i n g e x c e p t f o r P l o t 2 ( o r S2) i n w h i c h t i m e s e t was 6 h o u r s and c o u p l e d w i t h t h e f a c t t h a t i r r i g a t i o n was a p p l i e d i n i n t e r v a l s ( i n t e r m i t t e n t l y ) t o keep r u n o f f a t z e r o , i r r i g a t i n g t h i s p l o t o f t e n s t r e t c h e d t o a f t e r n o o n and a t t i m e s w i n d s p e e d was so h i g h t h a t i t had t o be c o n t i n u e d i n , t h e e v e n i n g . One row of p l a n t s e a c h ( a c r o s s f i e l d m a c h i n e r y rows) on e i t h e r s i d e of t h e s p r i n k l e r l a t e r a l p o s i t i o n was l e f t o u t e n t i r e l y from e x p e r i m e n t a l measurements b e c a u s e : ( i ) of e x c e p t i o n a l i n f l u e n c e o v e r them as compared t o t h e r e s t , o f s t a r t i n g a n d s t o p p i n g s p r i n k l e r o p e r a t i o n , and ( i i ) c o n s t a n t movement o f l a t e r a l ( f r o m e x p e r i e n c e ) damaged o r d i s t u r b e d t h e s e two rows u n i q u e l y . The a r e a s u s e d f o r p l a n t h a r v e s t s as w e l l as p l a n t and s o i l w a t e r d e t e r m i n a t i o n s a r e i l l u s t r a t e d i n F i g u r e 4. C r o p water use was d e t e r m i n e d by h y d r o l o g i c b a l a n c e a t e a c h s o i l s a m p l i n g s i t e . A p p l i e d w a t e r was m e a s u r e d w i t h c a t c h c a n s and i s r e p o r t e d i n A p p e n d i x B. P r e c i p i t a t i o n r e c o r d was o b t a i n e d from t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a w e a t h e r s t a t i o n . S o i l m o i s t u r e d e p l e t i o n by c r o p between i r r i g a t i o n s was measured by g r a v i m e t r i c d i f f e r e n c e between p r e - and p o s t - i r r i g a t i o n s a m p l i n g s . 44 S e a s o n a l a n d s h o r t e r t i m e p e r i o d s o f w a t e r use were d e t e r m i n e d u s i n g t h e e q u a t i o n : E T = I R R + P + D + A S - Q [ 3 . 3 ] w h e r e : E T = w a t e r use (eg s e a s o n a l , m o n t h l y ) , mm IRR= i r r i g a t i o n w a t e r a p p l i e d P = p r e c i p i t a t i o n D = d r a i n a g e A S = c h a n g e i n s t o r e d s o i l m o i s t u r e f o r t h e p e r i o d c o n s i d e r e d Q = s u r f a c e r u n o f f . In t h e a p p l i c a t i o n o f t h i s m o i s t u r e b a l a n c e e q u a t i o n , t h e f o l l o w i n g a s s u m p t i o n s were made: 1. A l l p r e c i p i t a t i o n e x c l u d e d r u n o f f — i . e . , a l l r a i n f a l l w a t e r p a s s e d t h r o u g h t h e r o o t z o n e 2 . D r a i n a g e , w h i c h a s sumes a p o s i t i v e v a l u e when t h e r e i s c o n t r i b u t i o n f rom g r o u n d w a t e r a n d a n e g a t i v e v a l u e when t h e r e i s d e e p p e r c o l a t i o n f r o m IRR o r P , was t a k e n a s z e r o u n d e r e i t h e r c i r c u m s t a n c e . B u t i n i r r i g a t i n g t h e r o o t zone t o f i e l d c a p a c i t y , d e e p p e r c o l a t i o n was u n a v o i d a b l e . W a t e r u se a n d h e n c e w a t e r u s e e f f i c i e n c y u n d e r t h e d i f f e r e n t i r r i g a t i o n s c h e d u l i n g m e t h o d s were d e t e r m i n e d by b a l a n c i n g t h e s e h y d r o l o g i c i n p u t s . 45 Figure 4 - Experimental Units and S o i l Sampling Sites " J L 0.60 m r OO E E ' o CNJ CC • CO CTl ro o i — i +-> C o 4- £ +-> CO t— Ol o S-_ J O-CQ • ZD -4-> OO S-O) 4— «—I J O WO o o o QROW -O—G O—Gr ROW P5 -&-Q—O O LEGEND: -X—X W3 W3 0 0 0 0 o 0 o o PI P3 P4 P2 W3 WO 3.66 m WQ_ W2 Wl (> o o o O OiO o-&-Q 0| O O O O i O Q r-o d> o o O O O (I WO W2 2.25 m 3.05 m Wl ROW LINE SOURCE SPRINKLER A r b i t r a r i l y randomized f e r t i l i z e r P l e v e l s . F ie ld machinery row or ienta t ion A l l s o i l samples taken along this l ine a f ter an i r r i g a t i o n were assumed to have received the same amount of water. WO x * WO Disturbed rows, not sampled for analysis An experimental unit with 12 plant stands, 2 p lants/hole 46 3.3.3 P h o s p h o r u s F e r t i l i z e r T r e a t m e n t s F i v e l e v e l s (0, 50, 60, 70 and 80 kg/ha) P 2 0 5 of s i n g l e s u p e r p h o s p h a t e (18% a c t i v e i n g r e d i e n t ) were a p p l i e d . T h i s was e q u i v a l e n t t o a p p l y i n g 0, 6.2, 7.4, 8.6 and 9.8 g / p l a n t s t a n d r e s p e c t i v e l y ( T a b l e I I I ) . F e r t i l i z e r was a p p l i e d on June 2 5 t h , 1983. G o i n g by t h e s t a n d a r d p r a c t i c e of row f e r t i l i z e r a p p l i c a t i o n , a h o l e a b o u t 5cm deep was dug r o u n d and 2.54cm away f r o m t h e c r o p s t a n d , m a n u a l l y . The a p p r o p r i a t e amount o f f e r t i l i z e r was a p p l i e d i n t h e r i n g and c o v e r e d back w i t h t o p s o i l . The P f e r t i l i z e r t r e a t m e n t s , h e n c e f o r t h r e f e r r e d t o as P1, P2, P3, P4 a n d P5, i n t h a t o r d e r , were r a n d o m i z e d a l o n g t h e p l o t and i n e f f e c t , were a t r i g h t a n g l e s t o t h e w a t e r v a r i a b l e . F i g u r e 4 shows an a r b i t r a r y r a n d o m i z a t i o n i n one r e p l i c a t e o f a p l o t . E a c h l e v e l o f f e r t i l i z e r was a p p l i e d on t h r e e rows — t h e l a t t e r r a n a c r o s s t h e l e n g t h o f t h e p l o t . F u r t h e r i n f o r m a t i o n on t h e f e r t i l i z e r t r e a t m e n t s i s as g i v e n i n T a b l e I I I . 47 T a b l e I I I - F e r t i l i z e r d a t a F E R T I L I Z E R FERT. RATE ACTUAL Wt o f FERT. Wt Wt of FERT LEVEL kg/ha 18% FERT. PER SUBPLOT pe r s t a n d -a . i . * (kg/ha) (g) (g) P1 0 0 0 0 P2 50 277 .8 222.2 6.2 P3 60 333 . 3 266.7 7.4 P4 70 388.9 311.1 8.6 P5 80 444 . 4 355.6 9.8 T o t a l amount u s e d = 11.6 kg * a . i . = a c t i v e i n g r e d i e n t 3.3.4 I r r i g a t i o n S c h e d u l i n g T e c h n i q u e s In S e c t i o n 3.1, c l i m a t i c , s o i l , and c r o p d a t a were u s e d t o e s t i m a t e e v a p o t r a n s p i r a t i o n (ET) and t h e l a t t e r was i n t u r n u t i l i z e d i n s u b s e c t i o n 3.1.2 t o d e r i v e i r r i g a t i o n i n t e r v a l s . The r e l a t i o n s h i p between t h e a c t u a l ET e x p e r i e n c e d by t h e c r o p and t h e maximum p o s s i b l e ET v a r i e s w i t h c l i m a t i c c o n d i t i o n s , c r o p d e v e l o p m e n t and s o i l m o i s t u r e . A c t u a l e v a p o t r a n s p i r a t i o n w i l l p r o c e e d a t a p p r o x i m a t e l y t h e maximum r a t e u n t i l p e r h a p s h a l f of t h e a v a i l a b l e s o i l m o i s t u r e has been d e p l e t e d ( J e n s e n e t a l , 1970; Stegman e t a l , 1981 and E n g l i s h and Nuss, 1982). Beyond t h a t p o i n t , a c t u a l ET w i l l f a l l p r o g r e s s i v e l y f a r t h e r 48 b e h i n d t h e maximum r a t e . The key a s s u m p t i o n u s e d i n s p e c i f y i n g t h e i r r i g a t i o n s c h e d u l e s i n t h i s s t u d y was t h a t t h e r a t i o of a c t u a l e v a p o t r a n s p i r a t i o n (ET ) t o maximum p o s s i b l e e v a p o t r a n s p i r a t i o n (ET*) w i l l e q u a l 1.0 u n t i l 50% ( a v a i l a b i l i t y c o e f f i c i e n t ) of t h e maximum a v a i l a b l e s o i l m o i s t u r e has been d e p l e t e d a f t e r w h i c h i t would d e c l i n e l i n e a r l y f r o m 1.0 t o 0 as a v a i l a b l e m o i s t u r e a p p r o a c h e s e x h a u s t i o n . On t h e b a s i s o f t h i s s i m p l i f i e d but w i d e l y u s e d s o i l m o i s t u r e d e p l e t i o n model, a r e l a t i o n s h i p between t i m e s e t , minimum i r r i g a t i o n i n t e r v a l , ET and Av.C was e s t a b l i s h e d . ET* was assumed c o n s t a n t f o r a p e r i o d o f s e v e r a l d a y s so t h a t t h e e l a p s e d t i m e or i r r i g a t i o n i n t e r v a l u n t i l 50% d e p l e t i o n ( T 5 0 ) o c c u r s w i l l be T 5 0 = M 0 / ( 2 E T * ) [ 3 . 4 ] where M 0 d e n o t e s t h e i n i t i a l s o i l m o i s t u r e . U s i n g a s e l e c t e d t i m e s e t o f 6 h r s . , and a s s u m i n g i r r i g a t i o n s c h e d u l e s i n w h i c h any ET d e f i c i t s were u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t e a c h month, i n c o n j u n c t i o n w i t h t h e s p e c i f i c a t i o n t h a t AWSC i n t h e r o o t zone would not be d e p l e t e d by more t h a n 50% between i r r i g a t i o n s d u r i n g a p e r i o d o f maximum e v a p o t r a n s p i r a t i o n demand, t h e d e s i g n f o r f u l l i r r i g a t i o n p r o d u c e d t h e s e i r r i g a t i o n i n t e r v a l s i n t h e c r o p p i n g months: June = 10 d a y s , J u l y = 8 d a y s , A u g u s t = 8 days and September = 10 d a y s . The f o l l o w i n g i r r i g a t i o n s c h e d u l e s and t h e i r s p e c i f i c a t i o n s were d e r i v e d f r o m t h e f u l l i r r i g a t i o n s y s t e m : 49 [ S 1 ] . . T h i s was t h e c o n t r o l . I t was a l l o t e d P l o t I ( F i g u r e 3 ) . T h i s p l o t was i r r i g a t e d t o f i e l d c a p a c i t y b e f o r e p l a n t i n g t o e n s u r e g e r m i n a t i o n and good s e e d l i n g e s t a b l i s h m e n t . I t r e c e i v e d no f u r t h e r i r r i g a t i o n w a ter but no a t t e m p t was made t o c o n t r o l t h e r a i n f a l l t h a t f e l l on i t , j u s t a s any o t h e r p l o t . [ S 2 ] . . A l l o t e d P l o t I I , t h i s was t h e i r r i g a t i o n s c h e d u l e s p e c i f i e d f o r t h e f u l l i r r i g a t i o n s y s t e m . The c o n f i g u r a t i o n of t h e f u l l i r r i g a t i o n s y s t e m was b a s e d on s t a n d a r d d e s i g n p r o c e d u r e s as d e s c r i b e d i n S e c t i o n 3.1 of t h i s c h a p t e r , and t h e key a s s u m p t i o n s and r e l a t i o n s h i p s d i s c u s s e d a b o v e . In summary, i t i s h e l p f u l t o m e n t i o n a g a i n t h a t t i m e s e t p e r i r r i g a t i o n was 6 h o u r s , t o wet 60cm ( o r 2 f t ) of s o i l t o FC w i t h 42.85mm ( 1 . 7 i n ) of i r r i g a t i o n water", s p r i n k l i n g a t a r a t e of 7.lmm/hr ( 0 . 2 8 i n / h r ) . The minimum i n t e r v a l between i r r i g a t i o n s (whenever t h e r e was no r a i n f a l l i n b e t w e e n ) was 10 d a y s i n J u n e , 8 d a y s i n J u l y , and 8 d a y s i n A u g u s t . E a r l y i n September, t h e F a l l r a i n s had s t a b i l i z e d , t h e s o i l was q u i t e m o i s t and t h e r e f o r e , no f u r t h e r w ater was a p p l i e d . In f a c t , no p l o t was i r r i g a t e d i n S e p t e m b e r . [ S 3 ] . . T h i s i s t h e f i r s t o f two h i g h - f r e q u e n c y d e f i c i t i r r i g a t i o n s . On t h e b a s i s of i n t e r v a l s e n u m e r a t e d i n S2, i f no r a i n f a l l e v e n t o c c u r r e d w i t h i n 50 any p a r t i c u l a r i n t e r v a l ( o r between any two s u c c e s s i v e i r r i g a t i o n s ) , t h e same amount of water would have been a p p l i e d as i n S2 but i n t h r e e s e p a r a t e and s m a l l e r amounts (14.3mm e a c h ) . However, when t h e r e was enough r a i n f a l l , s a y , soon a f t e r a p p l y i n g t h e f i r s t one t h i r d o f 42.85mm, t o c a r r y t h e c r o p t o t h e n e x t i n t e r v a l , 2/3 of 42.85mm of wa t e r and 4 man-hours w o u l d have been s a v e d i n c o m p a r i s o n t o S2 i n w h i c h t h e whole o f t h a t 42.85mm of water was a p p l i e d r e q u i r i n g 6 man-h o u r s p e r i r r i g a t i o n . T h i s i r r i g a t i o n s c h e d u l e was r a n on P l o t I I I . T e c h n i c a l l y , o n l y 16.5% = 1/3 o f 50% AWSC d e p l e t i o n was a l l o w e d u n d e r t h i s p r o c e d u r e . [ S 4 ] . . P l o t IV, a l s o c a l l e d ' S t a g e - o f - G r o w t h ' s c h e d u l i n g method, was i r r i g a t e d b a s e d on t h e l i f e s p a n of t h e c r o p and i n p a r t i c u l a r , t h e t y p e o f p l a n t p r o d u c t t o be h a r v e s t e d . The t e s t c r o p , Cowpea, was h a r v e s t e d f o r i t s d r y m a t t e r y i e l d and a c c o r d i n g l y , t h e t i m e f r o m p l a n t i n g t o h a r v e s t (90 d a y s ) , was b r o k e n i n t o t h r e e v e g e t a t i v e s t a g e s , s t r i c t l y on t h e b a s i s o f age: 1st s t a g e = emergence t o 30 d a y s ; 2nd s t a g e = 31-60 d a y s ; 3 r d s t a g e = 61-90 d a y s . The p l o t was i r r i g a t e d t o f i e l d c a p a c i t y by a p p r o p r i a t e l a t e r a l o v e r l a p p i n g on t h e 4 t h J u l y and on 5 t h A u g u s t . The t h i r d i r r i g a t i o n was s c h e d u l e d f o r 51 September but f o r t h e r e a s o n s t a t e d u nder S2, t h i s was not i m p l e m e n t e d . [ S 5 ] . . T h i s was t h e 2nd h i g h - f r e q u e n c y i r r i g a t i o n s c h e d u l e . I t was c o n d u c t e d on P l o t V and was i r r i g a t e d a t h a l f t h e c a l c u l a t e d i n t e r v a l i n S2 f o r e a c h month and o n l y one h a l f of t h e g r o s s water r e q u i r e d was a p p l i e d a t e a c h t i m e s e t i . e . , 3 h o u r s . In e s s e n c e , t h e c r o p was a l l o w e d t o d e p l e t e o n l y 1/2 of S2 i r r i g a t i o n d e p t h o r 25% of AWSC o r 1/2 o f 50% AWSC b e f o r e t h e n e x t i r r i g a t i o n became n e c e s s a r y . A l l p l o t s were i r r i g a t e d b e f o r e p l a n t i n g . T h e r e f o r e , i r r i g a t i o n w a t e r t r e a t m e n t s and s c h e d u l i n g p r o c e d u r e s s t a r t e d a f t e r s e e d l i n g emergence. B. ANALYTICAL PROCEDURES 3.4 The S o i l 3.4.1 S o i l A n a l y s e s S o i l s a m p l e s were t a k e n f r o m t h r e e r e p r e s e n t a t i v e p i t s a t t h e s i t e i m m e d i a t e l y b e f o r e t h e e x p e r i m e n t began and i m m e d i a t e l y a f t e r t h e c r o p was h a r v e s t e d . Samples were c o l l e c t e d from t h r e e d e p t h s ( T a b l e I ) . The f o l l o w i n g a n a l y s e s were p e r f o r m e d : i . S o i l pH ( s o i l i n c a l c i u m c h l o r i d e s o l u t i o n ) i i . A v a i l a b l e K, Mg and Ca by Morgan's e x t r a c t i o n method i i i . A v a i l a b l e P by Bray-1 method i v . O r g a n i c m a t t e r c o n t e n t by W a l k l e y - B l a c k method 52 v. A v a i l a b l e n i t r a t e - N v i . S u l p h a t e - S v i i . S o i l t e x t u r e . T hese a r e s t a n d a r d l a b o r a t o r y s o i l t e s t s and r e f e r e n c e s t o t h e p r o c e d u r e s a d o p t e d c o u l d be made t o B l a c k e_t a l , (1965) and L a v k u l i c h ( 1 9 8 3 ) . 3.4.2 S o i l M o i s t u r e D e t e r m i n a t i o n s D u r i n g E x p e r i m e n t S o i l s a m p l i n g f o r g r a v i m e t r i c m o i s t u r e c o n t e n t d e t e r m i n a t i o n s was u n d e r t a k e n f o r e a c h s a m p l i n g s i t e f o r two p r o f i l e l a y e r s — t h e t o p 0-30cm and 30-60cm by a u g e r i n g on t h e f o l l o w i n g b a s e s : • b e f o r e e v e r y i r r i g a t i o n • 4-5 h o u r s a f t e r i r r i g a t i n g P l o t s I I I and V • 2-3 d a y s a f t e r i r r i g a t i n g P l o t s I I and IV • 2-3 d a y s a f t e r e v e r y heavy r a i n f a l l . N o t e : P l o t s I I and .IV were i r r i g a t e d t o f i e l d c a p a c i t y , t h e r e f o r e , i t was n e c e s s a r y t o a l l o w m o i s t u r e d i s t r i b u t i o n i n t h e p r o f i l e f o r a p e r i o d of 2-3 d a y s . 53 3.5 The C r o p 3.5.1 C r o p Growth I n d i c a t o r s The p l a n t p e r f o r m a n c e a t d i f f e r e n t s t a g e s o f g r o w t h was a s s e s s e d u s i n g t h r e e growth i n d i c a t o r s . The f o l l o w i n g measurements were t a k e n on a w e e k l y b a s i s f o r s e v e n weeks b e g i n n i n g one week a f t e r s e e d l i n g e m ergence: 1. P l a n t h e i g h t 2. Number o f nodes p e r p l a n t 3. Number o f t r i f o l i a t e s p e r p l a n t . These g r o w t h i n d i c e s were c h o s e n out o f many o t h e r s recommended f o r cowpea b e c a u s e , t h e y were r e l a t i v e l y e a s y t o measure, r e q u i r i n g l i t t l e e q u i p m e n t . F u r t h e r m o r e , S i n h a (1977) e x p l a i n e d t h a t t h e r e a l i s a t i o n o f a h i g h l e a f s u r f a c e a r e a ( r e l a t e d t o t r i f o l i a t e c o u n t s p e r p l a n t ) as a d r y m a t t e r " s o u r c e " t o be m o b i l i s e d t o s i t e s o f a c c u m u l a t i o n o r " s i n k " as t h e c r o p m a t u r e s i s v i t a l ; and a c c o r d i n g t o Kassam ( 1 9 7 6 ) , s e e d y i e l d i n cowpea i s l a r g e l y d e p e n d e n t on t h e t o t a l number of nodes p r o d u c e d b e f o r e t h e o n s e t o f f l o w e r i n g and t h e number of pods s u b s e q u e n t l y p r o d u c e d and r e t a i n e d a t t h e s e n o d e s . Thus, an e x p l a n a t i o n f o r t h e u l t i m a t e y i e l d o b t a i n e d from a c o m b i n a t i o n of t r e a t m e n t s o r i g i n a t e s f r o m a d a t a s o u r c e of t h e s e p h y s i o l o g i c a l p e r f o r m a n c e s of t h e c r o p a t t h e v a r i o u s s t a g e s of i t s growth c y c l e . T h ese g r o w t h i n d i c a t o r s were m e a s u r e d on t h r e e h e a l t h y -l o o k i n g p l a n t s r e p r e s e n t a t i v e o f an e x p e r i m e n t a l u n i t . O n l y t h e mean of the r e c o r d e d v a l u e s was r e p o r t e d i n e a c h c a s e . 