A study of the environment and characteristics of tropical podzols in sarawak (east-Malaysia)

A study of the environment and characteristics of tropical podzols in sarawak (east-Malaysia)

Geoderma - Elsevier Publishing Company, Amsterdam Printed in The Netherlands A STUDY OF THE ENVIRONMENT AND CHARACTERISTICS OF TROPICAL PODZOLS IN SA...

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Geoderma - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

A STUDY OF THE ENVIRONMENT AND CHARACTERISTICS OF TROPICAL PODZOLS IN SARAWAK (EAST-MALAYSIA)

J.P. ANDRIESSE Soils Division, Department of Agriculture, Sarawak (East-Malaysia} (Received August 5, 1968) (Resubmitted November 6, 1968) SUMMARY A study of the e n v i r o n m e n t , p r o f i l e s and a n a l y t i c a l data of humus podzols o c c u r r i n g in the humid t r o p i c a l lowlands of S a r a w a k c o n f i r m s that the s o i l s a r e m o r p h o l o g i c a l l y s i m i l a r to those developing in t e m p e r a t e r e g i o n s . However, c a u s e s for the d e v e l o p m e n t a r e not i d e n t i c a l . T h e s e a r e for S a r a w a k r e l a t e d m a i n l y to kind of p a r e n t m a t e r i a l and topography. O r ganic s u r f a c e horizons develop b e c a u s e of the low d e c o m p o s i t i o n r a t e due to wet, p o o r l y d r a i n e d conditions c o m b i n e d with the o c c u r r e n c e of acid, highly lignitic l i t t e r , poor in b a s e s . The p o d z o l i z a t i o n p r o c e s s is s i m i l a r to that found in t e m p e r a t e r e g i o n s but does not need to include an " i n i t i a t i o n " p r o c e s s . The position of i l l u v i a l humus h o r i z o n s is in many c a s e s r e l a t e d to the b i s e q u e n t nature of the p a r e n t m a t e r i a l s and, in the a b s e n c e of this, to the ground w a t e r level, while the c a u s e s for the a c c u m u l a t i o n of humus in t h e s e h o r i z o n s a r e : lack of l a t e r a l flow of ground w a t e r to d r a i n off w a t e r r i c h in humic m a t e r i a l s and p e r i o d i c a l l y d r y i n g out of s u r f a c e h o r i z o n s m a i n l y through e v a p o r a t i o n .

INTRODUCTION T h a t podzols occur at low a l t i t u d e s in the t r o p i c s is a w e l l e s t a b l i s h e d fact. F o l l o w i n g d i s c o v e r i e s in the I n d o n e s i a n A r c h i p e l a g o by Hardon (1937) and R i c h a r d s (1941) many such o c c u r r e n c e s have been r e p o r t e d in the l a s t twenty y e a r s . In a r e c e n t p a p e r Klinge (1965) s u m m a r i z e s much of the p r e s ent knowledge on t h e s e s o i l s but it a p p e a r s that although much i n f o r m a t i o n has been a c c u m u l a t e d on the o c c u r r e n c e s of podzols in t r o p i c a l lowlands not many d e t a i l e d s t u d i e s have been c a r r i e d out. T h o s e p u b l i s h e d a r e m a i n l y s t u d i e s of s i n g l e p r o f i l e s or o c c u r r e n c e s and it is difficult to e s t i m a t e the r e l e v a n c e of the i n f o r m a t i o n outside the s i t e s studied. Although it is p o s s i b l e to a r r i v e at s o m e g e n e r a l conclusions by c o m p a r i n g the d a t a a v i l a b l e f r o m many w i d e l y - s c a t t e r e d t r o p i c a l c o u n t r i e s t h e r e s t i l l r e m a i n many unsolved q u e s t i o n s r e g a r d i n g the o r i g i n of podzols in t r o p i c a l a r e a s . T h i s p a p e r d e a l s with s t u d i e s which w e r e p a r t i c u l a r l y made to p r o v i d e an o v e r a l l p i c t u r e of the e n v i r o n m e n t a l conditions and the c h a r a c t e r i s t i c s Geoderma, 2 (1968/1969)

