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Effect of fluctuations in pH on the availability of K, Mg, Cu, Mn, Zn, and Fe Ulansky, Sherry M. 1989

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EFFECT OF FLUCTUATIONS IN pH ON THE AVAILABILITY OF K, Mg, Cu, Mn, Zn, AND Fe By SHERRY M. ULANSKY B . S c , The U n i v e r s i t y of B r i t i s h Columbia, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of S o i l Science) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH October 1989 (6) Sherry M. Ulansky, COLUMBIA 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date Odr- 19^9 . DE-6 (2/88) 11 ABSTRACT An e x p l o r a t o r y s t u d y was conducted t o de t e r m i n e the e f f e c t of f l u c t u a t i o n s i n pH on the a v a i l a b i l i t y of K, Mg, Cu, Mn, Zn, and Fe. Two s o i l s were s e l e c t e d f o r s t u d y which had d i f f e r i n g b u f f e r i n g c a p a c i t i e s based on t h e i r c l a y and o r g a n i c m a tter c o n t e n t s . The pH of s o i l samples was m a n i p u l a t e d w i t h e i t h e r HC1 or CaC0 3 and then a l l o w e d t o i n c u b a t e . M i n e r a l o g i c a l changes, as w e l l a s , c h e m i c a l a n a l y s i s i n d i c a t e d t h a t f l u c t u a t i o n s i n pH may have a m o d e r a t i n g e f f e c t on m i n e r a l w e a t h e r i n g and p r e c i p i t a t i o n r e a c t i o n s which i n t u r n i n f l u e n c e s n u t r i e n t r e l e a s e , f i x a t i o n , and a v a i l a b i l i t y . Changes i n o r g a n i c m a tter c o n t e n t w i t h f l u c t u a t i n g pH appear t o i n f l u e n c e the b e h a v i o u r of the t r a c e m e t a l s and may be r e s p o n s i b l e f o r the development of pH-dependent charge CEC. i i i TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS . i i i LIS T OF TABLES v i LI S T OF FIGURES v i i ACKNOWLEDGEMENTS v i i i INTRODUCTION 1 LITERATURE REVIEW 4 D i s c u s s i o n o f s o i l pH 4 D e f i n i t i o n o f pH 4 A p p l i c a t i o n of pH t o S o i l s 5 D e t e r m i n a t i o n of S o i l pH 8 F a c t o r s I n f l u e n c i n g S o i l pH 10 S u s p e n s i o n E f f e c t 10 D i l u t i o n E f f e c t 11 S a l t C o n c e n t r a t i o n . . 12 E f f e c t o f C a r b o n D i o x i d e 13 Sample D r y i n g 14 S e a s o n a l V a r i a t i o n 15 S o i 1 Buf f er i n g 16 B u f f e r i n g - D e f i n i t i o n 16 B u f f e r i n g Mechanisms C o n t r o l l i n g S o i l pH 17 O r g a n i c M a t t e r 17 C a r b o n D i o x i d e 18 C a r b o n a t e s 19 Aluminum 20 S o i l A c i d i t y 22 T o t a l and E x c h a n g e a b l e A c i d i t y 22 S o u r c e s o f Hydrogen Ions 23 S u r f a c e C h a r g e 23 C a r b o n i c A c i d 25 N i t r i f i c a t i o n 25 F e r t i 1 i z e r s 25 Aluminum H y d r o l y s i s 26 i v Page O x i d a t i o n of P y r i t e 26 N e u t r a l i z a t i o n of S o i l A c i d i t y 26 Liming - D e f i n i t i o n 26 Reactions of Lime in the S o i l 27 F a c t o r s A f f e c t i n g the Rate of Reaction of Lime 28 S o i l P r o p e r t i e s 28 Lime P r o p e r t i e s 30 Chemical Composition 30 P a r t i c l e S i z e 31 Surface Reactions on Lime P a r t i c l e s 32 P h y s i c a l P r o p e r t i e s of Limestone 32 S o i l pH and N u t r i e n t A v a i l a b i l i t y 33 Potassium 33 Magnesium 34 Trace Elements 36 Copper 36 Z i n c 37 Manganese 38 Iron 39 MATERIALS AND METHODS 40 S o i l Samples and Sample P r e p a r a t i o n 40 Incubation Study 41 A n a l y s i s of the Eluant 43 A n a l y s i s of the S o i l s 43 Chemical 43 M i n e r a l o g i c a l A n a l y s i s 44 S t a t i s t i c a l A n a l y s i s '....45 Te s t s f o r Normality and Homogeneity of Variance....45 E l u a t e Data 45 Soi 1 Data 46 RESULTS AND DISCUSSION 47 pH 48 Potassium 48 Magnesium 54 Copper 56 V Page Manganese 58 Z i n c 59 I ron 61 Phosphorus 64 C a t i o n Exchange C a p a c i t y 64 SUMMARY AND CONCLUSIONS 68 LITERATURE CITED 73 APPENDICES 80 A. S o i l P r o f i l e D e s c r i p t i o n s 80 B. Incubation Set-up 82 C. S o i l A c i d T i t r a t i o n 84 v i L I S T OF TABLES Page T a b l e 1 The pH o f a c a l c i u m c a r b o n a t e and a c a l c a r e o u s c l a y s u s p e n s i o n i n e q u i l i b r i u m w i t h c a r b o n d i o x i d e a t v a r y i n g p r e s s u r e s 14 T a b l e 2 Some p r o p e r t i e s o f t h e s o i l s u s e d f o r t h e s t u d y 40 T a b l e 3 pH i n w a t e r and 0.01 M C a C l j 49 T a b l e 4 A v a i l a b l e Mg, K, Cu, Mn , Zn and Fe 49 T a b l e 5 C l a y m i n e r a l o g y o f t h e c o a r s e c l a y f r a c t i o n 50 T a b l e 6 pH and c o n c e n t r a t i o n o f K, Mg, Cu, Mn, Zn, Fe, and A l i n t h e combined e l u a n t f o r e a c h t r e a t m e n t f o r t h e L a n g l e y s o i l 52 T a b l e 7 pH and c o n c e n t r a t i o n o f K, Mg, Cu, Mn, Zn, Fe, and A l i n t h e combined e l u a n t f o r e a c h t r e a t m e n t f o r t h e Lynden s o i l 53 T a b l e 8 A c i d ammonium o x a l a t e e x t r a c t a b l e Fe and A l 62 v i i L I S T OF FIGURES Page F i g u r e 1. G l a s s and c a l o m e l e l e c t r o d e w i t h l i q u i d j u n c t i o n 8 F i g u r e 2. Flow d i a g r a m f o r 1 c y c l e o f t h e i n c u b a t i o n s t u d y . . 4 1 F i g u r e 3. R e l e a s e o f Mg i n t o s o l u t i o n 57 F i g u r e 4. R e l e a s e o f Zn i n t o s o l u t i o n 60 F i g u r e 5. R e l e a s e o f Fe i n t o s o l u t i o n 61 ACKNOWLEDGEMENTS I w o u l d l i k e t o thank my t h e s i s s u p e r v i s o r , Dr. L.M. L a v k u l i c h , f o r h i s g u i d a n c e and s u p p o r t , as w e l l a s , t h e r e m a i n d e r o f my c o m m i t t e e , Dr. L.E. Lowe, Dr. A. Bomke, and Dr. G. B r a d f i e l d . The a d v i c e and a s s i s t a n c e o f E v e l i n e W o l t e r s o n and B e r n i e Von S p i n d l e r i n t h e l a b o r a t o r y was i n v a l u a b l e . The c o n t i n u e d s u p p o r t o f my l o n g - t i m e f r i e n d s M a r y - J a n e D o u g l a s , Kim Longmuir, Doreen Lynk, and Bob Ward; and my new f r i e n d s Dave M a k i , B r e n d a Hooge, H e l e n A r m s t r o n g , and Kim Daum was g r a t e f u l l y a p p r e c i a t e d . I w o u l d l i k e t o g i v e v e r y s p e c i a l t h a n k s t o my mother and my s i s t e r who have a l w a y s been t h e r e . F i n a l l y , I w o u l d l i k e t o s a y a s p e c i a l t h a n k - y o u t o R a l p h who i s not h e r e t o s h a r e i n t h e c o m p l e t i o n o f t h i s work, but who a l w a y s b e l i e v e d I c o u l d do i t . 1 INTRODUCTION S o i l a c i d i t y a r i s e s f r o m s e v e r a l d i f f e r e n t s o u r c e s . N a t u r a l s o u r c e s w h i c h c a u s e a c i d i f i c a t i o n o f s o i l s i n c l u d e C0 2 i n t h e s o i l s o l u t i o n , o r g a n i c a c i d s p r o d u c e d d u r i n g p l a n t d e c o m p o s i t i o n , t h e n i t r i f i c a t i o n o f ammonium, h y d r o l y s i s o f i r o n and aluminum, t h e d i s s o c i a t i o n o f f u n c t i o n a l g r o u p s o f o r g a n i c m a t t e r , and t h e o x i d a t i o n o f p y r i t e ( F e S ) . In r e c e n t y e a r s , man has d r a m a t i c a l l y i n c r e a s e d t h e a c i d i f i c a t i o n p r o c e s s t h r o u g h t h e use o f f e r t i l i z e r s and by i n d u s t r i a l e m i s s i o n s . F e r t i l i z e r s a r e t h e major s o u r c e of s o i l a c i d i f i c a t i o n on a g r i c u l t u r a l l a n d s i n B.C. ( H o y t , 1983). F e r t i l i z e r s a l t s c o n t r i b u t e t o a c i d i t y e i t h e r t h r o u g h n i t r i f i c a t i o n o r h y d r o l y s i s . A n t h r o p o g e n i c e m i s s i o n s o f s u l p h u r d i o x i d e and n i t r o g e n o x i d e s a c i d i f y t h e s o i l as e i t h e r a c i d r a i n o r by d i r e c t a b s o r p t i o n by t h e s o i l . S o i l a c i d i f i c a t i o n i s a c o n c e r n b e c a u s e o f i t s e f f e c t s on t h e c h e m i c a l p r o p e r t i e s o f s o i l s , n u t r i e n t r e l e a s e , l e a c h i n g and f i x a t i o n , as w e l l a s , s o i l w e a t h e r i n g . Some o f t h e e x p e c t e d e f f e c t s o f a c i d i f i c a t i o n on s o i l s have been summarized by McFee (1978) and A r t h u r and Wagner ( 1 9 8 2 ) : - l o w e r e d s o i l pH; - i n c r e a s e d r a t i o o f a c i d i c t o b a s i c c a t i o n s , and r e d u c e d c a t i o n e xchange c a p a c i t y ; - i n c r e a s e d r a t e o f l e a c h i n g o f n u t r i e n t c a t i o n s f r o m r o o t z o n e s ; 2 - l o s s o f p l a n t - a v a i l a b l e P t h r o u g h i t s p r e c i p i t a t i o n w i t h s o l u b l e A l ; - s o l u b i l i z a t i o n of t o x i c m e t a l s e.g. A l , N i , Cd, Hg, and Pb; - a l t e r a t i o n s i n n u t r i e n t c y c l e s as a r e s u l t o f d e c r e a s e d m i n e r a l i z a t i o n of i m p o r t a n t n u t r i e n t s (N, P, S ) ; - s u p p r e s s e d n o d u l a t i o n o f legumes and r e d u c e d s y m b i o t i c , a s s o c i a t i v e , and f r e e - l i v i n g n i t r o g e n f i x a t i o n ; - q u a n t i t a t i v e and q u a l i t a t i v e c h a n g e s i n m i c r o f l o r a l c o m m u n i t i e s ; - d e p r e s s e d a c t i v i t y of m y c o r r h i z a l a s s o c i a t i o n s ; - l o s s o f s i l i c a t e s t r u c t u r e and d e s t a b i 1 i z a t i o n o f c l a y m i n e r a l s ; - i m p r o v e d a v a i l a b i l i t y of p l a n t - e s s e n t i a l m i c r o n u t r i e n t s due t o i n c r e a s e d w e a t h e r i n g p r o c e s s e s ; - a m e l i o r a t i o n o f i r o n d e f i c i e n c i e s i n c a l c a r e o u s s o i l s . A m e l i o r a t i o n o f a c i d c o n d i t i o n s on a g r i c u l t u r a l s o i l s i s g e n e r a l l y a c c o m p l i s h e d by t h e a p p l i c a t i o n o f l i m e . The o b j e c t i v e i n l i m i n g a c i d s o i l s i s t o c r e a t e a more f a v o u r a b l e e n v i r o n m e n t f o r p l a n t g r o w t h , a l t h o u g h i t i s now w i d e l y a c c e p t e d t h a t t h e pH a t w h i c h maximum p l a n t y i e l d o c c u r s may v a r y d e p e n d i n g on s o i l c h a r a c t e r i s t i c s ( C u r t i n e t a l . , 1983). Some o f t h e e f f e c t s o f l i m i n g have been summarized by M a c K e n z i e ( 1 9 7 8 ) , C u r t i n and S m i l l i e ( 1 9 8 3 ) , and Coleman and Thomas ( 1 9 6 7 ) : - i n c r e a s e d s o i l pH; - r e d u c t i o n o f Fe, A l , and Mn c o n c e n t r a t i o n s ; 3 - i n d u c t i o n of Fe and Mn d e f i c i e n c y ; - i n c r e a s e Mo a v a i l a b i l i t y ; - p o t e n t i a l i n c r e a s e s in s o l u b l e phosphate; - i n c r e a s e d m i n e r a l i z a t i o n of o r g a n i c m a t t e r ; - enhanced decompos i t i on of s o i l o r g a n i c m a t t e r ; - i n c r e a s e s in the c a t i o n exchange c a p a c i t y ; - he lps to c r e a t e a s o i l which i s f r i a b l e , w e l l g r a n u l a t e d , r e s i s t a n t to compact ion , and i s permeable to a i r and water . The a b i l i t y of a s o i l to r e s i s t changes i n pH i s r e l a t e d to the b u f f e r i n g c a p a c i t y of the s o i l . A g r i c u l t u r a l p r a c t i s e s may r e s u l t in s o i l a c i d i f i c a t i o n through the use of a c i d p r o d u c i n g f e r t i l i z e r s . Lime i s then a p p l i e d to t r e a t the a c i d c o n d i t i o n s . These p r a c t i s e s o f t e n r e s u l t in wide f l u c t u a t i o n s in s o i l pH. Numerous r e s e a r c h e r s have i n v e s t i g a t e d the e f f e c t of e i t h e r a c i d i f i c a t i o n or l i m i n g upon n u t r i e n t a v a i l a b i l i t y and s o i l m i n e r a l o g y , however, l i t t l e i s known about the e f f e c t s of wide f l u c t u a t i o n s in pH on s o i l chemica l and m i n e r a l o g i c a l p r o p e r t i e s . Two s o i l s were s e l e c t e d for s tudy which had d i f f e r e n t b u f f e r i n g c a p a c i t i e s based on t h e i r c l a y and o r g a n i c matter c o n t e n t s . The o b j e c t i v e s of t h i s s tudy were: 1. To as ses s the e f f e c t s of f l u c t u a t i o n s in pH ( i . e . a c i d i c to a l k a l i n e ) on the r e l e a s e and f i x a t i o n of the p l a n t n u t r i e n t s K, Mg, C u , Mn, Z n , and Fe . 2. To as ses s s e l e c t i v e m i n e r a l o g i c a l changes r e s u l t i n g from f l u c t u a t i o n s in pH and i n v e s t i g a t e the r e l a t i o n s h i p between n u t r i e n t a v a i l a b i l i t y and m i n e r a l o g y . 4 DISCUSSION OF SOIL pH D e f i n i t i o n of pH The term pH was introduced by Sorensen i n 1909, and i s d e f i n e d as the negative logarithm to base 10 of the H+ ion a c t i v i t y or the r e c i p r o c a l of the H+ ion a c t i v i t y : pH = - l o g 1 0 a H + = l o g 1 0 1 where a i s the a c t i v i t y of H ions. A c t i v i t y i s r e l a t e d to c o n c e n t r a t i o n , M ( m o l a r i t y ) i n moles per l i t r e by the a c t i v i t y c o e f f i c i e n t t, so that a=%M. The concept of pH i s based on the ion product of pure water: H20 : »• H+ + OH-Kw = [H +][0H"] = 10"14 at 25 "C. By t a k i n g the - l o g of the above, the equation changes i n t o : - l o g [H +] - log [OH"] = 14. Since p=-log, then pH + pOH = 14. Thus, the pH of pure water at 25 °C i s 7 and when [H+]>[0H~] the s o l u t i o n i s a c i d i c and c o n v e r s e l y when [0H"]>[H+] the s o l u t i o n i s b a s i c . In accordance with s t r i c t thermodynamics, s i n g l e ions and s i n g l e ion a c t i v i t i e s do not e x i s t . T h e r e f o r e , exact ion a c t i v i t y measurements are not p o s s i b l e . The thermodynamic i m p o s s i b i l i t y of measuring the exact a c t i v i t y of a s i n g l e ion i s overcome by the o p e r a t i o n a l d e f i n i t i o n of pH: pH x = pHR + (E, - E.) k where pHR i s the assigned pH of a re f e r e n c e s o l u t i o n , E x and E R are the values of an e l e c t r o m o t i v e f o r c e (emf) of a c e l l 5 r e v e r s i b l e t o IT i o n s w i t h t h e e l e c t r o d e s immersed i n t h e unknown s o l u t i o n X and t h e r e f e r e n c e s o l u t i o n R, r e s p e c t i v e l y , and k i s a t e m p e r a t u r e - d e p e n d e n t c o n s t a n t , 0.059 a t 25 °C (Bache, 1979). A p p l i c a t i o n o f pH t o S o i l s The pH c o n c e p t , as d e s c r i b e d e a r l i e r , i s d e r i v e d f r o m a c h e m i c a l p o t e n t i a l and i s c h e m i c a l l y a s s o c i a t e d w i t h t h e H + i o n a c t i v i t y o f a s o l u t i o n . W h i l e H + a c t i v i t y i s not t h e same as c o n c e n t r a t i o n i t can be e q u a t e d w i t h t h e H + i o n c o n c e n t r a t i o n i f t h e s o l u t i o n i s v e r y d i l u t e . T h i s r e l a t i o n s h i p a p p e a r s t o r e m a i n v a l i d when d e a l i n g w i t h volumes w h i c h a r e l a r g e compared t o m o l e c u l a r d i m e n s i o n s and when t h e i n d i v i d u a l m o l e c u l e s and i o n s i n t h e s o l u t i o n a r e u n i f o r m l y d i s p e r s e d t h r o u g h o u t i t , b u t b e g i n s t o b r e a k down as t h e s a l t c o n t e n t of t h e s o l u t i o n i n c r e a s e s . T h i s a s s u m p t i o n may not be v a l i d f o r a s o i l w h i c h i s a m u l t i p h a s e h e t e r o g e n e o u s s y s t e m c o m p r i s e d o f s o l i d , c o l l o i d a l , l i q u i d , and g a s e o u s components i n v a r y i n g p r o p o r t i o n s . S o i l p a r t i c l e s a r e v e r y l a r g e compared t o m o l e c u l a r d i m e n s i o n s and a l s o c a r r y i o n s a t t a c h e d t o them. The i o n s as a r e s u l t , a r e not 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 t h e s o l u t i o n , w h i c h means i t may be more d e s i r a b l e t o t h i n k o f s o i l pH i n r e l a t i o n t o t h e p r o p e r t i e s o f t h e i o n i c a t m o s p h e r e a r o u n d t h e s o i l p a r t i c l e s ( R u s s e l l , 1973). The n e t n e g a t i v e c h a r g e o f a s o i l i s n e u t r a l i z e d by c a t i o n s . Some o f t h e s e c a t i o n s a r e r e t a i n e d i m m e d i a t e l y n e x t t o t h e c o l l o i d s u r f a c e s f o r m i n g t h e S t e r n - l a y e r , and some d i s s o c i a t e 6 i n t o t h e s o l u t i o n c l o s e t o t h e l a t t i c e s u r f a c e t o form t h e Gouy d i f f u s e d o u b l e l a y e r . The p r o p o r t i o n o f t h e c a t i o n s t h a t d i s s o c i a t e f r o m t h e S t e r n i n t o t h e Gouy l a y e r depends p a r t l y on th e c o n c e n t r a t i o n o f s a l t s d i s s o l v e d i n t h e w a t e r , and p a r t l y on t h e t i g h t n e s s w i t h w h i c h t h e y a r e h e l d t o t h e s u r f a c e . The l a y e r i s t h i c k e r t h e more d i l u t e t h e s o l u t i o n , t h e g r e a t e r t h e h y d r a t i o n o f t h e c a t i o n s , and t h e lower t h e v a l e n c y o f t h e c a t i o n s . C o n v e r s e l y , t h e more c o n c e n t r a t e d t h e s a l t s o l u t i o n , t h e t h i n n e r t h e d o u b l e l a y e r , and t h e c l o s e r a r e t h e c a t i o n c o n c e n t r a t i o n s j u s t o u t s i d e t h e S t e r n l a y e r t o t h o s e i n t h e b u l k s o l u t i o n . T h i s e f f e c t of s a l t s i s d i r e c t l y r e l a t e d t o some o f t h e l i m i t a t i o n s i n h e r e n t i n t h e c o n c e p t o f s o i l pH t o be d i s c u s s e d l a t e r . The c a t i o n c o n c e n t r a t i o n g r a d i e n t i n t h i s d i f f u s e d o u b l e l a y e r s e t s up an e l e c t r i c a l p o t e n t i a l d i f f e r e n c e between t h e s u r f a c e o f t h e c l a y p a r t i c l e and t h e b u l k o f t h e s o l u t i o n . T h i s means t h a t work must be done t o t r a n s f e r a c a t i o n f r o m t h e c l a y s u r f a c e t o t h e b u l k of t h e s o l u t i o n t h u s h i n d e r i n g t h e f r e e i n t e r c h a n g e o f c a t i o n s between t h e s e two r e g i o n s . The t h i c k e r t h e d o u b l e d i f f u s e l a y e r i s , t h e s m a l l e r w i l l be t h e c o n t r i b u t i o n o f t h e H + i o n s t h a t c an d i s s o c i a t e f r o m t h e c l a y s u r f a c e t o t h e H + i o n c o n c e n t r a t i o n o f t h e b u l k s o l u t i o n . As t h e d o u b l e l a y e r i s made more compact by a d d i n g an e l e c t r o l y t e t o t h e s y s t e m , t h e H + i o n c o n c e n t r a t i o n g r a d i e n t a c r o s s t h e d o u b l e l a y e r i s r e d u c e d , and t h e more f r e e l y w i l l t h e H + i o n s be a b l e t o move f r o m t h e c l a y s u r f a c e i n t o t h e s o l u t i o n . T h i s d e c r e a s e s t h e pH o f t h e 7 b u l k s o l u t i o n so t h a t i t more c l o s e l y a p p r o x i m a t e s t h e pH c l o s e t o t h e s u r f a c e o f t h e s o i l p a r t i c l e . A l s o , s i n c e a t a g i v e n c o n c e n t r a t i o n , a d i v a l e n t c a t i o n s u c h as Ca* 2 w i l l c o m p r e s s t h e d o u b l e l a y e r more s t r o n g l y t h a n a m o n o v a l e n t c a t i o n s u c h as K +, a s o i l w i l l have a lower pH i n a d i l u t e C a C l 2 s o l u t i o n t h a n i n a KC1 s o l u t i o n o f t h e same c o n c e n t r a t i o n . The pH o f t h e b u l k s o l u t i o n i s t h u s g r e a t e r t h a n t h e pH j u s t o u t s i d e t h e S t e r n l a y e r u n l e s s t h e s a l t c o n c e n t r a t i o n i n t h e s o l u t i o n i s h i g h ( R u s s e l l , 1973; Bohn, 1979). At l e a s t some c o n s i d e r a t i o n s h o u l d be g i v e n t o t h e p r o b l e m s a s s o c i a t e d w i t h t a k i n g a c o n c e p t , d e r i v e d f r o m a c h e m i c a l p o t e n t i a l , f o r a p p l i c a t i o n o n l y t o a s o l u t i o n and i m p o s i n g t h a t on a m u l t i p h a s e , h e t e r o g e n e o u s s y s t e m s u c h as s o i l . I t may be d i f f i c u l t t o d e f i n e t h e o r e t i c a l l y or t o d e t e r m i n e e x p e r i m e n t a l l y a u n i q u e pH v a l u e f o r a s o i l , as can be done w i t h an a c i d s u c h as H 2S0 4, and by t h e v e r y n a t u r e o f s o i l p r o b a b l y can not be assumed t o have one (Maimer, 1976; Bache, 1979). The H + i o n a c t i v i t y e x p r e s s e d by a " s o i l pH" r e a d i n g i s t h e r e s u l t o f an a c c u m u l a t i o n o f s e v e r a l c o n t r i b u t i n g f a c t o r s and not j u s t t h e a c t i v i t y o f t h e IT i o n s a l o n e as i n a p u r e s o l u t i o n . T h i s does not l e s s e n t h e u s e f u l n e s s o f t h e measurement however, s i n c e s o i l pH i s g e n e r a l l y an i n d i c a t o r o f a symptom of t h e s o i l s c o n d i t i o n as w e l l as b e i n g t h e c a u s e o f many of t h e r e a c t i o n s t h a t o c c u r . 