PlAP OF SAL?Tk' SOILS OF AFRICA

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G . Aubert

INTRODUCTION

The map of s a l t y s o i l s of A f r i c a w a s p r e p a r e d as p a r t of a j o i n t p r o j e c t of t h e Subcommission on S a l t - A f f e c t e d S o i l s of t h e 5 t h Commission (morphology, g e n e s i s , and c l a s s i f i c a t i o n of s o i l s ) of t h e I n t e r n a t i o n a l S o c i e t y of s o i l Science and t h e D i v i s i o n of N a t u r a l R e s o u r c e s of t h e Department of S c i e n c e s of UNESCO t o p r e p a r e s a l i n e s o i l s maps of t h e c o n t i n e n t s .

A s p a r t of t h a t p r o j e c t , two r e p o r t s have been p u b l i s h e d : A u s t r a l i a n - - S o i l s w i t h Saline and Sodic P r o p e r t i e s by K. H. N o r t h c o t e and J. K. M. Skene (1972) w i t h a map a t t h e scale of 1 t o 5 m i l l i o n , and S a l t A f f e c t e d S o i l s i n Europe by I. Szabolcs (1974) w i t h two maps, one a t a s c a l e of 1 t o - 5 m i l l i o n f o r t h e . e n t i r e c o n t i n e n t and t h e o t h e r a t a s c a l e of 1 t o 500;OOO f o r Hungary. The d r a f t map p r e s e n t e d a t t h e I n t e r n a t i o n a l S a l i n i t y Conference w i l l be r e v i s e d and p u b l i s h e d a f t e r a meeting of A f r i c a n s o i l s s c i e n t i s t s i n 1977. The scale w i l l b e 1 t o 5 m i l l i o n , t h e same as t h e d r a f t map. I n c o n f o r m i t y w i t h United N a t i o n s d i r e c t i v e s , Libya, Egypt, and t h e Sudan are i n c l u d e d i n t h e Near and Middle E a s t map p r e p a r e d by M. M. E l g a b a l y r a t h e r t h a n i n the . map of A f r i c a . PROCEDURE

We have u t i l i z e d t h e two 1 t o 5 m i l l i o n s o i l s maps of A f r i c a p r e p a r e d by J. d'Hoore (1964) and FAO/UNESCO (19751, as w e l l as numerous maps p u b l i s h ed a t t h a t scale o r a t a l a r g e r S c a l e f o r v a r i o u s A f r i c a n c o u n t r i e s . Addit i o n a l l y , several r e p o r t s and documents o f t h e United Kingdom Land Resources D i v i s i o n and O.R.S.T.O.M. have been u s e d . The a s s i s t a n c e provided by S. A. Radwanski and G. Murdoch of t h e Land Resources D i v i s i o n and by J. Boyer and A. Mori of O.R.S.T.O.M. h a s been p a r t i c u l a r l y h e l p f u l . F i r s t d r a z t s of t h e A f r i c a map w e r e p r e s e n t e d a t Subcommission m e e t i n g s i n Yerevan i n 1969 a n d - C a i r o i n 1972. A map of t h e savannah r e g i o n w a s p r e p a r e d f o r t h e m e e t i n g of t h e 4 t h , 5 t h , and 6 t h Commissions of t h e I n t e r n a t i o n a l S o c i e t y of S o i l Science at Tema-Accra i n 1975. S i n c e t h e b e g i n n i n g of 1976, d r a f t maps have been s e n t to t h e c h i e f s of s o i l services i n several c o u n t r i e s of -Africa f o r v e r i f i c a t i o n of t h e a c c u r a c y of t h e d e l i n e a t i o n s . Changes recomended by t h e reviewers have been i n c o r p o r a t e d i n t o t h e p r e s e n t map, which r e p r e s e n t s t h e combined e f f o r t s of *numerous colleagues.

1/ O f f i c e de l a Recherche S c i e n t i f i q u e e t Technique Outre-Mer, Bondy, France.