54 3.5.2 C r o p Y i e l d The c r o p was h a r v e s t e d f o r d r y m a t t e r p r o d u c t i o n on September 2 0 t h , 1983. O n l y t h e a b o v e - g r o u n d p l a n t p a r t was t a k e n . However, some p l a n t s were u p r o o t e d a t random from e a c h p l o t t o g i v e a g e n e r a l i d e a a b o u t t h e a c t u a l r o o t i n g d e p t h a c h i e v e d by t h e c r o p and t h e e x t e n t of n o d u l a t i o n when cowpea was i n n o c u l a t e d w i t h n o n - s p e c i f i c b a c t e r i a i n a f i e l d where t h e c r o p had n e v e r been c u l t i v a t e d b e f o r e . The a r e a l p a r t s h a r v e s t e d were w e i g h e d f r e s h i n t h e f i e l d b e f o r e d r y i n g f o r d r y m a t t e r measurement. 3.5.3 N u t r i e n t U p t a k e The c r o p was h a r v e s t e d a t t h e b o o t s t a g e a t w h i c h t i m e , some s t a n d s had a l r e a d y d e v e l o p e d f l o w e r buds. Most w o r k e r s on cowpea — t h e most a u t h o r i t a t i v e b e i n g S h a n t h a k u m a r i and S i n h a (1972 and 1973), a g r e e t h a t p h o t o s y n t h e s i s i n cowpea l e a v e s c e a s e s i m m e d i a t e l y p r i o r t o f l o w e r i n g . G r a i n f i l l i n g i s by t r a n s f e r of a s s i m i l a t e s from t h e s e l e a v e s , e s p e c i a l l y t e r m i n a l o n e s , and from n o d u l e s as w e l l a s f r o m t h e s t i l l p h o t o s y n t h e s i z i n g f r u i t w a l l . Hence, c o l l e c t i o n o f t e r m i n a l l e a v e s i m m e d i a t e l y p r i o r t o f l o w e r i n g g i v e s a good i n d e x o f u p t a k e of a p p l i e d n u t r i e n t . N i n e t e r m i n a l l e a v e s p e r e x p e r i m e n t a l u n i t (3 l e a v e s f r o m e a c h p l a n t c o l l e c t e d i n s u b s e c t i o n 2.5.2 above) were p r e s e r v e d f o r f o l i a r ( c h e m i c a l ) a n a l y s e s . The method of p r e p a r a t i o n and t o t a l a n a l y s e s of t h e l e a f s a m p l e s f o r N, P, K, Ca and Mg was by t h e P a r k i n s o n and A l l e n D i g e s t i o n f o r f o l i a g e ( L a v k u l i c h , 1983). 5 5 3 . 5 . 4 S t a t i s t i c a l C o m p a r i s o n of R e s u l t s A s t a t i s t i c a l a n a l y s i s was p e r f o r m e d t o d e t e r m i n e t h e main e f f e c t s and i n t e r a c t i o n s of i r r i g a t i o n amount o f w a t e r , p h o s p h o r u s f e r t i l i z e r and i r r i g a t i o n s c h e d u l e s on d r y m a t t e r p r o d u c e d by t h e c r o p . The r e s u l t s o f t h i s a n a l y s i s a r e p r e s e n t e d i n t h e form of ' A n a l y s i s o f V a r i a n c e ' (ANOVA) t a b l e i n c h a p t e r IV. S i g n i f i c a n c e l e v e l s of 5 and 1% were u s e d t o d e t e r m i n e y i e l d d i f f e r e n c e s between any two t r e a t m e n t means. P h o s p h a t e u p t a k e d a t a was a l s o s u b j e c t e d t o a s i m i l a r s t a t i s t i c a l a n a l y s i s . 56 IV. RESULTS AND DISCUSSION 4 . 1 S o i l P r o p e r t i e s R e s u l t s of a n a l y s e s of s o i l s a m p l e s t a k e n from t h e e x p e r i m e n t a l s i t e p r i o r t o p l a n t i n g a r e shown i n T a b l e IV. The s o i l i s s l i g h t l y a c i d i c down t o t h e 90cm p r o f i l e d e p t h a n a l y s e d . The a r e a i s w e l l - d r a i n e d , on a s l o p e o f 3-5%. O t h e r p e r t i n e n t s o i l p r o p e r t i e s a r e as g i v e n i n t h e same t a b l e . I t was n e c e s s a r y t o d e t e r m i n e t h e m o i s t u r e r e t e n t i o n p r o p e r t i e s o f t h e s o i l . F i g u r e 5 i s a p r e s e n t a t i o n of t h e s o i l m o i s t u r e c h a r a c t e r i s t i c c u r v e o f t h e plow l a y e r ( a p p r o x i m a t e l y 30cm o f t o p s o i l , C u r v e A ) . The p l o t o f t h e m o i s t u r e r e t e n t i o n c h a r a c t e r i s t i c of t h e s u b s o i l ( C u r v e B) was f o u n d t o f o l l o w t h i s one v e r y c l o s e l y and i s a l s o shown on t h e same f i g u r e . T a b l e IV - S o i l A n a l y s e s R e s u l t s B a s e d on Morgan S o i l T e s t i n g S y s t e m SAMPLE ORG. MATTER AVAILABLE NUTRIENTS IN SOIL DEPTH PH CONTENT N i t r a t e ~ N P K S u l f a t e - S TEXTURE ( c m ) . (%) ppm 0-30 5.9 1.35 75 12 14 nd SL 30-60 5.5 0.88 16 12 12 nd LS 60-90 5.3 0.75 25 nd nd nd SL nd = n o t d e t e c t e d i n s o i l sample; SL = sandy loam pH = s o i l , i n 0.01N C a l c i u m c h l o r i d e ( 1 : 1 ) ; LS = loamy sand R e s u l t s r e p r e s e n t t h e a v e r a g e v a l u e s f o r 3 p r o f i l e s . 57 I t c an be seen i n c u r v e A o f F i g u r e 5 t h a t t h e f i e l d c a p a c i t y ( s u c t i o n of about 30 KPa o r 0.3 b a r ) of t h e t o p l a y e r of s o i l i s 30.7% w h i l e t h e permanent w i l t i n g p o i n t ( s u c t i o n of 1500 KPa or 15 b a r s ) i s 12.1%. One o b v i o u s l i m i t a t i o n of t h i s c u r v e ( d e r i v e d from r e s u l t s of a l a b o r a t o r y m o i s t u r e r e t e n t i o n t e s t ) i s t h e f a c t t h a t whereas f i e l d s i t u a t i o n d u r i n g i r r i g a t i o n i s a s e r i e s o f w e t t i n g and d r y i n g c y c l e s , t h e l a b o r a t o r y t e s t was a s t r a i g h t - f o r w a r d d r y i n g e x e r c i s e . S u b s e q u e n t c o n v e r s i o n s of t h e c u r v e t o y i e l d a v a i l a b l e w a t e r d e p l e t i o n t h u s , o v e r l o o k h y s t e r e s i s e f f e c t and a t b e s t , t h e n , a r e a p p r o x i m a t i o n s w h i c h must be c o n t a i n e d w i t h as t h e y a l w a y s have been i n i r r i g a t i o n p r a c t i c e . The d r y b u l k d e n s i t y o f t h e s o i l was f o u n d t o be 1.01 g / c c . • S o i l t e s t v a l u e s f o r a v a i l a b l e P a t t h e end o f t h e e x p e r i m e n t a l r u n a r e shown i n T a b l e V. The a v a i l a b i l i t y t r e n d r e f l e c t s t h e r a t e of p h o s p h o r u s f e r t i l i z e r a p p l i e d . I t may be n o t e d t h a t t h e f e r t i l i z e r was b a n d - a p p l i e d and s u b s e q u e n t s a m p l i n g was from t h e s e h i g h l y c o n c e n t r a t e d s i t e s — a s o r t o f q u a s i - p o t e x p e r i m e n t . F i g u r e 6 i s a f u r t h e r i l l u s t r a t i o n of t h e same d a t a . A n a l y s i s f o r K, Ca, Mg and o r g a n i c m a t t e r c o n t e n t a r e a l s o r e p o r t e d ( T a b l e V I ) . Figure 5 - Moisture Retention Curves of Experimental Soil 60^, 0 -j-: , ! , , .—, , , . , 0 2 4 .6 8 10 12 14 16 SUCTION (bars) 59 T a b l e V -- Mean E x t r a c t a b l e . ( A v a i l a b l e ) P i n s o i l i n ppm F E R T I L I Z E R P L 0 T S TREATMENTS SI S2 S3 S4 S5 PI 64 98 36 72 40 P2 1 22 1 32 1 34 1 74 1 56 P3 1 24 151 1 77 1 20 1 78 P4 1 60 232 228 1 86 206 P5 186 186 204 1 66 238 4 . 2 C r o p D e v e l o p m e n t Weekly measurements of r a t e of stem e l o n g a t i o n , t r i f o l i a t e c o u n t s and number of nodes p e r p l a n t as f r om s e v e n days a f t e r emergence were c a r r i e d o u t . F o r a l l p l o t s , t h e number of t r i f o l i a t e l e a v e s was 3-5 a f t e r t h e f o u r t h week from s e e d l i n g emergence and number of nodes was 5-7 p e r p l a n t . However, on u n f e r t i l i z e d p l o t s , p l a n t s were a l i g h t e r g r e e n t h a n f e r t i l i z e d p l o t s . C o l d s o i l t e m p e r a t u r e s a n d / o r n i t r o g e n d e f i c i e n c y were s u s p e c t e d . T h a t t h e r e was no n i t r o g e n d e f i c i e n c y became e v i d e n t by t h e d i s a p p e a r a n c e of t h i s symptom a f t e r f i v e weeks, a t w h i c h t i m e , day t e m p e r a t u r e s were a s t a b l e 15 d e g . C e l s i u s o r o v e r and t h e p l a n t s had a l s o e s t a b l i s h e d t h e m s e l v e s . P l a n t s g r o w i n g u n d e r t h e w e t t e s t t r e a t m e n t c o m b i n a t i o n (S3W3) were most s t u n t e d 60 t h r o u g h o u t t h e s e 7 weeks compared t o t h e c o n t r o l and l e s s f r e q u e n t l y i r r i g a t e d p l o t s . The maximum r o o t d e p t h o b s e r v e d i n p r o f i l e p i t s was some 30cm, a b o u t 90 days a f t e r p l a n t i n g . N o n - i r r i g a t e d ( c o n t r o l ) p l a n t s e x h i b i t e d t h e g r e a t e s t r o o t e l o n g a t i o n , p e r h a p s , b e c a u s e of t h e a t t e m p t by t h e p l a n t s under t h i s t r e a t m e n t t o e x p l o r e t h e d e e p e r p r o f i l e l a y e r f o r m o i s t u r e , i n view of t h e o c c a s i o n a l l y m o i s t u r e - d e f i c i e n t t o p s o i l o f t h i s p l o t . A maximum r o o t d e p t h o f 60cm was e x p e c t e d f r om t h i s c r o p b u t i t was o b v i o u s t h a t m e c h a n i c a l impedance was not a r e s t r i c t i n g f a c t o r t o v e r t i c a l r o o t d e v e l o p m e n t on t h i s s i t e . The c o m b i n a t i o n o f i r r i g a t i o n and band a p p l i c a t i o n o f p h o s p h o r u s . f e r t i l i z e r m i g h t have c o n t r i b u t e d t o t h e s h a l l o w r o o t i n g a s i t was not n e c e s s a r y f o r t h e c r o p t o e x t e n d i t s r o o t s below 30cm of s o i l d e p t h i n t h i s g r o u p of p l o t s . 4.3 C r o p Y i e l d R e s p o n s e Dry m a t t e r y i e l d s i n r e l a t i o n t o t h e d i f f e r e n t c o m b i n a t i o n s of s e a s o n a l i r r i g a t i o n w a t e r , p h o s p h o r u s f e r t i l i z e r and i r r i g a t i o n s c h e d u l i n g t e c h n i q u e s a r e as i n T a b l e V I I . T a b l e V I I I c o n t a i n s t h e r e s u l t of t h e a n a l y s i s of v a r i a n c e (ANOVA) p e r f o r m e d f o r t h e y i e l d . The s t a t i s t i c a l t e s t r e v e a l e d t h a t b l o c k e f f e c t was not s i g n i f i c a n t . T h i s , i n s t a t i s t i c a l t e r m s , may be due t o two r e a s o n s ( S t e e l and T o r r i e , 1983): (a) t h e e x p e r i m e n t was not s u c c e s s f u l i n r e d u c i n g e r r o r v a r i a n c e by t h e g r o u p i n g of ure 6 — Extractable P in Soi 3CH 1 1 — I r P1 P2 P3 P4 P5 APPLIED PHOSPHATE (kg/ha) T a b l e V I - R e s u l t s o f S o i l C h e m i c a l A n a l y s e s ( a t e n d o f e x p e r i m e n t a l r u n ) F E R T I I I Z E R L E V E L S P 1 P 2 P 3 P 4 P 5 P L O T T E S T /! k B A B A B A B A B P H O S P H O R U S 5 2 . 0 7 6 . 0 1 2 8 . . 0 1 16 . 0 9 6 . 0 1 5 2 . . 0 1 0 8 . 0 2 1 2 , , 0 2 1 2 . 0 1 6 0 . . 0 P O T A S S I U M 0 . 17 0 . 2 0 0 . 16 0 . 2 2 0 . 16 0 . . 19 0 . 3 4 0 . . 1 2 0 . 5 6 0 . . 19 S 1 M A G N E S I U M 1 . 4 1 1 . 4 G 1 . . 7 7 1 . 4 6 1 . 4 1 1 . . 4 6 1 . 5 6 1 . . 16 1 . . 5 6 1 . . 4 6 C A L C I U M 1G . 6 4 17 . 2 4 19 . 6 6 1 6 . 9 4 1 6 . . 6 4 1 9 . 0 6 17 . . 8 5 1 4 , . 8 2 18 . . 7 6 18 . . 4 5 O R G . M A T T E R 2 . 5 2 . 4 2 . . 1 2 . 6 2 . . 1 2 , , 8 1 . . 9 2 , . 3 2 . . 7 2 . . 6 P 1 G 0 . 0 3 6 . 0 1 6 0 . 0 1 0 4 . 0 1 5 2 . . 0 1 5 0 . . 0 2 6 8 . . 0 1 9 6 , . 0 1 5 6 . . 0 1 7 6 . . 0 K 0 . 2 3 0 . 2 2 0 . 2 0 0 . 2 0 0 . . 2 2 0 . . 19 0 , 17 0 , . 2 0 0 . . 14 0 . . 14 S 2 M g 1 . G1 1 . 3 6 1 . . 5 1 1 . 4 1 1 . . 2 6 1 . . 7 1 1 . . 5 1 1 , . 4 1 1 . . 1 1 1 . . 2 6 C a 17 . 5 5 16 . 6 4 17 . . 2 4 1 8 . 4 5 1 6 . 6 4 18 , , 4 5 1 9 . . 0 6 1 6 , . 6 4 14 . . 2 2 17 . . 2 4 0 . M 2 . 4 2 . 4 1 . . 9 1 . 9 2 . . 5 2 . . 5 2 . 7 2 , . 5 2 . 1 2 . . 7 P 2 0 . 0 5 2 . 0 1 2 8 . . 0 1 4 0 . 0 1 7 2 . 0 1 8 1 , , 0 2 3 2 . . 0 2 2 4 , . 0 2 0 8 . 0 2 0 0 . . 0 K 0 . 16 0 . 14 0 . . 1 1 0 . 19 0 . 17 0 . . 2 3 0 . 2 0 0 , . 2 2 0 . 1 2 0 . . 2 3 S 3 M g 1 . 3 6 1 . 5 1 1 , , 16 1 . 7 1 1 . 3 6 1 . . 3 6 1 . 3 G 1 , . 3 6 1 . 0 1 1 . 5 6 C a 1 5 . 7 3 17 . 5 5 1 5 . . 7 3 16 . 0 3 1 6 . 6 4 14 . . 5 2 1 6 . 0 3 16 , . 9 4 1 2 . 1 0 18 . 4 5 0 . M 2 . 0 1 . 9 2 . 1 2 . 4 1 . 9 2 , , 1 2 . 2 2 , . 2 1 . 8 2 . . 2 T a b l e V I c o n t i n u e d P 6 4 . 0 8 0 . 0 2 0 4 . 0 1 4 4 . 0 1 4 0 . 0 1 0 0 . 0 2 2 8 . 0 1 4 4 . 0 1 3 6 . 0 1 9 6 . 0 K 0 . 16 0 . . 2 0 0 . 1 9 0 . 2 0 0 . . 14 0 , . 14 0 . 14 0 . . 14 0 . . 17 0 . . 19 M g 1 . 0 6 1 . . 1 1 1 . 5 1 1 . . 2 1 1 . . 0 6 0 . . 9 6 1 . 0 1 1 . . 2 1 1 . . 4 6 1 . 3 6 S 4 C a 1 3 . 0 1 1 5 . . 1 3 16 . 0 3 18 . 4 5 1 3 , , 9 2 1 3 . . 3 1 1 5 . 1 3 1 5 . . 1 3 17 . . 2 4 1 6 . . 6 4 O . M 2 . 3 2 , . 1 2 . 4 2 . 4 2 . . 5 2 . . 6 1 . 9 2 . 6 2 . 1 2 . 7 P 16 . . 0 6 4 . . 0 1 7 6 . . 0 1 3 6 . . 0 2 2 8 , , 0 1 2 8 . . 0 1 9 6 . 0 2 1 6 . . 0 2 2 4 . . 0 2 5 2 . . 0 K 0 . . 1 9 0 , . 1 9 0 . . 19 0 . . 16 0 . , 19 0 . . 2 2 0 . 2 2 0 . . 16 0 . . 2 2 0 . . 16 S 5 M g 1 . 1 1 1 . . 1 1 1 . 2 1 1 . . 0 1 0 . . 9 1 1 , . 0 1 1 . 2 1 1 . 2 1 1 . . 3 6 1 . . 1 1 C a 14 . 8 2 14 . . 8 2 1 5 . 1 3 1 5 . . 4 3 12 . 7 1 18 . . 1 5 16 . 0 3 16 . . 3 4 16 . . 9 4 1 5 . . 4 3 O . M 2 , . 1 1 . . 9 1 , . 9 2 . . 0 2 . 5 2 . 6 2 . . 1 2 . . 6 2 . 2 2 . . 2 N B : * P i n p p m * C a , K a n d M g i n m e q / 1 0 0 g s o i l * O r g a n i c m a t t e r , O . M , i n % * A a n d B a r e r e p l i c a t e s ( b l o c k s ) T a b l e V I I - M e a n D r y M a t t e r y i e l d ( m e t r i c t o n s / h a ) S 1 S 2 S 3 S 4 S 5 S U M W3 W2 W1 W3 W2 W1 W3 W2 W1 W3 W2 W1 W3 W2 W1 p 1 1 1 . . 8 1 1 . 6 1 1 . . 8 1 3 . . 5 1 2 . 2 9 . 7 9 . 3 1 0 . 3 13 . . 1 1 1 . . 3 1 1 . 3 9 . . 6 1 5 . . 7 14 . 4 14 . . 0 1 7 9 . 5 P 2 ' 1 2 . . 3 1 2 . . 2 1 1 . 7 14 . 9 1 3 . 7 1 0 . 9 1 1 . 9 1 3 . 6 1 5 . 8 1 1 . . 7 1 1 . 3 1 1 . 3 19 . 4 2 0 . . 9 16 . 9 2 0 8 . 6 P 3 1 3 . 6 14 . . 2 14 . 2 18 : 1 16 . 2 1 1 . 3 16 . 3 1 5 . 1 2 0 . 5 14 . 4 1 3 . 9 14 . 7 2 3 . 9 2 3 . . 8 1 8 . 7 2 4 9 . 0 P 4 14 . . 8 1 5 . . 2 15 . . 2 19 . 6 17 . 1 1 1 . 8 14 . 4 1 6 . 8 2 2 . 1 16 . . 1 14 . 3 14 . 6 2 6 . . 1 2 3 . . 6 2 2 . 3 2 6 4 . 3 P 5 1 5 . . 8 16 . . 2 1 5 . 6 19 . 8 18 . 0 1 2 . 6 13 . 5 1 7 . 9 2 2 . 5 1 5 . 2 1 5 . 0 14 . 0 2 4 . 3 2 3 . . 4 18 . 1 2 6 1 . 9 S U M 6 8 . . 3 6 9 . . 4 6 8 . 6 8 9 . 6 7 7 . 2 5 6 . 3 6 5 . 4 7 3 . 7 9 4 . 0 6 8 . 7 6 5 . 8 6 4 . 2 1 0 9 . 3 1 0 6 . . 1 9 0 . 0 S U M - 2 0 6 . , 4 - -- ... --- • 2 2 3 . 4 - ' -- --• --- - 2 3 3 . 1 -- --- --- 1 9 8 . 7 - -- --- - 3 0 0 . 5 -M E A N 6 8 . 8 7 4 . 5 7 7 . 7 6 8 . 2 1 0 0 . 2 S = I R R I G A T I O N S C H E D U L E W = I R R I G A T I O N W A T E R P = P H O S P H O R U S F E R T I L I Z E R T R E A T M E N T T a b l e V I I I - A n a l y s i s o f V a r i a n c e (ANOVA) o f D r y m a t t e r Y i e l d SOURCE OF DEGREE OF FREEDOM SUM OF VARIANCE F % CONTRIBUTION V A R I A T I O N SQUARES TEST BY S .S BLOCK r - 1 = 1 81 .4 8 1 . 4 0 . 84 n . s s . SCHEDULE s - 1 4 1 2 0 6 5 3 . 2 3 0 1 6 3 . 3 3 1 2 . . 5 9 * * 4 0 .8 w, I R R I G A T I O N w - 1 2 3 4 8 5 .6 1742 .8 18 . 0 6 * * 1 . . 2 p , PHOSPHORUS p - 1 4 9 1 130 .9 2 2 7 8 2 .7 2 3 6 . . 0 9 * * 3 0 . 8 SW ( s - 1 ) ( w - 1 ) = 8 5 2 0 5 8 .6 6 5 0 7 . 3 67 . 4 0 * * 17 . 6 PW ( p - 1 ) ( w - 1 ) = 8 9 0 1 . .4 112 . . 7 1 . 20 n . s 0 . . 3 SP ( s - 1 ) ( p - 1 ) = 16 11712 .6 7 3 2 . 0 7 . . 59 * * 4 . . 0 SWP ( s - 1 ) ( w - 1 ) ( p - 1 ) =32 8 2 4 8 . . 5 257 .8 2 . .67 * * 2 . 8 ERROR ( s w p - 1 ) ( r - 1 ) = 74 7 1 3 9 . . 0 96 . . 5 TOTAL ( s p w r - 1 ) = 149 2 9 5 4 1 0 . .9 A n a l y s i s o f v a r i a n c e o n r a w d a t a o f y i e l d f r o m 2 r e p l i c a t e s n . s = n o t s i g n i f i c a n t a t 5% l e v e l * * = s i g n i f i c a n t a t 1% l e v e l . 66 i n d i v i d u a l u n i t s i n t o b l o c k s , and (b) t h e u n i t s were e s s e n t i a l l y homogeneous t o b e g i n w i t h . C o n s i d e r i n g t h e s m a l l n e s s i n s i z e o f t h e e x p e r i m e n t a l a r e a (39 m by 14.6 m) and t h e d a t a on pH, a v a i l a b l e K, Mg & Ca and s o i l o r g a n i c m a t t e r c o n t e n t ( T a b l e s V and V I ) , one would be i n c l i n e d t o a c c e p t t h e l a t t e r r e a s o n a s t h e major f a c t o r r e s p o n s i b l e f o r t h e s t a t i s t i c a l i n s i g n i f i c a n c e o f t h e b l o c k t r e a t m e n t s . T h u s , t h e c o n f i g u r a t i o n s of t h e s u r f a c e s do n o t d i f f e r , i n d i c a t i n g t h a t t h e y a r e members o f t h e same p o p u l a t i o n o f s u r f a c e s and d i f f e r e n c e s i n r e s u l t s would be a t t r i b u t e d t o t h e t r e a t m e n t s . 