8 D e t e r m i n a t i o n of S o i l pH S o i l pH i s u s u a l l y measured e l e c t r i c a l l y w i t h a pH meter which i n v o l v e s a g l a s s H +-sensing e l e c t r o d e p a i r e d w i t h a r e f e r e n c e e l e c t r o d e a t t a c h e d to a meter f o r measuring emf, which has been shown to be p r o p o r t i o n a l to pH. It can a l s o be measured c o l o u r i m e t r i c a l l y w i t h the use of dyes or a c i d - b a s e i n d i c a t o r s which change c o l o u r a c c o r d i n g to the H*-ion a c t i v i t y . The c o l o u r i m e t r i c methods are g e n e r a l l y r e s e r v e d f o r f i e l d t e s t k i t s where the pH l e v e l i s to be approximated s i n c e the methods tend to be slower, l e s s p r e c i s e , and obscured from view by the s o i l p a r t i c l e s (McLean, 1982). F i g u r e 1. G l a s s and calomel e l e c t r o d e w i t h l i q u i d j u n c t i o n (Taken from van L i e r o p and Tran, 1978). Class electrode Calo«el ref. electrode Mercury KCl-Caloael (Hg 2Cl 2) Saturated KC1 salt bridge l iquid junction The e l e c t r o m e t r i c , method i n v o l v e s the use of a g l a s s e l e c t r o d e , s e n s i t i v e to changes in the a c t i v i t y of H* ions i n s o l u t i o n , and a calomel e l e c t r o d e ( F i g u r e 1). The g l a s s bulb c o n t a i n s a s o l u t i o n of d i l u t e h y d r o c h l o r i c a c i d (0.1 M) i n 9 c o n t a c t w i t h a s t a n d a r d Ag-AgCl ( o r H g-Hg 2Cl 2) e l e c t r o d e . The t h i n g l a s s membrane c o n t a c t s b o t h t h e i n s i d e s o l u t i o n o f c o n s t a n t H + c o n c e n t r a t i o n and t h e o u t s i d e s o l u t i o n o f unknown H + c o n c e n t r a t i o n . In e l e c t r o d e measurements, a s e c o n d e l e c t r o d e i s r e q u i r e d t o c o m p l e t e t h e e l e c t r i c a l c i r c u i t . G e n e r a l l y a c a l o m e l ( H g - H g 2 C l 2 ) e l e c t r o d e i s u s e d w h i c h i s c o m p r i s e d o f a g l a s s t u b e , w i t h a v e r y s m a l l h o l e a t t h e lower end, c o n t a i n i n g a s o l u t i o n o f s a t u r a t e d KC1 i n c o n t a c t w i t h a s t a n d a r d H g - H g 2 C l 2 ( o r A g - A g C l ) e l e c t r o d e . The s m a l l h o l e a t t h e end o f t h e t u b e i s a v e r y f i n e c a p i l l a r y o p e n i n g w h i c h i s u s u a l l y s e a l e d w i t h an a s b e s t o s f i b r e t o a l l o w e l e c t r i c a l c o n t a c t w i t h m i n i m a l l e a k a g e t o t h e o u t s i d e s o l u t i o n . T h i s pH d e t e r m i n a t i o n method can be i l l u s t r a t e d as f o l l o w s ( T a k e n f r o m van L i e r o p e t a l . , 1978): Ag A g C l G l a s s E l e c t r o d e 0.01 N HC1 G l a s s Sample pH M e t e r KC1 ( . a t ) C a l o m e l E l e c t r o d e H g 2 C l 2 Hg * J u n c t i o n p o t e n t i a l 10 If t h e e f f e c t i v e p r o t o n c o n c e n t r a t i o n ( a c t i v i t y ) o f a s o l u t i o n i n t o w h i c h t h e e l e c t r o d e i s p l a c e d d i f f e r s f r o m t h a t i n t h e r e f e r e n c e e l e c t r o d e b u l b , a p o t e n t i a l d i f f e r e n c e wi11 d e v e l o p a c r o s s t h e g l a s s b o u n d a r y . The p o t e n t i a l s a c r o s s t h e g l a s s membrane c a n be c l o s e l y c a l i b r a t e d t o t h e a p p r o x i m a t e v a l u e o f t h e H + a c t i v i t y . The p o t e n t i a l o f t h e pH e l e c t r o d e i s : E = E r e f - 0.059 l o g a t t e s t s o l u t i o n ) a H + ( s t a n d a r d s o l u t i o n ) where E i s t h e measured p o t e n t i a l and i s c o n v e r t e d t o pH u n i t s by t h e s c a l e o f t h e pH me t e r . E r e f i n c l u d e s a l l o t h e r p o t e n t i a l s , w h i c h a r e assumed t o r e m a i n c o n s t a n t , and w h i c h a r e n u l l i f i e d by s t a n d a r d i z i n g t h e s y s t e m w i t h a s t a n d a r d pH b u f f e r s o l u t i o n . F a c t o r s I n f l u e n c i n g S o i l pH S u s p e n s i o n E f f e c t When t h e c a l o m e l e l e c t r o d e i s p l a c e d i n t h e s e t t l e d s e d i m e n t o f a s o i l s u s p e n s i o n t h e measured s o l u t i o n pH i s lower t h a n i f t h e e l e c t r o d e i s p l a c e d i n t h e s u p e r n a t a n t . pH i s i n d e p e n d e n t o f t h e p o s i t i o n o f t h e g l a s s e l e c t r o d e i n t h e sample. T h i s d i f f e r e n c e i n t h e s e d i m e n t and s u p e r n a t a n t pH i s c a l l e d t h e s u s p e n s i o n e f f e c t . The d i f f e r e n c e i n pH a c c o r d i n g t o t h e p o s i t i o n o f t h e c a l o m e l e l e c t r o d e has been e x p l a i n e d by d i f f e r e n c e s i n t h e r e l a t i v e d i f f u s i o n r a t e s o f K + and CI" i n t h e KC1 b r i d g e (Coleman e t a l . , 1950; C l a r k , 1966; Peech e t a l . , 1953). In w a t e r t h e m o b i l i t i e s o f K + and CI" a r e a p p r o x i m a t e l y t h e same and t h u s each 11 c o n d u c t s a p p r o x i m a t e l y t h e same amount o f c u r r e n t . In t h e p r e s e n c e o f a n e g a t i v e l y c h a r g e d c o l l o i d t h e i o n m o b i l i t i e s c hange r e l a t i v e t o each o t h e r g i v i n g r i s e t o a l i q u i d j u n c t i o n p o t e n t i a l ( i . e . h i g h e r c a t i o n a c t i v i t y i n t h e c l a y s u s p e n s i o n t h a n i n t h e e q u i l i b r i u m d i a l y s a t e ) . McLean ( 1 9 8 2 ) , f o u n d t h a t a s u s p e n s i o n e f f e c t f o r Ca was o b s e r v e d i n Ca-K c l a y s y s t e m . To e l i m i n a t e t h e p o s s i b i l i t y o f a l i q u i d j u n c t i o n d e v e l o p i n g , a d o u b l e - e l e c t r o d e method was u s e d i n w h i c h t h e C a + 2 a c t i v i t i e s were computed from t h e d i f f e r e n c e s i n emf between two c e l l s t h a t c o n s i s t e d o f t h e same c a l o m e l e l e c t r o d e s . However, g r e a t e r a c t i v i t y o f C a + J was s t i l l m e a sured i n s u s p e n s i o n as compared t o t h e s u p e r n a t a n t . McLean s u g g e s t e d t h a t p e r h a p s t h e s u s p e n s i o n e f f e c t i s due t o r e a l d i f f e r e n c e s i n c a t i o n c o n c e n t r a t i o n s w i t h d i s t a n c e s from s o i l p a r t i c l e s . The s u s p e n s i o n e f f e c t can be m i n i m i z e d by s u s p e n d i n g t h e s o i l i n 0.01 M C a C l 2 ( W o o d r u f f , 1967) and by k e e p i n g t h e c a l o m e l e l e c t r o d e i n t h e c l e a r s u p e r n a t a n t . D i l u t i o n E f f e c t The pH o f s o i l s t e n d s t o i n c r e a s e as t h e y a r e d i l u t e d w i t h w a t e r . T h i s e f f e c t i s t h o u g h t t o be t h e r e s u l t o f d i l u t i o n o f t h e f i n i t e amount o f h y d r o l y z a b l e a c i d i t y , d i l u t i o n o f s a l t c a t i o n s t h a t d i s p l a c e A l + 3 and H + i n t o s o l u t i o n ( C o n y e r s e t a l . , 1988) o r t h e d e v e l o p m e n t o f a l a r g e r d i f f u s i o n p o t e n t i a l i n more c o n c e n t r a t e d s l u r r i e s (Thomas and H a r g r o v e , 1984). As t h e volume of s o l u t i o n r i s e s , d i l u t i o n o f t h e i n i t i a l amount o f a c i d i s 12 p a r t i a l l y c o m p e n s a t e d f o r by a d d i t i o n a l d i s s o c i a t i o n o f IT f r o m t h e s o i l (McLean, 1973). Van L i e r o p and T r a n ( 1 9 7 8 ) , c o n f i r m e d t h i s e f f e c t f o r w a t e r and 1 M KC1 when u s i n g s o i l t o s o l u t i o n r a t i o s between 2:1 and 1:8. However, when t h e pH was d e t e r m i n e d i n 0.01 M C a C l 2 o r S r C l 2 , t h e v a l u e s o b t a i n e d were i n d e p e n d e n t o f t h e s o i l t o s o l u t i o n r a t i o o v e r t h e ran g e o f 1:1 t o 1:8. S a l t C o n c e n t r a t i o n The pH v a l u e s o f s o i l s a r e g e n e r a l l y r e d u c e d when measured i n a s a l t s o l u t i o n (van L i e r o p , 1981; van L i e r o p and T r a n , 1978; D a v i e s , 1971). The e f f e c t o f s a l t c o n c e n t r a t i o n on s o i l pH seems t o be t h r o u g h t h e d i s r u p t i o n o f t h e d i s t r i b u t i o n o f e x c h a n g e a b l e c a t i o n s . In a c i d s o i l s , t h e r e d u c t i o n i n pH may a r i s e f r o m d i s p l a c e m e n t o f H + i o n s o r from t h e h y d r o l y s i s o f d e s o r b e d A l + 3 i o n s , t h e f i r s t s t a g e o f w h i c h can be r e p r e s e n t e d by: [ A 1 ( H 2 0 ) 6 ] + 3 + H 20 [ A l ( H 2 0 ) s O H ] + 2 + H 30 +. However, t h e pH o f an a c i d , s o i l - s a l t s o l u t i o n i s o f t e n lower t h a n c an be a c c o u n t e d f o r by t h e A l + 3 c o n c e n t r a t i o n o f t h e s o i l s o l u t i o n and t h e h y d r o l y s i s p r o d u c t s e x p e c t e d from t h a t s o l u t i o n A l + 3 c o n c e n t r a t i o n (Thomas and H a r g r o v e , 1984; Bache, 1974). T h i s anomaly may be due t o i n c r e a s e d h y d r o l y s i s o f A l due t o t h e r e -a d s o r p t i o n o f h y d r o x y - A l by c l a y ( R a g l a n d and Coleman, 1960) or fr o m a d d i t i o n a l s o u r c e s o f p r o t o n s , p r e s u m a b l y t h e c a r b o x y l g r o u p s i n humus as s u g g e s t e d by Bache ( 1 9 7 4 ) . 13 In s o i l s where t h e pH i n w a t e r i s above 5, t h e r e d u c t i o n i n pH i s not a l w a y s a c c o m p a n i e d by an i n c r e a s e i n s o l u t i o n A l * 3 (Coleman e t a l . , 1959). The a d d i t i o n a l p r o t o n s may come from o r g a n i c m a t t e r o r from t h e h y d r o l y s i s o f h y d r o x y - A l o r h y d r o x y -Fe l o c a t e d i n i n t e r l a y e r s , on p l a n a r s u r f a c e s , o r edges o f l a y e r s i l i c a t e s . In some s o i l s , a d d i t i o n a l p r o t o n s may be s u p p l i e d by a l l o p h a n e o r o t h e r amorphous a l u m i n o s i 1 i c a t e s (Thomas and H a r g r o v e , 1984). S c h o f i e l d and T a y l o r ( 1 9 5 5 ) , p r o p o s e d t h e use o f 0.01 M C a C l 2 s o l u t i o n f o r pH measurements o f t e m p e r a t e , n o n s a l i n e s o i l s . T h i s s o l u t i o n masks s m a l l d i f f e r e n c e s i n s a l t c o n c e n t r a t i o n w i t h o u t d i s p l a c i n g a l a r g e f r a c t i o n of t h e e x c h a n g e a b l e H* and A l * 3 . The pH, when measured i n C a C l 2 , i s much l e s s d e p e n d e n t on t h e s o i l t o s o l u t i o n r a t i o ( S c h o f i e l d e t a l . , 1955; R u s s e l l , 1973) and does not g i v e an a p p r e c i a b l e j u n c t i o n p o t e n t i a l w i t h t h e c a l o m e l e l e c t r o d e ( R u s s e l l , 1973). C o n y e r s and Davey ( 1 9 8 8 ) , f o u n d t h a t as t h e d i l u t i o n r a t i o d e c r e a s e d , r e g r e s s i o n s of w a t e r and C a C l 2 pH v a l u e s have a common i n t e r c e p t , s u g g e s t i n g t h a t C a C l 2 pH may a p p r o x i m a t e t h e pH o f t h e s o i l s o l u t i o n . D a v i e s ( 1 9 8 8 ) , f o u n d t h a t t h e two v a l u e s d i d not c o n v e r g e when c a l c i u m c a r b o n a t e s o c c u r r e d i n t h e s o i l . E f f e c t o f C a r b o n D i o x i d e The pH o f a s o i l i s a l s o a f f e c t e d by t h e C0 2 c o n c e n t r a t i o n o f t h e s o i l s o l u t i o n . C0 2 makes up about 0.03% by volume o f t h e a t m o s p h e r e , however, p l a n t r o o t and m i c r o b i a l r e s p i r a t i o n c an 14 i n c r e a s e i t s c o n c e n t r a t i o n i n t h e s o i l s o l u t i o n . C0 2 d i s s o l v e s i n w a t e r t o form c a r b o n i c a c i d : C0 2 + H 20 ; = » H 2C0 3 t h e r e s u l t b e i n g a r e d u c t i o n i n pH. C a r b o n i c a c i d l o w e r s t h e pH not o n l y by t h e H + i t c o n t a i n s but a l s o v i a t h e d i s s o l u t i o n of v a r i o u s compounds w h i c h i n c r e a s e s t h e l e v e l o f s a l t s . Lime (CaC0 3) i s a n o t h e r s o u r c e o f C0 2 w h i c h w i l l s e r v e t o r e d u c e pH. The pH of w a t e r c o n t a i n i n g CaC0 3 i n e q u i l i b r i u m w i t h t h e p a r t i a l p r e s s u r e o f C0 2 i n t h e a i r i s a b o u t 8.35. T a b l e 1 shows t h e r e d u c t i o n i n pH as t h e p a r t i a l p r e s s u r e o f C0 2 i s i n c r e a s e d . T a b l e 1. The pH of a c a l c i u m c a r b o n a t e and a c a l c a r e o u s c l a y s u s p e n s i o n i n e q u i l i b r i u m w i t h c a r b o n d i o x i d e a t v a r y i n g p r e s s u r e s (Taken from R u s s e l l , 1973). C a r b o n d i o x i d e pH o f c a l c i u m p r e s s u r e c a r b o n a t e pH o f c l a y i n atm. s u s p e n s i o n s u s p e n s i o n 0. ,00033 8. ,42 8. ,57 0. , 001 8. ,00 8. ,30 0. , 003 7. , 77 7. , 95 0. ,01 7. ,33 7. , 62 0. ,03 7. ,00 7. ,30 0. . 1 6. ,65 6. . 95 Sample D r y i n g A i r - d r y i n g a s o i l sample may g i v e a more a c i d pH t h a n a f i e l d - m o i s t sample ( C o l l i n s e t a l . , 1970; Davey e t a l . , 1988). Davey and C o n y e r s ( 1 9 8 8 ) , s u s p e c t e d t h e d i f f e r e n c e was due t o d i f f e r e n c e s i n s o i l m o i s t u r e c o n t e n t and hence a c h a n g e d s o i l t o 15 s o l u t i o n r a t i o . A c o r r e c t i o n f a c t o r was u s e d t o a c c o u n t f o r t h e e x t r a w a t e r i n t h e f i e l d - m o i s t sample, but t h i s f a i l e d t o a c c o u n t f o r t h e i n c r e a s e d a c i d i t y o f t h e a i r - d r y sample. They c o n c l u d e d t h a t t h e c h a n g e s i n pH on a i r - d r y i n g a r e not due s i m p l y t o c h a n g e s i n t h e m o i s t u r e c o n t e n t o f t h e sample, but may i n v o l v e m i c r o b i a l a c t i v i t y . S e a s o n a l V a r i a t i o n S e a s o n a l v a r i a t i o n s i n s o i l pH a r e known t o o c c u r (van L i e r o p , 1978; Davey e t a l . , 1988; R u s s e l l , 1973; Weaver e t a l . , 1979). E v i d e n c e i n d i c a t e s t h a t s o l u b l e s a l t s a r e p r o b a b l y r e s p o n s i b l e f o r most o f t h e s e a s o n a l v a r i a b i l i t y . The c o n c e n t r a t i o n o f s a l t s w i l l v a r y t h r o u g h o u t t h e y e a r d e p e n d i n g on s u c h f a c t o r s as t h e amount o f r a i n , f e r t i l i z a t i o n , and t h e r a t e o f n i t r i f i c a t i o n . As a r e s u l t t h e pH o f a s o i l i s o f t e n h i g h e r i n wet w e a t h e r or w i n t e r t h a n i n d r y w e a t h e r o r summer, and under c r o p t h a n f a l l o w . T h i s s e a s o n a l v a r i a t i o n however, i s u s u a l l y s m a l l e r t h a n t h e v a r i a t i o n s f r o m s i t e t o s i t e i n t h e same f i e l d . SOIL BUFFERING 16 B u f f e r i n g - D e f i n i t i o n In t h e c a s e o f an aqueous s o l u t i o n , b u f f e r s a r e t h o s e s y s t e m s w h i c h t e n d t o m a i n t a i n a c o n s t a n t pH l e v e l upon a d d i t i o n of an a c i d o r b a s e , or d e s p i t e d i l u t i o n . They u s u a l l y c o n s i s t o f m i x t u r e s o f e i t h e r a weak a c i d and a s a l t f ormed f r o m t h e a c i d and a s t r o n g b a s e , o r a weak base and a s a l t f ormed from t h e base and a s t r o n g a c i d . B u f f e r i n g i n s o i l c a n be d e f i n e d as t h e r e s i s t a n c e o f t h e s o i l t o v a r i a t i o n s i n pH and i s c h i e f l y due t o t h e c o l l o i d a l i n o r g a n i c and o r g a n i c f r a c t i o n s o f t h e s o i l . The c o l l o i d a l s o i l complex f u n c t i o n s as a s l i g h t l y i o n i z e d a c i d o r a s l i g h t l y i o n i z e d s a l t o f a weak a c i d ( G e s s a , 1979). In s o i l s most o f t h e a c i d g r o u p s become d i s s o c i a t e d ( d e p r o t o n a t e d ) between t h e pH v a l u e s o f 4.0 and 8.5. A l t h o u g h a s i m p l i f i c a t i o n , t h e H + i o n s c an be c o n s i d e r e d as p r o t o n s , w h i c h can be f r e e d w h a t e v e r t h e s t r u c t u r a l f o r m b e l o n g t o ( G e s s a , 1979). J a c k s o n ( 1 9 6 3 ) , s t a t e d t h a t f