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26 NOV. 1383 O.R.S. T.Q,PB, Fonds ~ ~ ~ ~ ~ ~

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CARTOGRAPHIC UNITS Cartographic u n i t s used on t h e map have been d e f i n e d a t p r e v i o u s m e e t i n g s of t h e Subcommission on S a l t Affected S o i l s . The u n i t s are:

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S a l i n e and A l k a l i S o i l s

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S a l i n e s o i l s and solonchaks A l k a l i s o i l s , i n c l u d i n g s o l o n e t z and s o l o d i z e d s o i l s

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Other C a r t o g r a p h i c C a t e g o r i e s Shown

1. P e r c e n t d i s t r i b u t i o n of s a l t a f f e c t e d s o i l s w i t h i n delineations 2. S a l t a f f e c t e d s o i l s of . l u n e t t e s 3 . S a l t a f f e c t e d mangrove s o i l s 4-. P o t e n t i a l l y s a l t a f f e c t e d s o i l s

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Associated w i t h p r e s e n t l y s a l t a f f e c t e d s o i l s Not a s s o c i a t e d w i t h p r e s e n t l y s a l t a f f e c t e d s o i l s O R I G I N OF SALT AFFECTED SOILS OF AFRICA

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S o i l s a f f e c t e d by s o l u b l e s a l t s o r by t h e i r i o n s occupy immense areas i n A f r i c a : more t h a n 650,000 Kru2 according t o t h e FAO/UNESCO map, and much more a c c o r d i n g t o t h e p r e s e n t map. The d i f f e r e n c e s are due t o t h e d i f ferences i n definitions. S a l t a f f e c t e d s o i l s are e s p e c i a l l y p r e v a l e n t i n n o r t h e r n Africa, South Africa, and Botswana, and are common i n t h e a r i d or s e m i a r i d zones of t r o p i c a l Africa ( S e n e g a l , M a u r i t a n i a , Upper V o l t a , Chad, n o r t h e r n Cameron, Swaziland, M a l a w i , Kenya, Zambia, Rhodesia, s o u t h e r n Angola, and s o u t h e r n Mozambique). Only Zaire, Congo, t h e C e n t r a l Afr i c a n Republic, Gabon, I v o r y Coast, and L i b e r i a a r e v i r t u a l l y f r e e of s u c h soils. The a f f e c t e d s o i l s are v e r y d i v e r s e i n c h a r a c t e r i s t i c s and i n o r i g i n . The s a l t s r e s p o n s i b l e f o r the formation of s a l t a f f e c t e d s o i l s , as i s p e r h a p s u s u a l i n a l a r g e c o n t i n e n t , are of marine o r c o n t i n e n t a l o r i g i n o r are c a r r i e d t o t h e - s u r f a c e i n a r t e s i a n waters. . S a l t A f f e c t e d S o i l s Caused by S a l t s of Marine O r i g i n I

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1. E x i s t i n g S e a s It i s p o s s i b l e t h a t c e r t a i n zones of s a l i n e s o i l s of Africa owe t h e i r e x i s t e n c e t o t i d a l w a t e r s . TO our knowledge, t h e y have n o t however been d e s c r i b e d o r mapped. I f t h e y e x i s t , t h e y c e r t a i n l y are of l i t t l e e x t e n t . On t h e o t h e r hand, i n numerous r e g i o n s w i t h a l t e r n a t i n g w e t and d r y p e r i o d s , t h e sea e x t e n d s f a r i n l a n d - sometinles more t h a n 100 km i n t h e e s t u a r i e s and d e l t a s and t h e i r t r i b u t a r i e s . I t s e f f e c t i s t o c o v e r t h e l a n d w i t h s a l t y w a t e r , which p e r s i s t s u n t i l f l o o d s d u r i n g the r a i n y s e a s o n push some o f . t h e s a l t y w a t e r back toward t h e sea. The s a l t s g r a d u a l l y move through t h e s o i l s and e n t e r t h e ground w a t e r system. One of the t y p i c a l examples of t h i s i s t h e pseudo-delta of Senegal. Others c a n be found i n Nadagascar, Tanzania, and Southwest A f r i c a .