4.3.1 E f f e c t o f I r r i g a t i o n on Y i e l d s In a f a c t o r i a l e x p e r i m e n t o f t h i s n a t u r e , s i m p l e e f f e c t s o f i n d i v i d u a l t r e a t m e n t s a r e n e v e r c o n c l u s i v e e s p e c i a l l y when t h e r e i s e i t h e r a f i r s t o r d e r i n t e r a c t i o n between any two f a c t o r s c o n s i d e r e d o r , as i n t h i s c a s e , where a l l t h e t h r e e f a c t o r s i n d i c a t e t h a t t h e i r i n f l u e n c e on y i e l d i s d e p e n d e n t on e a c h o t h e r . T h i s l a t t e r c a s e i n d i c a t e s t h a t t h e r e i s a s t a t i s t i c a l l y s i g n i f i c a n t s e c o n d o r d e r i n t e r a c t i o n (W x P x S ) . T h e r e was a p o s i t i v e r e s p o n s e t o i r r i g a t i o n w a ter (W) and t h i s was s t a t i s t i c a l l y s i g n i f i c a n t a t 1% l e v e l . B u t , a s m e n t i o n e d i n t h e p r e c e d i n g p a r a g r a p h , t h i s i s n o t c o n c l u s i v e s i n c e i t can be seen ( T a b l e V I I I ) t h a t t h e c o n t r i b u t i o n by sum o f s q u a r e s , SS, due t o t h i s e f f e c t i n t h e f a c t o r i a l a r r a n g e m e n t was a mere 1.2%. When compared t o s c h e d u l e and p h o p h o r u s w i t h r e s p e c t i v e l y 40.8% and 30.8% c o n t r i b u t i o n s , t h e p h y s i c a l i n t e r p r e t a t i o n of t h e e f f e c t o f i r r i g a t i o n on y i e l d becomes 67 c l e a r : t h a t i t was h i g h l y i n f l u e n c e d by o t h e r f a c t o r s and hence i m i g h t or might n o t be an i m p o r t a n t f a c t o r i f i t were t e s t e d s e p a r a t e l y i n a n o t h e r e x p e r i m e n t a l s e t - u p . In o t h e r words, l i m i t i n g t h e d i s c u s s i o n t o t h e r e s u l t of t h i s s t u d y a l o n e , i t i s e v i d e n t t h a t amount of i r r i g a t i o n w a t e r a f f e c t e d cowpea y i e l d , b u t b e c a u s e of i t s i n t e r a c t i v e e f f e c t s w i t h P and S d e t e c t e d by t h e e x p e r i m e n t a l d e s i g n , t h e p o s s i b i l i t y t h a t W c o u l d as w e l l have been an u n i m p o r t a n t f a c t o r i n cowpea p r o d u c t i o n i f t e s t e d s e p a r a t e l y and a l o n e c a n n o t be r u l e d o u t a t t h i s s i t e . In g e n e r a l , d r y m a t t e r y i e l d d i f f e r e n c e s between Wl and W3 t r e a t m e n t s were e v i d e n t w h i l e t h e y i e l d s o f W2 p l a n t s were n o t s i g n i f i c a n t l y d i f f e r e n t (0.05 l e v e l ) f r o m e i t h e r t h o s e o f W1 o r W3. I t s h o u l d be m e n t i o n e d h e r e t h a t S and W t r e a t m e n t s were so i n t i m a t e l y c o n n e c t e d ( n o t i n s t a t i s t i c a l s e n s e ) i n t h i s e x p e r i m e n t a l d e s i g n t h a t t h e y have t o be d i s c u s s e d a l o n g s i d e e a c h o t h e r and n o t i n i s o l a t i o n . As shown i n T a b l e V I I I , t h e SxW i n t e r a c t i o n was s i g n i f i c a n t a t 1% l e v e l . The c o n t r i b u t i o n by sum of s q u a r e s , o f t h i s i n t e r a c t i v e e f f e c t r a n k e d t h i r d (17.6 %•) and d e m o n s t r a t e d t h a t i t d i d s u b s t a n t i a l l y i n f l u e n c e t h e d i f f e r e n c e s i n y i e l d s . F u r t h e r m o r e , i t i s s t a t i s t i c a l l y e v i d e n t t h a t t h e s e two v a r i a b l e s d i d n o t a c t i n d e p e n d e n t l y i n t h e i r e f f e c t s . The h i g h c o n t r i b u t i o n by sum o f s q u a r e ( S S ) , of S t r e a t m e n t (40.8) was p r o b a b l y r e s p o n s i b l e f o r t h i s r e s u l t ; i t w i l l be a r g u e d l a t e r i n t h e t e x t t h a t S, t h e i r r i g a t i o n s c h e d u l i n g p r o c e d u r e t e n d e d t o e x e r t a g r e a t e r i m p a c t on d r y m a t t e r y i e l d of cowpea than d i d t h e i r r i g a t i o n amount of w a t e r , W. 6 8 S i n c e no a t t e m p t was made t o c o n t r o l t h e amount of r a i n f a l l t h a t f e l l on t h e d i f f e r e n t p l o t s d u r i n g t h e i r r i g a t i o n s e a s o n , t h e cowpea c r o p was s u b j e c t e d t o d i f f e r e n t w a ter s t r e s s and n o n s t r e s s p e r i o d s d e p e n d i n g on t h e i r r i g a t i o n s c h e d u l e . The ANOVA ( T a b l e V I I I ) i n d i c a t e d s i g n i f i c a n c e a t 0.01 l e v e l f o r S t r e a t m e n t . From a knowledge of t h e permanent w i l t i n g p o i n t ( F i g u r e 5 ) , and by c o m p a r i s o n w i t h t h e g r a v i m e t r i c d e t e r m i n a t i o n s ( A p p e n d i x E) t h r o u g h o u t t h e c r o p p i n g p e r i o d , i t was e v i d e n t t h a t S1 ( t h e c o n t r o l ) and S4 ( t h e S t a g e - o f - G r o w t h ) p l o t s were under m o i s t u r e s t r e s s s e v e r a l t i m e s e s p e c i a l l y between June 2 6 t h and J u l y 10th (4.9 mm r a i n f a l l t o t a l ) and t h r o u g h o u t A u g u s t (46.8.mm r a i n f a l l ) . D e s p i t e t h e a p p a r e n t water d e f i c i t s t h a t p r e v a i l e d i n p l o t s S1 and S4 e a r l y a f t e r g e r m i n a t i o n and i n A u g u s t , S1 s t i l l p r o d u c e d h i g h y i e l d s and p e r h a p s , i f i t were p o s s i b l e t o a l l o w t h e c r o p t o f l o w e r and f r u i t ( n i g h t s o i l t e m p e r a t u r e s of below 8 d e g . C i n September p r e c l u d e t h e s u c c e s s o f t h i s c o n t e m p l a t i o n ) , u l t i m a t e g r a i n y i e l d m i g h t not have d i f f e r e d s i g n i f i c a n t l y f r o m t h o s e o f t h e o t h e r s c h e d u l e s . G r o w t h r a t e of p l a n t was s l o w e r and, number o f t r i f o l i a t e s and nodes was r e l a t i v e l y l o w e r i n S1 and S4 when compared t o S2, S3 and S5. I t a p p e a r s t h e f o r m e r g r o u p f o l l o w e d t h e c o n c e p t of " c r o p c o n d i t i o n i n g " t o water' s t r e s s i n view of t h e h i g h d r y m a t t e r y i e l d s o b t a i n e d from them u l t i m a t e l y . T h e s e y i e l d s were however, l o w e r f o r a l l f e r t i l i z e r l e v e l s 7 b y c o m p a r i s o n w i t h y i e l d s f r o m S2, S3 and S5 p l a n t s . H s i a o ( 1 9 7 3 ) , s u g g e s t e d t h a t some r e d u c t i o n i n l e a f a r e a from s t r e s s e s i n t h e e a r l y v e g e t a t i v e p e r i o d of a c r o p w i l l o f t e n 69 m i n i m a l l y a f f e c t y i e l d of a r e p r o d u c t i v e o r g a n b e c a u s e ( i ) r e d u c e d l e a f a r e a can a l l o w g r e a t e r c a n o p y l i g h t p e n e t r a t i o n t o m a i n t a i n a s s i m i l a t i v e c a p a c i t y , and ( i i ) , a s s i m i l a t e s a c c u m u l a t e d d u r i n g t h e day may e x c e e d r e q u i r e m e n t s f o r g r o w t h a t n i g h t b e c a u s e of o t h e r g r o w t h - 1 i m i t i n g f a c t o r s ( s u c h a s t e m p e r a t u r e and n u t r i e n t d e f i c i e n c i e s ) . T h a t o p t i m a l l y -i r r i g a t e d (S2) and f r e q u e n t l y - i r r i g a t e d (S3 and S5) p l a n t s e x c e l l e d i n y i e l d u l t i m a t e l y i s i n a g r e e m e n t w i t h t h e c o n c l u s i o n of Stegman et. a_l ( 1 9 8 1 ) , t h a t maximum y i e l d s a r e u s u a l l y a s s o c i a t e d w i t h maximum s e a s o n a l p h o t o s y n t h e s i s w h i c h r e q u i r e s o p t i m a l l e a f a r e a d e v e l o p m e n t and t h e m a i n t e n a n c e of a h e a l t h y and p h o t o s y t h e t i c a l l y a c t i v e p l a n t d u r i n g t h e v e g e t a t i v e , r e p r o d u c t i v e and n a t u r a l m a t u r a t i o n s t a g e s . F u r t h e r m o r e , T a b l e V I I i l l u s t r a t e s t h a t w h i l e m a i n t a i n i n g t h e n o r m a l i r r i g a t i o n i n t e r v a l (S2) w o u l d p r o d u c e r e a s o n a b l e y i e l d T i m p r o v i n g on t h i s y i e l d c o u l d be a c h i e v e d by r e d u c i n g t h e a l l o w a b l e w a t e r d e p l e t i o n by t h e t e c h n i q u e o f i r r i g a t i n g more f r e q u e n t l y b u t w i t h h a l f t h e d e s i g n c o n s u m p t i v e use a t e a c h i r r i g a t i o n and t w i c e w i t h i n t h e d e s i g n i n t e r v a l ( S 5 ) . But t h e r e i s a l i m i t t o t h e f r e q u e n c y of i r r i g a t i o n a s f a r as t h i s c r o p i s c o n c e r n e d on t h i s e c o l o g i c a l s i t e . I n c r e a s i n g t h e f r e q u e n c y f r o m 50% a l l o w a b l e water d e p l e t i o n i n S2 t o 25% maximum s o i l w a t e r d e f i c i t i n S5 i n c r e a s e d y i e l d i n t h e o r d e r W3 > W2 > W1 bu t on f u r t h e r i n c r e a s e o f t h i s i r r i g a t i o n f r e q u e n c y t o 3 t i m e s w i t h i n t h e i n t e r v a l ( S 3 ) , t h e i d e n t i c a l y i e l d t r e n d s r e c o r d e d i n S2 and S5 were r e v e r s e d t o W1 > W2 > W3, p r o b a b l y b e c a u s e o f w a t e r l o g g i n g and t h e c h a i n of o t h e r s o i l c o m p l i c a t i o n s s e t f o r t h 70 once t h e s o i l was so t r e a t e d . In f a c t , n o t o n l y was t h e y i e l d l o w e r i n t h i s p l o t r e l a t i v e t o p l o t s 2 and 5, but i t a l m o s t came down t o t h e same o r d e r of m a g n i t u d e r e c o r d e d i n t h e c o n t r o l p l o t (S1) and, t h e s t a g e - o f - g r o w t h s c h e d u l i n g p r o c e d u r e (S4) w h i c h was a l s o c l e a r l y i n a d e q u a t e l y i r r i g a t e d . A l s o , t h e q u a l i t y of f o l i a g e was q u i t e p o o r , w h i c h i s a d e b i t on i t s a c c e p t a b i l i t y a s good f o r a g e . The c o n t r o l p l a n t s p r o d u c e d t h e most s u c c c u l e n t -l o o k i n g f o l i a g e f o l l o w e d by t h e S2 p l a n t s . T h e s e r e s u l t s d e m o n s t r a t e d t h a t on p h y s i o l o g i c a l g r o u n d s , t h e use of t o t a l w a ter q u a n t i t y t h a t p a s s e d t h r o u g h t h e r o o t z o ne (IRR + R + D) can have but l i m i t e d u s e f u l n e s s s i n c e (1) t h e t o t a l w a t e r q u a n t i t y i n t h i s c a s e m i g h t f a i l t o c o n s i d e r w a t e r d i s t r i b u t i o n o v e r t h e i r r i g a t i o n s e a s o n a s an i m p o r t a n t v a r i a b l e i n i t s own r i g h t and (2) t o t a l i r r i g a t i o n w a ter a p p l i e d c a n n o t be d i r e c t l y c o r r e l a t e d w i t h p l a n t w a t e r u s e . I n d e e d , C l e m e n t s ( 1 9 6 4 ) , u s i n g t i s s u e m o i s t u r e as a g u i d e f o r i r r i g a t i o n d u r i n g v e g e t a t i v e p h a s e o f s u g a r cane d e v e l o p m e n t , d e m o n s t r a t e d t h a t o n c e t i s s u e m o i s t u r e l e v e l d r o p p e d s i g n i f i c a n t l y a s a c o n s e q u e n c e o f i r r i g a t i o n water b e i n g a p p l i e d t o o l a t e , and a f t e r t h e m o i s t u r e s t r e s s has a l r e a d y t r i g g e r e d a d r o u g h t r e a c t i o n i n t h e p l a n t , s l o w e r g r o w t h and c a r b o h y d r a t e a c c u m u l a t i o n w i l l r e s u l t and, c o m p l e t e r e c o v e r y f r o m t h i s s e t b a c k i n g r o w t h i s d i f f i c u l t . The e f f e c t of water s t r e s s w i l l be f u r t h e r d i s c u s s e d under i t s i n t e r a c t i o n w i t h f e r t i l i z e r t r e a t m e n t s , but p o s s i b l e r e a s o n s f o r t h e f a i l u r e o f S1 and S4 p l a n t s t o y i e l d o p t i m a l l y have been o f f e r r e d by t h e p o i n t s b o r r o w e d from C l e m e n t s and s t a t e d a b o v e . 71 However, a s i g n i f i c a n t p o i n t t o r e i t e r a t e a t t h i s j u n c t u r e i s t h a t i r r i g a t i o n amount of w a t e r o r t o t a l e v a p o t r a n s p i r a t i o n was n o t as i m p o r t a n t i n i n f l u e n c i n g cowpea y i e l d a s t h e t i m e d i s t r i b u t i o n of t h e a p p l i e d w a t e r o r t h e s c h e d u l i n g p r o c e d u r e u n d e r w h i c h t h e c r o p was grown. 72 T a b l e IX - S x P I n t e r a c t i o n on Cowpea Y i e l d , t / h a F E R T I L I Z E R S C H E D U L E S LEVELS S1 S2 S3 S4 S5 PI 11.7 11.8 10.9 10.7 14.7 P2 12.1 13.2 13.8 11.5 19.0 P3 14.0 15.2 17.3 14.3 22. 1 P4 15.1 16.2 17.8 15.0 24.0 P5 15.9 16.8 18.0 14.8 21 .9 T a b l e X - P x W I n t e r a c t i o n on Cowpea Y i e l d , t / h a F E R T I L I Z E R W A T E R L E V E L S LEVELS W1 W2 W3 P I 11.7 11.9 12.3 P2 13.3 14.4 14.0 P3 15.9 16.6 17.3 P4 17.2 17.4 18.2 P5 16.6 18.1 17.7 3o- | Figure 7 - Interaction of S and P Treatments on Yield o Q I L J >-LxJ a 24 H P2 P3 P4 APPLIED FERTILIZER P (kg/ha) P5 Legend A SCHEDULE 1 X SCHEDULE 2 • SCHEDULE 3 EI SCHEDULE 4 S SCHEDULE 5 CO 74 Figure 8 - Examination of S x P Interaction on Y i e l d : (Mann-Whiteney non-parametric comparison) SlPl S1P2 S1P3 S1P4 S1P5. S2P1 S2P5 S3P1 S3P5 S4P1 S4P3 S4P5 S5P1 S5P2 S5P3 S5P4 S5P5 S lP l * X X X x *- X X X X X X X X x x S1P2 X X X X X X X x x X X X X X S1P3 X X X X X X X X X X x X * X S1P4 X X X X X X X x x x x S1P5 X X X X X X X X X X S2P1 S2P2 S2P4 S2P5 . X X X X X X X X X X x x X X X X X X X X X X X X X X x x S3P1 S3P5 X X X (x)x x x x x X X x x x x x x x x x X X X X S4P1 S4P5 ! 1 i X X X X x x x X X X X X X X x x x X X X X K X X X x x x x S5P1 S5P2 S5P3 S5P4 S5P5 X X X x r X * X = 5% s igni f icance level e . g . : An X corresponding to say, the i n t e r s e c t i o n point between treatment combinations S3P1 (ROW) and S4P3 (COLUMN) means that there i s a s i g n i f i c a n t difference at 5% leve l between these two y i e l d s ; ( X ) . 20-i cr .-C c o >-OH LxJ f— !< Cr: Q 14H 10 H 8 Figure 9 - Interaction of W and P Treatments on Yield PI P2 P3 P4 APPLIED PHOSPHATE (kg/ha) Legend A WATER LEVEL 1 X WATER LEVEL 2 • WATER LEVEL 3 P5 76 4.3.2 E f f e c t of P h o s p h o r u s F e r t i l i z e r on Y i e l d F o r a g e y i e l d i n c r e a s e d w i t h i n c r e a s i n g l e v e l o f p h o s p h a t e f e r t i l i z e r a p p l i e d and t h e s e i n c r e a s e s were f o u n d t o be s t a t i s t i c a l l y s i g n i f i c a n t a t 1% l e v e l . T h i s r e s u l t i s i n a greement w i t h t h a t of M a l i k (1974) who o b t a i n e d i n c r e a s e s i n cowpea f o r a g e y i e l d up t o an optimum dose o f 88 kg/ha P 2 0 5 . In t h i s e x p e r i m e n t , maximum y i e l d was o b t a i n e d u n d e r i r r i g a t i o n s c h e d u l e s S2, S4 and S5 w i t h 70 kg/ha P 2 0 5 a t w a t e r l e v e l W3 and a t W1 i n S3 w i t h t h e same d o s e o f f e r t i l i z e r . In t h e c o n t r o l , S1, y i e l d r e s p o n s e t o f e r t i l i z e r P was maximum a t W2 a t P5 f e r t i l i z e r l e v e l and, p e r h a p s , w i t h f u r t h e r i n c r e a s e i n p h o s p h a t e d o s e , the c r o p w o u l d s t i l l i m p r o v e i n f o r a g e p r o d u c t i o n . But f o r r e a s o n s t o be g i v e n l a t e r and e s p e c i a l l y i n t h e d i s c u s s i o n on i n t e r a c t i o n s t h a t w i l l f o l l o w s h o r t l y , i t i s r e a s o n a b l e t o say t h a t i n c r e a s i n g t h e p h o s p h o r u s f e r t i l i z e r l e v e l beyond 80 kg/ha P 2 0 5 i s u n j u s t i f i a b l e . The s u b s t a n t i a l r e s p o n s e o f cowpea t o P a p p l i c a t i o n i s i n a g r e e m e n t w i t h t h e works of T e w a r i (1965) i n N i g e r i a , M a l i k (1974) i n I n d i a , Kang and N a n g j u (1983) i n N i g e r i a and Rhoades (1980) i n U.S.A. E f f e c t of P on n o d u l a t i o n was n o t p a r t o f t h i s s t u d y but i t i s w o r t h y o f m e n t i o n h e r e t h a t a l t h o u g h a l l s e e d s were i n n o c u l a t e d w i t h n o n - s p e c i f i c R h i z o b i u m p r i o r t o p l a n t i n g , f e r t i l i z e d p l a n t s p r o d u c e d more t h r i v i n g n o d u l e s t h a n u n f e r t i l i z e d o n e s . Of g r e a t e r i m p o r t a n c e i s t h e p h o s p h o r u s k i n e t i c s t h a t c o u l d have e x e r t e d t r e m e n d o u s i n f l u e n c e on t h e y i e l d r e s p o n s e t o f e r t i l i z a t i o n . A c a r e f u l s t u d y o f T a b l e V I I f u r t h e r r e v e a l s t h a t w h i l e 77 c o n s i s t e n t y i e l d i n c r e a s e s w i t h p h o s p h o r u s f e r t i l i z e r were r e c o r d e d under i r r i g a t i o n s c h e d u l e s S2, S3 and S5, c o r r e s p o n d i n g r e s u l t s i n S1 and S4 were i r r e g u l a r and i n g e n e r a l , of l o w e r m a g n i t u d e s . T h i s r e l a t i v e l y p o o r r e s p o n s e of c r o p t o a p p l i e d f e r t i l i z e r P i n S1 and S4 may be due t o t h e i r r e g u l a r w a t e r s u p p l y w h i c h i s a s u b j e c t o f d i s c u s s i o n i n t h e n e x t s e c t i o n . I t c o u l d be n o t e d however, t h a t t h e f i n d i n g s i n t h i s s t u d y have p o i n t e d o u t t h e f a c t t h a t when water s u p p l y i s d e f i c i e n t o r i r r e g u l a r o r b o t h , c r o p u t i l i z a t i o n o f a p p l i e d f e r t i l i z e r n u t r i e n t would be d i f f i c u l t t o p r e d i c t . O f t e n , t h e r e s p o n s e c o u l d be low. 4.3.3 E f f e c t s o f F a c t o r I n t e r a c t i o n s on Cowpea Y i e l d T h e r e was a h i g h l y s i g n i f i c a n t i n t e r a c t i o n between i r r i g a t i o n s c h e d u l e (S) and p h o s p h o r u s f e r t i l i z e r ( P ) . I n e f f e c t , t h e s e f a c t o r s were d e p e n d e n t on one a n o t h e r i n i n f l u e n c i n g cowpea d r y m a t t e r p r o d u c t i o n , and i t f o l l o w s t h a t t h e s i m p l e e f f e c t s of any s i n g l e f a c t o r d i f f e r and t h e m a g n i t u d e of any s i m p l e e f f e c t d epends upon t h e l e v e l of t h e o t h e r f a c t o r of t h e i n t e r a c t i o n t e r m . The means of S x P y i e l d s ( o v e r a l l W) a r e p r e s e n t e d i n T a b l e IX, and F i g u r e 7 i s a g r a p h i c a l i l l u s t r a t i o n o f t h e same d a t a . R e f e r r i n g t o T a b l e IX o r F i g . 