u n d a m e n t a l l y , a l l a c i d i t y g r o u p s depend on p r o t o n d i s s o c i a t i o n , t h a t i s " e x c h a n g e a b l e H+", but d i f f e r o n l y i n t h e a c i d s t r e n g t h o f t h e f u n c t i o n a l g r o u p s . The s t a t e o f e q u i l i b r i u m between d i s s o c i a t e d and u n d i s s o c i a t e d f orms o f w e a k - a c i d g r o u p s o f t h e c o l l o i d a l complex can be r e p r e s e n t e d by: C o l l o i d a l - H + H 20 C o l l o i d a l - " + H 30 + Complex Complex Upon a d d i t i o n of a ba s e , b u f f e r i n g o c c u r s t h r o u g h t h e n e u t r a l i z a t i o n o f t h e H + i o n s w h i c h r e s u l t s i n a f u r t h e r 17 l i b e r a t i o n o f p r o t o n s f r o m t h e c o l l o i d a l complex, l e a v i n g t h e pH more o r l e s s t h e same. B u f f e r i n g w i t h r e s p e c t t o a c i d s i s a c c o m p l i s h e d by t h e r e a d s o r p t i o n o f p r o t o n s by t h e c o l l o i d a l complex w i t h t h e c o n c o m i t a n t r e l e a s e o f a d s o r b e d e x c h a n g e a b l e b a s e s i n t o s o l u t i o n . T h i s r e s u l t s i n a s h i f t i n t h e e q u i l i b r i u m o f t h e d i s s o c i a t e d f o r m o f t h e "weak a c i d " t o w a r d s t h e u n d i s s o c i a t e d f o r m a g a i n , l e a v i n g pH u n v a r i e d . B u f f e r i n g Mechanisms C o n t r o l l i n g S o i l pH As m e n t i o n e d p r e v i o u s l y , s o i l s behave i n many ways l i k e b u f f e r e d weak a c i d s . The f o l l o w i n g a r e t h e more i m p o r t a n t components of t h i s p H - r e g u l a t i n g b u f f e r s y s t e m i n s o i l s . O r g a n i c M a t t e r In g e n e r a l , s o i l o r g a n i c m a t t e r may be d e s c r i b e d as a c o n d e n s e d p o l y m e r i c , a r o m a t i c c o r e t o w h i c h a r e a t t a c h e d a l i p h a t i c s i d e c h a i n s s u c h as p e p t i d e s , s u g a r s , p h e n o l i c and c a r b o x y l i c a c i d s (Thomas and H a r g r o v e , 1984). The major r e a c t i v e g r o u p s r e s p o n s i b l e f o r a c i d i t y a r e c a r b o x y l s a t t a c h e d t o u n s a t u r a t e d r i n g s t h a t a r e a r r a n g e d i n p o l y m e r s o f v a r i a b l e and unknown s i z e s ( S c h n i t z e r e t a l . , 1964) and p h e n o l i c h y d r o x y l s (Thomas e t a l . , 1984; G e s s a , 1979). pK a v a l u e s f o r t h e d i s s o c i a t i o n c o n s t a n t s o f p u r e c a r b o x y l i c a c i d s a r e i n t h e r a n g e o f 4 t o 5 and f o r p h e n o l i c - O H s between 7.8 and 9.8. O r g a n i c m a t t e r g e n e r a l l y has h i g h e r pK a v a l u e s ( a b o u t 6) t h a n do t h e "pure a c i d s " i n d i c a t i n g a r e d u c t i o n i n a c i d 18 s t r e n g t h . S t u d i e s have shown t h a t t h i s a b normal weakness i s due t o A l and Fe on exchange s i t e s and t h a t r e m o v a l o f Fe and A l from o r g a n i c m a t t e r g i v e s an a p p a r e n t pK a o f 4 ( M a r t i n and Reeve, 1958; H a r g r o v e and Thomas, 1982). H a r g r o v e and Thomas ( 1 9 8 2 ) , a l s o f o u n d t h a t A l a l s o a p p e a r s t o change t h e b e h a v i o u r o f o r g a n i c m a t t e r f r o m t h a t o f a t y p i c a l p o l y p r o t i c a c i d t o a weaker m o n o p r o t i c a c i d . T h i s i m p l i e s t h a t o r g a n i c m a t t e r c o n t a i n s t r i - , d i - , and s i n g l e c a r b o x y l i c a c i d s and t h a t A l i s p r o b a b l y r e a c t i n g w i t h t h e p o l y c a r b o x y 1 i c a c i d s f i r s t , l e a v i n g t h e s i n g l e c a r b o x y l i c g r o u p s w h i c h behave as a m o n o p r o t i c a c i d . B u f f e r i n g by o r g a n i c m a t t e r c an be r e p r e s e n t e d by t h e f o l l o w i n g example f o r a c a r b o x y l i c a c i d ( P o s n e r , 1964): R.COOH + H 20 ; = i R.COCT + H 30 + pK, = 5.5+1.7. The r a n g e o v e r w h i c h t h e p r o t o n - d o n a t i n g f u n c t i o n a l g r o u p s e x e r t b u f f e r i n g a c t i o n i s a p p r o x i m a t e l y one pH u n i t on e i t h e r s i d e o f t h e pK, v a l u e . C a r b o n D i o x i d e C a r b o n d i o x i d e d i s s o l v e s i n w a t e r t o form weak c a r b o n i c a c i d ( H 2 C 0 3 ) : C 0 2 ( a q ) + H 20 ^ H 2C0 3. The c o n c e n t r a t i o n of c a r b o n i c a c i d i n s o l u t i o n f o l l o w s H e n r y ' s Law f a i r l y c l o s e l y , w h i c h r e l a t e s t h e p a r t i a l p r e s s u r e o f t h e gas t o i t s c o n c e n t r a t i o n i n s o l u t i o n : 19 K H = i H 2 C 0 3 l = IO'' 5 p r C02 where (H 2C0 3) r e f e r s t o t h e sum o f C0 2, H 2C0 3, and C 0 2 ( a q ) i n s o l u t i o n . As t h e c o n c e n t r a t i o n o f C0 2 i n c r e a s e s t h e pH o f d i s t i l l e d w a t e r w i l l d e c r e a s e . The c a r b o n i c a c i d d i s s o c i a t e s s u c c e s s i v e l y i n t o b i c a r b o n a t e and c a r b o n a t e i o n s as t h e pH i n c r e a s e s a c c o r d i n g t o t h e r e a c t i o n s : H 2C0 3 H + + HC0 3" pK= 6.4 HC0 3- ^ = ± H + + CO,-2 pK=10.3 T h i s r e s u l t s i n d i f f e r e n t aqueous C0 2 s p e c i e s b e i n g p r e s e n t d e p e n d i n g on t h e pH o f t h e s o l u t i o n . The a t m o s p h e r e c o n t a i n s about 0.03% C0 2 by volume, w h i c h g i v e s an e q u i l i b r i u m pH o f 5.63 i n w a t e r ; but h i g h e r C0 2 p a r t i a l p r e s s u r e s may o c c u r i n s o i l s due t o r e s p i r a t i o n o f p l a n t r o o t s and d e c o m p o s i t i o n o f o r g a n i c r e s i d u e s by s o i l m i c r o - o r g a n i s m s . The pH o f t h e s o i l - C 0 2 - w a t e r s y s t e m may d r o p t o as low as 4.4 due t o s o i l f a u n a l and f l o r a l r e s p i r a t i o n . C a r b o n a t e s C a l c i u m c a r b o n a t e ( c a l c i t e ) i s a major c o n s t i t u e n t of some s o i l s and can a l s o be added t o t h e s o i l by t h e p r a c t i c e o f l i m i n g . A s u s p e n s i o n o f c a l c i u m c a r b o n a t e i n p u r e w a t e r g i v e s a pH o f 9.6, and t h e s o l u b i l i t y r e a c t i o n c an be w r i t t e n a s : CaC0 3 + H 20 ; > C a + 2 + HC0 3" + OH" I n s o l u b l e S o l u b l e 20 CaC0 3 i s o n l y s p a r i n g l y s o l u b l e i n w a t e r . B o t h t h e HC0 3~ and OH" i o n s a r e a v a i l a b l e t o n e u t r a l i z e a c i d i t y as shown by: OH" + H + ; » H 20 H C O 3 - + H + H 2C0 3 C 0 2 ( g ) + H 20. T h i s s y s t e m i s a l s o i n e q u i l i b r i u m w i t h c a r b o n d i o x i d e . The e q u i l i b r i u m pH o f t h e c a r b o n a t e s y s t e m d r o p s f r o m 9.6 t o 8.4 when t h e c o n c e n t r a t i o n o f C0 2 i s 0.03% ( c o n c e n t r a t i o n i n t h e a t m o s p h e r e ) and t o a p p r o x i m a t e l y 6.7 when t h e a t m o s p h e r e c o n t a i n s 10% C0 2. A l u m i num When t h e pH o f t h e s o i l s o l u t i o n f a l l s t o 4 o r below, H + ( h y d r o n i u m ) i o n s c a u s e t h e d i s s o l u t i o n o f A l + 3 f r o m t h e e d g e s o f t h e m i n e r a l s t r u c t u r e o f c l a y s (McLean, 1976). Upon r e l e a s e , t h e A l + 3 i o n s become s i x f o l d c o o r d i n a t e d w i t h oxygen i n 0H 2 g r o u p s , i . e . A l ( - O H 2 0 5 + ) B . These 0H 2 g r o u p s a r e e s s e n t i a l l y A l s u b s t i t u t e d h y d r o n i u m i o n s , w i t h t h e A l r e p l a c i n g one H + f r o m e a c h o f t h e s i x h y d r o n i u m i o n s ( 0 H 3 + ) . T h i s A l s u b s t i t u t e d h y d r o n i u m i o n i s c a l l e d a l u m i n o h e x a h y d r o n i u m and i s d e s i g n a t e d as A1'6H 20 + 3. F o r c o n v e n i e n c e , t h e H 20 l i g a n d s w i l l be o m i t t e d f r o m t h e f o r m u l a e and t h e s t r u c t u r e w i l l be r e f e r r e d t o as A l + 3 h e n c e f o r t h . The A l + 3 i o n s e q u e n t i a l l y d i s s o c i a t e s H + i o n s as t h e pH i s i n c r e a s e d l e a v i n g OH i o n s i n p l a c e o f t h e 0H 2 g r o u p s . A l + J + H 20 . > A l ( O H ) * 2 + H 30 + A 1 ( 0 H ) + 2 + H 20 ; = i A l ( 0 H ) 2 + + H 30 + A l ( O H ) / + H20 ; = i A l ( 0 H ) 3 + H 30 + A 1 ( 0 H ) 3 + H 20 ; = s A l ( 0 H ) 4 - + H 30 + A1(0H) 4" + H 20 ^ = s A l ( 0 H ) s - + H 30 + A1(0H) 6" + H 20 ; = * A 1 ( 0 H ) 6 - + H 30 + 21 The d i s t r i b u t i o n o f A l s p e c i e s v a r i e s w i t h pH as shown above. T r i v a l e n t A l may r e m a i n i n s o l u t i o n , b u t most o f t h e i o n s a r e a d s o r b e d on t o s o i l c a t i o n e xchange s i t e s f r o m w h i c h t h e y a r e d i s p l a c e d w i t h o r d i n a r y u n b u f f e r e d s a l t s o l u t i o n s s u c h as 1 M KC1, i f t h e pH i s below 5. The h y d r o l y s i s p r o d u c t s o f A l * 3 r a p i d l y p o l y m e r i z e t o p r o d u c e p o l y m e r s o f t h e g e n e r a l f o r m u l a [ A l ( O H ) x ( H 2 0 ) B _ x ( 3 _ x > + ] n where n i s t h e a v e r a g e number o f aluminum i o n s per p o l y m e r (Bohn, 1979). T h e s e p o l y m e r s a r e s t r o n g l y r e t a i n e d by s o i l c l a y and o r g a n i c m a t t e r and a r e not g e n e r a l l y e x c h a n g e a b l e w i t h u n b u f f e r e d s a l t s o l u t i o n s ( H a r g r o v e and Thomas, 1982; P i o n k e and C o r e y , 1967; Thomas, 1975). B o t h t h e e x c h a n g e a b l e A l + 3 and t h e p o l y m e r i c n o n e x c h a n g e a b l e h y d r o x y - A l c o n t r i b u t e t o t h e b u f f e r i n g c a p a c i t y o f s o i l s . S e v e r a l p e o p l e have s u g g e s t e d t h a t t h e p o l y m e r i c f o r m s may i n many c a s e s be q u a n t i t a t i v e l y more i m p o r t a n t t h a n e x c h a n g e a b l e A l + 3 (Schwertmann and J a c k s o n , 1964; Coleman and Thomas, 1964; V o l k and J a c k s o n , 1964). H a r g r o v e and Thomas ( 1 9 8 2 ) , f o u n d t h a t A l was h y d r o x y l a t e d i n o r g a n i c m a t t e r even though t h e pH was g e n e r a l l y low (<4). A p p a r e n t l y A l i s h y d r o l y s e d t o a g r e a t e r e x t e n t a t low pH i n t h e p r e s e n c e o f o r g a n i c m a t t e r b e c a u s e t h e h y d r o l y t i c p r o d u c t r e a c t s w i t h c a r b o x y l g r o u p s more e x t e n s i v e l y t h a n w i t h c l a y and b e c a u s e t h e o r g a n i c m a t t e r s e r v e s as a s i n k f o r H + i o n s p r o d u c e d by h y d r o l y s i s . 22 SOIL ACIDITY T o t a l and E x c h a n g e a b l e A c i d i t y A s o i l m a t e r i a l h a v i n g a pH o f l e s s t h a n 7.0 i s c o n s i d e r e d t o be an a c i d s o i l . T h e r e a r e two components o f s o i l a c i d i t y , one w h i c h i s e x c h a n g e a b l e w i t h a n e u t r a l s a l t and a n o t h e r w h i c h i s t i t r a t a b l e , but not e x c h a n g e a b l e . E x c h a n g e a b l e a c i d i t y i s t h e component o f t o t a l a c i d i t y t h a t c an be e x t r a c t e d f r o m s o i l by a c o n c e n t r a t e d u n b u f f e r e d s a l t s o l u t i o n s u c h as 1.0 M KC1. E x c h a n g e a b l e a c i d i t y i s due a l m o s t e n t i r e l y t o monomeric A l + 3 i o n s ( B a c h e , 1988; Thomas and H a r g r o v e , 1984) . T i t r a t a b l e a c i d i t y , o r t o t a l a c i d i t y , i s t h e amount o f a c i d n e u t r a l i z e d a t a s e l e c t e d pH. I t i s now d e t e r m i n e d by e q u i l i b r a t i n g s o i l w i t h a t r i e t h a n o l a m i n e b u f f e r a t pH 8.2 i n 0.25 M B a C l j s o l u t i o n , and back t i t r a t i n g t h e u n r e a c t e d b u f f e r w i t h a c i d . I t s h o u l d be n o t e d t h a t s o i l t i t r a t i o n c u r v e s do not g e n e r a l l y show an i n f l e x i o n or end p o i n t , so t h e r e i s no a b s o l u t e pH a t w h i c h t o measure t o t a l a c i d i t y ( Bache, 1988). T i t r a t a b l e a c i d i t y i n c l u d e s e x c h a n g e a b l e a c i d i t y as w e l l as a c i d i t y g e n e r a t e d by pH dependent c h a r g e s upon an i n c r e a s e i n s o i l pH. 23 S o u r c e s o f Hydrogen Ions S u r f a c e C h a r g e S o i l a c i d i t y i s c l o s e l y l i n k e d t o t h e s u r f a c e s c h a r g e s on s o i l c o l l o i d s s i n c e p r o t o n t r a n s f e r r e a c t i o n s i n v o l v e c h a r g e t r a n s f e r . The f o l l o w i n g a r e t h e more i m p o r t a n t mechanisms i n v o l v e d i n t h e d e v e l o p m e n t o f s u r f a c e c h a r g e and t h e s u b s e q u e n t a c i d i t y a s s o c i a t e d w i t h i t : - Isomorphous s u b s t i t u t i o n - The s u b s t i t u t i o n o f one e l e m e n t f o r a n o t h e r w i t h i n t h e c r y s t a l l a t t i c e o f a l u m i n o s i 1 i c a t e c l a y m i n e r a l s r e s u l t s i n permanent n e g a t i v e c h a r g e s w h i c h a r e n e u t r a l i z e d by e x c h a n g e a b l e c a t i o n s . T h i s p o r t i o n o f t h e b u f f e r i n g c a p a c i t y o f t h e i n o r g a n i c s o i l component, known as t h e permanent c h a r g e c a t i o n e x c h a n g e c a p a c i t y ( C E C ) , does not i t s e l f c o n t r i b u t e t o a c i d - b a s e b u f f e r i n g . As pH d r o p s below about 5.5, H + a d s o r p t i o n by c l a y s b r e a k s A l - 0 bonds i n t h e c l a y s r e l e a s i n g A l + 3 . The A l + 3 i o n s a r e a d s o r b e d back o n t o t h e c l a y v i a t h e CEC and a r e p o t e n t i a l s o u r c e s o f H + i o n s t h r o u g h exchange r e a c t i o n s w i t h o t h e r c a t i o n s and h y d r o l y s i s . pH d e p e n d e n t c h a r g e i n o x i d e m i n e r a l s - H y d r o u s o r a n h y d r o u s , Fe and A l o x i d e s a r e p r e s e n t i n n e a r l y a l l s o i l s , o c c u r r i n g as d i s c r e t e c r y s t a l s , c o a t i n g s on t h e s u r f a c e s of a l u m i n o s i 1 i c a t e o r o t h e r m i n e r a l s , and as m i x e d g e l s w h i c h can i n c o r p o r a t e s i l i c a , H2P04"" and o r g a n i c c o l l o i d s . T hese o x i d e s d e v e l o p c h a r g e s by t h e a d s o r p t i o n o f p o t e n t i a l -d e t e r m i n i n g i o n s , m a i n l y H + and OH" by t h e scheme shown 24 below: The e f f e c t i v e CEC o f a l a y e r s i 1 i c a t e - s e s q u i o x i d e complex i s a r e s u l t o f t h e f a c t t h a t t h e n e g a t i v e c h a r g e o f t h e a l u m i n o s i 1 i c a t e does not v a r y w i t h pH w h i l e t h e o x i d e c h a r g e v a r i e s f r o m p o s i t i v e a t low pH t o n e g a t i v e a t h i g h pH. The r e s u l t i s t h a t t h e net c h a r g e can v a r y f r o m p o s i t i v e t o n e g a t i v e d e p e n d i n g on t h e pH as w e l l as t h e p o i n t o f z e r o net c h a r g e (PZNC). The a c i d i t y a s s o c i a t e d w i t h t h e s e m a t e r i a l s may be p r o d u c e d d u r i n g t h e i r f o r m a t i o n as w e l l as c o n t r i b u t i n g t o t h e r e d u c t i o n i n e x c h a n g e a b l e a c i d i t y by b l o c k i n g e x c h a n g e s i t e s . - pH d e p e n d e n t c h a r g e i n o r g a n i c m a t t e r - T h i s s o u r c e o f c h a r g e has been a t t r i b u t e d t o t h e d i s s o c i a t i o n o f c a r b o x y l i c a c i d and weaker a c i d p h e n o l i c g r o u p s as d i s c u s s e d e a r l i e r . T h e r e i s now e v i d e n c e w h i c h s u g g e s t s t h a t much of t h i s o r g a n i c pH-dependent c h a r g e may a r i s e f r o m t h e d e p r o t o n a t i o n o f o r g a n i c a l l y - c o m p l e x e d aluminum ( H a r g r o v e and Thomas, 1982 a.b) . 25 C a r b o n i c A c i d The p r o c e s s e s o f r e s p i r a t i o n by p l a n t r o o t s and t h e a e r o b i c d e c o m p o s i t i o n o f o r g a n i c m a t t e r by s o i l m i c r o - o r g a n i s m s u t i l i z e 0 2 and combine i t w i t h C from o r g a n i c s u b s t a n c e s t o form C0 2. C a r b o n i c a c i d i s formed t h r o u g h t h e c o m b i n a t i o n o f C0 2 and H 20 i n t h e s o i l s o l u t i o n as d i s c u s s e d e a r l i e r . N i t r i f i c a t i o n The b i o l o g i c a l o x i d a t i o n o f ammonium t o n i t r a t e i s a two s t e p p r o c e s s : 2 NH 4 + + 3 0 2 2 HN0 2 + 2 H + + 2 H 20 2 HNO, + 0, • . 2 NO,' + 2 H* Net 2 NH4+ + 4 0 2 2 N03" + 4 H + + 2 H 20. The n e t e q u a t i o n shows t h a t n i t r i f i c a t i o n o f NH4* t o N0 3" i s c o u p l e d w i t h t h e r e l e a s e o f H + t h u s b e i n g a s o u r c e o f ac i d i f i c a t i o n . N i t r o g e n s o u r c e s a v a i l a b l e f o r n i t r i f i c a t i o n come f r o m o r g a n i c m a t t e r and NH 4 + f e r t i l i z e r s . F e r t i l i z e r s NH 4 + c o n t a i n i n g f e r t i l i z e r s a c i d i f y s o i l s i n t h e manner d i s c u s s e d p r e v i o u s l y . M o n o c a l c i u m p h o s p h a t e f e r t i l i z e r h y d r o l y s e s i n wa t e r t o y i e l d p h o s p h o r i c a c i d : C a ( H 2 P 0 4 ) 2 CaHP0 4 + H 3P0 4. As w e l l as t h e p r o d u c t i o n o f H + i o n s , t h e c a t i o n s and a n i o n s p r o d u c e d by f e r t i l i z e r d i s s o l u t i o n may r e p l a c e t h e c a t i o n s on t h e s o i l e x c h a n g e complex. H + and A l + 3 i o n s may be d i s p l a c e d 26 r e s u l t i n g i n a f u r t h e r l o w e r i n g o f s o i l pH. Aluminum H y d r o l y s i s Monomeric A l * 3 i o n s d i s p l a c e d f r o m c l a y m i n e r a l s h y d r o l y z e i n s o l u t i o n as d i s c u s s e d e a r l i e r . The h y d r o l y s i s p r o d u c t s a r e r e a d s o r b e d , c a u s i n g i n c r e a s e d h y d r o l y s i s ( R a g l a n d and Coleman, 1960). Thomas and H a r g r o v e ( 1 9 8 4 ) , s t a t e t h a t " h y d r o l y s i s i s r e s p o n s i b l e f o r b o t h t h e low pH o f s o l u t i o n s c o n t a i n i n g A l + 3 i o n s and t h e n e u t r a l i z a t i o n o r b u f f e r c a p a c i t y o f s u c h s o l u t i o n s " . O x i d a t i o n o f P y r i t e O x i d a t i o n o f p y r i t e i s as f o l l o w s : 2 F e S 2 + 7 HjO + 7 1/2 0 2 : * 4 S 0 4 - 2 + 8 H + + 2 F e ( 0 H ) 3 . P y r i t e c a n o c c u r i n s t r i p mine s p o i l s as w e l l as a c i d - s u l f a t e s o i l s o r c a t c l a y s . C a t c l a y s a r e common i n m a r i n e f l o o d p l a i n s . NEUTRALIZATION OF SOIL ACIDITY L i m i n g - D e f i n i t i o n L i m i n g i s t h e p r a c t i c e o f a d d i n g l i m i n g m a t e r i a l s t o a c i d s o i l s f o r t h e p u r p o s e of c r e a t i n g o r m a i n t a i n i n g a f a v o u r a b l e e n v i r o n m e n t f o r p l a n t r o o t g r o w t h . C r e a t i o n o f a more s u i t a b l e e n v i r o n m e n t f o r p l a n t s may be a c c o m p l i s h e d e i t h e r by l i m i n g t o a more f a v o u r a b l e p r e - s e l e c t e d pH (McLean, 1970) o r by l i m i n g t o i n a c t i v a t e t o x i c e l e m e n t s (Kamprath, 1970 a , b ) . 