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a l a r g e a r e a , sea water i s i n c o n t a c t more o r less d i r e c t l y w i t h t h e ground w a t e r of t h e l i t o r a l zone. I n many a r i d and s e m i - a r i d c o a s t a l r e g i o n s , and sometimes i n humid areas, t h e p r o c e s s of evapo r a t i o n b r i n g s s a l i n e ground water t o t h e s u r f a c e by c a p i l l a r y movement from s h a l l o w w a t e r t a b l e s , l e a d i n g t o s t r o n g s o i l s a l i n i z a t i o n ( A l g e r i a , Morocco, S e n e g a l , S i e r r a Leone, Togo, Ghana, and s o u t h e r n Madagascar). I n humid t r o p i c a l r e g i o n s , t h e c o a s t a l s a l i n e s o i l s of e s t u a r i e s and d e l t a s which a r e r i c h i n o r g a n i c m a t t e r a c q u i r e s p e c i a l c h a r a c t e r i s t i c s where t h e =angroves are a s s o c i a t e d w i t h Avicennia o r Rhizophora v e g e t a t i o n . Where such s o i l s are d r a i n e d , t h e y become extremely a c i d i c . They a r e c l a s s i f i e d as T h i o n i c F l u v i s o l s and have been s t u d i e d i n S i e r r a Leone, s o u t h e r n S e n e g a l , t h e Gambia, Guinea B i s s a n , Guinea, L i b e r i a , N i g e r i a , Cameron, Gabon, Kenya, T a n z a n i a , Mozambique, and i n ' w e s t e r n Madagascar. S a l i n e s o i l s owing t h e i r o r i g i n t o present-day marine i n f l u e n c e s u s u a l l y are v e r y s a l i n e (solonchaks) and have d e c r e a s i n g s a l i n i t y w i t h depth. They-may show a weakly developed' s a l t c r u s t o r a hor i z o n of pseudo-sand ( v e r y f i n e a g g r e g a t e of s a l i n e - a l k a l i c l a y ) on t h e s u r f a c e . I n more humid r e g i o n s , s a l i n i t y may i n c r e a s e w i t h d e p t h , b u t t h i s i s r e l a t i v e l y rare.

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E x t i n c t Seas

Soils a f f e c t e d by s a l t s d e p o s i t e d from former seas d u r i n g t h e Quaternary p e r i o d are widespread i n Africa- S a l i n e p a r e n t m a t e r i a l h a s given rise t o s a l i n e s o i l s wherever t h e c l i m a t e i s f a v o r a b l e f o r s a l t accumulation. T h i s i s the. c a s e i n t h e lower v a l l e y of t h e Chelif River i n A l g e r i a , i n t h e pseudo-delta and t h e v a l l e y of t h e Senegal River, i n Southwest A f r i c a , and i n numerous o t h e r countries. Moreover, some g u l f s have become f i l l e d , l i t t l e by l i t t l e o v e r t h e l a s t 3,a03 to 4 , 0 0 0 , b y the a l l u v i a l and c o l l u v i a l d e p o s i t s . The sea w a t e r t h u s t r a p p e d g i v e s r i s e t o s a l i n e lakes a n d , ' f i n a l l y , t o s a l i n e ground w a t e r which o f t e n i s c l o s e t o t h e s o i l s u r f a c e , s u c h as i n t h e lower v a l l e y of t h e Medjerda River i n T u n i s i a . ' A s b e f o r e , the dominant i o n s are c h l o r i d e , s u l f a t e , and sodiÚm.-

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Transported Marine S a l t s Transported m a r i n e s a l t s c o n s t i t u t e a n o t h e r s o u r c e of soil s a l i n i t y i n c e r t a i n p a r t s of A f r i c a . I n s e v e r a l cases, t h e p r i n c i p a l s a l t d e p o s i t e d as t h e r e s u l t of l e a c h i n g of a n c i e n t marine r o c k s i s gypsum; o t h e r s i n c l u d e sodium and magnesium s u l f a t e and c h l o r i d e s . Rocks of T r i a s s i c and Miocene age i n n o r t h e r n A f r i c a and T r i a s s i c or Permian i n Rhodesia and South Africa a r e i m p o r t a n t s o u r c e s of s o l u b l e s a l t s and gypsum.