7, i t w i l l be o b s e r v e d t h a t f o r a l l i r r i g a t i o n s c h e d u l e s e x c e p t S5, y i e l d o b t a i n e d was o f t h e same o r d e r of m a g n i t u d e ( a p p r o x i m a t e l y 11.5 t/ha) a t P1. Beyond 0 kg/ha P 2 0 5 a l l s c h e d u l e s showed s h a r p r e s p o n s e t o P a p p l i e d up t o and i n c l u d i n g P3 and t h e n d i m i n i s h i n g r e t u r n s s e t 78 i n and so, d e p e n d i n g on t h e e c o n o m i c s of f e r t i l i z e r u s e , P3 m i g h t be c o n s i d e r e d an optimum f e r t i l i z e r l e v e l f o r S1 - S4. However, w i t h S5, maximum y i e l d was o b t a i n e d w i t h P4 and a l t h o u g h y i e l d d e c r e a s e d s l i g h t l y when t h e f e r t i l i z e r l e v e l was i n c r e a s e d by 10 kg, t h i s y i e l d was s t i l l h i g h e r t h a n t h o s e o b t a i n e d under any of t h e o t h e r s c h e d u l e s i r r e s p e c t i v e of t h e f e r t i l i z e r t r e a t m e n t l e v e l . I t i s i m p o r t a n t t o e x p l o r e t h i s r e s u l t f u r t h e r . F o r a g e p r o d u c t i o n i n S1 under z e r o i r r i g a t i o n , was s t i l l i n c r e a s i n g a t P5 ( F i g u r e 7 ) , and p e r h a p s w o u l d c o n t i n u e t o r e s p o n d g i v e n more p h o s p h a t e . But g i v e n t h e p r e s e n t 15% l e v e l o f c r o p r e c o v e r y of a p p l i e d p h o s p h o r u s ( L a r s e n , 1967; and H a g i n and T u c k e r , 1982) i t i s o b v i o u s t h a t a l o t of r e s i d u e s a r e l e f t i n s o i l a f t e r h a r v e s t . F o r a l l f e r t i l i z e r l e v e l s , S2, S3 and S5 p r o d u c e d h i g h e r y i e l d s t h a n no i r r i g a t i o n o r s t a g e i r r i g a t i o n . T h i s i s an i n d i c a t i o n t h a t t o improve f e r t i l i z e r u s e , i r r i g a t i o n i s e s s e n t i a l . S3 and S5 gave h i g h e r y i e l d t h a n S2, f u r t h e r i n d i c a t i n g t h a t f r e q u e n c y of i r r i g a t i o n w h i c h i s e s s e n t i a l l y m a i n t a i n i n g a h i g h a v a i l a b l e s o i l m o i s t u r e , c o u l d improve y i e l d a t a l l f e r t i l i z e r l e v e l s . However, a p p a r e n t w a t e r l o g g i n g and o t h e r i n j u r i o u s s o i l r e a c t i o n s w h i c h o c c u r r e d i n S3 l e d t o r e d u c e d y i e l d s , t h e r e b y d e m o n s t r a t i n g t h a t t h e r e a r e l i m i t s t o how f r e q u e n t l y t h i s c r o p c a n be i r r i g a t e d w i t h o u t n e g a t i v e e f f e c t s . P h o s p h o r u s has l o n g been c l a i m e d ( B l a c k , 1966) as a p o s s i b l e l i m i t i n g n u t r i e n t f o r d e e p r o o t d e v e l o p m e n t b e c a u s e o f i t s i m m o b i l i t y i n s o i l and i t s low l e v e l o f a v a i l a b i l i t y i n many 79 s u b s o i l s . P u p t a k e by c r o p i s s t r o n g l y a f f e c t e d by s o i l m o i s t u r e l e v e l , p a r t i c u l a r l y n e a r t h e permanent w i l t i n g p e r c e n t a g e ( P i e r r e ejt a l , 1965). The r e s u l t s d e s c r i b e d above f o l l o w t h i s f a c t . P l a n t s i n t h e n o n - i r r i g a t e d p l o t (S1) and t h e i n a d e q u a t e l y i r r i g a t e d p l o t (S4) were s u b j e c t e d t o s t r e s s a t s e v e r a l t i m e s w h i c h no d o u b t a f f e c t e d t h e i r p h o s p h o r u s n u t r i e n t u t i l i z a t i o n when compared t o t h e r e s t of t h e p l o t s . S i n c e t h e plow l a y e r was s u b j e c t t o p e r i o d i c d r y i n g i n S1 and S4, t h e p r e s e n c e of a v a i l a b l e P i n t h e s u b s o i l m ight have i m p r o v e d r o o t g r o w t h i n t h e l o w e r h o r i z o n s d u r i n g p e r i o d s of d e p r e s s e d u p t a k e ( b e c a u s e of m o i s t u r e d e f i c i t ) i n t h e s u r f a c e s o i l . T h e r e was e v i d e n c e of d e e p e r p l a n t r o o t p e n e t r a t i o n i n S1 a l t h o u g h n o t s u b s t a n t i a l . T h e r e i s r e a s o n t o b e l i e v e t h a t t h i s p r o n o u n c e d r o o t e l o n g a t i o n was more a t t r i b u t e d t o m o i s t u r e d e f i c i e n c y i n t h e upper l a y e r f o r c i n g r o o t s t o e x t e n d and e x p l o r e d e e p e r l a y e r s as a r e s u l t o f w h i c h r e s p o n s e t o P l e v e l s was e x h i b i t e d t o g r e a t e r s t r e n g t h s t h a n i t would o t h e r w i s e have been. But s i n c e t h e a p p l i e d f e r t i l i z e r n u t r i e n t was l o c k e d i n t h e s u r f a c e l a y e r , i t i s not s u r p r i s i n g t h e r e f o r e , why t h e o v e r a l l e f f o r t was n o t s u f f i c i e n t t o a c c o m p l i s h t h e y i e l d optimum g e n e r a t e d i n o t h e r p l o t s a t s i m i l a r f e r t i l i z e r l e v e l s . T h i s i s t h e phenomenon e x p l a i n e d by V e i t s (1972) as a n o t h e r form o f ' d r o u g h t ' . W i t h i n any p l o t as w e l l as between p l o t s , t h e r e was a s i g n i f i c a n t d i f f e r e n c e i n y i e l d between S1P1 and S1P5 or S5P5. The Mann-Whitney U - T e s t f o r n o n - p a r a m e t r i c c o m p a r i s o n s o f t r e a t m e n t means was a d o p t e d i n s t u d y i n g S x P i n t e r a c t i o n and 80 t h e r e s u l t s a r e shown i n F i g u r e 8. W x P i n t e r a c t i o n was n o t s i g n i f i c a n t and has a s i m i l a r meaning t o t h e S x P i n t e r a c t i o n . T h a t P x W i n t e r a c t i o n was n o t s i g n i f i c a n t means, s t a t i s t i c a l l y s p e a k i n g , t h a t t h e s e f a c t o r s a c t e d i n d e p e n d e n t of e a c h o t h e r i n i n f l u e n c i n g cowpea p e r f o r m a n c e ; t h e s i m p l e e f f e c t s o f any o f them were t h e same f o r a l l l e v e l s o f t h e o t h e r w i t h i n c h a n c e v a r i a t i o n , as m e asured by e x p e r i m e n t a l e r r o r ; and s i m p l e e f f e c t s were e q u a l t o t h e c o r r e s p o n d i n g main e f f e c t s and i n e f f e c t , a main e f f e c t i n t h i s f a c t o r i a l e x p e r i m e n t was e s t i m a t e d a s a c c u r a t e l y as i f t h e e n t i r e e x p e r i m e n t had been d e v o t e d t o t h a t f a c t o r . D a t a on W x P i n t e r a c t i v e e f f e c t s on c r o p y i e l d a r e p r e s e n t e d i n T a b l e X. U n l i k e i n S x P i n t e r a c t i o n , y i e l d was o f t h e same o r d e r of m a g n i t u d e a t any g i v e n f e r t i l i z e r l e v e l under a l l amounts of i r r i g a t i o n w a t e r ( F i g u r e 9 ) . T h e r e was an o b v i o u s i n c r e a s e i n y i e l d ( d r y m a t t e r a c c u m u l a t i o n ) w i t h i n c r e a s e i n f e r t i l i z e r P a p p l i e d up t o P4, when any one i r r i g a t i o n w a t e r l e v e l i s c o n s i d e r e d . But t h e s e were not s t a t i s t i c a l l y s i g n i f i c a n t . I n p r a c t i c e , i t i s l e f t t o t h e f a r m e r t o c h o o s e what l e v e l t o a p p l y o r what e c o n o m i c y i e l d i s a c c e p t a b l e . The c l o s e n e s s o f t h e c u r v e s i n F i g . 9, t h e i r s i m i l a r d i r e c t i o n of o r i e n t a t i o n a s P l e v e l s i n c r e a s e , and t h e c r i s s - c r o s s i n g o f W2 and W3 a r e c l e a r m a n i f e s t a t i o n s of t h e i n s i g n i f i c a n t d i f f e r e n c e s ( a t 0.05 l e v e l ) . A f u r t h e r s c r u t i n y o f T a b l e X r e v e a l s t h a t y i e l d s were h i g h e r under W3 t h a n W1 a t any g i v e n P l e v e l . A p o s s i b l e e x p l a n a t i o n i s t h a t w a t e r a v a i l a b i l i t y m a i n t a i n e d h i g h i n t e n s i t y 81 o f P i n s o i l s o l u t i o n as w e l l as e n s u r e d a c o n s i s t e n t P u p t a k e ( d i f f u s i o n f a c t o r ) c u l m i n a t i n g i n h i g h e r y i e l d u nder W3 r e l a t i v e t o t h e o t h e r two l e v e l s . A l s o , t h e o b s e r v e d i n c r e a s e i n y i e l d , a l b e i t not s i g n i f i c a n t s t a t i s t i c a l l y , w i t h i n c r e a s e i n P i n s o i l a t s a y , a s i n g l e i r r i g a t i o n w a t e r t r e a t m e n t l e v e l , W1, i s q u i t e i n o r d e r . T h i s r e s u l t o n l y shows t h a t s t r e s s due t o w a t e r d e f i c i e n c y as m o d i f i e d by s o i l f e r t i l i t y , f a v o r s p l a n t s grown under more f e r t i l i z e r q u a n t i t y t h a n t h e r e q u i r e d optimum; more q u a n t i t y t h a n t h e s t a t e d r e c o m m e n d a t i o n f o r a d e f i n e d a r e a , d i c t a t e s t h e n u t r i e n t r e p l e n i s h m e n t r a t e f r o m t h e s t o r e house i n t h e s o l i d p h a s e i n t o t h e s o i l s o l u t i o n . A d d i t i o n a l f a c t o r s s u c h as more e x t e n s i v e r o o t d e v e l o p m e n t u n d e r h i g h e r n u t r i e n t a v a i l a b i l i t y and hence u t i l i z a t i o n when r a i n water was s p o r a d i c a l l y a v a i l a b l e must have c o n t r i b u t e d t o w a r d s i n c r e a s e d y i e l d s w i t h i n c r e a s e d f e r t i l i z e r a p p l i e d a t any g i v e n l e v e l o f i r r i g a t i o n amount o f w a t e r . In f i e l d c r o p e x p e r i m e n t s a i m e d a t d e r i v i n g r e c o m m e n d a t i o n s f o r f e r t i l i z e r and i r r i g a t i o n w a t e r r e q u i r e m e n t , t h e v a l i d i t y o f t h e e x p e r i m e n t a l d e s i g n i s a l w a y s c r u c i a l ( S t e e l and T o r r i e , 1983). P i o n e e r s of t h e ' C o n t i n u o u s V a r i a b l e D e s i g n ' (Fox, 1973 and Bauder e_t a l , 1975) were c a u t i o u s o f t h e s t a t i s t i c a l l i m i t a t i o n s o f t h i s d e s i g n as a b a s i s f o r r e l i a b l e a g r o n o m i c r e c o m m e n d a t i o n s . T h i s was b e c a u s e of f i r s t l y , t h e f a c t t h a t t h e i n d i v i d u a l i r r i g a t i o n t r e a t m e n t s a r e a l w a y s s u r r o u n d e d by t h e a d j a c e n t t r e a t m e n t s and c a n n o t t h e r e f o r e be c o n s i d e r e d r a n d o m i z e d , and s e c o n d l y , t h e d e s i g n as g i v e n by Fox (1973) has s m a l l i n d i v i d u a l p l o t s i z e s ( F i g u r e 2) w h i c h a r e f e a r e d t o l e a d 82 t o more v a r i a b i l i t y . F o r t u n a t e l y , s u b s e q u e n t i n v e s t i g a t o r s (Hanks e_t a l , 1976) c o n c l u d e d t h a t when p r o p e r l y used, t h e d e s i g n can be a v e r y e f f e c t i v e r e s e a r c h t o o l and l a c k of r a n d o m i z a t i o n s o f t h e i r r i g a t i o n w a t e r l e v e l s and r e p l i c a t i o n s may not a l w a y s be a s e r i o u s l i m i t a t i o n c o n s i d e r i n g t h e s u b s t a n t i a l o p p o r t u n i t i e s t h e d e s i g n o f f e r s f o r s m a l l s y s t e m and e x p e r i m e n t a l d e s i g n s t u d i e s . The r e s u l t s o f t h i s s t u d y a r e no d o u b t , handy f o r r e c o m m e n d a t i o n s e x c e p t t h a t s u c h r e c o m m e n d a t i o n s would be w i t h i n t h e l i m i t a t i o n s of t h e c r o p and e n v i r o n m e n t f o r w h i c h t h e e x p e r i m e n t was c o n d u c t e d . 4.4 E f f e c t of I r r i g a t i o n on F e r t i l i z e r U t i l i z a t i o n D i g e s t e d cowpea f o l i a g e was p r e p a r e d f o r d e t e r m i n a t i o n s o f t o t a l N and P on an a u t o a n a l y s e r . A p p e n d i x C i s a p o r t i o n o f t h e p r i n t o u t from t h e a n a l y s e r w h i l e t h e n e c e s s a r y c o m p u t a t i o n s o f e l e m e n t a l c o n c e n t r a t i o n s i n l e a f a s e x t r a c t e d i n t o s o l u t i o n were done w i t h an A p p l e II c o m p u t e r u s i n g t h e a p p r o p r i a t e programme a v a i l a b l e i n t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a S o i l S c i e n c e L a b o r a t o r y . The r e s u l t s o f t h e c o m p u t a t i o n s as p r o d u c e d by t h e computer a r e p r e s e n t e d i n A p p e n d i x D. The r e s u l t s were r e a r r a n g e d ( T a b l e XI) t o c o n f o r m w i t h t h e e x p e r i m e n t a l d e s i g n and i n l i n e w i t h T a b l e V I I , f o r e a s e of i n t e r p r e t a t i o n . T a b l e X I I i s t h e a n a l y s i s o f v a r i a n c e t a b l e o b t a i n e d upon s t a t i s t i c a l e v a l u a t i o n of P - u p t a k e . P h o s p h o r u s u p t a k e f o l l o w e d a t r e n d s i m i l a r t o t h e d r y m a t t e r y i e l d e x c e p t t h a t n o n s i g n i f i c a n c e was o b t a i n e d f o r t h e S x W x P i n t e r a c t i v e e f f e c t on P u p t a k e . T h e d a t a of T a b l e X I a o f f e r s an i n d i c a t i o n 83 of i m p r o v e d P n u t r i e n t u p t a k e w i t h i r r i g a t i o n as w e l l as w i t h h e a v i e r r a t e s of f e r t i l i z e r . In g e n e r a l , t h e r e s u l t s of t h i s s t u d y s u g g e s t t h a t h e a v i e r r a t e s o f f e r t i l i z e r n u t r i e n t were r e q u i r e d t o s a t i s f y t h e needs o f t h e l a r g e r c r o p s p r o d u c e d by i r r i g a t i o n and t h a t n u t r i e n t d e f i c i e n c i e s d i d n o t o c c u r as a r e s u l t of m o i s t u r e s t r e s s . The f a v o r a b l e p e r f o r m a n c e of c r o p s t a n d s t h a t r e c e i v e d z e r o i r r i g a t i o n (S1) and "minimum" i r r i g a t i o n (S4) adds w e i g h t t o t h i s argument b e c a u s e t h e s e p l a n t s a p p a r e n t l y were a b l e t o a b s o r b s u f f i c i e n t n u t r i e n t t o c a p i t a l i z e on t h e o c c a s i o n a l p r e c i p i t a t i o n . However, F i g u r e s 10 and 11 w h i c h a r e g r a p h i c a l p l o t s o f T a b l e s X I I I and XIV r e s p e c t i v e l y , c l e a r l y l e a v e one w i t h t h e i m p r e s s i o n t h a t u nder z e r o i r r i g a t i o n o r a m i n i m a l one i n w h i c h t h e c r o p i s f o r c e d t o depend on u n p r e d i c t a b l e r a i n s d u r i n g summer, p h o s p h a t e u p t a k e c o u l d be as v a r i a b l e i n t h e f i e l d a s y i e l d i t s e l f — a p a t t e r n few f a r m e r s would t o l e r a t e . N i t r o g e n u p t a k e l e v e l s i n s a m p l e s from t h e v a r i o u s t r e a t m e n t c o m b i n a t i o n s were so c l o s e (3 - 5% N i n s o l i d , T a b l e s X I I and X I I I ) t h a t t h e i n s i g n i f i c a n t d i f f e r e n c e s a r e o b v i o u s u n d e r a l l s c h e d u l e s and a t a l l f e r t i l i z e r l e v e l s r e c e i v i n g t h e d i f f e r e n t amounts o f w a t e r . A g a i n , v a r i a t i o n s e x i s t i n p l o t s 1 and 4. T h e s e r e s u l t s of N u p t a k e c o u l d be: (a) a p r o o f t h a t N was not d e f i c i e n t i n s o i l and so d i d n o t a f f e c t P u p t a k e and (b) due t o t h e f a c t t h a t a l l p l a n t s were c a p a b l e o f f i x i n g N g i v e n t h e a p p r o p r i a t e b a c t e r i a l i n n o c u l a t i o n . Upon d i g e s t i o n , t h e p l a n t e x t r a c t was d i l u t e d and a n a l y s e s f o r e l e m e n t a l K, Ca and Mg were done by d i r e c t l y r e a d i n g on an 84 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r and d a t a i n T a b l e X l b a r e a p r e s e n t a t i o n of t h e r e s u l t . The c o n c e n t r a t i o n s of t h e s e e l e m e n t s i n l e a f were w i t h i n s u c h a c l o s e r a n g e t h a t i t was not n e c e s s a r y t o s u b j e c t t h e d a t a t o s t a t i s t i c a l a n a l y s i s . B e s i d e s , t h e r e i s no l i t e r a t u r e t o s u g g e s t t h a t u p t a k e o f P by cowpea i s c o r r e l a t e d t o any o f t h e s e e l e m e n t s . The i n c r e a s e d u p t a k e of P by p l a n t s r e c e i v i n g s u c c e s s i v e l y h i g h e r p h o s p h a t e a p p l i c a t i o n s a p a r t f r o m b e i n g s i g n i f i c a n t l y d i f f e r e n t (0.01 l e v e l ) , i s a f i n d i n g t h a t i s c o n s i s t e n t w i t h t h e d r y m a t t e r y i e l d r e s u l t s and a g r e e s w i t h works o f M a l i k ( 1 9 7 4 ) , Z i s k a and H a l l ( 1 9 8 3 ) and Kang and N a n g j u ( 1 9 8 3 ) . The o b s e r v a t i o n t h a t t h e r e l a t i v e u p t a k e o f n i t r o g e n was not r e l a t e d t o p h o s p h o r u s a p p l i c a t i o n was u n u s u a l as i t i s known ( B l a c k , 1966; V e i t s , 1972; and Sumner and B o s w e l l , 1981) t h a t p h o s p h o r u s and n i t r o g e n a r e c o m p l e m e n t a r y . However, t h e r e i s no i n f o r m a t i o n t o c o n f i r m t h a t t h i s N-P r e l a t i o n s h i p i s t r u e f o r cowpea. T a b l e X I a - P l a n t A n a l y s e s R e s u l t s . T o t a l N & P r e a d o n A u t o A n a l y s e r f r o m o r i g i n a l d i g e s t w i t h o u t d i l u t i o n . TOTAL P (ppm i n s o l u t i o n ) P L 0 T S FERT. I I I I I I I V V LEVELS W3 W2 W1 W3 W2 W1 W3 W2 W1 p 1 64 . . 