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of humus podzols ( h e r e a f t e r c a l l e d p o d z o l s ) o c c u r r i n g in S a r a w a k , the northw e s t e r n p a r t of Borneo from which the e a r l i e r now c l a s s i c d i s c o v e r i e s were r e p o r t e d by Hardon (1937) and R i c h a r d s (1941). These studies involved the c o l l e c t i n g of data obtained on p o d z o l s during a 7 - y e a r s routine s u r v e y i n g p e r i o d , and the e x t r a c t i n g of s a l i e n t f a c t o r s involved in the g e n e s i s of t h e s e s o i l s , the following up by d e t a i l e d investigations in s e l e c t e d a r e a s to c o n f i r m c e r t a i n indications drawn from the e a r l i e r o b s e r v a t i o n s and the p r o c e s s i n g of a n a l y t i c a l data f r o m p r o f i l e s , carefully s e l e c t e d with a view to e s t a b l i s h the norm than the exception. ENVIRONMENTAL CONDITIONS C li,~ ate C h a r a c t e r i s t i c f e a t u r e s a r e a heavy r a i n f a l l , a c o m p a r a t i v e l y uniform high t e m o e r a t u r e ( a v e r a g e daily t e m p e r a t u r e 25.5°C) and high humidity (84% daily a v e r a g e ) . Mean annual r a i n f a l l is between 2,500 and 5,000 m m , f a i r l y well d i s t r i b u t e d , with no s i n g l e month in any l o c a l i t y w h e r e the r a i n fall is below 100 mm. Thus, a c c o r d i n g to M o h r ' s (1944) r a i n f a l l c l a s s i f i c a tion the c l i m a t e is " c o n t i n u o u s l y wet". Altitude Although podzols do o c c u r above an altitude of 1,000 m, in o r d e r to omit p o s s i b l e effects f r o m c l i m a t i c v a r i a t i o n s in the development of podzols at different a l t i t u d e s only those o c c u r r i n g under a t r u e t r o p i c a l c l i m a t e , thus, below 330 m, have been studied. The l a t t e r a r e found at a l l heights from s e a level to 330 m.

Parent materials T h e s e can be divided into two d i s t i n c t groups: (1) m a t e r i a l of a l l u v i a l origin - old (mainly P l e i s t o c e n e ) t e r r a c e deposits (both r i v e r i n e and marine); (2) m a t e r i a l of s e d e n t a r y o r i g i n - s a n d s t o n e s and c o n g l o m e r a t e s (mainly of T e r t i a r y age). The f i r s t group of m a t e r i a l s commonly c o n s i s t s of a l t e r n a t i n g l a y e r s of quartz sand and q u a r t z g r a v e l frequently i n t e r l a y e r e d with sandy c l a y s . M i n e r a l o g i c a l a n a l y s e s show that the s o u r c e of t h e s e t e r r a c e m a t e r i a l s can in many p l a c e s be t r a c e d to T e r t i a r y sandstone and c o n g l o m e r a t e s and the p a r e n t m a t e r i a l s of a l l u v i a l o r i g i n can, t h e r e f o r e , be r e g a r d e d as v e r y s i m i l a r to p a r e n t m a t e r i a l s of s e d e n t a r y origin, but even m o r e i m p o v e r i s h e d . A l l u v i a l p a r e n t m a t e r i a l . The t e r r a c e m a t e r i a l s a r e e x t r e m e l y poor in b a s e s and s e s q u i o x i d e s ( p a r t i c u l a r l y iron) and c o n s i s t l a r g e l y of c r y s t a l line quartz; the c l a y s a l s o c o n s i s t m a i n l y of quartz with s u b o r d i n a t e m i c a s , k a o l i n i t e (fire c l a y s ) and i l l i t e . The heavy m i n e r a l a s s o c i a t i o n of the sand f r a c t i o n i n v a r i a b l y shows c o n c e n t r a t i o n s of z i r c o n , t o u r m a l i n e and t i t a n i -