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While a n c i e n t r o c k s may b e cov r e d w i t h s a l i n e s o i l s , t h e i r more s i g n i f i c a n t e f f e c t on s a l i n i t y i s f e l t i n d e p r e s s i o n s o r i n l o w e r p a r t s of v a l l e y systems i n t o which r u n o f f w a t e r d r a i n s . S a l t s brought t o t h e s u r f a c e by e v a p o r a t i o n d u r i n g d r y p e r i o d s are dep o s i t e d and accumulate a t lower l e v e l s when f l o o d s sweep a c r o s s t h e drainageways.

U t i l i z a t i o n of s a l i n e ground w a t e r from t h e s e g e o l o g i c m a t e r i a l s f o r i r r i g a t i o n i s a n o t h e r s o u r e e of s a l t a f f s c t e d s o i l s when d r a i n a g e s s p o o r and t h e c l i m a t e i s a r i d . Even when t h e r a i n f a l l i s as much as 1,000 mm, low l y i n g impermeable s o i l s may b e s a l i n e i f a n n u a l e v a p o r a t i o n i s h i g h , a s i t commonly i s i n t r o p i c a l A f r i c a . T r a n s p o r t e d marine s a l t s a r e o f t e n t h e s o u r c e of s o i l s a l t s i n Morocco , A l g e r i a , T u n i s i a , n o r t h e r n N i g e r i a , Swaziland , Bo'tswana, and South Africa.

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Accumulation of s o l u b l e s a l t s c a n b e t h e r e s u l t of a p e d o g e n e t i c p r o c e s s i n v o l v i n g seepage of r a i n w a t e r . S a l t s h a v e been shown t o accumulate b e n e a t h calcrete h o r i z o n s i n Morocco and A l g e r i a . Breaki n g up o r removal of calcrete t o p e r m i t c u l t i v a t i o n of l a n d r i s k s t h e p o s s i b i l i t y t h a t t h e u n d e r l y i n g salts w i l l t h e n rise t o t h e . s o i l surface. Commonly, t h e s o i l s a f f e c t e d by t r a n s p o r t e d m a r i n e s a l t s are solonchaks w i t h a c a t i o n exchange complex dominated by calcium and magnesium o r by sodium. T h e r e sometimes i s a more o r less t h i c k s a l t c r u s t o r a pseudo-sand h o r i z o n of f i v e t o s i x c e n t i meters i n t h i c k n e s s on t h e s u r f a c e . That i s t h e case i n A l g e r i a , Southwest A f r i c a , and South A f r i c a . A l k a l i s o i l s h a v i n g a similar o r i g i n o c c u r i n Morocco, A l g e r i a , and nor;hern N i g e r i a . S o l o n e t z s o i l s have been noted i n Botswana where r a i n f a l l i s g r e a t e r . The s o i l s a l t s g e n e r a l l y are l a r g e l y c h l o r i d e s b u t s u l f a t e s o f t e n are Öf g r e a t l o c a l importance, as i n t h e f o r m a t i o n of g y p s i f e r o u s s o i l s . The dominant c a t i o n n o r m a l l y i s sodium b u t i t may b e t h e calcium-magnesium combination. Hydroscopic s o i l s composed p r i n c i p a l l y of c a l c i u m and magnesium c h l o r i d e s have b e e n observed i n t h e Hauts P l a t e a u x of A l g e r i a , w h e r e t h e y a r e weakly a l k a l i n e , and i n t h e S a b i V a l l e y of Rhodesia, where t h e y are a c i d i c . I n zones where t h e s a l t a f f e c t e d s o i l s have a m a r i n e s a l t o r i g i n , t h e i r e x t e n s i o n , whether n a t u r a l o r due t o i r r i g a t i o n o r o t h e r management p r a c t i c e s , i s growing. The s p r e a d of s a l t a f f e c t e d s o i l s i s g r a d u a l l y outward froxi a s o u r c e o r i s of a s p o t l y n a t u r e . S a l t Affected S o i l s Caused by S a l t s of C o n t i n e n t a l O r i g i n Weathering of r o c k s c o n t a i n i n g sodium m i n e r a l s ( f e l d s p a r s , amphiboles, e t c . ) produces s o l u b l e sodium s a l t s , p r i n c i p a l l y c a r b o n a t e s and b i c a r b o n a t e s , o f t e n s u l f a t e s , sometimes s i l i c a t e s , and r a r e l y c h l o r i d e s . Such r o c k s a r e numerous and o f t e n of g r e a t e x t e n t i n A f r i c a . Under c e r t a i n c o n d i t i o n s