29 56 . 20 58 . . 24 65 . 0 4 51 . 97 77 . . 7 1 58 . . 1 1 50 . 4 3 6 0 . . 33 5 0 . . 4 1 5 0 . . 54 P2 42 . . 50 78 . 75 47 . . 32 68 . 6 5 61 . 5 2 68 . . 4 8 52 . . 16 45 . 0 6 70 . 7 1 64 . . 24 57 . . 8 6 P3 69 . 59 73 . 8 0 72 . , 50 72 . 2 4 5 3 . . 5 6 65 . . 3 6 82 . . 56 64 . . 0 2 86 . . 8 3 74 . . 93 67 , , 59 P4 8 0 . . 0 5 97 . . 6 0 78 . . 46 73 . 0 4 67 . 4 5 55 , . 37 74 , . 42 5 0 . 13 88 . . 23 84 . . 56 72 . . 8 9 CO P5 4 0 . 52 72 . 36 69 . . 75 79 . 18 59 . 6 6 59 . , 6 9 75 , , 27 44 . 24 66 . 0 2 79 . . 18 74 . . 46 TOTAL N (% i n p l a n t s o l i d ) I I I I I I I V V P1 4 . 3 2 4 . 8 9 4 . 2 2 4 . 0 2 3 . 8 4 2 . 7 0 5 . 3 0 4 . 4 4 4 . 2 7 4 . 1 0 4 . 0 9 P2 1 . 1 1 3 . 9 9 5 . 0 7 4 . 7 6 4 . 5 0 4 . 4 2 4 . 4 5 3 . 8 6 3 . 7 8 4 . 2 0 4 . 2 8 P3 4 . 5 0 4 . 4 2 4 . 8 4 4 . 5 6 4 . 2 4 5 . 2 3 4 . 2 4 4 . 6 8 3 . 5 8 3 . 9 4 4 . 2 2 P4 3 . 6 8 3 . 8 1 4 . 7 4 4 . 2 2 4 . 7 9 4 . 7 6 3 . 8 7 2 . 5 0 5 . 0 4 3 . 7 5 4 . 3 3 P5 3 . 5 9 4 . 4 8 3 . 4 3 4 . 1 4 . 4 . 9 4 4 . 9 0 4 . 2 1 3 . 6 5 5 . 1 0 3 . 9 8 4 . 3 8 T a b l e X l b - P l a n t A n a l y s e s R e s u l t s c o n t d . T o t a l K, Mg a n d Ca i n % p l a n t o f f o l i a g e . FERT. I I I I I I I V V LEVEL ELEMENT W3 W2 W1 W3 W2 W1 W3 W2 W1 K 2 . 8 5 1 .92 2 . 4 7 2 . 6 3 2 . 1 9 2 . 9 9 2 . 3 7 1 .62 3 . 3 7 3 . 0 2 3 . 2 0 P1 Mg 0 . 5 8 0 . 9 9 0 . 6 2 0 . 6 2 0 . 6 2 0 . 6 5 0 . 3 2 0 . 3 8 0 . 9 8 0 . 5 3 0 . 7 5 Ca 0 . 9 0 1 . 4 5 1 . 1 5 1 . 0 0 0 . 8 3 0 . 9 5 0 . 6 5 0 . 6 8 1 . 0 0 0 . 8 5 0 . 8 0 K 0 . 5 3 3 . 9 7 1 . 7 4 2 . 5 3 2 . 9 3 3 . 0 2 2 . 0 0 1 .74 0 . 5 9 3 . 5 5 2 . 7 2 P2 Mg 0 . 0 8 0 . 9 8 0 . 5 3 0 . 7 3 0 . 6 5 1 .22 0 . 3 8 0 . 2 8 1 .07 0 . 8 8 0 . 7 7 Ca 0 . 3 3 1 . 0 2 1 . 0 3 0 . 9 0 1 . 0 3 0 . 8 8 1 . 2 8 0 . 6 5 1 . 0 8 1 .05 0 . 7 5 K 3 . 3 2 3 . 4 4 3 . 2 5 3 . 5 5 3 . 3 9 2 . 3 2 3 . 5 5 2 . 3 5 3 . 5 5 2 . 9 3 1 . 8 2 P3 Mg 1 . 0 5 0 . 6 8 1 . 1 2 0 . 8 8 0 . 6 5 0 . 4 0 0 . 8 8 0 . 4 7 1 . 0 3 0 . 8 5 0 . 6 2 Ca 1 . 2 3 0 . 8 0 1 . 3 8 2 . 2 3 1 . 1 0 1 . 0 0 2 . 2 3 0 . 9 0 1 . 1 0 1 . 1 8 1 .08 K 3 . 7 7 2 . 9 9 3 . 8 2 4 . 8 5 2 . 9 4 2 . 1 3 3 . 3 7 1 .87 3 . 0 2 3 . 0 7 3 . 1 7 P4 Mg 1 . 1 5 0 . 4 7 0 . 8 3 0 . 9 5 0 . 6 5 0 . 8 5 0 . 8 5 0 . 4 0 0 . 8 0 0 . 7 5 0 . 8 0 Ca 0 . 9 0 1 . 5 2 0 . 8 5 1 .03 0 . 8 8 0 . 6 8 1 . 3 3 0 . 8 3 1 . 0 0 0 . 6 5 0 . 9 5 K 2 . 9 0 2 . 7 2 2 . 8 0 3 . 9 7 2 . 1 3 2 . 3 7 1 . 6 2 1 . 6 0 2 . 3 0 3 . 4 9 2 . 5 4 P5 Mg 0 . 8 5 0 . 5 8 0 . 6 8 0 . 8 8 0 . 5 3 0 . 3 8 0 . 4 5 0 . 5 3 0 . 4 3 0 . 6 8 0 . 8 5 Ca 1 . 8 0 0 . 5 0 1 . 1 0 1 .08 0 . 5 8 0 . 8 5 1 . 0 0 0 . 9 5 1 . 1 0 0 . 8 3 0 . 9 3 87 T a b l e X I I - A n a l y s i s o f V a r i a n c e f o r P - U p t a k e SOURCE d . f SUM OF SQUARES MEAN SQUARES F - TEST T r e a t m e n t ( 3 6 4 3 1 . 6 5 ) S 2 458.77 229.39 10. 92 ** w 4 9369.81 2342.45 111. 55 ** p 4 9853.81 2463.45 117. 31 ** S x P 8 630.05 78. 76 3. 75 ** S x W 8 1746.35 218.36 10. 40 P x W 1 6 13696.04 856.00 40. 76 ** S x W x P 32 676.32 21.14 1 . 01 ns B l o c k 1 0.01 0.01 0. 00 ns E r r o r 74 1546.98 20.91 T o t a l 1 49 37978.63 A l l v a l u e s a r e u n c o d e d ** = s i g n i f i c a n t a t 0.01 l e v e l ns = n o t s i g n i f i c a n t a t 0.05 l e v e l F = T r e a t m e n t Mean s q u a r e / E r r o r Mean s q u a r e . 88 T a b l e X I I I - S x P I n t e r a c t i o n on P - U p t a k e , ppm F E R T I L I Z E R I R R I G A T I 0 N L E V E L S S C H E D U L E S S1 S2 S3 S4 S5 P1 6 4 . 29 5 9 . 8 3 6 2 . 6 0 5 0 . 4 3 5 3 . 7 6 P2 4 2 . 50 64 .91 6 0 . 7 2 4 5 . 0 6 6 4 . 2 7 P3 6 9 . 59 7 2 . 8 5 6 7 . 1 6 6 4 . 0 2 7 6 . 4 5 P4 8 0 . 05 8 3 . 0 3 6 5 . 7 5 5 0 . 1 3 81 . 8 9 P5 4 0 . 52 7 3 . 7 6 6 4 . 8 7 4 4 . 2 4 73 . 22 T a b l e X I V - P X W I n t e r a c t i o n on P - u p t a k e , ppm F E R T I L I Z E R I R R I G A T I O N WATER L E V E L S L E V E L S W1 W2 W3 P1 5 7 . 6 8 60 . 22 5 6 . 6 4 P2 5 6 . 7 5 5 7 . 0 2 63 .21 P3 71 . 2 0 6 9 . 2 8 6 9 . 5 6 P4 70 .11 69 .71 7 6 . 6 9 P5 62 .71 58 . 68 5 6 . 5 6 9 0 - i Figure 10 - Interaction of S and P Treatments on P-Uptake PI P2 P3 P4 FERTILIZER LEVEL, Kg/ha Legend A SCHEDULE 1 X SCHEDULE 2 • SCHEDULED H SCHEDULE 4 £ SCHEDULE 5 oo VO P-UPTAKE PER PLANT, ppm ID • X > r — ST AT AT CO n m m CD TV X ) •r- r~ r~ i in m m < < < n m m [— M — * CO o I cr CD 06 91 4.5 Water Use E f f i c i e n c y C r o p water use e f f i c i e n c y (WUE) may be d e f i n e d as t h e amount of d r y m a t t e r p r o d u c e d p e r u n i t volume of water t a k e n up by t h e c r o p from t h e s o i l . T h i s t e r m i s e s s e n t i a l l y t h e r e c i p r o c a l o f t r a n s p i r a t i o n r e f e r r e d t o ( B r i g g s and S h a n t z , 1913) as t h e mass o f w a t e r t r a n s p i r e d p e r u n i t mass o f d r y m a t t e r p r o d u c e d . In t h i s work, w a t e r use e f f i c e n c y (ton/ha/mm) i s e x p r e s s e d a s : WUE = Y/ET [4.1] where: WUE i s as d e f i n e d a b o v e , Y = d r y m a t t e r y i e l d , t o n / h a , ET = s e a s o n a l w a t e r u s e , mm. S e a s o n a l ET was e s t i m a t e d from e q u a t i o n [ 3 . 3 ] . In p r i n c i p l e , AS i n t h e l a t t e r e q u a t i o n c o u l d be computed from h i s t o r i c d a t a b e f o r e i r r i g a t i n g . I n p r a c t i c e however, t h e s e c o m p u t a t i o n s were p e r f o r m e d a t t h e end of. t h e i r r i g a t i o n p e r i o d t o f a c i l i t a t e e s t i m a t i o n of WUE u n d e r t h e d i f f e r e n t i r r i g a t i o n r e g i m e s and s c h e d u l e s . R e s u l t s o f s u c h c o m p u t a t i o n s a r e p r e s e n t e d i n T a b l e XV. I t can be seen i n e q u a t i o n 4.1 t h a t WUE may be i n c r e a s e d by (a) i n c r e a s i n g y i e l d and m a i n t a i n i n g e q u a l water use o r (b) m a i n t a i n i n g e q u a l y i e l d and d e c r e a s i n g w a t e r u s e . M a x i m i z i n g w a t e r use e f f i c i e n c y p e r se i s n o t t h e o b j e c t i v e i n t h i s s t u d y but an "optimum" WUE — maximum y i e l d r e l a t i o n s h i p , s u b j e c t t o 92 l o c a l c o n s t r a i n t s o f water a v a i l a b i l i t y , i s a d e s i r a b l e t o o l f o r water p l a n n e r s i n a s s e s s i n g f u t u r e w a t e r r e q u i r e m e n t s of t h e c r o p i n q u e s t i o n . From T a b l e XV, i t i s a p p a r e n t t h a t w h i l e t h e c o n t r o l p l o t (S1) gave t h e h i g h e s t w ater use e f f i c e n c y , i.ts y i e l d was much l o w e r than t h o s e o f S2, S3 and S5 a t W1-W3 water l e v e l s t h o u g h t h e l a t t e r were o b t a i n e d a t r e l a t i v e l y low WUE's. H i g h e s t i r r i g a t i o n w a ter use e f f i c i e n c i e s a s s o c i a t e d w i t h i n d i v i d u a l i r r i g a t i o n t r e a t m e n t s o c c u r r e d e i t h e r when i r r i g a t i o n w a ter was a p p l i e d d u r i n g d r y p e r i o d s when l i t t l e o r no r a i n f a l l o c c u r r e d o r when l o n g e r i r r i g a t i o n i n t e r v a l s were u s e d d u r i n g p e r i o d s when a p p r e c i a b l e r a i n f a l l o c c u r r e d and g e n e r a l l y i n v o l v e d t r e a t m e n t s t h a t i n c u r r e d some m o i s t u r e s t r e s s ( a s i n d i c a t e d by g r a v i m e t r i c s a m p l i n g o f s o i l ) a nd s l i g h t t o m o d e r a t e y i e l d r e d u c t i o n s . M a i n t a i n i n g a d e q u a t e l y w a t e r e d t r e a t m e n t s f o r h i g h y i e l d s , as e x p e c t e d ( M u s i c k and Dusek, 1971; and S t e w a r t e_t a l , 1980), l o w e r e d i r r i g a t i o n water e f f i c i e n c y a s s o c i a t e d w i t h some i n d i v i d u a l i r r i g a t i o n s l i k e S2W2, S2W3, and S5W1 t o S5W3. M a x i m i z i n g i r r i g a t i o n WUE i n c o m b i n a t i o n w i t h s e a s o n a l r a i n f a l l p e r m i t t e d a p p l y i n g fewer i r r i g a t i o n s a s was d e m o n s t r a t e d by t h e r e s u l t s of S3W1 and S5W1 i n T a b l e XV; i f t h i s c r o p were grown f o r g r a i n s , t h i s p r a c t i c e would have t e n d e d t o f a v o r e a r l y i r r i g a t i o n c u t o f f d a t e when s i g n i f i c a n t r a i n f a l l t h a t o c c u r r e d as September began, would have been s u f f i c i e n t f o r g r a i n f i l l i n g . T a b l e XV - Y i e l d , E v a p o t r a n s p i r a t i o n a n d W a t e r Use E f f i c i e n c y R e s u l t s . I R R I G A T I O N SEASONAL SEASONAL SCHEDULE I R R I G A T I O N INTERVALS N o . o f RAINFALL DEPTH ( d a y s ) I R R I G . ( P ) , mm (mm) J - J - A S1 S2 S4 W1 3 1 . 2 W2 1 0 6 . 8 W3 1 6 9 . 8 1 0 - 8 - 8 W1 1 0 . 4 S3 W2 3 5 . 6 W3 5 6 . 6 1 6 9 . 8 3 0 W1 1 5 . 6 S5 W2 5 3 . 4 W3 8 4 . 9 4 - 4 10 UUN 1 3 . 1 JUL 78 . 1 AUG 4 6 . 8 SEP 8 7 . 7 TOT. 2 2 6 . 5 SEASONAL ET, mm 138 . 8 W1 3 2 6 . 0 W2 7 7 9 . 6 W3 1 1 5 7 . 6 W1 2 4 2 . 8 W2 4 9 4 . 8 W3 7 0 4 . 8 4 7 8 . 4 W1 2 6 3 . 6 W2 5 6 6 . 0 W3 8 1 8 . 0 Y IELD ( Y ) ( t / h a ) 13 . 76 1 1 . 26 1 5 . 4 4 1 7 . 9 2 1 8 . 8 0 14 . 74 13 . 0 8 13 . 24 18 . 0 0 21 . 2 2 21 . 9 2 WATER USE EFFIC IENCY 99 . 1 34 . 5 1 9 . 8 1 5 . 5 77 . 4 29 . 8 1 8 . 6 27 . 7 68 . 3 37 . 5 26 . 8 CO CO N o t e : W a t e r Use E f f i c i e n c y i s d e f i n e d a s k g / h a / m m a s g i v e n o n p a g e 9 0 94 95 The h i g h y i e l d r e s p o n s e (and h i g h WUE) o b t a i n e d from s c h e d u l e d i r r i g a t i o n c o m b i n a t i o n s S3W1 and S5W1 a r e i n d i c a t i v e o f t h e f a c t t h a t i t i s not n e c e s s a r y t o l a r g e l y rewet th e s o i l p r o f i l e t o o b t a i n e f f i c i e n t use o f i r r i g a t i o n water and t h a t s e a s o n a l i r r i g a t i o n water r e q u i r e m e n t s may be r e d u c e d somewhat by u s i n g s m a l l e r i r r i g a t i o n s and r e l y i n g t o some e x t e n t , on e x p e c t e d s e a s o n a l r a i n f a l l as s u g g e s t e d by R a w l i n s and R a a t s ( 1 9 7 5 ) , and Hobbs and Krogman ( 1 9 7 8 ) . I t i s i m p o r t a n t t o add h e r e t h a t under s u r f a c e i r r i g a t i o n c o n d i t i o n s , a f a r m e r may n o t be a b l e t o s u b s t a n t i a l l y r e d u c e t h e s i z e of i r r i g a t i o n s and s t i l l o b t a i n r e a s o n a b l y good water d i s t r i b u t i o n o v e r t h e f i e l d b u t use of s p r i n k l e r s makes t h i s p r a c t i c e f e a s i b l e . Low w ater use e f f i c i e n c y v a l u e s i n S2 were due p r i n c i p a l l y t o t h e o c c u r e n c e s o f heavy r a i n s on two o c c a s i o n s a day a f t e r f u l l i r r i g a t i o n i n e a c h c a s e . On t h e o t h e r hand, i r r i g a t i n g when t h e s o i l was modera~tely wet f r o m r e c e n t r a i n f a l l o r t h e p r e v i o u s i r r i g a t i o n c o u l d be r e s p o n s i b l e f o r t h e low v a l u e s i n S3, p a r t i c u l a r l y i n W3 t r e a t m e n t . 4.6 C r o p P r o d u c t i o n F u n c t i o n s The e c o n o m i c e v a l u a t i o n o f t h e p o t e n t i a l use and d e v e l o p m e n t o f w a t e r r e s o u r c e s r e q u i r e s , i n t e r a l i a , e s t i m a t e s o f t h e m a r g i n a l v a l u e p r o d u c t o f w a t e r . T h e s e m a r g i n a l v a l u e s a r e o f t e n needed t o a s s e s s t h e b e n e f i t s ( o r o t h e r w i s e ) f r o m p o t e n t i a l p r o j e c t s t o s u p p l y i r r i g a t i o n w a t e r and i n d e t e r m i n i n g t h e optimum a l l o c a t i o n of e x i s t i n g w a t e r s u p p l i e s and c o m p e t i n g u s e s . E s t i m a t e s o f t h e m a r g i n a l p r o d u c t of water i m p l y some 96 knowledge of t h e p r o d u c t i o n f u n c t i o n of w a t e r . The e x i s t e n c e of s u c h knowledge i s o f two o b v i o u s a d v a n t a g e s : ( i ) i t s i m p o r t a n c e i n t h e s t u d y o f i r r i g a t i o n p o l i c y , and ( i i ) t h e s y s t e m a t i c h a n d l i n g o f c l i m a t e as a major component of e r r o r i n c r o p p r o d u c t i o n f u n c t i o n s and as a s o u r c e o f p r o d u c t i o n i n s t a b i l i t y i n a g r i c u l t u r e . A l t h o u g h s i m p l e models have been d e v e l o p e d w h i c h a l l o w p r e d i c t i o n of u l t i m a t e y i e l d a s r e l a t e d t o s o i l w a t e r c o n d i t i o n s , t h e f u n c t i o n s a r e i n r e a l i t y complex and i n t e r r e l a t e d t o many o t h e r f a c t o r s of w h i c h s o i l f e r t i l i t y and c l i m a t e a r e v e r y i m p o r t a n t . Many o f t h e s e p r o d u c t i o n f u n c t i o n s a r e s t a t i s t i c a l i n n a t u r e and t h u s , o f t e n s i t e - o r i e n t e d (Hanks and H i l l , 1980). In t h i s s t u d y , p h y s i c a l l y - o r i e n t e d , w i d e l y a p p l i c a b l e and s i m p l e m o d els d e v e l o p e d t o p r e d i c t y i e l d a s r e l a t e d t o w a t e r use and f e r t i l i z e r u t i l i z a t i o n , f i r s t a s s i n g l e , i n d e p e n d e n t f a c t o r s and s e c o n d l y , as d e p e n d e n t v a r i a b l e s were a d o p t e d and f i t t e d t o t h e e x p e r i m e n t a l d a t a . 4.6.1 Y i e l d - W a t e r Model When r e g r e s s i o n e q u a t i o n s were d e v e l o p e d r e l a t i n g c r o p y i e l d t o c r o p e v a p o t r a n s p i r a t i o n , d i f f e r e n t r e l a t i o n s f o r d i f f e r e n t l o c a t i o n s were f o u n d w h i c h i n d i c a t e t h a t t h e r e a r e s i t e d i f f e r e n c e s w i t h r e s p e c t t o s o i l and c l i m a t e (Hanks and H i l l , 1980). Thus, t h e e q u a t i o n s c o u l d not be u s e d t o p r e d i c t r e s u l t s a t d i f f e r e n t s i t e s . M o r e o v e r , even a t t h e same l o c a t i o n , t h e same e q u a t i o n d o e s n o t h o l d t r u e from one y e a r t o a n o t h e r . 97 F o r i r r i g a t i o n management p u r p o s e s , r e l a t i o n s t h a t c a n be t r a n s f e r r e d ( t h a t a r e c o n s t a n t ) f r o m s i t e t o s i t e a r e needed t o p r e d i c t f u t u r e s i t u a t i o n s . One o f t h e most s i m p l e models i s t h a t o f S t e w a r t ( S t e w a r t e t a l , 1 977): Y/Y„ = 1 - i% ET D = 1 (1 - E T / E T m ) [ 4 . 2 ] i n w h i c h Y = a c t u a l d r y m a t t e r y i e l d Y m = maximum ( p o t e n t i a l ) y i e l d when ET = E T m ; ET i s a c t u a l s e a s o n a l e v a p o t r a n s p i r a t i o n a n d E T m i s maximum s e a s o n a l ET. ETjj = e v a p o t r a n s p i r a t i o n d e f i c i t = 1 - E T / E T m , and Pg = s l o p e of r e l a t i v e y i e l d Y/Y m vs ET . S i g n i f i c a n t q u a l i t i e s o f t h i s model ( S t e w a r t e_t a l , 1977) a r e : (a) ET c a n be measured o r e s t i m a t e d ( I n t h i s s t u d y , w a ter b a l a n c e e q u a t i o n [ 3 . 