27 B a r b e r ( 1 9 8 4 ) , d e f i n e s an a g r i c u l t u r a l l i m i n g m a t e r i a l as a m a t e r i a l whose Ca and Mg compounds a r e c a p a b l e o f n e u t r a l i z i n g s o i l a c i d i t y . T hese m a t e r i a l s i n c l u d e q u i c k l i m e (CaO), h y d r a t e d l i m e [ Ca(OH)j] , l i m e s t o n e ( b o t h c a l c i t i c and d o l o m i t i c ) , m a r l , s h e l l s , and b y p r o d u c t s s u c h as s l a g . L i m e s t o n e i s t h e major m a t e r i a l u s e d . I t ' s c o m p o s i t i o n may be c a l c i t e ( C a C 0 3 ) , d o l o m i t e ( C a C 0 3 » M g C 0 3 ) , o r a m i x t u r e o f b o t h o f t h e s e two m a t e r i a l s . R e a c t i o n s o f Lime i n t h e S o i l The r e a c t i o n s w h i c h o c c u r i n t h e s o i l upon a d d i t i o n o f t h e p r e v i o u s l y m e n t i o n e d compounds can be g e n e r a l i z e d as f o l l o w s : CaO + H 20 5 = ^ C a ( O H ) 2 C a ( O H ) 2 + C02 : » CaC0 3 + H 20. F o r t h e r e m a i n d e r o f t h i s d i s c u s s i o n , t h e t e r m " l i m e " w i l l be assumed t o mean c a l c i t e ( C a C 0 3 ) . Upon i n c o r p o r a t i o n i n t o an a c i d s o i l , t h e s u r f a c e m o l e c u l e s on t h e l i m e p a r t i c l e s b e g i n t o d i s s o l v e . The f o l l o w i n g r e a c t i o n s may o c c u r : CaC0 3 + H 20 C a + 2 + HC0 3" + 0H-OH" + H + H 20 HC0 3" + H + H 2C0 3 H 20 + C0 2. I t s h o u l d be n o t e d t h a t t h e a c i d i t y i s n e u t r a l i z e d by t h e c a r b o n a t e and not by t h e c a l c i u m i o n . The c a l c i u m a c t s as an e x c h a n g e a b l e i o n w h i c h d i s p l a c e s t h e a c i d i c c a t i o n s , H + and A l + 3 , f r o m t h e s o i l c o l l o i d s i n t o t h e s o i l s o l u t i o n . O r d i n a r i l y t h e r e l e a s e o f a c i d i c c a t i o n s would r e s u l t i n a d e c r e a s e i n s o i l pH, 28 but b e c a u s e o f t h e q u a n t i t y o f C0 3" i o n s ( f r o m t h e l i m e ) and OH" ( f r o m t h e w a t e r ) , t h e net e f f e c t i s u s u a l l y an i n c r e a s e i n pH, s i n c e t h e a c i d H 2C0 3 i s weaker t h a n t h e b a s e C a ( O H ) 2 . The g e n e r a l i z e d o v e r a l l r e a c t i o n o f l i m e w i t h an a c i d s o i l may be w r i t t e n as 2 A l - S o i l + 3 CaC0 3 + H 20 » 3 C a - S o i l + 2 A 1 ( 0 H ) 3 + 3 C0 2. The p r o d u c t s o f a c o m p l e t e d l i m i n g r e a c t i o n a r e e x c h a n g e a b l e Ca and p r e c i p i t a t e d A 1 ( 0 H ) 3 . In an i d e a l i z e d c a s e , t h e s o i l pH w i l l be a bout 8.3 and t h e r e w i l l be 100% base s a t u r a t i o n . However, under f i e l d c o n d i t i o n s , s o i l s a r e g e n e r a l l y l i m e d t o pH v a l u e s no h i g h e r t h a n about 6.5 w h i c h r e s u l t s i n a b a s e s a t u r a t i o n of l e s s t h a n 100%. M o s c h l e r e t a l . ( 1962), f o u n d t h a t t h e Tatum s i l t loam had a base s a t u r a t i o n o f 66.3% when l i m e d t o pH 6.7 w i t h 8.8 t / h a o f c a l c i t e . F a c t o r s A f f e c t i n g t h e R ate o f R e a c t i o n o f Lime S o i l P r o p e r t i e s The r a t e o f s o l u t i o n o f l i m e s t o n e p a r t i c l e s i s v e r y s e n s i t i v e t o pH and t o t h e s p e e d w i t h w h i c h t h e OH" i o n i s removed as w e l l as t h e r a t e o f d i f f u s i o n o f C a + J away f r o m t h e p a r t i c l e s u r f a c e . T h i s i m p l i e s t h a t t h e d i f f u s i o n o f t h e a c i d i o n s i n t h e s o i l s o l u t i o n t o w a r d s t h e c a r b o n a t e d e t e r m i n e s t h e s p e e d o f r e m o v a l o f t h e OH" i o n , t h e r e b y a f f e c t i n g t h e r e a c t i o n r a t e o f t h e c a r b o n a t e . A c i d s o i l s c o n t a i n v a r y i n g amounts of H + and A l + 3 i o n s . D e p e n d i n g on t h e pH of t h e s o i l , t h e A l * 3 may become h y d r o x y l a t e d t o form A 1 ( 0 H ) + 2 and A 1 ( 0 H ) 2 + The l a t t e r i o n s may 29 be c o m p l e x e d by o r g a n i c m a t t e r ( H a r g r o v e and Thomas, 1982), a d s o r b e d as monomers o r p o l y m e r s by t h e e x c h a n g e complex, o r may a l s o p o l y m e r i z e on c l a y s u r f a c e s as c o n t i n u o u s l a y e r s o r as i s l a n d s o f n o n e x c h a n g e a b l e a c i d i t y (Schwertmann and J a c k s o n , 1964). The f a c t o r s t h a t d e t e r m i n e t h e r a t e and e x t e n t o f A l + 3 h y d r o l y s i s w i l l a f f e c t t h e r e a c t i o n r a t e o f l i m e . Coleman and Thomas ( 1 9 6 4 ) , f o u n d t h a t h y d r o l y s i s o f A l + 3 was i n c r e a s e d by t h e a d d i t i o n o f s a l t s . However, a d s o r p t i o n o f t h e h y d r o x y - A l compounds r e d u c e s t h e h y d r o l y s i s r e s u l t i n g i n a l o n g e r p e r i o d o f t i m e f o r n e u t r a l i z a t i o n t o o c c u r . H a r g r o v e and Thomas ( 1 9 8 4 ) , f o u n d as t h e amount o f o r g a n i c a l l y c o m p l e x e d h y d r o x y - A l i n c r e a s e d , t h e r e a c t i v i t y of l i m e d e c r e a s e d . B o t h o r g a n i c m a t t e r and c l a y m i n e r a l s c o n t a i n -OH g r o u p s w h i c h w i l l i o n i z e H + as t h e pH i s i n c r e a s e d . T h i s s o u r c e o f H + comes f r o m t h e pH-dependent c h a r g e d g r o u p s . The h y d r o l y s i s r a t e o f CaC0 3 i s d i r e c t l y r e l a t e d t o t h e r a t e a t w h i c h t h e OH" i o n s a r e removed from t h e s o i l s o l u t i o n as w e l l as t h e p a r t i a l p r e s s u r e of C0 2 i n s o l u t i o n . As l o n g as t h e r e a r e H* i o n s i n s o l u t i o n , CaC0 3 w i l l c o n t i n u e t o d i s s o l v e and h y d r o l y z e . The r a t e a t w h i c h t h e s o i l i s a f f e c t e d by l i m e depends upon t h e c o n c e n t r a t i o n g r a d i e n t f o r b o t h t h e C a + 2 and OH" i o n s and t h e d i s t a n c e t h e d i f f u s i n g i o n s must t r a v e l i n t h e s o i l . S i n c e t h e c o n c e n t r a t i o n o f OH" and C a + 2 i o n s a t t h e c a l c i t e p a r t i c l e s u r f a c e r e m a i n s n e a r l y c o n s t a n t t h e c o n c e n t r a t i o n g r a d i e n t i s d e t e r m i n e d by i o n c o n c e n t r a t i o n s i n t h e s o i l s o l u t i o n . A g r e a t e r 30 c o n c e n t r a t i o n d i f f e r e n c e between two p o i n t s w i l l a c c e l e r a t e d i f f u s i o n . S e v e r a l e n v i r o n m e n t a l f a c t o r s a l s o i n f l u e n c e t h e r a t e o f CaC0 3 d i s s o l u t i o n : - T e m p e r a t u r e - The r e a c t i o n w i l l be f a s t e r as t e m p e r a t u r e i n c r e a s e s . - S o i l T e x t u r e - C l a y e y s o i l s have l e s s t o r t u o u s d i f f u s i o n p a t h s as compared t o sandy s o i l s . - M o i s t u r e - As t h e m o i s t u r e c o n t e n t i n c r e a s e s l a r g e r q u a n t i t i e s o f i o n s can d i f f u s e more e a s i l y s i n c e t h e d i f f u s i o n pathway becomes l a r g e r . Lime P r o p e r t i e s The n e u t r a l i z a t i o n of s o i l a c i d i t y o c c u r s on t h e s u r f a c e of t h e l i m e p a r t i c l e s . The r a t e o f t h e s e r e a c t i o n s a r e a l s o a f f e c t e d by s e v e r a l c h e m i c a l and p h y s i c a l p r o p e r t i e s o f t h e l i m e s t o n e p a r t i c l e . T hese f a c t o r s must a l s o by c o n s i d e r e d . C h e m i c a l C o m p o s i t i o n The r a n g e i n c o m p o s i t i o n i n l i m e s t o n e v a r i e s f r o m p u r e c a l c i t e (CaC0 3) t o p u r e d o l o m i t e (CaC0 3'MgC0 3) . P u r e c a l c i t e c o n t a i n s 400 g kg" 1 (40.0%) o f Ca. Pure d o l o m i t e has e q u a l m o l e c u l a r r a t i o s o f CaC0 3 and MgC0 3, and on a w e i g h t b a s i s , i t c o n t a i n s 217 g k g - 1 (21.7%) o f Ca and 131 g k g - 1 (13.1%) o f Mg. The d i v i d i n g l i n e between what i s c a l c i t e and what i s d o l o m i t e i s not v e r y c l e a r . I t i s g e n e r a l l y a c c e p t e d t h a t c a l c i t e r e f e r s 31 t o m a t e r i a l s w i t h v e r y l i t t l e Mg w h i l e t h e Mg c o n t e n t o f d o l o m i t i c l i m e s t o n e may v a r y from p u r e d o l o m i t e t o l i m e s t o n e w i t h a few grams o f Mg per k i l o g r a m . Mg i n l i m e s t o n e g e n e r a l l y r e d u c e s t h e r a t e a t w h i c h t h e l i m e s t o n e d i s s o l v e s . As a r e s u l t , d o l o m i t i c l i m e s t o n e s w i l l r e a c t more s l o w l y w i t h s o i l a c i d s t h a n w i l l c a l c i t e . B a r b e r ( 1 9 8 4 ) , summarized d a t a r e p o r t e d i n t h e l i t e r a t u r e and f o u n d t h a t c a l c i t e d i s a p p e a r e d a t a r a t e a p p r o x i m a t e l y t w i c e t h a t o f d o l o m i t e . The c h e m i c a l e f f e c t i v e n e s s o f a l i m e s t o n e i s g i v e n by i t s c a l c i u m c a r b o n a t e e q u i v a l e n t ( C C E ) . T h i s i s d e f i n e d as t h e a c i d -n e u t r a l i z i n g c a p a c i t y o f a l i m i n g m a t e r i a l e x p r e s s e d as a w e i g h t p e r c e n t a g e o f c a l c i u m c a r b o n a t e . Pure d o l o m i t e has a CCE o f 108%. T h e r e f o r e , d o l o m i t e on a w e i g h t b a s i s w i l l n e u t r a l i z e 1.08 t i m e s as much as t h e same w e i g h t of c a l c i u m c a r b o n a t e . P a r t i c l e S i z e The s p e c i f i c s u r f a c e a r e a i n c r e a s e s w i t h a r e d u c t i o n i n p a r t i c l e d i a m e t e r . B e c a u s e c a l c i t e i s o n l y s p a r i n g l y s o l u b l e t h e l a r g e r t h e s u r f a c e a r e a i n c o n t a c t w i t h t h e s o i l s o l u t i o n t h e f a s t e r t h e r a t e of d i s s o l u t i o n w i l l be. B a r b e r ( 1984), c o n c l u d e s t h a t t h e p e r s i s t e n c e o f l i m e p a r t i c l e s i n t h e s o i l i s p r o p o r t i o n a l t o t h e i r d i a m e t e r . In t e r m s o f l e n g t h of t i m e t o c o r r e c t s o i l pH and l o n g e v i t y of t h e l i m e a p p l i c a t i o n , f i n e r p a r t i c l e s w i l l r e a c t more r a p i d l y t h a n c o a r s e r o n e s , but t h e p e r s i s t e n c e o f t h e e f f e c t may be 32 s h o r t e r . Meyer and V o l k ( 1 9 5 2 ) , f o u n d t h a t l i m e p a r t i c l e s f i n e r t h a n 100-mesh r e a c t e d soon a f t e r a p p l i c a t i o n , b u t d e c r e a s e d i n e f f e c t i v e n e s s a f t e r 18 months. M a t e r i a l s v a r y i n g i n p a r t i c l e s i z e f r o m 20 t o 60-mesh i n d u c e d l i t t l e i n i t i a l c h a nge i n pH, but a f t e r 18 months a p p r o a c h e d t h e e f f e c t i v e n e s s o f f i n e r s i z e d p a r t i c l e s . S u r f a c e R e a c t i o n s on Lime P a r t i c l e s S e v e r a l compounds may be a d s o r b e d by c a l c i t e o r p r e c i p i t a t e d o n t o t h e p a r t i c l e w h i c h may impede d i s s o l u t i o n . B a r r o w s e t a l . ( 1 9 6 6 ) , f o u n d t h a t when l i m e was added t o h i g h - P s o i l s , c o a t i n g s f o r m e d t h r o u g h t h e combined p r o c e s s e s o f w e a t h e r i n g , r e c r y s t a 1 1 i z a t i o n of c a l c i t e , and a d s o r p t i o n and/or p r e c i p i t a t i o n o f p h o s p h a t e s from t h e s o i l . In s o i l s c o n t a i n i n g A l + 3 , Fe + 3 , and Mn*4, a p r e c i p i t a t i o n o f A 1 ( 0 H ) 2 + , A 1 ( 0 H ) 3 , F e ( 0 H ) 3 o r Mn0 2 may o c c u r on t h e p a r t i c l e s u r f a c e ( M a c K e n z i e , 1978). P h y s i c a l P r o p e r t i e s o f L i m e s t o n e L i m e s t o n e s v a r y from dense, h i g h l y c r y s t a l l i n e m a t e r i a l s t o s o f t , p o r o u s o n e s . Lahav and B o l t ( 1 9 6 4 ) , f o u n d s i g n i f i c a n t v a r i a t i o n s i n l i m e s t o n e p r o p e r t i e s , and c o n s e q u e n t l y v a r i a b l e r e a c t i o n s i n s o i l s . 33 SOIL pH AND NUTRIENT AVAILABILITY P o t a s s i u m The f orms o f s o i l K a r e m i n e r a l , n o n e x c h a n g e a b l e , e x c h a n g e a b l e and t h a t p r e s e n t i n t h e s o i l s o l u t i o n ( M a r t i n e t a l . , 1985). The l a r g e s t p o r t i o n o f s o i l K i s p r e s e n t as p a r t o f t h e c r y s t a l l i n e s t r u c t u r e of p r i m a r y and s e c o n d a r y m i n e r a l s s u c h as m i c a s , f e l d s p a r s , and t h e m i c a c e o u s m i n e r a l s o f t h e c l a y f r a c t i o n . N o n e x c h a n g e a b l e K i s d i s t i n c t f r o m m i n e r a l K i n t h a t i t i s not bonded c o v a l e n t l y w i t h i n t h e c r y s t a l s t r u c t u r e o f m i n e r a l s , but i s bound c o u l o m b i c a l l y between t h e a d j a c e n t t e t r a h e d r a l l a y e r s o f s o i l m i c a s , v e r m i c u l i t e and i n t e r g r a d e c l a y m i n e r a l s . One t o f o u r p e r c e n t o f t h e t o t a l K i s p r e s e n t i n e x c h a n g e a b l e form and i s a d s o r b e d t o s o i l c o l l o i d s . The K f o u n d i n t h e s o i l s o l u t i o n makes up a s m a l l p e r c e n t a g e o f t h e e x c h a n g e a b l e f r a c t i o n . The a v a i l a b i l i t y o f K i s dependent upon two f a c t o r s . A dynamic e q u i l i b r i u m e x i s t s between n o n e x c h a n g e a b l e and e x c h a n g e a b l e forms w i t h t h e l e v e l o f K i n t h e s o i l s o l u t i o n a f f e c t i n g t h e r e l e a s e o f n o n e x c h a n g e a b l e K. A s e c o n d f a c t o r c o n t r o l l i n g t h e m a g n i t u d e o f K r e l e a s e f r o m t h e n o n e x c h a n g e a b l e form i s t h e t y p e o f c l a y m i n e r a l p r e s e n t . S o i l s c o n t a i n i n g l a r g e q u a n t i t i e s o f k a o l i n i t e and low c h a r g e montmori1 I o n i t e w i l l c o n t a i n low q u a n t i t i e s of n o n e x c h a n g e a b l e K as compared t o s o i l s c o n t a i n i n g v e r m i c u l i t i c and m i c a c e o u s m i n e r a l s . The i n f l u e n c e o f pH on K c o n c e n t r a t i o n i s r e l a t e d t o b o t h o f t h e s e f a c t o r s . 34 The K c o n c e n t r a t i o n i n s o l u t i o n g e n e r a l l y i n c r e a s e s w i t h a d e c r e a s e i n pH (Nemeth and Grimme, 1972; R i c h , 1964). T h i s i n d i c a t e s l e s s s t r o n g K b o n d i n g , p r o b a b l y b e c a u s e o f i n c r e a s i n g c o m p e t i t i o n f r o m A l and t h e h y d r o x y - A l compounds f o u n d i n a c i d s o i l s f o r K s e l e c t i v e s i t e s . In s o i l s w i t h a p p r e c i a b l e o r g a n i c m a t t e r , t h e h i g h s e l e c t i v i t y o f o r g a n i c m a t t e r f o r A l and i t s h y d r o x y compounds may r e s u l t i n most o f t h e A l b e i n g o r g a n i c a l l y bound (Nemeth e t a l . , 1972) l e a v i n g t h e K - s e l e c t i v e s i t e s f r e e l y a c c e s s i b l e . In t h i s c a s e , t h e K c o n c e n t r a t i o n may not i n c r e a s e w i t h a r e d u c t i o n i n pH. L i m i n g an a c i d s o i l r e s u l t s i n d e c r e a s e d a v a i l a b i l i t y o f K ( B a r t l e t t e t a l . , 1969; Magdoff e t a l . , 1980; York e t a l . , 1953). T h i s e f f e c t has been e x p l a i n e d by a f r e e i n g o f K s e l e c t i v e s i t e s t h r o u g h t h e n e u t r a l i z a t i o n of A l and h y d r o x y - A l compounds w h i c h were p r e v i o u s l y b l o c k i n g them (Nemeth e t a l . , 1972) o r t o a l o w e r e d p e r c e n t K s a t u r a t i o n o f t h e exch a n g e complex c a u s e d by th e i n c r e a s e d CEC w i t h an i n c r e a s e i n pH ( B a r t l e t t e t a l . , 1969; Magdoff e t a l . , 1980). Magnesium Magnesium i s an i m p o r t a n t c o n s t i t u e n t o f many p r i m a r y and s e c o n d a r y a l u m i n o s i 1 i c a t e m i n e r a l s . The Mg i n t h i s f o r m i s not v e r y s o l u b l e . Magnesium may a l s o be p r e s e n t as t h e c a r b o n a t e (CaC0 3» MgC0 3) i n s o i l s c o n t a i n i n g d o l o m i t e . T h i s f o r m i s s l i g h t l y s o l u b l e . S o i l Mg o r i g i n a t e s f r o m t h e w e a t h e r i n g o f t h e s e m i n e r a l s and i s p r e s e n t i n t h e s o i l as an e x c h a n g e a b l e i o n i n 35 n e u t r a l t o s l i g h t l y a c i d c o n d i t i o n s . L i t t l e i n f o r m a t i o n i s a v a i l a b l e on t h e i n f l u e n c e s o f a c i d t r e a t m e n t s on s o i l Mg v a l u e s . R u s s e l l ( 1 9 8 8 ) , s t a t e s t h a t i n a c i d s o i l s , e x c h a n g e a b l e Mg i s o f t e n p r e s e n t i n s m a l l amounts and may be d e f i c i e n t . Shuman e t a l . ( 1 9 8 3 ) , f o u n d t h a t w i t h a 0.02 u n i t r e d u c t i o n i n pH, Mg l e v e l s were l o w e r e d s l i g h t l y o v e r t h e 3 y e a r s o f t h e s t u d y . I t a p p e a r s t h a t as pH d e c r e a s e s , H* and A l + 3 d i s p l a c e Mg i o n s f r o m exchange s i t e s w i t h t h e r e s u l t t h a t s o i l s o l u t i o n c o n c e n t r a t i o n i n c r e a s e s ( C u r t i n and S m i l l i e , 1983) t h e s e Mg i o n s a r e t h e n s u b j e c t t o l o s s v i a l e a c h i n g i f not t a k e n up by p l a n t s . A p p l i c a t i o n of l i m e t o t r e a t s o i l a c i d i t y u s u a l l y r e s u l t s i n s u b s t a n t i a l r e d u c t i o n s i n e x c h a n g e a b l e Mg ( G r o v e e t a l . , 1981; M e y e r s e t a l . , 1988; Grove e t a l . , 1985; Sumner e t a l . , 1978). R e d u c t i o n i n e x c h a n g e a b l e Mg has been r e f e r r e d t o as Mg " f i x a t i o n " . I t has been s u g g e s t e d t h a t Mg f i x a t i o n i s d i r e c t l y r e l a t e d t o t h e e x c h a n g e a b l e A l c o n t e n t o f t h e s o i l ( G r o v e e t a l . , 1981; M e y e r s e t a l . , 1988) o r t o t h e p r e s e n c e o f an i l l - d e f i n e d A l f r a c t i o n t h a t i s e x t r a c t a b l e w i t h a c i d i f i e d NH 40Ac ( G r o v e and Sumner, 1985). The mechanism s u g g e s t e d f o r t h i s f i x a t i o n i n v o l v e s t h e o c c l u s i o n of Mg i n t o amorphous A l p o l y m e r s formed by t h e p r e c i p i t a t i o n o f e x c h a n g e a b l e A l upon l i m i n g ( G r o v e e t a l . , 1981; M e y e r s e t a l . , 1988). 