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of c l i m a t e and t o p o g r a p h y , t h e i r weathering p r o d u c t s may accumulate and l e a d t o t h e f o r m a t i o n of s a l i n e o r s o d i c s o i l s .

I n a r i d r e g i o n s , c l o s e d b a s i n s such a s Lake Chad a r e p r a c t i c a l l y a l w a y s s a l i n e due t o t h e s a l t s b r o u g h t i n by streams d r a i n i n g i n t o the b a s i n s . The p r i n c i p a l s a l t a f f e c t e d s o i l s a r e s o l o n c h a k s , f r e q u e n t l y w i t h s e l t c r u s t s on t h e s u r f a c e o r s t r o n g l y s a l i n e h o r i z o n s a t s h a l l o w d e p t h s , as i n Chad, Kenya, Botswana, and Mali, among o t h e r s . Many of t h e s a l t c r u s t s c o n t a i n i n g c o n c e n t r a t e d sodium c a r b o n a t e o r sodium c h l o r i d e are e x p l o i t e d commercially. I n marshy areas where the s o i l s a r e r i c h i n o r g a n i c m a t t e r , as i n c e r t a i n branches of Lake Chad, a v a r i e t y of s a l t a f f e c t e d s o i l s may be formed (Cheverry, 1974).

In subhumid r e g i o n s and i n c e r t a i n s e m i a r i d r e g i o n s where e x c e s s water accumulates, -the s i t u a t i o n i s d i f f e r e n t . Here, t h e d i s s o l v e d sodium accumulates as exchangeable sodium í n t h e B h o r i z o n , due t o v e r k i c a l or h o r i z o n t a l l e a c h i n g . The s o i l s which r e s u l t are s o l o n e t z , s o l o d i z e d s o i l s , o r s o l o d i z e d p l a n o s o l s . Formation of t h e s e s o l o d i z e d s o i l s i s f a v o r e d by hydromorphic c o n d i t i o n s and h i g h s o i l t e m p e r a t u r e . They are found i n Upper V o l t a , n o r t h e r n Togo, n o r t h e r n N i g e r i a , n o r t h e r n Cameroon, Chad, Malawi, Botswana, Rhodesia, Swaziland, L e s o t h o , and o t h e r t r o p i c a l c o u n t r i e s . S o l o n e t z and s o l o d i z e d s o i l s are very s u s c e p t i b l e t o w a t e r e r o s i o n . They are g e n g r a l l y a s s o c i a t e d w i t h h i g h e r l y i n g Leached Tropi c a l F e r r u g i n o u s S o i l s (Chromic o r F e r r a l i t i c L u v i s o l s ) , w h i c h o f t e n are hydromorphic, and lower l y i n g v e r t i s o l s (Bocquier, 1971). .