3 ] was u s e d as a b a s i s f o r e s t i m a t e of ET, T a b l e X V ) . (b) ET^ was d e t e r m i n e d by t h e e q u a t i o n where growth was c o n s i d e r e d n ot t o be l i m i t e d by w a t e r ( c ) Model can be a p p l i e d d i r e c t l y i f t o t a l d r y m a t t e r i s d e s i r e d p r o d u c t and t i m i n g of i r r i g a t i o n i s not of p r a c t i c a l i m p o r t a n c e [ T i m i n g h e r e r e f e r s t o c u l t i v a t i o n f o r g r a i n so t h a t i r r i g a t i o n t i m i n g a s r e l a t e d t o g r o w t h s t a g e , i n f l u e n c e s u l t i m a t e y i e l d so . much so t h a t t h e s i m p l e r e l a t i o n a c c o u n t i n g o n l y f o r s e a s o n a l ET may not h o l d w e l l a s a p r e d i c t i v e t o o l ] . 98 Hanks (1980) s u g g e s t s t h a t by e q u a t i o n [ 4 . 2 ] , t h e r a t i o E T / E T m , where Y/Y m i s z e r o , c a n be shown t o a p p r o x i m a t e t h e p o r t i o n of ET t h a t i s due t o e v a p o r a t i o n (E) d i r e c t l y from t h e s o i l ; and t h e p o r t i o n of ET t h a t i s t r a n s p i r a t i o n (T) i s a p p r o x i m a t e d by the f r a c t i o n (1 - 1 //30) . T h u s , fi0 i s a l w a y s ^ 1.0. In e f f e c t , P0 = 1 p o i n t s t o t h e f a c t t h a t no e v a p o r a t i o n o c c u r r e d from t h e s o i l s u r f a c e w h i l e a (56 of 1.5 would mean t h a t o n e - t h i r d o f the ET was E and t w o - t h i r d s T, w h i c h i s a t r u e i n t e r p r e t a t i o n of t h e m a t h e m a t i c a l r e l a t i o n s h i p between t h e f a c t o r s i n t h e e q u a t i o n . F i g u r e 12 i l l u s t r a t e s d r y m a t t e r v s ET. B e c a u s e o n l y f o u r p o i n t s p e r d a t a s e t c o u l d be p l o t t e d , model v a l i d a t i o n i n t h e f o r m o f e q u a t i o n 4.2 would have f u r t h e r e n t a i l e d r e d u c i n g t h i s t o t h r e e w h i c h might i n t u r n r e d u c e t h e p r e d i c t i v e power and t h e r e l i a b i l i t y o f any g r a p h o b t a i n e d f r o m 3 p o i n t s . R a t h e r t h a n do t h a t , t h e model was r e p o r t e d i n F i g . 12 as d a t a t h a t r e f l e c t p r i m a r i l y t h e maximum y i e l d p o t e n t i a l and t h e r a t e of y i e l d i n c r e a s e p e r u n i t w a t e r use t h a t i s a t t a i n a b l e f o r a g i v e n s e t of p r o d u c t i o n i n p u t s ( s u c h a s n o n - l i m i t i n g n u t r i e n t l e v e l under a g i v e n i r r i g a t i o n s c h e d u l e ) . A c c o r d i n g t o S t e w a r t e_t a l , ( 1 9 8 0 ) , a p l o t o f t h e r a t i o of ET/ET„, a c h i e v e s a g r e a t e r g e n e r a l i z a t i o n of y i e l d f u n c t i o n . However, s i n c e t h i s e x p e r i m e n t i s c o n s i d e r e d s i t e - s p e c i f i c , no s e r i o u s a c c u r a c y was l o s t by t h e d i r e c t l i n e a r r e g r e s s i o n . R e g r e s s i o n a n a l y s i s c o u l d be p e r f o r m e d f o r S3 as w e l l , but b e c a u s e of t h e n e g a t i v e r e s p o n s e t o ET under t h i s s c h e d u l e , S2 and S5 a r e p r e f e r r e d f o r p r e d i c t i v e p u r p o s e s . Under S2, t h e 99 m e a s u r e d ET of 326 and 779 mm gave p r e d i c t e d y i e l d s of 11.6 and 15.2 t o n s / h a r e s p e c t i v e l y , u s i n g t h e r e g r e s s i o n l i n e i n F i g . 12. M e a s u r e d y i e l d s f o r t h i s p l o t were 11.3 and 15.4 t o n s / h a r e s p e c t i v e l y . S i m i l a r c o m p a r i s o n s c o u l d be made u s i n g t h e S5 c u r v e . 4.6.2 Y i e l d - F e r t i l i z e r F u n c t i o n a l R e l a t i o n s h i p One y i e l d e q u a t i o n t h a t h as been' f r e q u e n t l y u s e d t o q u a n t i t a t i v e l y r e l a t e c r o p y i e l d r e s p o n s e t o s o i l n u t r i e n t o r a p p l i e d f e r t i l i z e r i s t h e M i t s c h e r l i c h e q u a t i o n (ASA, 1975 and H a g i n and T u c k e r , 1982). The M i t s c h e r l i c h f o r m u l a t i o n i s an e x p o n e n t i a l y i e l d - n u t r i e n t r e s p o n s e f u n c t i o n d e v e l o p e d o r i g i n a l l y f o r a s i n g l e g r o w t h - l i m i t i n g n u t r i e n t , s u c h as P. M i t s c h e r l i c h s t a t e d i n h i s law o f e f f e c t s o f g r o w t h f a c t o r s t h a t ( H a g i n and T u c k e r , 1982) t h e i n c r e a s e i n y i e l d (y) r e s u l t i n g from i n c r e a s e i n s u p p l y o f a p l a n t n u t r i e n t (x) i s p r o p o r t i o n a l t o t h e d e c r e m e n t f r o m t h e maximum y i e l d (A) t h a t c a n be p r o d u c e d when t h e s u p p l y o f t h e n u t r i e n t i s i n c r e a s e d i n d e f i n i t e l y (x > ° o ) . Where a l l o t h e r g r o w t h c o n d i t i o n v a r i a b l e s c an be t a k e n a s c o n s t a n t , o r n o n l i m i t i n g , t h e f o l l o w i n g f u n c t i o n a l r e l a t i o n s h i p h o l d s : dy/dx = K(A-y) [4.3] where: A i s d e f i n e d a s a b o v e , 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 , 1 00 y i s t h e y i e l d r e s p o n s e , and x i s t h e s o i l - P s u p p l y . By i n t e g r a t i o n o f t h e above e q u a t i o n and i n s e r t i o n o f p r o p o r t i o n a l i t y f a c t o r , c [c = InK = 0.434K], t h e l o g a r i t h m i c f o r m i s l o g ( A - y ) = l o g A - cx [ 4 . 4 ] . By t h i s e q u a t i o n , a s x i s s u c c e s s i v e l y i n c r e a s e d , y becomes a s y m t o t i c t o A. I t has t o be e m p h a s i z e d h e r e t h a t t h e e q u a t i o n assumes t h a t when o n l y one n u t r i e n t f a c t o r , x, i s l i m i t i n g , and th e r e m a i n i n g g r o w t h f a c t o r s a r e c o n s t a n t , t h e d i f f e r e n t i a l i n c r e a s e , dy, o f y i e l d , y, w i t h r e s p e c t t o t h e d i f f e r e n t i a l i n c r e a s e , dx, of t h e l i m i t i n g n u t r i e n t f a c t o r , x, i s p r o p o r t i o n a l t o t h e maximum y i e l d A ( a t t a i n a b l e when x i s p r e s e n t i n e x c e s s and y e t not h a r m f u l amount) minus t h e a c t u a l y i e l d . In p r a c t i c e , t h e n u t r i e n t i s c o m i n g from two s o u r c e s : t h e f e r t i l i z e r , x and t h e a v a i l a b l e n u t r i e n t p r e s e n t i n t h e s o i l b e f o r e f e r t i l i z a t i o n , b ( H a g i n and T u c k e r , 1982). I n c o r p o r a t i n g t h i s d e v e l o p m e n t i n t o E q . [4.4] m o d i f i e s i t t o : l o g ( A - y ) = l o g A - c ( x + b) [ 4 . 5 ] . A s u b s e q u e n t a p p r o x i m a t i o n d i f f e r e n t i a t e d t h e p r o p o r t i o n a l i t y f a c t o r , c i n t o t h e p o r t i o n ( c , ) o r i g i n a t i n g f r o m t h e s o i l and t h e one coming from t h e f e r t i l i z e r ( c ) so t h a t t h e e q u a t i o n t o o k t h e fo r m : 101 l o g ( A - y ) = logA - (cx + c,b) [ 4 . 6 ] . The M i t s c h e r l i c h f u n c t i o n or some m o d i f i c a t i o n t h e r e o f , i s used f o r e s t i m a t i n g f e r t i l i z e r r e q u i r e m e n t s t h r o u g h o u t the w o r l d . I t has been found (ASA, 1975 ; and Hagin and Tu c k e r , 1982) t o d e s c r i b e w e l l the r e l a t i o n between a p p l i e d n u t r i e n t and y i e l d i n the p r a c t i c a l range of s o i l c o n d i t i o n s and r a t e s of a p p l i c a t i o n . To f u r t h e r j u s t i f y t he c h o i c e of t h i s model, i t c o u l d be added t h a t phosphorus ( t h e t e s t n u t r i e n t i n t h i s s t u d y ) , as w e l l as p o t a s s i u m uptake p a t t e r n s by c r o p s a r e s a i d t o f o l l o w the M i t s c h e r l i c h concept due p r i n c i p a l l y t o t h e i r r e l a t i v e i m m o b i l i t y i n s o i l ; the same i s not t r u e f o r n i t r o g e n . Model V a l i d a t i o n . The model i n the form p r e s e n t e d i n E q u a t i o n [4.5] w i l l s u f f i c e f o r t h i s paper: l o g ( A - y ) = logA - c ( x + b) [ 4 . 5 ] . W h i l e Hagin and Tucker (1982) recommended t h a t b i n the above e q u a t i o n be t h a t v a l u e o b t a i n e d from s o i l a n a l y s i s , A n a l o g i d e s and Rendig (1972) p r e f e r r e d the s t a n d a r d s e t by M i t s c h e r l i c h — t h a t b be c a l c u l a t e d . To be of p r a c t i c a l s i g n i f i c a n c e , r e s u l t s from b o t h methods s h o u l d agree c l o s e l y o t h e r w i s e i t would be d i f f i c u l t t o a c c e p t the a c c u r a c y of the model as s u f f i c i e n t f o r p r e d i c t i v e p u r p o s e s . To f i t the d a t a t o the model i n t h i s a n a l y s i s , c and b were c a l c u l a t e d and the l a t t e r was found t o agree r e a s o n a b l y w i t h 1 02 s o i l t e s t v a l u e s ( E q u a t i o n [ 4 . 9 ] o r F i g u r e 1 4 ) . The e x p r e s s i o n s u s e d f o r t h e c o m p u t a t i o n s a r e : c = 1/x l o g [ ( A - y 0 ) / ( A - y ) ] [ 4 . 7 ] b = [ l o g A - l o g ( A - y 0 ) ] / c [ 4 . 8 ] where y 0 i s t h e y i e l d o b t a i n e d w i t h P1 f e r t i l i z e r l e v e l . The f o l l o w i n g p o i n t s s h o u l d be kept i n mind i n t h e model f i t t i n g and a r e h e l p f u l i n u n d e r s t a n d i n g t h e r e s u l t s p r e s e n t e d h e r e i n : ( i ) b, t h e amount o f x o r p h o s p h o r u s o r i g i n a l l y p r e s e n t i n t h e s o i l was assumed t o be u n i f o r m l y a v a i l a b l e i n a l l p l o t s . A c c o r d i n g l y , a v e r a g e P from s o i l t e s t v a l u e ( T a b l e V) was u s e d . ( i i ) A, t h e maximal p o t e n t i a l y i e l d was a s s i g n e d t h e y i e l d o b t a i n e d when p h o s p h a t e was a d e q u a t e ( i r r i g a t i o n s c h e d u l e and i r r i g a t i o n w a t e r d e p t h h e l d c o n s t a n t w h i l e i t was assumed t h a t o t h e r g r o w t h p r o c e s s e s r e m a i n e d unchanged and n i t r o g e n was a d e q u a t e f o r a t t a i n i n g t h e y i e l d A ) . ( i i i ) I t was c o n s i d e r e d t h a t p r e d i c t i o n s f r o m S2, S3 and S5 would g i v e more r e l i a b l e y i e l d s s i n c e t h e e x t r a c t a b l e P l e v e l s i n S1 and S4 ( T a b l e V) were not c o m p a r a b l e t o t h o s e of t h e f o r m e r p l o t s . F u r t h e r m o r e , t h e M i t s c h e r l i c h e q u a t i o n s t i p u l a t e s t h a t t h e model be a d o p t e d o n l y when a l l o t h e r f a c t o r s a r e a d e q u a t e e x c e p t t h e l i m i t i n g one on t e s t . To do j u s t i c e t o t h i s c o n c e p t , S1 and S4 c o u l d not be r e g a r d e d as 103 a d e q u a t e l y i r r i g a t e d . Thus, d a t a p r e s e n t e d i n T a b l e XVI a r e a v e r a g e s f o r S2, S3 and S5 w h i c h s a t i s f y t h e c o n d i t i o n s s t a t e d i n a p p l i c a t i o n o f t h e m o d e l . The r e l a t i o n s h i p between P a d d e d t o t h e s o i l and Bray-1 e x t r a c t a n t (0.03N NH aF i n 0.25N HC1) - s o l u b l e P was l i n e a r f o r the t e s t s o i l ( F i g u r e 13). The l i n e a r r e g r e s s i o n was h i g h l y 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 i n t h i s sandy loam, t h e f r a c t i o n o f s o i l P e x t r a c t e d by Ammonium f l o u r i d e i s d i r e c t l y p r o p o r t i o n a l t o t h e a d d e d . W i t h t h i s c o n f i d e n c e e s t a b l i s h e d , t h e s o i l t e s t v a l u e i n ppm was u s e d d i r e c t l y i n model v a l i d a t i o n . In T a b l e XVI , P added (Column 4) r e f e r s t o t h a t amount o f P i n s o i l a d d i t i o n a l t o t h e a v a i l a b i l i t y b e f o r e t h e d i f f e r e n t l e v e l s o f f e r t i l i z e r P were a p p l i e d , o r (x-b) ppm. b i s P a v a i l a b l e i n t h e s o i l p r i o r t o a p p l i c a t i o n o f P - f e r t i l i z e r t r e a t m e n t s . T a b l e XVI - C o m p a r i s o n of A c t u a l and C a l c u l a t e d Y i e l d s o f Cowpea F E R T I L I Z E R SOIL TEST MEASURED ADDED P CALCULATED TREATMENT VALUE YIELD (x-b) YIELD LEVEL (ppm) ( t / h a ) (ppm) ( t / h a ) P1 58=b 11.5 0 11.5 P2 140 15.3 82 17.2 P3 169 18.2 111 18.0 P4 222 19.3 164 18.8 P5 239 1 8 . 9 181 19.0 104 Figure 13 - Rpplied P as Related to Bray-1 Extractable P H 1 i 1 i 1 1 1 i i i 1 i i i i 0.0 13.33 26.66 40.0 53.33 66.66 80.0 93.33 RDDED PHOSPHATE. Kg/ha 105 Figure 14 - f l i tscher l ich Equation and Yield Curves - 0 ACTUAL YIELD CURVE CALCULATED CURVE log(19.3 - y). = l o g l 9 . 3 - 0.0066(X + 58.6) ~ i r 64.0 96.0 128.0 160.0 P ADDED TO SOIL (ppm) — i 1 1 192.0 224.0 1 06 The e q u a t i o n d e s c r i b i n g t h e y i e l d c u r v e of F i g u r e 14 i s : l o g d 9 . 3 - y) = l o g l 9 . 3 - 0 . 0 0 6 6 U + 58.6) [ 4 . 9 ] . I t i s i n t e r e s t i n g t o n o t e t h a t c a l c u l a t e d b (58.6 ppm) a g r e e d s u f f i c i e n t l y w i t h a v e r a g e s o i l t e s t v a l u e o f 58.0 ppm shown i n T a b l e XVI. The "maximum y i e l d " , A = 19.3 t / h a , was e s t i m a t e d by e x t r a p o l a t i o n o f t h e y i e l d c u r v e t o i t s a s y m p t o t e i n a c c o r d a n c e w i t h E q u a t i o n [4.4] and by a d o p t i o n o f t h e method o f A n a l o g i d e s and R e n d i g ( 1 9 7 2 ) . The t h e o r i t i c a l y i e l d v a l u e s c a l c u l a t e d by means of E q u a t i o n [ 4 . 9 ] a r e r e p o r t e d i n T a b l e XVI a l s o . E x c e p t f o r P2, p r e d i c t e d y i e l d s f r o m t h e M i t s c h e r l i c h e q u a t i o n d e s c r i b e d w e l l , t h e r e l a t i o n between a p p l i e d P and y i e l d i n t h e p r a c t i c a l r a n g e of t h e e x p e r i m e n t a l r e s u l t s d i s c u s s e d h e r e i n . F i g u r e 14 f u r t h e r c l a r i f i e s t h i s r e s u l t . Of c o u r s e , t h e e q u a t i o n f a i l e d t o d e t e c t t h e y i e l d d e p r e s s i o n c a u s e d by f e r t i l i z e r l e v e l - P 5 and r e p o r t e d i n column 3. T h i s i s not u n u s u a l a s M i t s c h e r l i c h d i d ad m i t ( H a g i n and T u c k e r , 1982) t h a t t h e e q u a t i o n does n o t f i t d a t a o b t a i n e d a t b o t h e x t r e m e ends of t h e y i e l d c u r v e . In g e n e r a l a l s o (ASA, 1975) an e x p o n e n t i a l f u n c t i o n , s u c h a s t h e one i n q u e s t i o n , n e v e r r e a c h e s an a b s o l u t e maximum nor can i t p r e d i c t t o x i c i t y l e v e l o f an a p p l i e d n u t r i e n t . I t must be c o n c e d e d h e r e t h e n , t h a t t h e ra n g e and l e v e l s o f f e r t i l i z e r t r i a l i n t h i s e x p e r i m e n t were not wide (0 - 80 kg/ha P 2 0 5 ) and many (5 l e v e l s ) enough t o c a p t u r e s u f f i c i e n t l y a c c u r a t e l y , t h e s e n s i t i v i t y zone — t h e l e v e l of P t h a t i s t o x i c t o t h e c r o p or a p o i n t a t w h i c h 1 07 y i e l d s i g n i f i c a n t l y b e g i n s t o d e c r e a s e as a r e s u l t of e x c e s s n u t r i e n t , f o r b o t h t h e e x p e r i m e n t a l and c a l c u l a t e d y i e l d - a d d e d n u t r i e n t c u r v e s . The M i t s c h e r l i c h e q u a t i o n has o f r e c e n t been r a d i c a l l y e x p a n d e d t o i n c o r p o r a t e not o n l y two o r more n u t r i e n t s a t a t i m e but s e v e r a l o t h e r c r o p p r o d u c t i o n v a r i a b l e s . I t c o u l d be a p p l i e d i n t h i s s i t u a t i o n t o o , t o r e l a t e t h e s i m u l t a n e o u s e f f e c t s o f P and i r r i g a t i o n w a t e r on y i e l d b u t , p a r t l y b e c a u s e of t h e r e a s o n s t a t e d i n t h e p r e c e d i n g p a r a g r a p h and p a r t l y b e c a u s e t h e r e were o n l y t h r e e w a t e r t r e a t m e n t l e v e l s (W1, W2 and W3), t h e r e l i a b i l i t y of any f u n c t i o n a l r e l a t i o n s h i p d e r i v e d t h e r e f r o m f o r p r e d i c t i v e p u r p o s e s w o u l d be v e r y d o u b t f u l and w o r t h l i t t l e more t h a n a m a t h e m a t i c a l e x e r c i s e . The A m e r i c a n S o c i e t y o f Agronomy (ASA, 1975) s t a t e s : " any g i v e n f a c t o r t h a t d e c r e a s e s t h e g r o w t h r a t e ( o f a c r o p ) o v e r the g r o w t h p e r i o d must be r e f l e c t e d i n t h e same way a s any o t h e r f a c t o r t h a t r e d u c e s t h e r a t e i n t h e same amount. T h e r e f o r e , i f a r a t e r e d u c t i o n o c c u r s , whether c a u s e d by v a r i a t i o n s i n l e v e l s o f an e s s e n t i a l n u t r i e n t , weeds, s u n l i g h t , m o i s t u r e or s o i l c o m p a c t i o n , t h e r e l a t i o n s h i p between c a u s a t i v e a g e n t s and g r o w t h s h o u l d r e m a i n unchanged and s u b j e c t t o t h e same m a t h e m a t i c a l t r e a t m e n t s . " T h u s , t h e S t e w a r t model f o r r e l a t i n g i r r i g a t i o n water t o y i e l d ( E q . 4.2) and t h e M i t s c h e r l i c h f u n c t i o n d e s c r i b i n g t h e r e l a t i o n s h i p between P added and cowpea d r y m a t t e r y i e l d ( E q . 