36 T r a c e E l e m e n t s P l a n t a v a i l a b i l i t y o f t r a c e m e t a l s i s o f t e n r e p o r t e d t o be a s s o c i a t e d w i t h t h e d i s t r i b u t i o n o f t h e s e e l e m e n t s among c e r t a i n s o i l f r a c t i o n s . V a r i o u s f r a c t i o n a t i o n schemes have been u s e d t o s t u d y t h e a v a i l a b i l i t y and d i s t r i b u t i o n among s o l u t i o n , e x c h a n g e a b l e , o r g a n i c a l l y c o m p lexed, and o x i d e ( F e , Mn, A l ) bound fo r m s o f t h e s e e l e m e n t s . V a r i a t i o n s i n s e v e r a l s o i l p a r a m e t e r s s u c h as pH, redox c o n d i t i o n s , e xchange c a p a c i t y , o r g a n i c m a t t e r c o n t e n t , t e x t u r e , o x i d e c o n t e n t , and c l a y m i n e r a l o g y a l l may i n d u c e s h i f t s i n t h e s e p o o l s , w h i c h may r e s u l t i n d e f i c i e n c i e s or t o x i c i t i e s . I t i s i m p o r t a n t t o remember t h a t any o r a l l o f t h e s e f a c t o r s may i n f l u e n c e t r a c e m e t a l b e h a v i o u r and t h a t d i s c u s s i o n o f a l l o f t h e a f o r e m e n t i o n e d i s beyond t h e s c o p e o f t h i s p a p e r . C opper S o i l o r g a n i c m a t t e r forms v e r y s t a b l e c o m p l e x e s w i t h Cu. S e v e r a l s t u d i e s have shown t h a t t h e b u l k o f s o i l Cu i s p r e s e n t as C u + 2 - o r g a n i c m a t t e r c o m p l e x e s i n s o l u t i o n , o v e r t h e pH r a n g e 4.0-7.7 and t h a t a change i n pH does not c a u s e a t r a n s f o r m a t i o n of t h i s f o r m t o a n o t h e r ( S i m s , 1986; S a n d e r s , 1982; M c B r i d e e t a l . , 1979; J e f f e r y e t a l . , 1983; Hodgeson e t a l . , 1966). S a n d e r s ( 1 9 8 2 ) , f o u n d t h a t Cu c o n c e n t r a t i o n s i n t h e s o i l s o l u t i o n d e c r e a s e d o n l y s l i g h t l y as t h e s o l u t i o n pH i n c r e a s e d , b u t t h e f r e e Cu* 2 i o n c o n c e n t r a t i o n s i n s o l u t i o n d e c r e a s e d c o n s i d e r a b l y . T h i s i s a t t r i b u t e d t o t h e r e l e a s e o f a l k a l i - s o l u b l e o r g a n i c a c i d s 37 as pH i n c r e a s e s . At h i g h pH v a l u e s , as w e l l as under a c i d c o n d i t i o n s , h i g h e r p r o p o r t i o n s o f C u + 2 i o n may be p r e s e n t i n s o l u t i o n due t o c o m p e t i t i o n f o r o r g a n i c m a t t e r by Ca and A l i o n s . Copper o c c l u d e d i n Fe o x i d e s i s a s e c o n d a r y r e s e r v o i r o f s o i l Cu ( S i m s , 1986). T h i s f r a c t i o n a l s o e x h i b i t s l i t t l e v a r i a t i o n w i t h s o i l pH. Z i nc S o i l Zn may be p r e s e n t i n t h e s o i l s o l u t i o n as i o n i c o r o r g a n i c a l l y c o m p l e x e d s p e c i e s ; on exchange s i t e s ; o c c l u d e d i n o x i d e s and h y d r o x i d e s o f A l , Fe, and Mn; and e n t r a p p e d i n p r i m a r y and s e c o n d a r y m i n e r a l s ( I y e n g a r , S.S. e t a l . , 1981). At pH v a l u e s l e s s t h a n 5.5, t h e e x c h a n g e a b l e f r a c t i o n i s t h e dominant p o o l o f p l a n t a v a i l a b l e Zn* 2 ( S i m s , 1986; F.G. V i e t s , 1962). T h e r e may be s i g n i f i c a n t c o m p e t i t i o n o f A l * 3 w i t h Zn* 2 f o r e x c h a n g e s i t e s under a c i d i c c o n d i t i o n s , r e s u l t i n g i n an i n c r e a s e o f Zn* 2 i n t h e s o i l s o l u t i o n ( M c B r i d e e t a l . , 1979). The s o l u b i l i t y of Zn* 2 d e c r e a s e s m a r k e d l y w i t h i n c r e a s i n g s o i l pH ( J e f f e r y e t a l . , 1983; H a r t e r , 1983; Sims, 1986; M c B r i d e e t a l . , 1979). In t h e pH r a n g e 5.5 t o 7.0, Zn* 2 s o l u b i l i t y i s r e d u c e d by e i t h e r a d s o r p t i o n or o c c l u s i o n by p r e c i p i t a t e d Fe and A l o x i d e s (Shuman, 1979; Sims, 1986; K a l b a s i e t a l . , 1978). Sims ( 1 9 8 6 ) , f o u n d t h a t i n s o i l s w i t h s u f f i c i e n t o r g a n i c m a t t e r , c o m p l e x a t i o n by o r g a n i c l i g a n d s under h i g h pH c o n d i t i o n s (pH 7.7) may r e d u c e o c c l u s i o n o r p r e c i p i t a t i o n o f Zn* 2 by s o i l o x i d e s t h e r e b y i n c r e a s i n g s o l u t i o n l e v e l s . 38 Manganese Manganese e x i s t s i n s o i l s as w a t e r - s o l u b l e Mn; as e x c h a n g e a b l e Mn + 2; o r g a n i c a l l y c omplexed; and as v a r i o u s o x i d e s of Mn + 3 o r Mn + 4 ( J a r v i s , 1984; G e e r i n g e t a l . , 1969). The d i s t r i b u t i o n between m o b i l e Mn*2 and t h e i n s o l u b l e o x i d e s o f Mn 30 4 and Mn0 2, i s c o n t r o l l e d l a r g e l y by d r a i n a g e and redox c o n d i t i o n s , but a l s o by pH ( J a r v i s , 1984). At pH 5.0, p l a n t a v a i l a b l e Mn i s p r e s e n t i n s o l u t i o n and on t h e e x c h a n g e complex as t h e f r e e d i v a l e n t i o n Mn + 2 ( S a n d e r s , 1983; Sims, 1986; G e e r i n g e t a l . , 1969). Manganese s o l u b i l i t y d e c l i n e s w i t h an i n c r e a s e i n pH. As pH i s i n c r e a s e d , e x c h a n g e a b l e Mn + 2 d e c r e a s e s and i s t r a n s f o r m e d i n t o o r g a n i c f o r m s o r i s p r e c i p i t a t e d o r o c c l u d e d by s o i l o x i d e s ( e i t h e r Mn o r Fe) ( S i m s , 1986; Shuman, 1979; C h r i s t e n s e n e t a l . , 1950; S i n h a e t a l . , 1978). S o l u b i l i z a t i o n o f o r g a n i c a c i d s under a l k a l i n e c o n d i t i o n s may r e s u l t i n i n c r e a s e d c o m p l e x a t i o n o f Mn i n h i b i t i n g i t s p r e c i p i t a t i o n o r o c c l u s i o n by s o i l o x i d e s (Page, 1962). C h r i s t e n s e n e t a l . ( 1950), f o u n d t h a t e x c h a n g e a b l e Mn i n c r e a s e d i n l i m e d s o i l s when o r g a n i c m a t t e r was added as compared t o s o i l s w i t h o u t an a d d i t i o n o f o r g a n i c m a t t e r . S a n d e r s ( 1 9 8 3 ) , f o u n d however, t h a t o r g a n i c m a t t e r c o m p l e x i n g was not s t r o n g enough t o p r e v e n t p r e c i p i t a t i o n o r a d s o r p t i o n a t h i g h pH v a l u e s . 39 I r o n The s o l u b i l i t y o f Fe i n s o i l s i s l a r g e l y c o n t r o l l e d by t h e s o l u b i l i t y o f h y d r o u s F e ( I I I ) o x i d e s ( L i n d s a y , 1972): F e + 3 + 3 O H " , — F e ( O H ) 3 ( 9 ) K i p-10_„. 4. L i k e A l * 3 , Fe* 3 h y d r o l y s e s i n s o l u t i o n so t h a t h y d r o l y s e d s p e c i e s w i l l c o n s t i t u t e a major p a r t o f t h e Fe* 3 i n s o l u t i o n : Fe* 3 + H 20 FeOH* 2 + H* FeOH* 2 + H 20 : Fe(OH) 2* + H* Fe(OH) 2* + H 20 , = » F e ( O H ) 3 + H* F e ( O H ) 3 l o s e s f o u r more water m o l e c u l e s and p r e c i p i t a t e s as FeOOH ( g o e t h i t e ) . The v a r i o u s f o r m s o f Fe i n t h e s o i l s o l u t i o n a r e p r o b a b l y Fe* 3 and h y d r o x y - F e complex a s s o c i a t i o n s w i t h o r g a n i c m o l e c u l e s , c o l l o i d a l Fe, and f r e e h y d r o x y s p e c i e s ( U r e n , 1984). T o t a l Fe* 3 i n s o l u t i o n v a r i e s w i t h pH and r e a c h e s a minimum i n t h e pH ra n g e o f 6.5 t o 8.0 ( L i n d s a y , 1972; L o e p p e r t e t a l . , 1985, 1984). L i n d s a y ( 1 9 7 2 ) , s t a t e s t h a t t h e Fe* 3 a c t i v i t y i n s o l u t i o n d e c r e a s e s 1 , 0 0 0 - f o l d f o r e a c h u n i t i n c r e a s e i n pH due t o t h e p r e c i p i t a t i o n o f Fe* 3 as an o x i d e . The r e a c t i o n between Fe* 3 and l i m e c an be summarized as f o l l o w s ( L o e p p e r t e t a l . , 1984): 2 Fe* 3 + 3 CaC0 3 + H 20 » 2 FeOOH + 3 Ca* 2 + 3 C0 2. 40 MATERIALS AND METHODS S o i l Samples and Sample P r e p a r a t i o n The s o i l s u s e d were from t h e L a n g l e y a r e a o f B r i t i s h C o l u m b i a . C l a y and o r g a n i c m a t t e r c o n t e n t were t h e two s o i l p r o p e r t i e s u s e d f o r s o i l s e l e c t i o n s i n c e b o t h o f t h e s e p r o p e r t i e s i n f l u e n c e t h e b u f f e r i n g c a p a c i t y o f a s o i l . S o i l s w i t h e x t r e m e d i f f e r e n c e s i n t h e s e p r o p e r t i e s were d e s i r e d s o as t o r e p r e s e n t b o t h t h e h i g h and low ends o f t h e s c a l e . The L a n g l e y s o i l s e r i e s , a Humic L u v i c G l e y s o l , and t h e Lynden s o i l s e r i e s , an O r t h i c H u m o - F e r r i c P o d z o l , were c h o s e n ( s e e A p p e n d i x A f o r p r o f i l e d e s c r i p t i o n s ) . R e l e v a n t s o i l p r o p e r t i e s a r e g i v e n i n T a b l e 2. T a b l e 2. Some p r o p e r t i e s of t h e s o i l s u s e d f o r t h e s t u d y . S o i l C l a s s i f i c a t i o n T e x t u r e C l a y O r g a n i c C a r b o n pH L a n g l e y Humic L u v i c S i l t y C l a y 53 7.31 5.8 G l e y s o l Loam Lynden O r t h i c H u m o - F e r r i c Loamy Sand 6 2.15 4.2 P o d z o l The t o p 20 cm o f s i t e s w h i c h had not been under c u l t i v a t i o n were sa m p l e d . One l a r g e b u l k sample was t a k e n f r o m e a c h s i t e . E a c h s o i l sample was a i r - d r i e d and c r u s h e d t o p a s s t h r o u g h a 2 mm s t a i n l e s s s t e e l s c r e e n . A f t e r c r u s h i n g , e a c h s o i l was t h o r o u g h l y m i x e d by p a s s i n g t h e sample t h r o u g h a s t a i n l e s s s t e e l 41 s o i l s e p a r a t o r s e v e r a l t i m e s and m i x i n g t h e sample on a l a r g e p i e c e o f brown p a p e r . I n c u b a t i o n S t u d y To a c c o m p l i s h t h e o b j e c t i v e s o f t h i s r e s e a r c h t o a s s e s s t h e e f f e c t s o f a l t e r n a t i n g c y c l e s o f pH f l u c t u a t i o n s , t h e pH o f e a c h s o i l was a r t i f i c i a l l y a l t e r e d i n t h e l a b by t h e a d d i t i o n o f e i t h e r l i m e o r a c i d . The s a m p l e s were t h e n i n c u b a t e d f o r 3 weeks a t room t e m p e r a t u r e and f i e l d c a p a c i t y m o i s t u r e c o n t e n t . At t h e end o f t h i s t i m e t h e s o i l s were e l u t e d t w i c e i n o r d e r t o s i m u l a t e l e a c h i n g and p r e v e n t t h e a c c u m u l a t i o n o f s a l t s , e a c h t i m e s a v i n g t h e s u p e r n a t a n t l i q u i d ( F i g u r e 2 ) . At t h i s p o i n t t h e r e were 2 s u p e r n a t a n t e l u t i o n s per sample and t h e o r i g i n a l s o i l w h i c h had been i n c u b a t e d . T h i s t h e n c o m p l e t e d 1 c y c l e i n t h e e x p e r i m e n t . F i g u r e 2. Flow d i a g r a m f o r 1 c y c l e o f t h e i n c u b a t i o n s t u d y . »Incubate f o r 3 weeks Sample + Amendment-S o i l Sample-Repeat =Centr i f uge4= zAdd d i s t i l e d w a t e r Shake o v e r n i g h t D e c a n t e l u a n t and s a v e f o r a n a l y s i s The s o i l was t r e a t e d a g a i n as d e s c r i b e d above f o r a n o t h e r 5 c y c l e s t o make a t o t a l o f 6 c o m p l e t e c y c l e s . The 2 s u p e r n a t a n t e l u t i o n s were k e p t s e p a r a t e and s a v e d f o r a n a l y s i s l a t e r . 42 At t h e c o m p l e t i o n o f t h e 6 c y c l e s o f i n c u b a t i o n s , t h e t r e a t e d s o i l s were a i r - d r i e d , and c r u s h e d t o p a s s a 2 mm s t a i n l e s s s t e e l s i e v e . F o r a more d e t a i l e d d e s c r i p t i o n o f t h e i n c u b a t i o n s e t - u p r e f e r t o A p p e n d i x B. T i t r a t i o n c u r v e s were us e d t o d e t e r m i n e t h e amount o f a c i d r e q u i r e d t o lower t h e pH o f b o t h s o i l s t o a minimum o f 3.5 ( A p p e n d i x C) . From t h e s e c u r v e s i t was d e c i d e d t o use 2.5 me o f HC1 per 50 g of s o i l . A l i m e r e q u i r e m e n t d e t e r m i n a t i o n was c o n d u c t e d t o d e t e r m i n e t h e amount of CaC0 3 r e q u i r e d t o r a i s e t h e pH of t h e s o i l s t o a t l e a s t 6.5, by t h e method of L e w i s and Hardy ( 1 9 4 3 ) . From t h i s d e t e r m i n a t i o n i t was d e c i d e d t o use 40 me o f CaC0 3 per 50 g s o i l . The i n c u b a t i o n c o n s i s t e d o f 4 t r e a t m e n t s w i t h 12 r e p l i c a t e s p e r t r e a t m e n t : - T r e a t m e n t 1 - C o n t i n u o u s l y t r e a t e d w i t h d i s t i l l e d w a t e r . - T r e a t m e n t 2 - C o n t i n u o u s l y t r e a t e d w i t h HC1. - T r e a t m e n t 3 - C o n t i n u o u s l y t r e a t e d w i t h CaC0 3. - T r e a t m e n t 4 - A l t e r n a t i n g t r e a t m e n t s w i t h e i t h e r HC1 o r CaC0 3 i . e . c y c l e s 1,3,and 5 were a c i d i f i e d and c y c l e s 2,4,and 6 were l i m e d . T r e a t m e n t s 1, 2, and 3 were t h e c o n t r o l s . At t h e b e g i n n i n g o f ea c h c y c l e o f t h e i n c u b a t i o n , t h e above d e s c r i b e d amendments were added t o e a c h s o i l e.g. 2.5 me o f HC1 per 50 g o f s o i l was added t o t h e s o i l sample i n t r e a t m e n t 2 so t h a t t h e s o i l r e c e i v e d a t o t a l of 6 t r e a t m e n t s w i t h a c i d f o r t h a t t r e a t m e n t . T r e a t m e n t 3 r e c e i v e d l i m e a t o t a l of 6 t i m e s and t r e a t m e n t 4 was a c i d i f i e d 43 3 t i m e s and l i m e d 3 t i m e s . A n a l y s e s o f t h e E l u a n t The f i r s t and s e c o n d e l u t i o n s o b t a i n e d f r o m t h e i n c u b a t i o n s were a n a l y z e d s e p a r a t e l y . pH was measured u s i n g a Accumet (Model 810) pH m e t e r , by p l a c i n g t h e e l e c t r o d e d i r e c t l y i n t o t h e s o l u t i o n . Ca, Mg, K, Cu, Mn, Zn, Fe, and A l l e v e l s i n t h e s o l u t i o n were measured by f l a m e 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 r y on a P e r k i n Elmer 306 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 . A l l t h e e l e m e n t s , e x c e p t A l , were r e a d u s i n g an a i r - a c e t y l e n e f l a m e . A n i t r o u s o x i d e f l a m e was u s e d w i t h A l . P was d e t e r m i n e d c o l o u r i m e t r i c a l l y i n t h e s o l u t i o n u s i n g t h e a s c o r b i c a c i d method f o r c o l o u r d e v e l o p m e n t ( O l s e n e t a l . , 1954). The s a m p l e s were r e a d on a G i l f o r d s p e c t r o p h o t o m e r . F o r t h e p u r p o s e s of d a t a a n a l y s i s and p r e s e n t a t i o n o f t h e r e s u l t s , t h e d a t a f r o m t h e f i r s t and s e c o n d e l u t i o n s were combined t o g i v e one v a l u e . A n a l y s e s o f t h e S o i l s C h e m i c a l A n a l y s e s were p e r f o r m e d on t h e s a m p l e s w h i c h had been t r e a t e d d u r i n g t h e i n c u b a t i o n , as w e l l a s , s o i l s a m p l e s w h i c h had not r e c e i v e d any t r e a t m e n t s . S o i l pH was measured i n a 1:2 s o i l t o s o l u t i o n s u s p e n s i o n u s i n g b o t h w a t e r and 0.01 M C a C l 2 , by t h e method o f L a v k u l i c h 44 ( 1 9 7 8 ) . A v a i l a b l e l e v e l s o f Ca, Mg, K, Cu, Mn, Zn, and Fe were d e t e r m i n e d by e x t r a c t i n g w i t h 1.0 M NH 4HC0 3-0.005 M d i e t h y l e n e t r i a m i n e p e n t a a c e t i c a c i d (DTPA), a d j u s t e d t o pH 7.6, as d e s c r i b e d by S o l t a n p o u r and Schwab ( 1 9 7 7 ) . E l e m e n t a l l e v e l s were d e t e r m i n e d by 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 r y u s i n g an a i r - a c e t y l e n e f l a m e . Fe and A l l e v e l s were d e t e r m i n e d by e x t r a c t i o n w i t h a c i d ammonium o x a l a t e s o l u t i o n , by t h e method o f McKeague and Day ( 1 9 6 6 ) . E l e m e n t a l c o n c e n t r a t i o n s were measured by f l a m e 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 r y . A n i t r o u s o x i d e f l a m e was u s e d t o r e a d A l . T o t a l e xchange c a p a c i t y was d e t e r m i n e d by l e a c h i n g w i t h u n b u f f e r e d 1.0 M N a C l , by t h e method o f Chapman ( 1 9 6 5 ) . M i n e r a l o g i c a l A n a l y s e s C o m p o s i t e s a m p l e s f o r e a c h o f t h e f o u r t r e a t m e n t s were p r e p a r e d by m i x i n g 4 g o f s o i l f r o m e a c h one o f t h e r e p l i c a t e s w i t h i n e a c h t r e a t m e n t . The c o m p o s i t e s a m p l e s were t h o r o u g h l y mixed on a s h e e t o f brown paper and t h e n s t o r e d i n p l a s t i c b o t t l e s . A n a l y s e s were p e r f o r m e d on b o t h t h e t r e a t e d s o i l s , f o r w h i c h t h e c o m p o s i t e s a m p l e s were u s e d , and t h e u n t r e a t e d s o i l s . M i n e r a l o g i c a l a n a l y s i s was c a r r i e d o u t by s t a n d a r d x - r a y d i f f r a c t i o n p r o c e d u r e s ( W h i t t i g , 1965). C a r b o n a t e s and s e s q u i o x i d e s were removed by t h e method o f Kunze (1965) and 45 o r g a n i c m a t t e r was removed by t h e method o f A n d e r s o n ( 1 9 6 3 ) . The s a m p l e s were s e p a r a t e d i n t o t h e 2.0-0.2 m i c r o m e t e r s i z e f r a c t i o n by t h e method o f K i t t r i c k and Hope ( 1 9 6 3 ) . S t a t i s t i c a l A n a l y s e s T e s t s f o r N o r m a l i t y and Homogeneity o f V a r i a n c e s The e l u a n t and t h e s o i l s d a t a were a n a l y z e d s e p a r a t e l y . P r o b a b i l i t y p l o t s f o r b o t h t h e e l u t i o n and s o i l s d a t a were c o n s t r u c t e d f o r a l l o f t h e a n a l y z e d v a r i a b l e s . The p l o t s r e v e a l e d S - c u r v e s f o r a l m o s t a l l o f t h e v a r i a b l e s . Some v a r i a b l e s had d i s c o n t i n u o u s d i s t r i b u t i o n s , b ut t h e g e n e r a l t r e n d was s t i l l t h a t of an S - c u r v e . T h i s t y p e o f c u r v e i n d i c a t e d n o n n o r m a l i t y ( Z a r , 1984). V a r i a n c e s were c a l c u l a t e d f o r a l l o f t h e v a r i a b l e s f o r b o t h t h e s o i l s and t h e e l u t i o n d a t a . B a r t l e t t ' s t e s t f o r h o m o g e n e i t y of v a r i a n c e s , as w e l l a s , v i s u a l i n s p e c t i o n i n d i c a t e d h e t e r o g e n e i t y o f v a r i a n c e s . S i n c e t h e a s s u m p t i o n s o f n o r m a l i t y o f t h e p o p u l a t i o n and h o m o g e n e i t y o f v a r i a n c e s f o r use o f p a r a m e t r i c s t a t i s t i c s were v i o l a t e d i t was d e c i d e d t o use n o n p a r a m e t r i c t e s t s . E l u a t e D a t a F r e i d m a n ' s t e s t was u s e d t o t e s t f o r d i f f e r e n c e s between t r e a t m e n t s o v e r t i m e ( Z a r , 1984). The W i l c o x o n p a i r e d - s a m p l e t e s t was u s e d t o i d e n t i f y t h e t r e a t m e n t s w h i c h had s i g n i f i c a n t d i f f e r e n c e s ( Z a r , 1984). The 46 pairs which were tested were: treatment 1 x treatment 4; treatment 2 x treatment 4; and treatment 3 x treatment 4. S o i l Data Differences between treatments were i d e n t i f i e d with the Kruskal-Wal1 i s analysis of variance (Zar, 1984). Treatments with s i g n i f i c a n t differences were i d e n t i f i e d with the Wilcoxon paired-sample test (Zar, 1984). Four combinations were tested: treatment 1 x treatment 4; treatment 2 x treatment 4; treatment 3 x treatment 4; and treatment 5 x treatment 4. 