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Numerous v e r t i s o l s h a v e a B h o r i z o n w i t h a v e r y h i g h exchangeable sodium p e r c e n t a g e and p o s s e s s t h e p h y s i c a l p r o p e r t i e s of a l k a l i s o i l s . They o c c u r i n T u n i s i a , Kenya, Tanzania, Uganda, Zambia, L e s o t h o , Swaziland, and South Africa and have been c l a s s i f i e d as a l k a l i soils. S a l t Affected S o i l s Caused by S a l i n e A r t e s i a n Waters S a l t s i n a r t e s i a n w a t e r s o r i g i n a t e e i t h e r from s a l i n e s e d i m e n t s o r by a l t e r a t i o n of n a t i v e r o c k s . Most of t h e waters are m o d e r a t e l y o r s l i g h t l y s a l i n e ; t h e y become c o n c e n t r a t e d by e v a p o r a t i o n i n s o i l s t r a t a a t or n e a r t h e s u r f a c e . The c h o t t s of A l g e r i a and T u n i s i a (Belkhodja, 1972). are examples of vast areas where s a l t c r u s t s have formed as the r e s u l t of a r t e s i a n waters r i s i n g tc t h e s u r f a c e . S a l t a f f e c t e d s o i l s of Africa are q u i t e v a r i a b l e i n t h e i r c h a r a c t e r i s Some of them are u n l i k e l y t o increase i n area, oche= t i c s and o r i g i n . are l i k e l y t o d o s o . S a l i n i z a t i o n i s a t h r e a t i n many p l a c e s where sal i n i t y i s n o t p r e s e n t l y a problem, which makes i t i m p o r t a n t t o s t u d y t h e p r o c e s s of s a l i n i z a t i o n and t o maintain c o n t i n u e d s u r v e i l l a n c e of s u s c e p t i b l e areas. The FAO/United Nations Environment Program on s o i l degradat i o n and d e s e r t i f i c a t i o n , w i t h . t h e c o l l a b o r a t i o n of Unesco and o t h e r n2t i o n a l and i n t e r n a t i o n a l o r g a n i z a t i o n s , can b e t h e s o u r c e of v e r y v a l u a b l e i n f o r m a t i o n on s o i l s and l a n d management throughout t h e world. It n e e d s cont inued s u p p o r t .

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A d r a f t map of t h e d i s t r i b u t i o n of s a l t - a f f e c t e d s o i l s of A f r i c a has b e e n prepared on b e h a l f of the subcommission on S a l t - A f f e c t e d S o i l s o f t h e Causes of s a l t ISSS. A f i n a l d r a f t i s t o be prepared i n t h e near f u t u r e . accumulation i n A f r i c a n s o i l s were d i s c u s s e d and a b i b l i o g r a p h y o f p e r t i nent references w a s presented.

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Maignien, R. 1965. C a r t e d e s s o l s du Niger (l/l,ooo,ooo), Cartes e t Notices O.R.S.T.O.M., Bondy, F r a n c e , no 24. M i t c h e l l , A. 1967. A r e c o n n a i s s a n c e a s s e s s m e n t o f t h e i r r i g a t i o n p o t e n t i a l of s o i l s along t h e main r i v e r s o f E a s t e r n Bechuanaland. D i v i s i o n of Overseas Surveys, Tolworth, S u r r e y , England. Murdoch, G . 1970. D i s t r i b u t i o n of s a l i n e and a l k a l i s o i l s i n some A f r i c a c o u n t r i e s . 2nd Approximation. M.R. 80, D i v i s i o n of Overseas S u r v e y s , Tolworth, S u r r e y , England. N o r t h c o t e , K.H., and J.K.M. Skene. 1972. A u s t r a l i a n s o i l s w i t h s a l i n e and s o d i c p r o p e r t i e s . C . S . I . R . O . S . , Melbourne, A u s t r a l i a , Soil P u b l i c a t i o n No. 27, 62 p. Carte pkdologique d e s s o l s du Benin (1/200,000), O.R.S.T.O.M. 1976. O.R.S.T.O.M., Bondy, F r a n c e .

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