4.5) would be a c c e p t e d as a d e q u a t e f o r t h e p u r p o s e s o f p r e d i c t i o n , and, b e c a u s e of t h e l i m i t a t i o n s of t h e e x p e r i m e n t 1 08 b e i n g r e p o r t e d . In a s u b s e q u e n t a n a l y s i s o f t h e s e g r o w t h f a c t o r s w i t h t h e a i m of a r r i v i n g a t t h e i r recommended l e v e l s f o r t h i s c r o p and t h e s i t e where t h e e x p e r i m e n t was c o n d u c t e d , p u r e l y q u a l i t a t i v e s t a t e m e n t s would be made but t h e q u a n t i t a t i v e t r e a t m e n t s of t h e m o d e l s i n t h i s s e c t i o n must be f r e q u e n t l y b o r n e i n mind. W h i l e t h e i n f o r m a t i o n o b t a i n e d from t h i s e x p e r i m e n t and s u b s e q u e n t model v a l i d a t i o n s c a n be u s e d t o p r e d i c t t h e s e p a r a t e e f f e c t s o f i r r i g a t i o n and p h o s p h o r u s t r e a t m e n t s on cowpea c r o p y i e l d and p h o s p h o r u s c o n t e n t , t h e e f f e c t s of t h e c o m b i n e d i n f l u e n c e s o f t h e two e f f e c t s a r e l e s s e a s i l y p r e d i c t e d , p a r t i c u l a r l y i n v i e w o f t h e h i g h l y s i g n i f i c a n t i n t e r a c t i o n s between f a c t o r s as p r e s e n t e d i n t h e a n a l y s i s o f v a r i a n c e t a b l e s . W i t h t h i s t r e a t m e n t , t h e f o u r t h o b j e c t i v e o f t h i s t h e s i s has been a d d r e s s e d t o t h e e x t e n t t h a t t h e e x p e r i m e n t a l d a t a c o u l d a l l o w . -~ 1 09 V. SUMMARY AND CONCLUSIONS 5.1 Summary An i r r i g a t i o n - f e r t i 1 i z e r e x p e r i m e n t was c o n d u c t e d w i t h cowpea ( V i g n a u n g u i c u l a t a [ L . ] Walp.) on t h e U n i v e r s i t y of B r i t i s h C o l u m b i a r e s e a r c h f a r m . Water was a p p l i e d f r o m a l i n e -s o u r c e s p r i n k l e r l a t e r a l d e s i g n e d w i t h i t e r a t i v e c o n s i d e r a t i o n s o f t h e d e s i r a b l e i r r i g a t i o n s y s t e m c h a r a c t e r i s t i c s and t h e s i z e and d e s i g n o f t h e e x p e r i m e n t a l a r e a . The p u r p o s e of t h e e x p e r i m e n t was t o d e t e r m i n e t h e r e s p o n s e of t h i s c r o p t o p h o s p h o r u s f e r t i l i z e r under d i f f e r e n t i r r i g a t i o n w a ter r e g i m e s i n an e c o l o g i c a l e n v i r o n m e n t where t h e c r o p i s r a r e l y c u l t i v a t e d . F e r t i l i z e r P was b a n d - a p p l i e d . The p l a n t measurements made were l e a f number, p l a n t h e i g h t and number o f nodes p e r p l a n t on a w e e k l y b a s i s f o r t h e f i r s t s e v e n weeks a f t e r s e e d l i n g e m e r g e n c e . I r r i g a t i o n s c h e d u l i n g p r o c e d u r e s were t h e r i g i d i n t e r v a l - t y p e , b u t , g r a v i m e t r i c s o i l m o i s t u r e d e t e r m i n a t i o n s were t a k e n t h r o u g h o u t t h e c r o p p i n g s e a s o n t o a i d i n s c h e d u l i n g and p r o v i d e i n d i c a t i o n s as t o w h e t h e r or n o t p l a n t s e x p e r i e n c e d any w a t e r d e f i c i t and hence s t r e s s d u r i n g any p a r t of t h e i r r i g a t i o n c y c l e s . The p l a n t r e s p o n s e d a t a t a k e n d u r i n g t h e s e v e n weeks s t a t e d a b o v e , t h e y i e l d and n u t r i e n t c o n t e n t o f p l a n t f o l i a g e h a r v e s t e d a f t e r 90 d a y s o f g r o w t h were f o u n d t o be i n p a r a l l e l w i t h d a t a r e p o r t e d i n t h e q u o t e d l i t e r a t u r e . The d r y m a t t e r y i e l d -f e r t i l i z e r P r e s p o n s e c u r v e was c u r v i l i n e a r ( c o n v e x ) and f i t t e d n i c e l y t o t h e M i t s c h e r 1 i c h ' s model — t h e "law of d i m i n i s h i n g r e t u r n s . " A l i n e a r r e g r e s s i o n f i t was n o t n e c e s s a r y . However, 1 1 0 i n f i t t i n g t h e y i e l d - s e a s o n a l water use d a t a t o t h e S t e w a r t (1977) y i e l d - e v a p o t r a n s p i r a t i o n m o d e l , i t became u s e f u l . C u r v e s of b o t h model v a l i d a t i o n s c o u l d be a u s e f u l t o o l i n y i e l d p r e d i c t i o n s under a g i v e n s e t o f a g r o n o m i c management s p e c i f i c a t i o n s . T h e r e was n o t enough d a t a f o r f i t t i n g i r r i g a t i o n w a ter and f e r t i l i z e r P j o i n t l y t o a m u l t i v a r i a t e m o d e l . T h e r e i s no d e n y i n g t h e t r u t h t h a t t h e e x p e r i m e n t was d e s i g n e d so t h a t p r o p e r a c c o u n t c o u l d be t a k e n o f t h e l i k e l i h o o d t h a t any f u n c t i o n a l r e l a t i o n s h i p e x a m i n e d w i t h t h e s e q u a n t i t i e s of t e s t v a r i a b l e s (5 l e v e l s P, 3 l e v e l s W, and 5 l e v e l s S) m i g h t be l i m i t e d t o t h e g i v e n r a n g e of e x p e r i m e n t a l c o n d i t i o n s . E x t r a p o l a t i o n t o o t h e r l e v e l s , c r o p s and f o r t h a t m a t t e r , a n o t h e r e n v i r o n m e n t i s c o n s e q u e n t l y , l i m i t e d . I t was d e m o n s t r a t e d t h a t i r r i g a t i o n i s n e c e s s a r y f o r optimum cowpea y i e l d even t h o u g h t h e t o t a l w a t e r s u p p l y ( i r r i g a t i o n p l u s r a i n f a l l ) d u r i n g t h e i r r i g a t i o n s e a s o n c o u l d e x c e e d t h e s e a s o n a l water r e q u i r e m e n t o f t h e c r o p . In t h i s s t u d y , the d i s t r i b u t i o n of r a i n f a l l was i m p o r t a n t and c r i t i c a l i n t h e c o n t r o l p l o t . R e d u c i n g t h e amount o f i r r i g a t i o n w a t e r a p p l i e d p e r i r r i g a t i o n s e t by i r r i g a t i n g more f r e q u e n t l y w i t h l e s s water was f o u n d a d v a n t a g e o u s b e c a u s e i n a d d i t i o n t o i m p r o v i n g y i e l d i n c o m p a r i s o n t o n o r m a l i n t e r v a l s c h e d u l e s a t any g i v e n f e r t i l i z e r l e v e l , w a ter was e v i d e n t l y s a v e d ( T a b l e XV) as water use e f f i c i e n c y was i n c r e a s e d . I r r i g a t i n g t h r e e t i m e s w i t h i n t h e n o r m a l i r r i g a t i o n c y c l e ( e . g e v e r y 3 d a y s i n J u l y ) l e f t t h e s o i l p e r s i s t e n t l y e x c e s s i v e l y wet; t h i s p a r t l y a c c o u n t s f o r t h e d e p r e s s e d y i e l d under t h i s s c h e d u l i n g t e c h n i q u e '(S3). 1 1 1 H i g h f r e q u e n c y i r r i g a t i o n , t h u s , o f f e r e d many o p p o r t u n i t i e s w i t h r e g a r d t o w a t e r c o n s e r v a t i o n b e s i d e s i m p r o v i n g y i e l d b u t , a l i m i t e x i s t s . F o r t h i s c r o p on t h e t e s t s i t e , S 5 , i r r i g a t i n g t w i c e ( e v e r y 4 d a y s i n J u l y ) w i t h i n t h e i n t e r v a l w i t h h a l f t h e d e s i g n c o n s u m p t i v e use (43mm/day) gave maximum y i e l d a t any P l e v e l , when a l l t r e a t m e n t p l o t s a r e under c o n s i d e r a t i o n . Methods f o r d e t e r m i n i n g when cowpeas s h o u l d be i r r i g a t e d t o a c h i e v e r e d u c t i o n s i n water use w h i l e m a i n t a i n i n g d r y m a t t e r y i e l d optimum can be f u r t h e r d e v e l o p e d from t h e s e f i n d i n g s . In t h i s s t u d y , t h e i m p o r t a n c e o f i r r i g a t i o n water as a f a c t o r l i m i t i n g t h e y i e l d r e s p o n s e of cowpea a p p e a r e d t o be e r a s e d by t h e e f f e c t o f f a c t o r , s c h e d u l i n g p r o c e d u r e . In an i r r i g a t i o n a r e a where u n c e r t a i n r a i n f a l l ( s u c h a s i s t h e c a s e from l a t e S p r i n g t o l a t e Summer i n V a n c o u v e r ) v a r i e s t h e number of i r r i g a t i o n r e q u i r e d , t h e d e c i s i o n on when t o a p p l y t h e s e i r r i g a t i o n s i s o b v i o u s l y c r i t i c a l i f water i s t o be u s e d e f f i c i e n t l y . W i t h o u t t h e a i d o f i n f o r m a t i o n on s o i l w a t e r s u p p l i e s and c r o p - s o i l s t u d i e s s u c h a s t h i s , d e c i d i n g when t o i r r i g a t e and how much w a t e r t o a p p l y a t e a c h i r r i g a t i o n s e t , under t h e s e c o n d i t i o n s must be v e r y d i f f i c u l t . T h i s p r o b a b l y l e a d s f a r m e r s t o i r r i g a t i n g w i t h more water and i n some c a s e s , more f r e q u e n t l y t h a n e s s e n t i a l , t o be on t h e s a f e s i d e . S c h e d u l i n g i r r i g a t i o n s i n s u c h a r e a s b a s e d on c o n c e p t s i n v e s t i g a t e d i n t h i s work would e n s u r e t h a t i r r i g a t i o n s a r e a p p l i e d o n l y when n e c e s s a r y and w i t h j u s t some f r a c t i o n o f t h e d e s i g n i r r i g a t i o n d e p t h , l e a v i n g room f o r s p o r a d i c y e t s i g n i f i c a n t r a i n s and t h e r e b y m a x i m i z i n g t h e e f f i c i e n c y o f 1 1 2 i r r i g a t i o n w a ter u s a g e . As e x p e c t e d , cowpea b e i n g a legume, was f o u n d t o r e s p o n d w e l l t o f e r t i l i z a t i o n of s o i l w i t h p h o s p h o r u s . The d a t a o b t a i n e d might be l i m i t e d t o t h e s o i l - c r o p - c l i m a t i c c o n d i t i o n s i n q u e s t i o n but i t d i d r e p o r t t h e i m p o r t a n c e of m a i n t a i n i n g a h i g h l e v e l of P f e r t i l i z a t i o n i n o r d e r t o o b t a i n t h e y i e l d p o t e n t i a l w i t h optimum s o i l m o i s t u r e . F o r a l l p l o t s ( i r r i g a t i o n s c h e d u l e s ) , 0 and 50 kg/ha p h o s p h a t e p r o d u c e d f o r a g e t h a t was l o w e r i n q u a n t i t y t h a n p l a n t s f e r t i l i z e d w i t h 60, 70, and 80 kg/ha of t h e same n u t r i e n t . T h e s e a r e i n d i c a t i o n s t h a t t h e f o r m e r two l e v e l s o f P namely, P1 and P2, were m a r k e d l y low. F u r t h e r m o r e , i t was o b s e r v e d t h a t y i e l d s i n the c o n t r o l p l o t a t a l l P l e v e l s were q u i t e c l o s e , p o i n t i n g t o t h e f a c t t h a t i f no i r r i g a t i o n i s c o n t e m p l a t e d , t h e r e i s no need t o a p p l y f e r t i l i z e r ; o n ce c r o p i s t o be i r r i g a t e d , t h e need f o r p h o s p h o r u s f e r t i l i z e r a d d i t i o n t o e n s u r e optimum y i e l d becomes a p p a r e n t as i s n o t i c e a b l e i n t h e s u p e r i o r y i e l d s of i r r i g a t e d p l o t s o v e r t h e c o n t r o l (S1) and t h e s t a g e -o f - g r o w t h p l o t (S4) a t any s p e c i f i e d P l e v e l . Some d e d u c t i v e c o n c l u s i o n s c o u l d be drawn from t h e y i e l d -f e r t i l i z e r d a t a as a b a s i s f o r p h o s p h o r u s f e r t i l i z e r r e commendation f o r f o r a g e p r o d u c t i o n f r o m cowpea i n t h i s c l i m a t i c s i t u a t i o n . R e f e r r i n g t o t h e t a b l e s and g r a p h s on d r y m a t t e r y i e l d and P-Uptake by cowpea, i t i s c l e a r t h a t t h e r e i s no s i g n i f i c a n t d i f f e r e n c e between P4 and P5 — i n t h r e e out of t h e f i v e s c h e d u l i n g r e g i m e s , t h e l a t t e r P l e v e l was l e s s p r o d u c t i v e . 1 1 3 From an a g r o n o m i c p o i n t o f v i e w , i t c o u l d be s a i d t h a t P5 must have been t h e t o x i c l e v e l o r a t l e a s t i t s e t up s o i l r e a c t i o n s d e t r i m e n t a l t o t h e c r o p . E c o n o m i c a l l y s p e a k i n g , s i n c e a t t h i s l e v e l , t h e c r o p d i d n o t i n d i c a t e c o n v i n c i n g l y t h a t i t c a n p r o d u c e more d r y m a t t e r o r a b s o r b P a t l e a s t up t o P4 l e v e l p l a n t s , i t i s n o t e c o n o m i c a l l y j u s t i f i a b l e a p p l y i n g 80 kg/ha p h o s p h a t e . Based on e n v i r o n m e n t a l c o n s i d e r a t i o n s , i t i s o b v i o u s t h a t a p p l i c a t i o n of as much as 35 kg/ha P (P5) w i l l r e s u l t i n a l o t o f P r e s i d u e s i n s o i l ( w i t h o u t h i g h e r y i e l d t h a n P4) a t s i t e o f a p p l i c a t i o n e i t h e r as t h e a p p l i e d m o n o c a l c i u m f o r m or some t r a n s f o r m e d p r o d u c t s t h e r e o f . C r o p r e c o v e r y o f a p p l i e d f e r t i l i z e r P i s a low 15-25% ( V e i t s , 1972; and H a g i n and T u c k e r , 1982) w h i l e t h i s e l e m e n t has a low m o b i l i t y i n s o i l ( W i l l i a m , 1971 and H a g i n and T u c k e r , 1982) b u t c a n be c a r r i e d w i t h r u n o f f o r s u b d r a i n d i s c h a r g e s t o c o n s t i t u t e a s o u r c e o f p o l l u t i o n e l s e w h e r e remote f r o m t h e f a r m l o c a t i o n . The f o r e g o i n g d e d u c t i v e s t a t e m e n t s l e a v e no b a s i s f o r not r e j e c t i n g P5 as an e x c e s s i v e l e v e l . S i n c e e x p e r i m e n t a l d a t a s u g g e s t c o m p a r a b l e y i e l d s between P4 and P5 b u t s i g n i f i c a n t l y h i g h e r y i e l d s t h a n P1 P3, P4 (30.6 kg/ha P o r 70 kg/ha p h o s p h a t e ) c a n be c o n s i d e r e d an optimum l e v e l o f f e r t i l i z e r f o r i r r i g a t e d cowpea p r o d u c t i o n u n d e r t h e c r o p - s o i l s i t u a t i o n s t u d i e d . 1 1 4 5.2 C o n c l u s i o n I t must be r e i t e r a t e d t h a t i r r i g a t i o n on t h e s t u d y p l o t s was s u p p l e m e n t a r y . None of t h e d a t a i n d i c a t e d p l a n t d e p e n d e n c e on i r r i g a t i o n f o r growth, but f o r optimum d r y m a t t e r y i e l d i n t h e p r e s e n c e of f e r t i l i z e r , i r r i g a t i o n became i m p o r t a n t . The d a t a d e m o n s t r a t e d b e n e f i c i a l p l a n t g r o w t h r e s p o n s e s from i r r i g a t i o n . The a n a l y s i s of r e s u l t s s u p p o r t s t h e view t h a t t h e t i m e of a p p l i c a t i o n of i r r i g a t i o n w a t e r i s more i m p o r t a n t i n d e t e r m i n i n g i t s p r o d u c t i v i t y t h a n t h e t o t a l q u a n t i t y u s e d . The i m p o r t a n c e of t h i s s t u d y may be a c k n o w l e d g e d from two p e r s p e c t i v e s : The r e s u l t s 1* A l l o w t h e f a r m e r t o e s t i m a t e y i e l d s i f r a i n f a l l d o es not come r e g u l a r l y d u r i n g t h e g r o w i n g s e a s o n o r 2* G i v e t h e f a r m e r t h e management a l t e r n a t i v e s t o s e l e c t t h e e n e r g y , w a t e r a p p l i c a t i o n , p h o s p h a t e f e r t i l i z e r a n d y i e l d l e v e l s t h a t s h o u l d p r o d u c e maximum or optimum p r o d u c t i o n l e v e l s of cowpea on h i s f a r m . Poor r a i n f a l l d i s t r i b u t i o n w i t h r e s p e c t t o e v a p o t r a n s p i r a t i o n d i s t r i b u t i o n and low w a t e r - h o l d i n g c a p a c i t i e s of most s o i l s c a u s e i r r i g a t i o n t o be needed i n many s e m i - a r i d and humid a r e a s , p a r t i c u l a r l y i n i n t e n s i v e a g r i c u l t u r e . R a i n f a l l o c c u r s s p o r a d i c a l l y d u r i n g t h e g r o w i n g s e a s o n and d i s r u p t s any p r e p l a n n e d s c h e d u l e o r f a l l s a few d a y s a f t e r a f u l l i r r i g a t i o n r e g i m e . In o r d e r t o make e f f e c t i v e use of 1 1 5 r a i n f a l l t h a t o c c u r s d u r i n g t h e i r r i g a t i o n s e a s o n , i t i s i m p o r t a n t t h a t l e s s i r r i g a t i o n w a t e r be a p p l i e d t h a n t h e s o i l w i l l h o l d w i t h i n t h e r o o t i n g zone so t h e r e w i l l be room t o h o l d a r e a s o n a b l e amount of r a i n f a l l w i t h o u t deep p e r c o l a t i o n o c c u r r i n g . F o r t h e cowpea u s e d i n t h i s i n v e s t i g a t i o n , t h e e v i d e n c e g e n e r a t e d from t h e s t u d y i s t h a t t h e c r o p be i r r i g a t e d w i t h 21.59mm p e r i r r i g a t i o n s e t o f 3 h o u r s . T h i s i s h a l f t h e d e s i g n e d a p p l i c a t i o n d e p t h o f 43.18mm (1.7 i n . ) f o r a s e t t i m e o f 6 h o u r s and an a v e r a g e i r r i g a t i o n i n t e r v a l o f 9 d a y s f o r t h e r e l e v a n t c r o p p i n g months of June t o September, i n c l u s i v e . F o r r e a s o n s a l r e a d y d i s c u s s e d i n s e c t i o n 5.1, and under t h e p r e c e d i n g r e commendations of i r r i g a t i o n amount and s c h e d u l e , t h e f e r t i l i z e r P l e v e l r e q u i r e m e n t t h a t w i l l p r o d u c e t h e b e s t c o m b i n a t i o n of p r o d u c t i o n v a r i a b l e s f o r optimum cowpea d r y m a t t e r y i e l d and f e r t i l i z e r u t i l i z a t i o n i s 30.6 kg/ha P. 5.3 Recommendations f o r F u r t h e r S t u d i e s T h i s s t u d y was u n d e r t a k e n t o a s c e r t a i n t h e p e r f o r m a n c e o f cowpea w i t h t h e p u r p o s e of o p t i m i z i n g i r r i g a t i o n w a ter and p h o s p h o r u s f e r t i l i z e r l e v e l s t h a t w i l l p r o d u c e maximum y i e l d . The r e s u l t s o f t h e e x p e r i m e n t i l l u s t r a t e t h a t a complex r e l a t i o n s h i p e x i s t s between w a t e r management p r a c t i c e s and f e r t i l i z e r r e s p o n s e . A b e t t e r u n d e r s t a n d i n g of t h i s r e l a t i o n s h i p i s i m p o r t a n t i f t h e o b j e c t i v e of d e v e l o p i n g s o i l a nd w ater management p r a c t i c e s l e a d i n g t o more e f f i c i e n t use o f f e r t i l i z e r P and water i s t o be a c c o m p l i s h e d . I n t h i s r e s p e c t , f u r t h e r i n v e s t i g a t i o n s i n t h e f o l l o w i n g d i r e c t i o n s become 116 i m p e r a t i v e : 1. I t i s not known f r o m t h e s e s t u d i e s w hether t h e l e v e l s of t h e s o i l and p l a n t p a r a m e t e r s w h i c h were e s t a b l i s h e d a s i n d i c a t o r s o f i r r i g a t i o n n eed and optimum y i e l d a r e a p p l i c a b l e f o r cowpeas i n o t h e r s o i l and e n v i r o n m e n t a l c o n d i t i o n s i n B r i t i s h C o l u m b i a . C o n s e q u e n t l y , as i n many f i e l d e x p e r i m e n t s , i t i s i m p o r t a n t t o e v a l u a t e s i m i l a r t r e a t m e n t s as i n t h i s s t u d y , i n t h e d i f f e r e n t e n v i r o n m e n t a l c o n d i t i o n s p r e s e n t i n t h i s a r e a f o r w h i c h r e c o m m e n d a t i o n s a r e t o be made. L i k e w i s e , f o r an o b v i o u s r e a s o n , namely, t h a t c o n d i t i o n s v a r y . f r o m y e a r t o y e a r , i t i s i m p o r t a n t t o know t h e e f f e c t of y e a r s on t h e d i f f e r e n t t r e a t m e n t c o m b i n a t i o n s s i n c e r e c o m m e n d a t i o n s a r e u s u a l l y made f o r f u t u r e y e a r s . B o t h t e m p o r a l and s p a t i a l r e p e t i t i o n s w o u l d go a l o n g way t o w a r d s i m p r o v i n g t h e scope o f i n f e r e n c e made from t h e e x p e r i m e n t so f a r . 