47 RESULTS AND DISCUSSION I n t r o d u c t i o n The a c c u r a c y w i t h w h i c h t h e i n c u b a t i o n s t u d y d e s c r i b e d e a r l i e r m o d e l s f i e l d c o n d i t i o n s i s q u e s t i o n a b l e and t h e d a t a o b t a i n e d s h o u l d not be e x t r a p o l a t e d t o any p a r t i c u l a r f i e l d s i t u a t i o n . I t was assumed t h a t t h e s o i l s had r e a c h e d e q u i l i b r i u m i n t h e 3 weeks o f e a c h c y c l e , a l t h o u g h t h i s may not be t r u e . B e c a u s e t h e pH o f t h e s o i l was not measured a f t e r t h e s o i l was t r e a t e d w i t h i t s r e s p e c t i v e amendment i t i s d i f f i c u l t t o a c c u r a t e l y c o r r e l a t e c h a n g e s i n n u t r i e n t a v a i l a b i l i t y w i t h f l u c t u a t i o n s i n pH. The r e a d e r s h o u l d b e a r i n mind t h a t t h e d a t a p r e s e n t e d below w i l l o n l y be u s e d t o t r y and e l u c i d a t e t h e t r e n d s and s u g g e s t mechanisms r e s p o n s i b l e f o r c h a n g e s i n n u t r i e n t a v a i l a b i l i t y w i t h f l u c t u a t i o n s i n pH. 48 pH The s o i l pH v a l u e s i n w a t e r and 0.01 M C a C l 2 f o r a l l 4 t r e a t m e n t s a t t h e c o m p l e t i o n o f t h e i n c u b a t i o n s t u d y , as w e l l as t h e u n t r e a t e d s o i l a r e shown i n T a b l e 1. T r e a t m e n t 1 and t h e u n t r e a t e d s o i l had a p p r o x i m a t e l y t h e same pH. A c i d i f i c a t i o n r e d u c e d t h e pH a p p r o x i m a t e l y 2 pH u n i t s f o r b o t h t h e L a n g l e y and t h e Lynden s o i l , as compared w i t h t h e u n t r e a t e d s o i l . L i m i n g i n c r e a s e d t h e pH about 3 u n i t s . The l a s t amendment added t o t r e a t m e n t 4 ( a l t e r n a t i n g t r e a t m e n t ) was l i m e . The f i n a l s o i l pH was a b o u t 2.5 u n i t s h i g h e r t h a n t h e u n t r e a t e d s o i l f o r t h e L a n g l e y s o i l and about 2.8 u n i t s h i g h e r f o r t h e L y n d e n . The f i n a l s o i l pH v a l u e s f o r each of t h e t r e a t m e n t s , as w e l l as f o r t h e u n t r e a t e d s o i l s , were s i g n i f i c a n t l y d i f f e r e n t f r o m t r e a t m e n t 4 w i t h t h e e x c e p t i o n o f t h e C a C l 2 pH r e a d i n g o f t r e a t m e n t 3 i n t h e Lynden s o i 1 . P o t a s s i u m A c i d i f i c a t i o n r e s u l t e d i n an i n c r e a s e i n w a t e r s o l u b l e K ( T a b l e s 3 and 4) and a d e c r e a s e i n a v a i l a b l e s o i l K ( T a b l e 2 ) . T h i s i s i n agreement w i t h t h e r e s u l t s o f Nemeth and Grimme (1972) and R i c h ( 1 9 7 4 ) . S o l u t i o n A l l e v e l s a l s o i n c r e a s e w i t h a c i d i f i c a t i o n . T hese i o n s may compete w i t h K i o n s f o r K s e l e c t i v e e x c h a n g e s i t e s r e s u l t i n g i n a d e c r e a s e i n s o i l l e v e l s and an i n c r e a s e i n s o l u t i o n c o n c e n t r a t i o n s . T a b l e 5 shows t h a t t h e m i n e r a l o g y o f t h e c o a r s e c l a y f r a c t i o n o f b o t h s o i l s 49 Table 3. pH i n water and 0.01 M C a C l 2 . Values are averages of the r e p l i c a t e s f o r each treatment. S o i l Treatment 1 Treatment 2 Treatment 3 Treatment 4 Untreated Langley s o i l P H W « t e r 5.01* 3.13* 7.90* 7.52 4.92* P H C a C 1 2 4.17* 3.09* 7.44* 7.34 4.18* Lynden s o i l P H W « t e r 4.98* 2.98* 7.95* 7.79 4.83* P H C a C 1 2 4.35* 3.21* 7.51 7.47 4.26* Treatment was s i g n i f i c a n t l y d i f f e r e n t from treatment 4 ( i . e . the a l t e r n a t i n g treatment) at<*=0.05. Table 4. A v a i l a b l e Mg, K, Cu, Mn, Zn, and Fe as e x t r a c t e d w i t h 1.0 M NH 4HC0 3 - 0.005 M DTPA at pH 7.6 and c a t i o n exchange c a p a c i t y . Average values f o r the 12 r e p l i c a t e s are shown. Treatment A v a i l a b l e C a t i o n s (ppm) K Mg Ca Cu Mn Zn Fe CEC (meq/100g) Langley s o i l 1 83. .1* 4. .63* 63 .3* 0. .96* 52, .67* 1.55* 113. .8* 21.8* 2 27. .5* 0. .00 1. 6* 0. ,69* 6, .85* 0.68* 165. ,6* 20.4* 3 64. .8* 0. .00 180 .2* 1, .35 61 , .40* 0.40* 37. .5* 26.9* 4 49. .8 0. ,00 152 .1 1. ,25 75, .35 0.33 48. ,1 31.8 Untreated 97. .3* 7. .77* 88 .8* 0. ,83* 26. .79* 1.73* 78. .1* 22.3* Lynden s o i l 1 10 .2* 2. .35* 21 .8* 0, .40 19 .88 0.28* 45. .4* 5.6* 2 3. .7 0. .00 0 .0* 0. .35 1. .71* 0.14 153. .8* 5.2* 3 7 .3* 0. ,00 141 .5 0. .41 38 .77* 0.07 15. .2* 8.3 4 3, .8 0. ,00 152 .7 0. .37 18 .04 0.10 26. .0 8.5 Untreated 9 .8* 1 . .38* 4 .2* 0, .22* 11 .29* 0.38* 55. .6* 5.2* * Treatment i s s i g n i f i c a n t l y d i f f e r e n t from treatment 4 ( i . e . the a l t e r n a t i n g treatment) atot=0.05. 50 T a b l e 5. C l a y m i n e r a l o g y o f t h e c o a r s e c l a y f r a c t i o n (0.002 0.00002 mm) f o r t h e L a n g l e y and Lynden s o i l s e r i e s . T r e a t m e n t C l a y M i n e r a l o g y L a n g l e y S o i l U n t r e a t e d C h l o r i t e , H y d r o u s M i c a , Q u a r t z P l a g i o c l a s e F e l d s p a r s Montmori1 I o n i t e , C h l o r i t e , I n t e r s t r a t i f i e d M o n t m o r i 1 l o n i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s C h l o r i t e , H y d r o u s M i c a , I n t e r s t r a t i f i e d Montmori1 I o n i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s C h l o r i t e , Montmori1 I o n i t e , H y d r o u s M i c a , Q u a r t z , P l a g i o c l a s e F e l d s p a r s C h l o r i t e , H y d r o u s M i c a , I n t e r s t r a t i f i e d Montmori1 I o n i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s Lynden S o i l Unt r e a t e d 1 2 3 V e r m i c u l i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s V e r m i c u l i t e , H y d r o u s M i c a , C h l o r i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s C h l o r i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s C h l o r i t e , H y d r o u s M i c a , Q u a r t z , P l a g i o c l a s e F e l d s p a r s V e r m i c u l i t e , C h l o r i t e , Q u a r t z , P l a g i o c l a s e F e l d s p a r s was changed by a c i d i f i c a t i o n . Montmori1 I o n i t e i n t h e L a n g l e y s o i l , and v e r m i c u l i t e and h y d r o u s m i c a i n t h e Lynden s o i l , have been t r a n s f o r m e d t o e i t h e r an i n t e r s t r a t i f i e d m i n e r a l o r c h l o r i t e 51 by t h e p r e c i p i t a t i o n o f p o l y m e r i z e d h y d r o x y - A l compounds i n t h e i n t e r l a y e r s p a c e s (Grimme, 1979). L i m i n g r e d u c e d a v a i l a b l e K f o r b o t h s o i l s ( T a b l e 3 ) . The r e d u c t i o n due t o l i m i n g may have been t h e r e s u l t o f e i t h e r t h e f r e e i n g o f K s e l e c t i v e s i t e s t h r o u g h t h e n e u t r a l i z a t i o n o f A l compounds (Nemeth e t a l . , 1972) o r t o a l o w e r e d p e r c e n t K s a t u r a t i o n o f t h e exchange complex ( B a r t l e t t e t a l . , 1969; M agdoff e t a l . , 1980) due t o t h e i n c r e a s e o f t h e CEC. Water s o l u b l e K i n c r e a s e d s l i g h t l y upon l i m i n g f o r t h e f i r s t c y c l e o f t h e i n c u b a t i o n i n t r e a t m e n t 3, but t h e n d e c l i n e d ( T a b l e s 3 and 4 ) . York e t a l . ( 1 9 5 3 ) , f o u n d t h a t t h e p r e s e n c e o f e x c e s s CaC0 3 r e d u c e d t h e e x c h a n g e a b l e K w h i l e i n c r e a s i n g t h e c o n c e n t r a t i o n o f K i n a water e x t r a c t . B o t h s o i l s had h i g h e r K l e v e l s i n t h e a l t e r n a t i n g t r e a t m e n t as compared t o t h e a c i d t r e a t m e n t b u t , i n o n l y t h e L a n g l e y s o i l was t h e d i f f e r e n c e s i g n i f i c a n t . The p r e s e n c e o f v e r m i c u l i t e i n b o t h t r e a t m e n t s 2 and 4 ( T a b l e 5) o f t h e Lynden s o i l may e x p l a i n t h e l a c k o f a s i g n i f i c a n t d i f f e r e n c e s i n c e v e r m i c u l i t e has a h i g h a f f i n i t y f o r K and i s known t h e f i x K i n an u n a v a i l a b l e f o r m (Grimme, 1979). The l i m e t r e a t m e n t had h i g h e r K l e v e l s t h a n t h e a l t e r n a t i n g t r e a t m e n t . The d i f f e r e n c e was s i g n i f i c a n t f o r b o t h s o i l s . I t w o u l d a p p e a r t h a t pH f l u c t u a t i o n s r e d u c e d K a v a i l a b i l i t y as compared t o l i m i n g a l o n e . T h i s may be t h e r e s u l t of t h e i n c r e a s e i n CEC i n t h e a l t e r n a t i n g t r e a t m e n t . An i n c r e a s e i n CEC r e s u l t s i n l o w e r e d K a v a i l a b i l i t y due t o a r e d u c t i o n i n p e r c e n t 52 Table 6. pH and concentration of K, Mg, Cu, Mn, Zn, Fe, and Al in the combined eluate for each treatment over the six cycles of the incubation for the Langley s o i l . Average values of the 12 replicates are shown. Treatment Cycle pH K Mg Ca Cu Mn Zn Fe Al Concentration (ppm) 1 1 4.6 21.4* 7.5* 15.6* 0.1 0.8* 0.0* 30.0* 8.5 2 5.1* 24.3 9.7* 14.5* 0.0** 2.2* 0.1 47.6* 60.1* 3 4.4* 18.6 3.4* 2.9* 0.1 5.0* 0.2* 100.1* 129.7* 4 5.2* 27.5* 6.2* 0.6* 0.1* 6.8* 0.4* 152.3* 228.4* 5 5.0* 20.2 2.8* 0.2* 0.2* 3.8* 0.2* 105.8* 148.3* 6 5.3* 19.3* 2.3* 0.2* 0.1* 3.3* 0.3* 99.1* 136.8* 2 1 4.7 75.1* 97.6 327.2 0.0 45.0 0.2 0.0 3.0 2 4.1* 66.2* 90.9* 443.3* 0.0 127.5* 0.7* 1.6* 21.4 3 3.1* 43.5* 54.4* 304.2* 0.1* 124.2* 1.1* 5.0 93.9* 4 4.1* 26.7* 20.3* 98.7* 0.0* 54.6* 0.9* 0.8* 188.6* 5 3.3* 18.8* 9.7* 44.1* 0.1* 35.5* 0.7* 1.1* 263.3* 6 3.0* 14.0* 5.2* 31.8* 0.0 20.1* 0.7* 1.6* 326.7* 3 1 8.2* 22.7* 17.8* 235.8* 0.0 0.9* 0.0* 8.0* 9.4* 2 7.3 23.2 11.3* 134.0* 0.1* 2.6* 0.1* 66.4* 78.2* 3 6.7 19.9 7.7* 101.8* 0.2* 2.0* 0.1* 67.3* 128.0* 4 7.4* 25.4* 9.2 59.1* 0.1* 2.6* 0.3* 111.1* 182.5* 5 6.9 15.3* 6.4* 59.7* 0.2* 1.7* 0.1* 55.3* 89.8* 6 6.3 16.0* 6.1* 55.1* 0.1* 1.9* 0.2* 61.6* 94.9* 4 1 4.6 54.9 96.7 316.8 0.0 47.0 0.3 0.0 2.9 2 7.4 22.2 20.4 344.2 0.0 5.0 0.0 16.2 19.4 3 6.8 18.4 26.6 877.7 0.1 11.9 0.0 5.3 10.3 4 7.6 14.3 9.1 247.2 0.0 3.5 0.2 58.5 90.4 5 7.5 21.0 40.3 2858.8 0.1 26.9 0.0 2.3 2.6 6 7.0 8.8 11.4 912.9 0.0 8.1 0.1 10.0 12.4 * Treatment i s significantly different from treatment 4 (i.e. alternating treatment) at«=0.05. 53 pH and the concentration of K, Mg, Cu, Mn, Zn, Fe, and Al in the eluate for each treatment i the six cycles of the incubation for the Lynden s o i l . Average values of the 12 replicates shown. Treatment Cycle PH K Mg Ca Cu Mn Concentrat :ion (ppm)-1 1 4.6* 5.4* 0. 2* 0.7* 0.0* 2. .8* 2 4.8* 4.7* 1. 0 3.8* 0.0 2. .0 3 5.8* 3.1 1. 0* 0.9* 0.0* 2. .3* 4 4.9* 3.0* 1. 1* 0.4* 0.0* 5 .3* 5 4.8* 2.1 0. 8* 0.3* 0.0* 4 .5* 6 5.3* 3.4* 0. 9* 0.2* 0.1 6 .4* Zn Fe Al 0.1* 28.2* 31.2* 0.1* 39.3* 46.3* 0.1* 23.8* 34.9* 0.1* 47.5* 58.1* 0.1* 27.6* 33.4* 0.3* 74.8* 92.9* 2 3.8* 3 2.9* 4 3.8* 5 3.3 6 2.8* 23.0 10.5 8.2* 3.3* 3.3* 1.0* 2.2* 0.7 1.9 0.8* 1.6* 1.3* 108.8 0.0* 11.8* 0.0* 4.6* 0.1* 1.2* 0.0 0.6* 0.1* 0.9* 0.1* 123.8* 1.4* 76.7* 1.2* 24.9* 0.7* 8.6* 0.5* 7.1* 0.5* 5.2* 0.5* 2.5 96.3* 13.4 232.1* 2.3 265.0* 1.5* 276.6* 1.3* 330.5 1.7* 340.8* 3 1 7.6* 2 7.3 3 6.5 4 7.4 5 7.5 6 7.8* 7.1* 1.7* 3.7 2.2* 1.8* 1.7 1.3* 1.1* 1.7 1.5* 2.0* 1.4* 87.3* 0.0* 68.4* 0.0 63.2* 0.1 45.9* 0.0 58.3* 0.1* 45.9* 0.1* 1.0* 0.0* 3.0* 0.1* 2.5* 0.1* 1.8 0.0* 3.0* 0.1* 3.4* 0.2* 10.6* 13.5* 23.8* 31.7* 23.7* 40.9* 15.6 22.9 29.1* 36.8* 35.4* 56.5* 4 1 3.8 2 7.2 3 6.6 4 7.4 5 6.8 6 7.4 23.1 10.6 3.3 0.9 3.0 1.6 1.6 0.7 1.9 1.8 0.7 0.3 109.9 0.1 231.8 0.0 724.4 0.1 103.4 0.0 825.0 0.1 303.3 0.1 148.1 1.5 1.9 0.1 7.5 0.0 1.7 0.0 18.3 0.1 2.7 0.1 1.9 110.3 11.4 14.5 1.6 4.4 17.6 24.9 0.0 0.0 4.7 6.7 Treatment i s s i g n i f i c a n t l y different from treatment 4 ( i . e . alternating treatment) at«=0.05. 54 K s a t u r a t i o n o f t h e exchange complex ( B a r t l e t t e t a l . , 1969; M a gdoff e t a l . , 1980). T a b l e 5 a l s o shows p r e s e n c e o f v e r m i c u l i t e i n t h e Lynden s o i l and i n t e r s t r a t i f i e d m i n e r a l s i n t h e L a n g l e y s o i l . B o t h o f t h e s e m i n e r a l s a r e c a p a b l e o f f i x i n g K i n t o u n a v a i l a b l e forms t h u s d e c r e a s i n g a v a i l a b l e K. D e c r e a s e d K l e v e l s i n t h e a l t e r n a t i n g t r e a t m e n t may a l s o be e x p l a i n e d by h i g h e r s o l u t i o n K l e v e l s i n t h e a c i d i f i e d c y c l e s o f t h i s t r e a t m e n t w h i c h a r e removed i n t h e s i m u l a t e d l e a c h i n g . The lower l e v e l s i n b o t h s o i l and w a t e r e x t r a c t e d K may r e f l e c t t h e i n a b i l i t y o f t h e s o i l s t o r e - e s t a b l i s h e q u i l i b r i u m between n o n e x c h a n g e a b l e and e x c h a n g e a b l e forms o f K i n t h e l e n g t h o f t i m e t h e i n c u b a t i o n s t u d y was c a r r i e d o u t . The Lynden s o i l may have c o m p a r a t i v e l y lower l e v e l s of K t h a n t h e L a n g l e y s o i l due t o t h e d i f f e r e n c e i n t e x t u r e . Nemeth e t a l . ( 1 9 7 2 ) , f o u n d t h a t f o r a g i v e n amount o f e x c h a n g e a b l e K, t h e K c o n c e n t r a t i o n i n t h e s o i l s o l u t i o n w i l l be h i g h e r t h e lower t h e c l a y and s i l t c o n t e n t o f a s o i l a r e . The K i n t h i s s o i l , due t o t h e lower c l a y c o n t e n t , i s not o n l y more p r o n e t o l e a c h i n g t h a n t h e L a n g l e y s o i l , but t h e s o i l may a l s o have a lower p o t e n t i a l t o r e p l e n i s h K l o s t due t o l e a c h i n g . Magnesium T a b l e 4 shows t h a t no Mg was e x t r a c t e d from t h e s o i l f o r T r e a t m e n t s 2, 3, and 4 f o r b o t h s o i l s by t h e e x t r a c t i o n method employed. The u n t r e a t e d s o i l had a h i g h e r Mg c o n c e n t r a t i o n t h a n d i d t r e a t m e n t 1 o f t h e L a n g l e y s o i l . The s i t u a t i o n was r e v e r s e d 55 f o r t h e Lynden s o i l i n t h a t t r e a t m e n t 1 had a h i g h e r Mg c o n c e n t r a t i o n t h a n t h e u n t r e a t e d s o i l . A c i d i f i c a t i o n o f b o t h s o i l s r e s u l t e d i n an i n c r e a s e i n s o l u t i o n Mg as compared t o t h e o t h e r t r e a t m e n t s ( T a b l e s 6 and 7) c o u p l e d w i t h a d e c l i n e i n s o i l Mg. T h i s i s i n a c c o r d a n c e w i t h t h e f i n d i n g s o f C u r t i n and S m i l l i e ( 1 9 8 3 ) . The d i s a p p e a r a n c e o f h y d r o u s m i c a i n t h e Lynden s o i l , and montmori1 I o n i t e i n t h e L a n g l e y , i n f a v o u r of f o r m a t i o n of e i t h e r i n t e r s t r a t i f i e d m i n e r a l s o r c h l o r i t e may r e s u l t i n a d e c r e a s e o f e x c h a n g e s i t e s a v a i l a b l e t o r e t a i n b a s i c c a t i o n s a g a i n s t l e a c h i n g l o s s e s . O n l y t h e L a n g l e y s o i l d e m o n s t r a t e d a r e d u c t i o n i n CEC, as compared t o t h e u n t r e a t e d s o i l , w i t h a c i d i f i c a t i o n . A p p l i c a t i o n of l i m e s u b s t a n t i a l l y r e d u c e d a v a i l a b l e Mg, but s o l u t i o n l e v e l s a p p e a r e d t o be h i g h e r t h a n t h e w a t e r t r e a t e d s a m p l e s ( t r e a t m e n t 1). P e r h a p s a c o m b i n a t i o n o f Mg f i x a t i o n as p r o p o s e d by G r o v e e t a l . (1985) and M e y e r s e t a l . (1988) as w e l l as Mg b e i n g d i s p l a c e d o f f t h e exchange complex by Ca a c c o u n t f o r t h e low s o i l l e v e l s and i n c r e a s e s i n s o l u t i o n c o n c e n t r a t i o n s . C u r t i n and S m i l l i e ( 1 9 8 3 ) , a l s o f o u n d t h a t l i m i n g i n i t i a l l y i n c r e a s e d Mg i n s o l u t i o n . I t i s d i f f i c u l t t o s u g g e s t how t h e a l t e r n a t i n g c y c l e s o f a c i d i f i c a t i o n and l i m i n g a f f e c t e d t h e s o i l f r o m t h e r e s u l t s p r e s e n t e d s i n c e t h i s t r e a t m e n t a l s o had no a v a i l a b l e Mg. P e r h a p s a c o m b i n a t i o n o f Mg l o s t d u r i n g t h e s i m u l a t e d l e a c h i n g as w e l l as some f i x a t i o n d u r i n g t h e l i m e t r e a t m e n t s o f t h e i n c u b a t i o n can a c c o u n t f o r t h e r e s u l t s . 