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ferous m i n e r a l s in which r u t i l e is dominant, followed by a n a t a s e a n d b r o o k i t e . The content of opaque m i n e r a l s is usually high ( m o r e than 60%) and i l m e n i t e is dominant, although l e u c o x e n e and v a r i o u s i n t e r g r a d e s b e tween i l m e n i t e and non-opaque t i t a n i f e r o u s m i n e r a l s a r e commonly a l s o present. The thickness of the a l l u v i a l d e p o s i t s v a r i e s but is g e n e r a l l y g r e a t e r in m a r i n e than in r i v e r i n e d e p o s i t s . G r a v e l beds a r e uncommon in the f o r m e r , which g e n e r a l l y c o n s i s t s of a l t e r n a t i n g s a n d s and c l a y s . In the c a s e of r i v e r i n e deposits in which g r a v e l and b o u l d e r beds commonly o c c u r l a y e r s of contrasting t e x t u r e f r e q u e n t l y a l t e r n a t e within a r a n g e of as l i t t l e as 5 ft, while in the m a r i n e d e p o s i t s individual l a y e r s m a y r e a c h a t h i c k n e s s of m o r e than 20 ft. The heavy m i n e r a l a s s o c i a t i o n s of p a r e n t m a t e r i a l s of humus p o d z o l s is d i s t i n c t l y different f r o m t h o s e found in m a t e r i a l on which i r o n p o d z o l s f o r m . The l a t t e r only occur on s p e c i f i c p a r e n t m a t e r i a l s . P r o f i l e 4 ( s e e Fig.6) i l l u s t r a t e s that a significant d i f f e r e n c e i s the high c o n c e n t r a t i o n of h o r n b l e n d e in the p a r e n t m a t e r i a l of the i r o n podzol which p r e s u m a b l y f o r m s the s o u r c e of the i r o n oxides which following w e a t h e r i n g of the hornblende a r e r e l e a s e d . I r o n poazols being of v e r y m i n o r i m p o r t a n c e in S a r a w a k a r e f u r t h e r not c o n s i d e r e d in this study.

Sedentary parent material. T h e s a n d s t o n e s a r e g e n e r a l l y c o a r s e to m e d i u m - t e x t u r e d and highly q u a r t z i t i c as is the c a s e with the c o n g l o m e r a t e s . In c e r t a i n a r e a s the s a n d s t o n e s m a y have as the only w e a t h e r a b l e m i n e r a l a m o d e r a t e content of o r t h o c l a s e but s i n c e r e d - y e l l o w po0zolic s o i l s and poctzols occur in a s s o c i a t i o n on t h e s e s a n d s t o n e s and d e p o s i t s with and without o r t h o c l a s e o c c u r l i k e w i s e , the fact whether the p a r e n t m a t e r i a l s of the podzols do or do not contain o r t h o c l a s e cannot be e s t a b l i s h e d . Heavy m i n e r a l a s s o c i a t i o n s a r e v e r y s i m i l a r to those found in the t e r r a c e m a t e r i a l s . At low l e v e l s it is frequently v e r y difficult to d i s t i n g u i s h between t e r r a c e d e p o s i t s and T e r t i a r y s e d i m e n t s and p a r t i c u l a r l y so when the t e r r a c e morphology has been d i s t u r b e d by e r o s i o n a n d / o r the T e r t i a r y r o c k s in the a r e a a r e known to be p o o r l y consolidated. A l t e r n a t i n g beds of s a n d s t o n e , c o n g l o m e r a t e s and c l a y / s i l t d e p o s i t s g e n e r a l l y a p p e a r to be c o n s i d e r a b l y l e s s thick than those g e n e r a l l y found in the t e r r a c e m a t e r i a l s . Although, a c c o r d i n g to Jenny (1941, p.54) "the exact evaluation of the c o m p o s i t i o n of the p a r e n t m a t e r i a l i n v o l v e s c o n s i d e r a b l e s p e c u l a t i o n and is the s o u r c e of much u n c e r t a i n t y in the i n v e s t i g a t i o n of s o i l - f o r m i n g p r o c e s s e s " , t h e r e can be l i t l l e doubt that in S a r a w a k a l l p a r e n t m a t e r i a l s of p o d z o l s a r e e s s e n t i a l l y c h a r a c t e r i s e d by a high t o t a l quartz content, v e r y low w e a t h e r a b l e m i n e r a l content, low to v e r y low sequioxide content and a v e r y low clay content. They have f u r t h e r in common that they a r e e i t h e r u n c o n s o l i d a t e d or p o o r l y c o n s o l i d a t e d s e d i m e n t a r y m a t e r i a l s which show g e n e r a l l y s t r o n g t e x t u r e c o n t r a s t s between the d i f f e r e n t s t r a t a . Topography T h e m o s t outstanding f a c t o r in the topography of land on which po0zols have f o r m e d is that it is a l w a y s flat o r v e r y gently sloping. Slopes a r e genGeoderma, 2 (1968/1969)