2. The n e c e s s i t y of c o o r d i n a t i o n of i r r i g a t i o n a nd f e r t i l i z a t i o n p r a c t i c e s was d e m o n s t r a t e d by c o m p a r i n g y i e l d s of P f e r t i l i z a t i o n and i r r i g a t i o n f r e q u e n c y t r e a t m e n t s w i t h t h e y i e l d s r e s u l t i n g when e i t h e r v a r i a b l e was i n c r e a s e d i n d e p e n d e n t l y . To f u r t h e r i mprove t h e r e l i a b i l i t y and s c o p e of i n f e r e n c e of t h e e x p e r i m e n t , a number o f r e f i n e m e n t s a r e s u g g e s t e d : f i r s t , i t i s n e c e s s a r y t o i n c r e a s e t h e number o f i r r i g a t i o n w a t e r l e v e l s ; s e c o n d l y , w i t h i n t h e p r e s e n t r a n g e o f 50 - 70 kg/ha o f 117 p h o s p h a t e , t h e i n c r e m e n t a l r a t e s h o u l d be made 5 kg i n s t e a d of t h e 10 kg i m p l e m e n t e d i n t h e s t u d y b e i n g r e p o r t e d now; and l a s t l y , but n o t t h e l e a s t i m p o r t a n t , t h e r e i s a need t o i n c r e a s e t h e number of r e p l i c a t e s . U n f o r t u n a t e l y , t h e s e s u g g e s t e d r e f i n e m e n t s a r e l i m i t e d l a r g e l y by t h e f u n d s and t i m e a v a i l a b l e f o r e x p e r i m e n t a t i o n ; t h e c o m p a c t n e s s o f t h e s p e c i f i e d i r r i g a t i o n s y s t e m a n d hence e x p e r i m e n t a l d e s i g n r e d u c e t h i s f e a r and i mprove t h e f e a s i b i l i t y of t h e s e s u g g e s t e d s t u d i e s . 1 18 LITERATURE CITED A l l e n , W.H and J.R. Lambert, 1971. A p p l i c a t i o n of t h e p r i n c i p l e o f c a l c u l a t e d r i s k t o s c h e d u l i n g of s u p p l e m e n t a l i r r i g a t i o n . I . C o n c e p t s . A g r i c . M e t e o r o l . 8:193-201. A m e r i c a n S o c i e t y of Agronomy, 1975. S o i l T e s t i n g : C o r r e l a t i n g and i n t e r p r e t i n g t h e a n a l y t i c a l r e s u l t s . ASA S p e c i a l P u b l i c a t i o n No.29., M a d i s o n , W i s c o n s i n 53711. A n a l o g i d e s , D and V.V. R e n d i g , 1972. F u n c t i o n a l r e l a t i o n s h i p s between y i e l d r e s p o n s e and s o i l p h o s p h o r u s s u p p l y . I . C h o i c e o f t h e i n d e p e n d e n t v a r i a b l e . P l a n t and S o i l 37:545-559. B a u d e r , J.W., R . J . Hanks and D.W. James, 1975. C r o p p r o d u c t i o n f u n c t i o n d e t e r m i n a t i o n s as i n f l u e n c e d by i r r i g a t i o n and n i t r o g e n f e r t i l i z a t i o n u s i n g a c o n t i n u o u s v a r i a b l e d e s i g n . S o i l S c i . S o c . o f Amer. J . 39:1187-1192. Begg, J . E and N.C. T u r n e r , 1976. C r o p water d e f i c i t s . A d v a n c e s i n Agronomy 28:161-297, Ac e d e m i c P r e s s I n c . , New Y o r k & Lo n d o n . B l a c k , C.A., D.D. E v a n s , J . L . W h i t e , L . E . E n s m i n g e r and F.E. C l a r k , 1965. Methods of S o i l A n a l y s e s , P a r t s 1 and 2 . ASA, M a d i s o n , W i s c o n s i n . 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Huges, 1976. Some e f f e c t s o f e n v i r o n m e n t a l s t r e s s on s e e d of cowpea. P l a n t a n d S o i l 44:527-546. Sumner, M.E and F.C. B o s w e l l , 1981. A l l e v i a t i n g N u t r i e n t S t r e s s . I n : A r k i n , G.F and H.M. T a y l o r ( e d s . ) ; M o d i f y i n g t h e Root E n v i r o n m e n t t o Reduce C r o p S t r e s s . ASAE Monograph No. 4, S t . J o s e p h , M i c h i g a n 49085. T e w a r i , P.G., 1 965. E f f e c t s o f n i t r o g e n , p h o s p h o r u s and p o t a s s i u m on n o d u l a t i o n of cowpea. E x p t . A g r i c . 1:257-259. T e w a r i , P.G., 1966. . E f f e c t o f p l a n t i n g d a t e on n o d u l a t i o n and d r y m a t t e r y i e l d of cowpea i n N i g e r i a . E x p t . A g r i c . 2:45-47. T u r k , K . J and A.E. H a l l , 1980. D r o u g h t a d a p t i o n t o cowpea: P a p e r s I - I V , A g r o n . J . 72:413-439. Unger, P.W., H.V. Eck and J . T . M u s i c k , 1981. A l l e v i a t i n g P l a n t Water S t r e s s . I n : M o d i f y i n g t h e Root E n v i r o n m e n t t o Reduce C r o p S t r e s s , by A r k i n , G.F and T a y l o r H.M ( e d i t o r s ) . ASAE Monograph No. 4. V e i h m e y e r F . J and A.H. H e n d r i c k s o n , 1950. S o i l m o i s t u r e i n r e l a t i o n t o p l a n t g r o w t h . A n n u a l Rev. P l a n t P h y s i o l . V o l 1:285-304. 1 25 V e i h m e y e r F . J and A.H. H e n d r i c k s o n , 1955. Does t r a n s p i r a t i o n d e c r e a s e a s t h e s o i l m o i s t u r e d e c r e a s e s . T r a n s . Amer. Geophys. U n i o n 36:425-428. V e i t s , F.G. J r . , 1962. F e r t i l i z e r s and t h e e f f i c i e n t use o f w a t e r . A d v a n c e s i n Agronomy 14:223-264. V e i t s , F.G. J r . , 1966. I n c r e a s i n g w a t e r use e f f i c i e n c y by s o i l management. In P i e r r e e_t §_1 1965: P l a n t E n v i r o n m e n t and E f f i c i e n t Water Use. Amer. S o c . A g r o n . and S o i l S c i . S o c . o f Amer., M a d i s o n , W i s c o n s i n . V e i t s , F.G. J r . , 1972. Water D e f i c i t s and N u t r i e n t A v a i l a b i l i t y . In K o z l o w s k i , T.T. ( e d . ) ; Water D e f i c i t s a n d P l a n t Growth V o l . I I I . Academic P r e s s . W i l l i a m , C.H., 197 1 R e a c t i o n of S u r f a c e - a p p l i e d s u p e r p h o s p h a t e w i t h s o i l . I . The f e r t i l i z e r s o l u t i o n and i t s i n i t i a l r e a c t i o n w i t h s o i l . I I . Movement o f t h e p h o s p h o r u s a n d s u l p h u r i n t o t h e s o i l . A u s t r a l i a n J . S o i l R es. 9:83-106. W o o d r u f f , C.M., M.R. P e t e r s o n , D.H. S c h n a r r e and C.F. C r o m w e l l , 1972. I r r i g a t i o n S c h e d u l i n g w i t h P l a n n e d S o i l M o i s t u r e D e p l e t i o n . ASAE Paper No. 72-722, S t . J o s e p h , M i c h i g a n 94085. Z i s k a , L.K and A.E. H a l l , 1983. S o i l and p l a n t measurements f o r d e t e r m i n i n g when t o i r r i g a t e cowpeas grown u n d e r p l a n n e d - w a t e r - d e f i c i t s . I r r i g a t i o n S c i e n c e 3:247-257. APPENDIX A - METEOROLOGICAL DATA USED IN IRRIGATION DESIGN UBC., Vancouver Weather Stat ion, Lat. 49 15 r , Lone . 123 15 W, 87 m. (1951 - 1980 Normals) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR UNITS MAX. TEMP. 5.1 7.7 8.8 11.7 15.2 18.1 20.7 20.3 17.5 13.1 8.6 6.3 12.8 °C MIN. 0.6 2.5 3.0 5.2 8.3 11.4 13.2 13.2 11.1 7.5 3.7 2.0 6.8 °C DAILY TEMP. 2.9 5.1 6.0 8.5 11.8 14.8 16.9 16.8 14.3 10.3 6.2 4.1 9.8 °C RAINFALL 152.3 126.7 112.0 68.9 59.5 43.0 37 0 52.5 72 .2 133.4 158.4 187.1 1203. 0 mm SNOW 20.5 6.1 4.0 0.2 0.0 0.0 0 0 0.0 0 0 0.0 3.1 20.8 54. 7 mm TOTAL 172.8 132.8 116.0 69.1 59.5 43.0 37 0 52.5 72 .2 133.4 162.5 207.9 1257. 7 mm PRECIP. Vancouver International A i r p o r t , B.C. Lat. 49 11 N, Long. 123 10 W, Elevation 3m Alt i tude (1951 - 1980 Normals) REL. HUMID. 80 78 72 68 65 66 65 68 72 78 72 83 73 VAPOUR PRESS. 0.68 0.77 0.78 0.89 1.09 1.31 1.51 1.55 1.39 1.13 0.86 0.77 1.06 SEA-LEVEL PRESS.101.64 101.67 101.61 101.7 101.7 101.7 101.8101.7 101.7 101.7 101.7 101.6 101.69 % KPa KPa OO 0 0 _ J C 3 •Z. 0 0 >> — s-CC o o o o UJ • cc c o Source: Canadian Climate Normals, Min i s t ry of Environment, Ottawa. 127 APPENDIX B - CATCH CAN LAYOUT FOR WATER DISTRIBUTION TEST CATCH CAN 3.66 m 2.74 m - SPRINKLER LATERAL LINE * Evaporation losses were neither cont ro l l ed nor measured. * S p r i n k l e r operation during the t e s t (and Experiment) was 206.7 KPa 128 A P P E N D I X B c o n t d . S P R I N K L E R UNIFORMITY T E S T R E S U L T Time of catch = 50 min Diameter of can = 10.5 cm 13 11 10 76 100 155 205 285 300 .22 154 225 IRRIGATION LINE-SOURCE 205 316 300 966 202 305 275 648 MEAN VOLUME OF CATCH(ml) DEPTH OF WATER CAUGHT/can (cm) IRRIGATION DEPTH FOR TIME SET OF 6hr. (mm) 225 305 275 174 118 175 168 156 SPRINKLER 22 244.25 154.25 2.83 1.78 169.8 106.8 W3 W2 28 61 68 44.75 0.52 31.2 Wl SPRINKLER COEFFICIENT OF UNIFORMITY (Chr i s t iensen , 1942) = 48%. (Note that there was no l a t e r a l overlap, therefore , the coe f f i c i ent of uni formity given above i s apparently the water d i s t r i b u t i o n pattern pecul iar to the system). APPENDIX C - PLANT ANALYSIS: AUTOANALYSER PLOT APPENDIX D - AUTO ANALYSER N AND P DATA UIC rO«IIT I C I l l LAIOHATOAT - AUTOAMAITIIH N AND f DATA N * t r i C B 0*T« ICT: h T H H l l H I A N O N I H A I C A T I O N T H C A Tn tNT t N» : INI : 41.11; If I : M I 4 I N I m r » u i N T ) o t . ( i rrn tHC t * n r n % N ( I N %r i t * 0 L I & I S O L I D • n * N O r r n i tOlM. I I VATt» m n i K T t t n r L mil I . I CB I . I I • i . eoo •. n o 1.601 i . o e « . u e . 0 0 1 . 1 0 0 . SO 0 . 1 0 0 i i o . %tri l o . i o r r n • Ann. H VOL. NT 100. 100. 10 0. I 80 , 106. t,.w tot. l»1 IOILI LAIOAATUAT AHAbtit* DATA SET: KTI«#«»«t*l f of.w H IAR1C AT I OH TKIATHtNTS • H I : 41 . 00 J INI : it . IH If I : A W T O A M A L 1 S I K M A N D t P A T * . 10.06] If 1: I i . i n n r l : tut R C M I C N T loo.oo r r n N i SAnrLC wt UH « UN I O L 1 0 1 1 0 1 1 0 1 a n A M P a n m r K i i t K T 1 0 0 / 1 0 1 1 0 I 1 1 1 0 0 / 1 0 10 WATER • * * . * . 4 1 4 . t » 1 . 1 * 1 . 1 0 1 . 1 1 1 I . J 1 1 1. M l I . I l l 1. I l l 1.0*1 4.111 * . I M I . t l I 4.111 I . 0 • • . 1 0 4 . 1 1 ) . 1 0 1 . 1 4 1 . T T T . 1 6 1 .Tit .111 .10 1 .1*1 .6*1 r r n H tOLM. 1 TT . 1 141.1 It . T i t • . I 110.1 411.1 «Tt . 4 If 1 . I 4 1 1 . 1 1 1 4 . 1 4 1 1 . 1 4 1 4 . 1 4 1 1 . 1 1 1 1 . 1 0. I r r n r lAnri IOLM. ttAII 0.16; I I M : i 16.06 r r n r * * n r * . . I I rn VOL. I'T i i . I I i l : i i i i . l i T I . II l i . t i T I . 11 I T . 4 I II . IT I I . IT T» . TI 11.11 11.11 1 I . »» T l . l t 4f . II 11.14 IT . 4 4 .11 I .100 I . 000 I .01* I . 610 1 . I l l I . I l l I . 000 I. I l l 1.001 I . 111 I . 010 I . I l l 1.610 I . 000 • T i l I . o n 1 .600 I . I l l 1 0 0 . 0 0 1 6 6.00 1 0 0.06 I 6 6.0O 1 0 0 . 0 6 100.66 110.06 116.66 1 0 0 . 0 0 166.66 106.10 160.00 100.00 I I I . 11 100.06 116.00 106.66 116.60 16 .60 11.11 10.10 l l . l t 10.10 T l . l t II . 10 10.06 40.00 14.00 41.00 T0.lt 11.61 11.10 11.61 1 4 . 06 41.10 IT . 10 4 1.16 I .16 CO o WIC fORtlT I 0 I L 1 LA I OH ATOM 1 . AVTOANA IT11» H ANO t DATA ANAlTItH 0 ATA i(T; MTK0414I01 r A N D M IHMICATION THC ATntN.Tl »Nl : (4.16; SN1 : 41.10; * r I : It >N1 I IN) AtfAlltNT 111.61 r m N ; L I N I w n r i t \ H I I N sr * I N 10 L I O I 10 L I 0 I . 1 tT . 1 4 * . T i l . 060 • r i : l l . l l ; » i r i • r i A N D i n rrn H • O I H . 114. M O . 111.4 111 . I 4*4.4 4 I T , I 410.1 10 f . T 111 . I 411.4 111 . I 40 1 . T • TO . 1 JIT . I 114.0 I.I »6M : I . II MriINT I Ann. ftAlO 1.106 1.606 I . Oil 1 . 1 6 0 t . 6 6 6 1 . 6 6 0 1 . 0 0 0 1 . 0 6 0 1 . 0 0 0 I . I 0 0 1 . 6 0 0 1 . 0 0 6 1 . 0 6 6 1 . 0 0 0 1 . 0 0 0 I . 0 6 O 1 . 0 6 0 1 . 1 0 4 -1 0 0 . t o o . t o o . 100 . t o o . 1 6 0 . 1 0 0 . 1 0 1 . 1 6 6 . 1 0 0 . 1 0 6 . 1 6 0 . VIC rOHIAT BO IL ft LAlORATORT - AVTOANALfXth M AHD t DATA ANA L T 1 [ ft DATA 6 IT : fTTI 4 I 11 4 I 0 I r ANO H 1ARICATIOH. TJtfATHCHTa 4I.S0> » M i t . i l l a n : IT .toi t o r n t . i i * t a r n I H I t I N I A C f M c i t N T l i t . o i r r n H i l l N t l A n r i t %H I I N %r i I N a n A N O a n a i r R t a i K T r r n N r r n r l A n r t • O L N . riAta a O L N . I I . O O r r n •Anrt N VOL. N T 1 I 1 V A T C A 1.111 I • 11 I 4.111 1 . 711 I . I II 6.666 6.011 .111 . T i l .ITT .TOT . 1 7 1 0.100 t . I l l S 1 1 . I 4 1 1 . 1 4 1 1 . 1 • T T . I 1 1 1 . 1 • . • 0.0 1 . III 111.01 I . I l l . 1 0 0.00 1 . O i l 1 6 6 . 0 0 I . 010 110.11 I . 010 110.00 1 . l i t 1 0 0.00 1.010 1 0 0 . 1 0 I T N T TT . 10 41.11 4 1.11 T I .00 41.10 11.11 14.11 IT.II 14.10 1f.II I.10 I . I t t . I t I . II 131 A P P E N D I X E - SOME G R A V I M E T R I C MOISTURE DETERMINATION R E S U L T S So i l samples were taken with an auger . Sampling depths: 0-30cm 30-60cm. Gravimetric s o i l moisture contents recorded as % PLOT I PLOT IV PLOT II 0 cm „ S.S 30 cm — / I 60 cm July 4 t i . jRRIGATION ' WI W2 W3 Whole plot i r r i g a t e d to F ie ld capacity. Note special l a t e r a l moveme-nts made to ensure 'overlap' along the main. 0 -30 cm 30 -60 cm 32.61 30.25 W3 W2 WI July 3th s I r r i g a t i o n to f i e l d capacity. 15th J u l y , 3 days a f ter the rains from 10 to 12th July 1983. 0 - 30 cm 30 - 60 cm 35.47 33.20 36.18 30.68 34.81 29.31 26 - 28/7 0-30 30-60 rai n 47.70 50.60 50.60 49.18 July 25 th : I r r i g a t i o n . 40.6 50.6 40.55 51.37 45.46 44.6 33.8 36.5 38.4 35.5 40.5 36.9 Showers on 2/8 Moisture determinations on the fo l lowing day - 3/8. 29.37 31.50 28.67 29.92 28.86 29.2 30.6 28.5 36.07 40.14 35.82 34.63 35.51 36.11 35.4 35.0 Before i r r i g a t i n g other plots on 6/8/83. After i r r i g a t i o n on 6/8 28.06 28.51 49.7 48.7 52.7 52.7 4 9 . 1 4 3 . 0 30.78 34.19 50.1 50.6 50.8 48.4 51.6 48.8 No ra in since August 2nd . 20.58 22.48 25.44 28.93 Before i r r i g a t i n g on 14/8/83. 24.20 25.14 26.5 25.5 23.1 24.8 28.0 29.8 30.7 30.9 30.1 31.0 After i r r i g a t i o n 34.2 35.1 35.6 36.9 31.2 33.34 30.0 31.7 38.4 36.7 -33.0 34.3 132 A P P E N D I X E c o n t d . PLOT III PLOT V WI W2 W3 W3 W2 WI WI W2 W3 July 15th, 3 days after rains of July 10-l2th 32.11 35.76 30.76 34.80 33.85 32.30 W3 W2 WI 32.86 34.80 Before i r r i g a t i o n of plots III and V on July 22nd 25.4 26.0 25.1 25.9 28.53 29.6 29.4 27.7 30.9 29.2 28.1 26.7 26.7 30.4 28.5 31.7 30.4 31.7 34.5 30.1 30.3 29.7 33.5 32.6 After i r r i g a t i o n of both plots on the same day. 33.1 34.6 30.3 31.0 35.0 32.3 32.9 36.6 43.8 44.5 36.6 31.5 39.2 38.9 39.7 39.3 30.3 30.5 38.6 39 .140 .5 40.0 38.9 39.6 July 25th, 1983. Before i r r i g a t i o n 20.8 30.1 28.1 31.7 30.6 34.7 Af ter i r r iga t ion 28.1 32.8 39.1 33.81 31.7 42.5 31.1 27.3 21.35 32.9 30.4 30.80 40.4 33.7 42.5 34.8 28.23 32.45 Plot V not scheduled to be i r r iga ted . Note: Appendix E i s only i l l u s t r a t i v e and not the whole bulk of gravimetr ic s o i l moisture determinations ca r r i ed out during the experiment. Whenever there was a r a i n f a l l event, s o i l sampling was implemented by taking samples from two depths and on e i ther side of each l a t e r a l set in any p l o t ; however, when a p lo t was i r r i g a t e d , i t was necessary to estimate s o i l moisture contents at WI, W2 and W3 on both rep l i ca te s of a plot (Figure 4 of Chapter 3) . 

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