56 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 i n t h e a c i d i f i e d c y c l e s o f t r e a t m e n t 4 t h e r e i s an i n c r e a s e i n s o l u t i o n Mg ( T a b l e s 6 and 7) w h i l e i n t h e l i m i n g c y c l e s , s o l u t i o n Mg d e c r e a s e s . F i g u r e 3 shows t h e p a t t e r n of r e l e a s e o f Mg i n t o s o l u t i o n f o r t h e d u r a t i o n of t h e i n c u b a t i o n s t u d y f o r t h e L a n g l e y s o i l . The L a n g l e y s o i l g e n e r a l l y m a i n t a i n e d f a i r l y h i g h l e v e l s o f s o l u t i o n Mg r e g a r d l e s s of t h e t r e a t m e n t . T h i s was not t h e c a s e f o r t h e Lynden s o i l . T h e r e was a d r a m a t i c d r o p i n s o l u t i o n Mg a f t e r t h e f i r s t c y c l e , w i t h l e v e l s r e m a i n i n g somewhat lower t h a n any o f t h e o t h e r t r e a t m e n t s ( T a b l e 7 ) . The d i f f e r e n c e between t h e two s o i l s may have a g a i n been due t o m i n e r a l o g y and t e x t u r e d i f f e r e n c e s w h i c h may a f f e c t t h e a b i l i t y o f t h e s o i l t o r e p l e n i s h l o s t Mg. Copper O n l y t h e u n t r e a t e d sample o f t h e Lynden s o i l was s i g n i f i c a n t l y d i f f e r e n t from t r e a t m e n t 4 ( T a b l e 4 ) . Copper i s g e n e r a l l y p r e s e n t as an o r g a n i c complex w h i c h i s not r e a d i l y t r a n s f o r m e d i n t o a n o t h e r f r a c t i o n by pH f l u c t u a t i o n s ( S i m s , 1986; S a n d e r s , 1982; M c B r i d e e t a l . , 1979; J e f f e r y e t a l . , 1983). I t a p p e a r s t h a t t h i s was t h e c a s e f o r t h i s s o i l . L i m i n g i n c r e a s e d Cu i n t h e L a n g l e y s o i l . O n l y t r e a t m e n t s 1, 2, and t h e u n t r e a t e d s o i l a r e d i f f e r e n t f r o m t r e a t m e n t 4. T r e a t m e n t s 3 and 4 b o t h had s i m i l a r Cu c o n c e n t r a t i o n s w h i c h were h i g h e r t h a n t h e o t h e r t r e a t m e n t s . I t a p p e a r s t h a t l i m i n g has i n c r e a s e d t h e Cu c o n c e n t r a t i o n f o r t h i s s o i l . M c B r i d e and B l a s i a k ( 1 9 7 9 ) , f o u n d an i n c r e a s e i n Cu c o n c e n t r a t i o n above pH 57 F i g u r e 3. Release of Mg i n t o s o l u t i o n f o r treatments 1, 2, 3 and 4 f o r the Langley s o i l . l O O-i Mg C O N C ' N . (ppm) 5 0 -0 \ \ \ \ W A T E R A C I D I F I E D LIMED ALTERNATING (ACID w i t h L I M E ) \ A. \ \ \ V . — • - -• __z 1 2 3 4 5 C Y C L E (I CYCLE = 3 W E E K S ) T " 7.0. They a t t r i b u t e d t h i s to the a l k a l i s o l u b i l i z a t i o n of o r g a n i c a c i d s with an i n c r e a s e i n pH and the f a c t t h a t at pH 8 complexed Cu*2 made up 99.9% of the t o t a l s o l u b l e Cu*2. The Langley s o i l has a higher o r g a n i c matter content than the Lynden s o i l so t h i s may account f o r the d i f f e r e n c e i n the behaviour of Cu between the two s o i l s . The i n c r e a s e i n Cu i n the Langley s o i l f o r treatments 3 and 4 may a l s o of been the r e s u l t of e x t r a c t i o n of Cu from o r g a n i c matter by the DTPA i n the e x t r a c t i n g s o l u t i o n ( J a h i r u d d i n et a l . , 1986). Cu c o n c e n t r a t i o n s i n s o l u t i o n appeared to be u n a f f e c t e d by 58 c h a n g e s i n pH ( T a b l e s 6 and 7 ) . Manganese A c i d i f i c a t i o n r e s u l t e d i n a l a r g e i n c r e a s e i n s o l u t i o n Mn f o r b o t h s o i l s ( T a b l e s 6 and 7 ) . T h i s i s i n agreement w i t h t h e r e s u l t s o f S a n d e r s (1983) and Sims ( 1 9 8 6 ) . L i m i n g d e c r e a s e d t h e c o n c e n t r a t i o n o f Mn i n s o l u t i o n f o r b o t h s o i l s ( T a b l e s 6 and 7 ) , but i n c r e a s e d t h e s o i l c o n c e n t r a t i o n s ( T a b l e 4 ) . T h i s i s c o n s i s t e n t w i t h t h e r e s u l t s o f Sims (1986) and Shuman (1979) who f o u n d t h a t Mn i n s o l u t i o n d e c r e a s e d upon l i m i n g and was t r a n s f o r m e d i n t o e i t h e r o r g a n i c f o r m s o r was p r e c i p i t a t e d o r o c c l u d e d by s o i l o x i d e s . The i n c r e a s e o f s o i l Mn w i t h l i m i n g f o r t r e a t m e n t s 3 and 4 ( t h e l a s t c y c l e o f t h e i n c u b a t i o n s t u d y was a l i m e t r e a t m e n t ) was u n e x p e c t e d s i n c e t h e r e s u l t s o f s e v e r a l s t u d i e s have shown t h a t e x c h a n g e a b l e Mn d e c r e a s e s w i t h an i n c r e a s e i n pH ( J a h i r u d d i n e t a l . , 1986; Sims, 1986; Shuman, 1979). The a l t e r n a t i n g t r e a t m e n t had a Mn c o n c e n t r a t i o n c l o s e r t o t h e w a t e r t r e a t m e n t t h a n t h e l i m e t r e a t m e n t f o r t h e Lynden s o i l ( T a b l e 4 ) . T h i s may o f been due t o a l a r g e r p r o p o r t i o n o f t h e t o t a l a v a i l a b l e Mn b e i n g l o s t i n t h e s i m u l a t e d l e a c h i n g . The a c i d i f i e d c y c l e s o f t r e a t m e n t 4 showed an i n c r e a s e i n s o l u t i o n Mn w h i c h was r e d u c e d upon l i m i n g f o r b o t h s o i l s ( T a b l e s 6 and 7 ) . The a c i d i f i e d c y c l e s o f t r e a t m e n t 4 i n t h e L a n g l e y s o i l g e n e r a l l y had s i g n i f i c a n t l y lower s o l u t i o n l e v e l s o f Mn t h a n d i d t r e a t m e n t 2, w h i l e t h e l i m e d c y c l e s o f t h e t r e a t m e n t had 59 h i g h e r l e v e l s t h a n d i d t r e a t m e n t 3. E x c e p t f o r c y c l e s 1 and 5, t h e Lynden s o i l had s i g n i f i c a n t l y lower Mn l e v e l s i n s o l u t i o n i n t r e a t m e n t 4 t h a n d i d e i t h e r t r e a t m e n t 2 o r 3. Z i n c A c i d i f i c a t i o n i n c r e a s e d t h e s o l u t i o n c o n c e n t r a t i o n o f Zn f o r b o t h s o i l s ( T a b l e s 6 and 7) and l i m i n g a p p e a r e d t o d e c r e a s e t h e Zn i n s o l u t i o n . The s o l u b i l i t y o f Zn has been f o u n d by s e v e r a l p e o p l e t o d e c r e a s e w i t h an i n c r e a s e i n s o i l pH ( J e f f e r y e t a l . , 1983; H a r t e r , 1983; Sims, 1986; M c B r i d e e t a l . , 1 9 7 9 ) . E x c e p t f o r t h e f i r s t c y c l e i n t h e a l t e r n a t i n g t r e a t m e n t , a l t e r n a t i n g t r e a t m e n t s w i t h e a c h c y c l e d i d not seem t o a f f e c t t h e p a t t e r n o f Zn r e l e a s e d i n t o s o l u t i o n ( T a b l e s 6 and 7 ) , but t h i s t r e a t m e n t d i d a p p e a r t o have t h e l o w e s t s o l u t i o n l e v e l s o f Zn f o r t h e p e r i o d o f t h e i n c u b a t i o n . F i g u r e 4 shows t h e p a t t e r n of Zn r e l e a s e d i n t o s o l u t i o n w i t h e a c h t r e a t m e n t f o r t h e i n c u b a t i o n s t u d y . L i m i n g t h e L a n g l e y s o i l d i d r e d u c e a v a i l a b l e Zn ( T a b l e 4) as f o u n d by J e f f e r y e t a l ( 1 9 8 3 ) , H a r t e r ( 1 9 8 3 ) , Sims ( 1 9 8 6 ) , and M c B r i d e e t a l . ( 1 9 7 9 ) . A c i d i f i c a t i o n a l s o r e d u c e d e x c h a n g e a b l e Zn ( T a b l e 4 ) . The r e d u c t i o n i n s o i l Zn w i t h a c i d i f i c a t i o n may o f been due t o c o m p e t i t i o n o f A l + 3 and H + w i t h Z n + J f o r e x c h a n g e s i t e s ( M c B r i d e e t a l . , 1979) r e s u l t i n g i n lower e x c h a n g e a b l e l e v e l s o f Zn and h i g h e r s o l u t i o n l e v e l s . The Zn c o n c e n t r a t i o n i n t h e a l t e r n a t i n g t r e a t m e n t was s i g n i f i c a n t l y d i f f e r e n t f r o m a l l o t h e r t r e a t m e n t s and had t h e l o w e s t s o i l 60 l e v e l . F i g u r e 4. R e l e a s e o f Zn i n t o s o l u t i o n f o r t r e a t m e n t s 1, 2, 3, and 4 f o r t h e L a n g l e y s o i l . W A T E R C Y C L E ( I C Y C L E = 3 W E E K S ) The L y n d e n s o i l d i d not show any s i g n i f i c a n t d i f f e r e n c e s between s o i l Zn f o r t r e a t m e n t s 2, 3, and 4. L i m i n g d i d however r e d u c e Zn l e v e l s w h i l e a c i d i f i c a t i o n d i d r e s u l t i n h i g h e r s o l u t i o n l e v e l s . 61 I r o n Water s o l u b l e Fe r e l e a s e d i n t o s o l u t i o n d e c r e a s e s w i t h a c i d i f i c a t i o n f o r b o t h s o i l s ( T a b l e s 6 and 7) as compared t o Fe l e v e l s i n t h e w a t e r t r e a t m e n t , b u t t h e r e i s a c o n c u r r e n t i n c r e a s e i n e x c h a n g e a b l e l e v e l s ( T a b l e 4 ) . I r o n w h i c h i s b e i n g s o l u b i l i z e d a t low pH may be r e t a i n e d by t h e e x c h a n g e complex o f t h e s o i l c o l l o i d s r e s u l t i n g i n l o w e r e d s o l u t i o n l e v e l s . I r o n d i d n o t b e h a v e l i k e t h e o t h e r t r a c e m e t a l s when t h e s o i l was a c i d i f i e d i n t h a t s o l u t i o n l e v e l s d e c r e a s e d ( F i g u r e 5) w h i l e w i t h Mn and Zn t h e r e was an i n c r e a s e i n s o l u t i o n . F i g u r e 5. R e l e a s e o f Fe i n t o s o l u t i o n f o r t r e a t m e n t s 1, 2, 3, and 4 f o r t h e L a n g l e y s o i l . Fe C O N C ' N - 1 0 0 -(ppm) 1 5 0 -5 0 -ACIDIFIED ALTERNATING 0 0 5 6 C Y C L E ( I C Y C L E =3 W E E K S ) 62 L i m i n g g e n e r a l l y r e s u l t e d i n a r e d u c t i o n o f Fe i n s o l u t i o n . A d d i t i o n o f l i m e t o a s o i l c a u s e s t h e n e u t r a l i z a t i o n and p r e c i p i t a t i o n o f F e + 3 as FeOOH w h i c h r e d u c e s Fe a v a i l a b i l i t y ( L o e p p e r t e t a l . , 1985; 1984). The i n c r e a s e o f Fe i n s o l u t i o n i n t h e f o u r t h c y c l e o f t r e a t m e n t 3 i n t h e L a n g l e y s o i l may be a t t r i b u t e d t o t h e i n c r e a s e o f c o m p l e x e d Fe by o r g a n i c m a t t e r w h i c h has been s o l u b i l i z e d by l i m i n g ( C u r t i n and S m i l l i e , 1983). Note t h a t t h e r e i s a l s o an i n c r e a s e i n A l i n s o l u t i o n . T h i s e l e m e n t i s a l s o known t o r e m a i n i n s o l u t i o n t h r o u g h c o m p l e x a t i o n by o r g a n i c m a t t e r (Thomas, 1975). The p a t t e r n s of Fe e x t r a c t e d f r o m e a c h t r e a t m e n t by b o t h t h e 1.0 M NH 4HC0 3 - 0.005 M DTPA and t h e a c i d ammonium o x a l a t e s o l u t i o n were g e n e r a l l y t h e same, o n l y t h e r e l a t i v e amounts e x t r a c t e d d i f f e r e d ( T a b l e s 4 and 8 ) . T a b l e 8. A c i d ammonium o x a l a t e e x t r a c t a b l e Fe and A l . A v e r a g e v a l u e s o f t h e 12 r e p l i c a t e s a r e shown. T r e a t m e n t Fe A l % L a n g l e y s o i l 1 1.22* 1. 03* 2 1 .23* 0.99* 3 0.91* 0. 74* 4 1.06 0. 89 U n t r e a t e d 1 . 12* 0. 94* Lynden s o i l 1 0. 70 0. 88* 2 0.85* 0. 82* 3 0.61* 0.71* 4 0.71 0. 78 U n t r e a t e d 1 . 16* 1 .35* * T r e a t m e n t i s s i g n i f i c a n t l y d i f f e r e n t t h a n t r e a t m e n t 4 ( i . e . a l t e r n a t i n g t r e a t m e n t ) at *=0.05. 63 A c i d i f i c a t i o n i n c r e a s e d Fe and l i m i n g d e c r e a s e d Fe a v a i l a b i l i t y r e l a t i v e t o t h e w a t e r t r e a t m e n t . The a l t e r n a t i n g t r e a t m e n t had h i g h e r Fe l e v e l s t h a n d i d t r e a t m e n t 3 even tho u g h b o t h t r e a t m e n t s had been l i m e d i n t h e l a s t c y c l e o f t h e i n c u b a t i o n . The d i f f e r e n c e may be due t o d i f f e r e n c e s i n t h e l e n g t h o f t i m e t h a t t h e s o i l had been i n c o n t a c t w i t h t h e l i m e . P e r h a p s t h e l o n g e r c o n t a c t t i m e w i t h l i m e i n t r e a t m e n t 3 has r e s u l t e d i n t r a n s f o r m a t i o n o f t h e amorphous f o r m s i n t o more c r y s t a l l i n e o n e s w h i c h a r e not e x t r a c t a b l e by t h e methods employed. A c i d ammonium o x a l a t e i s s p e c i f i c f o r o n l y t h e amorphous f o r m s o f Fe (McKeague and Day, 1966; McKeague, 1967; McKeague e t a l . , 1971). The L a n g l e y s o i l had h i g h e r l e v e l s o f ammonium o x a l a t e e x t r a c t a b l e Fe a f t e r t h e i n c u b a t i o n s t u d y t h a n d i d t h e Lynden s o i l even though t h e l e v e l s were s i m i l a r i n t h e u n t r e a t e d s a m p l e s ( T a b l e 5 ) . T h i s may be a r e s u l t o f d i f f e r e n t f o r m s o f Fe b e i n g p r e s e n t ( i . e . amorphous v e r s u s more c r y s t a l l i n e ) i n t h e two s o i l s . L o e p p e r t and C l a r k e ( 1 9 8 4 ) , f o u n d t h a t t h e p r e s e n c e o f o r g a n i c m a t t e r i n f l u e n c e s t h e c r y s t a l 1 i n i t y o f t h e o x i d e p h a s e formed. The L a n g l e y s o i l has a h i g h e r o r g a n i c m a t t e r c o n t e n t t h a n t h e Lynden s o i l and so more Fe may be p r e s e n t i n t h e amorphous p h a s e . T h i s d i f f e r e n c e i n o r g a n i c m a t t e r c o n t e n t s may a l s o a c c o u n t f o r t h e d i f f e r e n c e s i n Fe r e l e a s e d i n t o s o l u t i o n ( T a b l e s 3 and 4) between t h e two s o i l s and t h e h i g h l e v e l s of Fe i n s o l u t i o n o f t h e L a n g l e y s o i l even i n t h e l i m e t r e a t m e n t s . C u r t i n and S m i l l i e ( 1 9 8 3 ) , a l s o f o u n d h i g h s o l u t i o n l e v e l s o f Fe 64 w i t h l i m i n g and a t t r i b u t e d i t t o t h e i n c r e a s e o f c o m p l e x e d Fe by o r g a n i c m a t t e r w h i c h was s o l u b i l i z e d by l i m i n g . P h o s p h o r u s The c o n c e n t r a t i o n o f P r e l e a s e d i n t o s o l u t i o n and e x t r a c t e d by t h e NH 4HC0 3 - DTPA e x t r a c t a n t was n e g l i g i b l e . T h e r e were no s i g n i f i c a n t d i f f e r e n c e s between P l e v e l s w i t h t r e a t m e n t s and so t h e r e s u l t s were not p r e s e n t e d . T hese r e s u l t s were p r o b a b l y due t o p r o b l e m s w i t h t h e e x t r a c t a n t s u s e d and not an i n d i c a t i o n t h a t P was unchanged w i t h pH. C a t i o n Exchange C a p a c i t y A c i d i f i c a t i o n t e n d e d t o d e c r e a s e t h e CEC f o r b o t h s o i l s and l i m i n g i n c r e a s e d CEC, r e l a t i v e t o t h e wa t e r and u n t r e a t e d s a m p l e s ( T a b l e 4 ) . P r a t t and B l a i r ( 1 9 6 2 ) , a l s o f o u n d t h a t CEC i n c r e a s e d s l o w l y w i t h an i n c r e a s e i n pH t o 5.0, and t h e n more r a p i d l y t o pH 8.0. They c o n c l u d e d t h a t t h e pH dependent CEC was due t o b o t h o r g a n i c m a t t e r and c l a y . S i n c e CEC was measured w i t h an u n b u f f e r e d s a l t s o l u t i o n , i t was e x p e c t e d t h a t t h e CEC measured w o u l d be dep e n d e n t on s o i l pH (Bhumbla e t a l . , 1965; G r o v e e t a l . , 1982). The dependence o f CEC on s o i l pH i s a f u n c t i o n o f t h e c o n s t i t u e n t m i n e r a l s and o r g a n i c m a t t e r . If t h e pH i s l e s s t h a n 5.0, A l + 3 w i l l be t h e p r e d o m i n a n t c a t i o n (McLean, 1976). Some A l + 3 i o n s may be i n s o l u t i o n , but most w i l l be a d s o r b e d on exch a n g e s i t e s f r o m w h i c h t h e y a r e e a s i l y d i s p l a c e d w i t h an o r d i n a r y u n b u f f e r e d s a l t 65 s o l u t i o n s u c h as 1 M KC1. As t h e pH i s i n c r e a s e d , A 1 ( 0 H ) + 2 o r A1(0H)2+ i s formed e i t h e r b e f o r e o r a f t e r t h e i o n s a r e a d s o r b e d t o t h e e x c h a n g e s i t e s . T h e s e i o n s p o l y m e r i z e as c o n t i n u o u s l a y e r s o r d i s c o n t i n u o u s i s l a n d s on c h a r g e d s u r f a c e s o r on t h e i n t e r l a y e r s u r f a c e s o f c l a y m i n e r a l s , o r t h e y complex w i t h s o i l o r g a n i c m a t t e r . The p o l y m e r i z e d h y d r o x y - A l l a y e r i s a m u l t i v a l e n t - c a t i o n i c l a y e r w h i c h i s t o o l a r g e and s t r o n g l y a d s o r b e d t o be d i s p l a c e d i n normal c a t i o n e x c h a n g e (McLean, 1976). T h i s s e r v e s t o d e c r e a s e CEC. M i n e r a l o g i c a l a n a l y s i s s u g g e s t s an i n c r e a s e i n t h e amount of p o l y m e r i z e d h y d r o x y - A l compounds formed w i t h a c i d i f i c a t i o n due t o t h e d i s a p p e a r a n c e o f montmori1 I o n i t e i n t h e L a n g l e y s o i l and v e r m i c u l i t e i n t h e Lynden s o i l , and t h e m a i n t e n a n c e o f c h l o r i t e ( T a b l e 5 ) . T h i s may a c c o u n t f o r t h e s l i g h t r e d u c t i o n i n CEC. Schwertman and J a c k s o n ( 1 9 6 4 ) , f o u n d t h a t i n t e r l a y e r s of h y d r o x y - A l were p r e s e n t i n b o t h montmori1 I o n i t e and v e r m i c u l i t e a l t h o u g h t h e r a t e and e x t e n t o f i n t e r l a y e r f o r m a t i o n was h i g h e r w i t h v e r m i c u l i t e p o s s i b l y b e c a u s e o f a h i g h e r l a y e r c h a r g e . L i m i n g i n c r e a s e d t h e CEC o f b o t h s o i l s ( T a b l e 4 ) . W i t h c o n t i n u e d a d d i t i o n o f l i m e A l + 3 h y d r o l y s i s c o n t i n u e s , u n t i l t h e A l p r e c i p i t a t e s as A 1 ( 0 H ) 3 , f r e e i n g t h e e x c h a n g e s i t e s f o r b a s i c c a t i o n s t h e r e b y , i n c r e a s i n g t h e net CEC. T a b l e 5 shows t h e r e a p p e a r a n c e o f montmori1 I o n i t e and t h e d i s a p p e a r a n c e o f t h e i n t e r s t r a t i f i e d m i n e r a l s i n t h e L a n g l e y s o i l and t h e r e a p p e a r a n c e of h y d r o u s m i c a i n t h e Lynden s o i l . T h i s s u g g e s t s t h e 66 n e u t r a l i z a t i o n and p r e c i p i t a t i o n o f A l and i t s hydroxy-compounds as A 1 ( 0 H ) 3 . An i n c r e a s e i n CEC may a l s o be t h e r e s u l t o f d e p r o t o n a t i o n of f u n c t i o n a l g r o u p s on o r g a n i c m a t t e r ( M a r t i n and Reeve, 1958; H a r g r o v e and Thomas, 1982). H e l l i n g e t a l . ( 1 9 6 4 ) , f o u n d t h a t o r g a n i c m a t t e r was t h e major c o n t r i b u t o r t o t h e pH d e p e n d e n t CEC f o r a g r o u p o f W i s c o n s i n s o i l s . T r e a t m e n t 4 had t h e h i g h e s t CEC o f b o t h s o i l s , b u t i t was o n l y s i g n i f i c a n t f o r t h e L a n g l e y s o i l . The c l a y m i n e r a l o g y o f t h e L a n g l e y s o i l was t h e same f o r t r e a t m e n t s 2 and 4, y e t t h e CEC i n t r e a t m e n t 4 i s s i g n i f i c a n t l y h i g h e r . The d i f f e r e n c e may be due t o o r g a n i c m a t t e r , but s i n c e o r g a n i c m a t t e r c o n t e n t was not measured i t i s d i f f i c u l t t o p r o p o s e any c o n c r e t e t h e o r i e s . P e r h a p s f l u c t u a t i o n s i n pH have c a u s e d i n c r e a s e d m i c r o b i a l a c t i v i t y r e s u l t i n g i n h i g h e r l e v e l s o f o r g a n i c m a t t e r . L i m i n g t h e L a n g l e y s o i l i n t r e a t m e n t 3 r e d u c e d t h e i n t e r s t r a t i f i e d m i n e r a l s w i t h t h e r e a p p e a r a n c e o f montmori1 I o n i t e . T h i s i m p l i e s t h e n e u t r a l i z a t i o n and p r e c i p i t a t i o n of A l compound from i n t e r l a y e r s p a c e s , t h u s f r e e i n g e x change s i t e s . However, t r e a t m e n t 4 has a s i g n i f i c a n t l y h i g h e r e x c h a n g e c a p a c i t y d e s p