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e r a l l y l e s s than 5 °, p a r t i c u l a r l y on s u m m i t a r e a s of t e r r a c e s . I n v a r i a b l y w h e r e the slope s t e e p e n s , a s on d i s s e c t e d p o r t i o n s of t e r r a c e r e m n a n t s , d i f f e r e n t soil types a r e found. C o m m o n l y the m i c r o r e l i e f of the t e r r a i n is h u m m o c k y showing low u n d u l a t i o n s w h e r e s u r p l u s r a i n w a t e r m a y collect. P o d z o l s on c o n s o l i d a t e d s e d i m e n t s a r e only found on d i p s l o p e a r e a s with v e r y g e n t l e slopes. T h e s a m e low u n d u l a t i o n s a s found on the t e r r a c e s i t e s o c c u r h e r e . The s t e e p e s t r e c o r d e d s l o p e for such a r e a s w h e r e podzols have f o r m e d is 8 °. A c r o s s - s e c t i o n of such a d i p s l o p e a r e a is shown on F i g . 1 .

Drainage P r e s e n t d r a i n a g e is l a r g e l y c o n t r o l l e d by d u r i p a n s which have f o r m e d d u r i n g the p o d z o l i z a t i o n p r o c e s s and, t h e r e f o r e , does not r e f l e c t the d r a i n a g e s i t u a t i o n b e f o r e the p o d z o l s f o r m e d . It is c o n s i d e r e d that, i n i t i a l l y , m o s t of the r a i n w a t e r was a b l e to p e r c o l a t e q u i c k l y t h r o u g h the p e r v i o u s s t r a t a it b e i n g s l o w e d down only by e i t h e r a l a y e r of fine t e x t u r e d m a t e r i a l or by g r o u n d w a t e r . R u n - o f f was slow b e c a u s e of the flat to a l m o s t fiat topography, and m o s t s u r p l u s r a i n w a t e r would u l t i m a t e l y d r a i n off l a t e r a l l y o v e r a s u b s u r f a c e clay l a y e r . T h e p e n e t r a t i o n depth of t h i s w a t e r would be d e p e n d e n t on the t h i c k n e s s of the p e r v i o u s s t r a t a , or the d i s t a n c e to a r e g u l a r or p e r c h e d g r o u n d w a t e r table. At p r e s e n t , on the o t h e r hand, the p e n e t r a t i o n depth is l a r g e l y i n f l u e n c ed by the i l l u v i a l B - h o r i z o n . W h e r e a h a r d p a n o c c u r s the w a t e r will not p e r colate d e e p e r than the p a n and if the s l o p e is s u f f i c i e n t , flows off l a t e r a l l y over it. P e r c h e d w a t e r t a b l e s f o r m in t i m e s of high r a i n f a l l on such p a n s and the s o i l s a r e s a t u r a t e d up to the s u r f a c e , r e s u l t i n g in s t a g n a n t w a t e r on flat a r e a s a n d a n i n c r e a s e i n the r u n - o f f r a t e on s l o p i n g land. In the c a s e of soft i l l u v i a l h o r i z o n s the p e r c o l a t i o n r a t e is only s l o w e d clown and although s a t u r a t i o n m a y o c c u r the s i t u a t i o n is n e v e r so e x t r e m e l y wet as i s the c a s e when h a r d p a n s e x i s t . T h e c o n n e c t i o n b e t w e e n slope, d r a i n a g e a n d soil t e x t u r e is, t h e r e f o r e , a n i m p o r t a n t one and w i l l be d i s c u s s e d l a t e r .