i t e t h e p r e s e n c e o f i n t e r s t r a t i f i e d m i n e r a l s and t h e a b s e n c e of montmori1 I o n i t e . pH d e p e n d e n t c h a r g e c o n t r i b u t e d by o r g a n i c m a t t e r may a c c o u n t f o r t h e d i f f e r e n c e . A c i d i f i c a t i o n o f t h e Lynden s o i l r e s u l t e d i n t h e d i s a p p e a r a n c e t o v e r m i c u l i t e and h y d r o u s m i c a ( T a b l e 5) w i t h a r e s u l t i n g s l i g h t d e c r e a s e i n CEC ( T a b l e 4 ) . A g a i n , t h e p r e s e n c e 67 of c h l o r i t e s u g g e s t s t h e f o r m a t i o n o f p o l y m e r i z e d A l compounds i n i n t e r l a y e r s p a c e s . T h i s i s i n agreement w i t h t h e r e s u l t s o f Schwertmann and J a c k s o n ( 1 9 6 4 ) . L i m i n g i n c r e a s e d t h e CEC o f t h e Lynden s o i l . T h i s i s p r o b a b l y due t o t h e n e u t r a l i z a t i o n o f A l o r t h e pH d e p e n d e n t c h a r g e c o n t r i b u t e d by o r g a n i c m a t t e r . T r e a t m e n t 4 o f t h e Lynden s o i l had t h e h i g h e s t CEC, but i t was not s i g n i f i c a n t l y d i f f e r e n t f r o m t r e a t m e n t 3. T a b l e 5 shows t h e p r e s e n c e o f v e r m i c u l i t e i n t r e a t m e n t 4, but not i n t r e a t m e n t 3. T r e a t m e n t 3 c o n t a i n s h y d r o u s m i c a w h i c h i s not p r e s e n t i n t r e a t m e n t 4. I t a p p e a r s from t h e s e r e s u l t s , t h a t l i m i n g and f l u c t u a t i o n s i n pH may have t h e same i n f l u e n c e on ex c h a n g e c a p a c i t y . T h i s may r e f l e c t t h e lower o r g a n i c m a t t e r o f t h i s s o i l , i m p l y i n g t h a t o n l y t h e m i n e r a l o g y i n f l u e n c e d t h e CEC. 68 SUMMARY AND CONCLUSIONS T h i s s t u d y was i n t e n d e d as an e x p l o r a t o r y s t u d y t o determine how f l u c t u a t i o n s i n pH ( i . e . a c i d t o a l k a l i n e ) would a f f e c t the a v a i l a b i l i t y of s e v e r a l p l a n t n u t r i e n t s and how any m i n e r a l o g i c a l changes i n f l u e n c e d n u t r i e n t a v a i l a b i l i t y . P o t a s s i u m - C l a y m i n e r a l o g y appears t o be the predominant f a c t o r i n f l u e n c i n g K a v a i l a b i l i t y . The presence of v e r m i c u l i t e i n the Lynden s o i l , and montmori1 I o n i t e and i n t e r s t r a t i f i e d s m e c t i t e s i n the L a n g l e y s o i l l a r g e l y d e t ermined K a v a i l a b i l i t y by the f i x a t i o n of K i n t o an u n a v a i l a b l e form. There appeared t o be a r e l a t i o n s h i p between pH and m i n e r a l o g y which was a d e t e r m i n i n g f a c t o r i n the f i x a t i o n of t h i s element. The f l u c t u a t i o n s i n pH seemed t o keep the K l e v e l s i n t e r m e d i a t e between th o s e of an a c i d or a l k a l i n e s o i l by i n f l u e n c i n g the c l a y m i n e r a l o g y . Magnesium - There was no s i g n i f i c a n t d i f f e r e n c e between the a c i d , l i m e , and a l t e r n a t i n g t r e a t m e n t s f o r s o i l Mg. F l u c t u a t i o n s i n pH r e s u l t e d i n a r e l e a s e of Mg i n t o s o l u t i o n w i t h a c i d i f i c a t i o n which was reduced w i t h l i m i n g . Copper - F l u c t u a t i o n s i n pH d i d not s i g n i f i c a n t l y i n c r e a s e s o i l Cu as compared t o the l i m e o n l y t r e a t m e n t . The c o n c e n t r a t i o n of Cu was h i g h e r i n the l i m e d and a l t e r n a t i n g 69 t r e a t m e n t s , t h a n t h e w a t e r and a c i d t r e a t e d s o i l s , as w e l l a s , t h e u n t r e a t e d sample f o r t h e L a n g l e y s o i l . The a l t e r n a t i n g t r e a t m e n t was s i g n i f i c a n t l y d i f f e r e n t f r o m o n l y t h e u n t r e a t e d sample of t h e Lynden s o i l . Manganese - L i m i n g and pH f l u c t u a t i o n s r e s u l t e d i n u n e x p e c t e d l y h i g h l e v e l s o f s o i l Mn, e s p e c i a l l y f o r t h e L a n g l e y s o i 1 . A c i d i f i c a t i o n r e s u l t e d i n i n c r e a s e d s o l u t i o n l e v e l s o f Mn w h i c h were r e d u c e d w i t h l i m i n g . The a l t e r n a t i n g t r e a t m e n t d i s p l a y e d t h i s r e l e a s e p a t t e r n t h r o u g h o u t t h e s t u d y . Z i n c - T h e r e was no s i g n i f i c a n t d i f f e r e n c e s between t r e a t m e n t s f o r t h e Lynden s o i l . F l u c t u a t i o n s i n pH r e s u l t e d i n t h e l o w e s t l e v e l s of a v a i l a b l e Zn i n t h e L a n g l e y s o i l , as compared w i t h , t h e a c i d i f i e d and l i m e d t r e a t m e n t s . T h i s d i f f e r e n c e was s i g n i f i c a n t . T h e r e a p p e a r e d t o be no p a t t e r n o f r e l e a s e o f Zn i n t o s o l u t i o n w i t h f l u c t u a t i o n s i n pH. I r o n - A c i d i f i c a t i o n i n c r e a s e d Fe s o l u b i l i t y w h i c h was t h e n d e c r e a s e d w i t h l i m i n g . F l u c t u a t i o n s i n pH r e s u l t e d i n s i g n i f i c a n t l y more a v a i l a b l e Fe t h a n was p r e s e n t i n a l i m e d s o i l , but l e s s t h a n an a c i d s o i l . 70 C a t i o n Exchange C a p a c i t y - F l u c t u a t i o n s i n pH r e s u l t e d i n a s i g n i f i c a n t l y h i g h e r CEC i n t h e L a n g l e y s o i l t h a n f o r any o t h e r t r e a t m e n t . The CEC o f t h e a l t e r n a t i n g t r e a t m e n t o f t h e Lynden s o i l was t h e h i g h e s t , but t h i s v a l u e was not s i g n i f i c a n t l y d i f f e r e n t f r o m t h e l i m e t r e a t m e n t . Changes i n e xchange c a p a c i t y a r e i m p o r t a n t s i n c e n u t r i e n t c a t i o n s can be a d s o r b e d by t h e e x c h a n g e complex and r e t a i n e d a g a i n s t l e a c h i n g . M i n e r a l o g y - F l u c t u a t i o n s i n pH p r o d u c e d m i n e r a l o g i c a l c h a n g e s s i m i l a r t o t h o s e p r o d u c e d by a c i d i f i c a t i o n . The r e s u l t s o f t h i s s t u d y have i m p l i c a t i o n s f o r t h e management of a g r i c u l t u r a l s o i l s . F l u c t u a t i o n s i n pH have e f f e c t e d s e v e r a l o f t h e c h e m i c a l and m i n e r a l o g i c a l p r o p e r t i e s o f t h e s o i l w h i c h i n t u r n i n f l u e n c e t h e b e h a v i o u r and s u b s e q u e n t a v a i l a b i l i t y o f t h e n u t r i e n t e l e m e n t s s t u d i e d . R e l e a s e i n t o s o l u t i o n - A l t h o u g h t h e r e may be no s i g n i f i c a n t d i f f e r e n c e s between t r e a t m e n t s f o r a v a i l a b l e l e v e l s o f a n u t r i e n t , t h e r e s t i l l may be i n c r e a s e s o r d e c r e a s e s i n s o l u t i o n l e v e l s w i t h a c i d i f i c a t i o n o f l i m i n g . The b e h a v i o u r o f e ach i n d i v i d u a l e l e m e n t w i t h pH c h a n g e s w i l l v a r y a c c o r d i n g t o t h e c h e m i s t r y o f t h e e l e m e n t i n v o l v e d . 71 - M i n e r a l o g y - T h e r e i s some i n d i c a t i o n t h a t f l u c t u a t i o n s i n pH may have a m o d e r a t i n g e f f e c t , as compared w i t h t h e p r o c e s s o f a c i d i f i c a t i o n o r t o c h a n g e s i n d u c e d by l i m i n g . T h i s may be o f p a r t i c u l a r i m p o r t a n c e i n d e t e r m i n i n g K a v a i l a b i l i t y i f a s o i l c o n t a i n s K - f i x i n g m i n e r a l s . Changes i n m i n e r a l o g y may a l s o r e s u l t i n c h a n g e s i n CEC. - O r g a n i c M a t t e r - Changes i n o r g a n i c m a t t e r c o u l d o n l y be i n f e r r e d by t h e b e h a v i o u r o f c e r t a i n m e t a l s i n t h e s t u d y e.g. A l . The a v a i l a b i l i t y o f t h e t r a c e m e t a l s Cu, Mn, Zn, and Fe may be i n f l u e n c e d by c h a n g e s i n o r g a n i c m a t t e r w i t h f l u c t u a t i o n s i n pH. I t i s e v i d e n t t h a t a c o m b i n a t i o n o f s e v e r a l c o m p l i c a t e d i n f l u e n c i n g f a c t o r s may i n f l u e n c e n u t r i e n t a v a i l a b i l i t y . C o n t i n u e d i n v e s t i g a t i o n i s r e q u i r e d t o f u r t h e r e l u c i d a t e t h e t r e n d s and mechanisms r e s p o n s i b l e f o r t h e a v a i l a b i l i t y o f t h e p l a n t n u t r i e n t s K, Mg, Cu, Mn, Zn and Fe. If a n o t h e r s t u d y , w i t h s i m i l a r o b j e c t i v e s i s t o be u n d e r t a k e n , t h e r e a r e s e v e r a l p o i n t s t o c o n s i d e r w h i c h may h e l p t o e l u c i d a t e t r e n d s and mechanisms: 1. M e a s u r e c h a n g e s i n o r g a n i c m a t t e r c o n t e n t o v e r t i m e . 2. Add d i f f e r i n g c o n c e n t r a t i o n s o f o r g a n i c m a t t e r t o t h e same s o i l t o m o n i t o r how o r g a n i c m a t t e r c o n c e n t r a t i o n 72 i n f l u e n c e s n u t r i e n t s h i f t s i n t o v a r i o u s p o o l s a c c o r d i n g to pH. Measure redox p o t e n t i a l to he lp e x p l a i n t r a c e metal b e h a v i o u r . 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B i o s t a t i s t i c a l P r e n t i c e - H a l l , I n c . , New J e r s e y . Ana 1ys i s. 718 p. Sec o n d E d i t i o n . 80 APPENDIX A P r o f i l e D e s c r i p t i o n f o r t h e L a n g l e y S o i l S e r i e s Humic L u v i c G l e y s o l d e v e l o p e d f r o m deep f i n e - t e x t u r e d , s t o n e -f r e e , m a r i n e s e d i m e n t s . Hor i z o n D e p th (cm) D e s c r i p t i o n Ap 0-22 (10 YR 2/1 m); S i C l ; m o d e r a t e , medium, g r a n u l a r ; f r i a b l e ; a b undant r o o t s ; a b r u p t b o u n d a r y . Ah 22-37 (10 YR 2/2 m); S i C l ; weak, medium, s u b a n g u l a r b l o c k y , weak f i n e g r a n u l a r ; f r i a b l e , a b undant r o o t s ; c l e a r b o u n d a r y . Aeg 37-47 BA 47-57 B t g 57-85 BC 85-100 (2.5 YR 4/2 m); S i C l ; weak t o mo d e r a t e , medium, s u b a n g u l a r b l o c k y ; f r i a b l e ; a b undant r o o t s ; few, f i n e , f a i n t ; c l e a r b o u n d a r y . (5.0 YR 5/2 m); s t r o n g , medium, medium, a n g u l a r p l e n t i f u l r o o t s S i C l ; m o d e r a t e t o p r i s m a t i c ; s t r o n g , b l o c k y ; v e r y f i r m ; common, medium, p r o m i n e n t (10 YR 5 / 8 ) ; few, t h i n , i n many v o i d s , c h a n n e l s and on some v e r t i c a l and h o r i z o n t a l ped f a c e s ; g r a d u a l b o u n d a r y . (5.0 YR 4.5/1 m); C I ; s t r o n g , medium, p r i s m a t i c ; s t r o n g , medium, a n g u l a r b l o c k y ; v e r y f i r m ; few r o o t s ; common, f i n e , p r o m i n e n t (10 YR 5 / 7 ) ; common, t h i n , i n many v o i d s , c h a n n e l s and on some v e r t i c a l and h o r i z o n t a l ped f a c e s ; g r a d u a l b o u n d a r y . (5.0 YR 4/1 m); C I ; m o d e r a t e , medium, p r i s m a t i c ; m o d e r a t e , medium, a n g u l a r b l o c k y ; v e r y f i r m ; common, f i n e , d i s t i n c t (10.0 YR 4.5/5); common, t h i n , on ped f a c e s -u n s p e c i f i e d ; g r a d u a l b o u n d a r y . Cg, 100-127 (5.0 Y 4.5/1 m); C I ; s t r o n g , medium t o c o a r s e , a n g u l a r b l o c k y , pseudo; v e r y f i r m , common, f i n e , p r o m i n e n t 81 (7.5 YR 4.5/5); d i f f u s e b o u n d a r y . Cg 2 127+ (5.0 Y 5/1 m); C I ; s t r o n g , medium t o c o a r s e , a n g u l a r b l o c k y , pseudo; v e r y f i r m ; common, f i n e p r o m i n e n t (7.5 YR 4.5/5.0). P r o f i l e D e s c r i p t i o n f o r t h e Lynden S o i l S e r i e s O r t h i c H u m o - F e r r i c P o d z o l d e v e l o p e d f r o m deep, c o a r s e - t e x t u r e d ( s a n d y ) , m a i n l y g l a c i o f l u v i a l and some l i t t o r a l d e p o s i t s . H o r i z o n Depth D e s c r i p t i o n (cm) L 4-0 O r g a n i c . Ae 0-1 (10.0 YR 5/1 m); LS; s i n g l e g r a i n ; l o o s e ; a b r u p t b o u n d a r y . Bf, 1-22 (7.5 YR 3/2 m); LS; v e r y weak, f i n e t o medium, g r a n u l a r ; v e r y f r i a b l e , s o f t ; p l e n t i f u l r o o t s ; g r a d u a l b o u n d a r y . B f 2 22-44 (10.0 YR 4/3 m); LS; s i n g l e g r a i n ; l o o s e ; p l e n t i f u l r o o t s . CI 44-87 C o a r s e s a n d ; s i n g l e g r a i n ; s l i g h t l y h a r d , w e a k l y cemented, d i s c o n t i nuous. C2 87+ C o a r s e s a n d ; s i n g l e g r a i n ; l o o s e . 82 APPENDIX B I n c u b a t i o n S e t - u p 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 o f t h e a i r - d r i e d s a m p l e s were c a r r i e d o u t by t h e method o f G a r d n e r ( 1 9 6 5 ) . The m o i s t u r e c o n t e n t a t f i e l d c a p a c i t y was d e t e r m i n e d by t h e method o f R i c h a r d s ( 1 9 6 7 ) . From t h i s d a t a t h e w e i g h t o f w a t e r r e q u i r e d t o m a i n t a i n t h e m o i s t u r e c o n t e n t e q u i v a l e n t t o f i e l d c a p a c i t y f o r 50 g o f s o i l was c a l c u l a t e d . A t i t r a t i o n c u r v e was us e d t o d e t e r m i n e t h e amount o f a c i d r e q u i r e d t o lower t h e pH o f b o t h s o i l s t o a p p r o x i m a t e l y 3.5 (See t h e p r o c e d u r e a t t h e end o f t h i s s e c t i o n ) . From t h e s e c u r v e s i t was d e c i d e d t o use 2.5 me o f HC1. A l i m e r e q u i r e m e n t d e t e r m i n a t i o n was done t o d e t e r m i n e t h e amount o f CaC0 3 r e q u i r e d t o r a i s e t h e pH o f t h e s o i l s t o 6.5, by t h e method o f L e w i s and Hardy ( ). From t h i s d e t e r m i n a t i o n i t was d e c i d e d t o use 40 me o f CaC0 3. The i n c u b a t i o n c o n s i s t e d o f 4 t r e a t m e n t s w i t h 12 r e p l i c a t e s . T h e r e were 3 c o n t r o l t r e a t m e n t s : - T r e a t m e n t 1 - D i s t i l l e d w a t e r ; - T r e a t m e n t 2 - 2.5 me HC1; - T r e a t m e n t 3 - 40 me CaC0 3. The f o u r t h t r e a t m e n t c o n s i s t e d o f a l t e r n a t i n g a p p l i c a t i o n s o f HC1 and CaC0 3. The i n c u b a t i o n was s e t up as f o l l o w s : 1. 50 g o f s o i l was w e i g h e d i n t o t a r e d 250 ml a c i d washed p l a s t i c b o t t l e s and t h e w e i g h t o f e a c h r e c o r d e d . 2. The amendment was added t o t h e s a m p l e s i . e . a c i d i f i e d o r 1imed. 3. Enough d i s t i l l e d w a t e r was added t o form a s a t u r a t e d p a s t e . T h i s m o i s t u r e c o n t e n t was c h o s e n t o e n s u r e a d e q u a t e m i x i n g o f t h e sample and amendment. T h i s was e s p e c i a l l y i m p o r t a n t w i t h t h e Lynden s o i l s i n c e i t ' s m o i s t u r e c o n t e n t a t f i e l d c a p a c i t y was o n l y 9 g o f w a t e r per 50 g o f o v e n - d r y s o i l . 4. The w e i g h t of e a c h sample was a g a i n r e c o r d e d . 5. The s a m p l e s were i n c u b a t e d a t room t e m p e r a t u r e , i n a c o n s t a n t t e m p e r a t u r e room f o r 3 weeks. D u r i n g t h e i n c u b a t i o n : i . The l i d s o f t h e p l a s t i c b o t t l e s were not s c r e w e d on, b ut were o n l y p l a c e d g e n t l y on t o p o f t h e b o t t l e s t o p r e v e n t c o n t a m i n a t i o n by a i r b o r n e d u s t p a r t i c l e s w h i l e s t i l l a l l o w i n g a e r a t i o n . i i . The s a m p l e s were a l l o w e d t o d r y t o f i e l d c a p a c i t y . Samples were t h e n m a i n t a i n e d a t t h a t m o i s t u r e c o n t e n t . 83 6. At t h e end o f t h e 3 week i n c u b a t i o n p e r i o d , d i s t i l l e d w a t e r was added u n t i l a 1:2 s o i l t o s o l u t i o n r a t i o (by w e i g h t ) was r e a c h e d . The s a m p l e s were s h a k e n on a r e c i p r o c a l s h a k e r f o r 16 h o u r s a t room t e m p e r a t u r e and t h e n c e n t r i f u g e d a t 2,400 rpm f o r 20 m i n u t e s . The s u p e r n a t a n t was d e c a n t e d o f f , f i l t e r e d t h r o u g h Whatman #42 f i l t e r p a p e r and s a v e d f o r a n a l y s i s . 7. S t e p 6 was t h e n r e p e a t e d a g a i n k e e p i n g t h e s e c o n d e l u t i o n s e p a r a t e f r o m t h e f i r s t . 8. S t e p s 2 t h r o u g h 7 were r e p e a t e d 5 more t i m e s t o g i v e a t o t a l o f 6 i n c u b a t i o n c y c l e s . 9. At t h e c o m p l e t i o n o f t h e i n c u b a t i o n s , t h e s o i l s were a i r -d r i e d and c r u s h e d t o p a s s a 2 mm s t a i n l e s s s t e e l s i e v e . 84 APPENDIX C S o i l A c i d T i t r a t i o n P a r t A 1. 100 ml of d i s t i l l e d w a t e r was added t o 5 g of a i r d r y s o i l . T h i s was a l l o w e d t o s t a n d f o r 30 m i n u t e s . 2. The s o i l - w a t e r m i x t u r e s were t i t r a t e d w i t h 6 N HC1, m e a s u r i n g t h e pH w i t h e a c h a l i q u o t o f a c i d added. The t i t r a t i o n was c a r r i e d o u t t o a p p r o x i m a t e l y pH 2.5. 3. A rough b u f f e r c u r v e was p l o t t e d . From t h i s c u r v e a p p r o x i m a t e volumes t o a c i d t o be added i n i n c r e m e n t s were c a l c u l a t e d i . e . t h e n e x t s t e p i n v o l v e d e q u i l i b r a t i n g a l i q u o t s o f a c i d w i t h t h e s o i l sample. The above p r o c e d u r e was done i n d u p l i c a t e . P a r t B 1. 5 g o f a i r d r y s o i l was w e i g h e d i n t o 12 250 ml a c i d washed f1 a s k s . 2. P r o g r e s s i v e l y l a r g e r a l i q u o t s o f 6 N HC1, as c a l c u l a t e d above, was added t o each f l a s k . Enough d i s t i l l e d w a t e r was added t o b r i n g t h e t o t a l volume of l i q u i d added t o 100 ml. To one f l a s k o n l y 100 ml o f w a t e r was added. 3. The f l a s k s were s t o p p e r e d w i t h r u b b e r s t o p p e r s and s h a k e n f o r 15 hours*. 4. A f t e r t h e s h a k i n g , C0 2 was b u b b l e d t h r o u g h t h e s u s p e n s i o n f o r 3 m i n u t e s . A f t e r t h i s , a i r was b u b b l e d t h r o u g h t h e s u s p e n s i o n f o r 30 m i n u t e s . 5. The s u s p e n s i o n was a l l o w e d t o s e t t l e f o r 1 hour and t h e n t h e pH was r e a d . * Dunn ( 1 9 4 3 ) , showed t h a t e q u i l i b r i u m was r e a c h e d i n f o u r d a y s w i t h o u t s h a k i n g , but i n 8 h o u r s w i t h s h a k i n g . Wiens ( 1 9 7 0 ) , u s e d a 15 hour s h a k i n g p e r i o d and f o r e a s e o f a n a l y s i s t h a t t i m e was u s e d i n t h i s d e t e r m i n a t i o n . 

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