Vegetatio~z A l l n a t u r a l v e g e t a t i o n on podzols is c h a r a c t e r i s e d by l o w l a n d heath f o r e s t ( l o c a l l y c a l l e d kerangas - a t e r m m e a n i n g s o i l which c a n n o t s u p p o r t h i l l r i c e } which on a i r p h o t o g r a p h s shows a d e n s e , e v e n to s l i g h t l y u n e v e n , canopy with g e n e r a l l y i n d i s t i n g u i s h a b l e s m a l l c r o w n s . T h e n u m b e r of s p e c i e s in this f o r e s t type can be c o n s i d e r a b l e a n d t h e r e is wide l o c a l v a r i a t i o n , d e p e n d i n g m u c h on d r a i n a g e . T h e r e is, o n t h e o t h e r hand, v e r y l i t t l e v a r i a t i o n b e t w e e n the v e g e t a t i o n found on p o d z o l s d e v e l o p e d in t e r r a c e m a t e r i a l s and on t h o s e o c c u r r i n g on T e r t i a r y s e d i m e n t a r y r o c k s . B r u n i g (1963} r e f e r r i n g to a s p e c i f i c a r e a , which is, h o w e v e r , t y p i c a l for t h i s type of f o r e s t , r e c o r d s the following s p e c i e s : Eugeissonia insignis: on b e t t e r d r a i n e d p l a c e s ; Eugeissonia minor: throughout; Pinanga spp.: s c a t t e r e d , l o c a l l y c o m m o n ; 204

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I

a

s

Licuala bidentata: s c a t t e r e d , l o c a l l y common; Teysmanniana altifrons: on b e t t e r d r a i n e d s i t e s . Common other s p e c i e s a r e : Tristania spp., Whiteodendron moultonianum, Palaquium spp., Payena spp.; Cratoxylon glaucum, Melanorrhoea spp., Ploiarium alternifolium, Calophyllum spp., Xylocarpus borneesis, Lauraceae, Hopea spp., Parastemon spicata, Xantophyllum spp., Linthocarpus spp., Garcinia spp., Ternstroemia spp,, Euphorbiaceae, Aetoxylon sympetalum, Kokoona ovato-lanceolate, Pseudosindora leiocarpa. Other r e p o r t s also mention: Casuarina sumatrana which l i k e Whiteodendron moultonianum, tends to b e c o m e dense when a r e a s within or n e a r its n a t u r a l r a n g e a r e d i s t u r b e d . Lowland heath f o r e s t a r e a s when d i s t u r b e d through f i r e or cultivation, f r e q u e n t l y develop into open p a r k l a n d in which Casuarina sumatrana and Whiteodendron a r e dominant, the ground c o v e r c o n s i s t i n g m a i n l y of s e d g e s , m o s s e s , p i t c h e r p l a n t s and o r c h i d s (Wall, 1966, p.95) much like the "padang" v e g e t a t i o n d e s c r i b e d by Hardon (1937).

GENERAL

P R O F I L E CHARACTERISTICS

A l l p o d z o l s under n a t u r a l v e g e t a t i o n have d a r k brown thick organic s u r f a c e h o r i z o n s . The acid~ highly l i g n i t i c , c o a r s e o r g a n i c d e b r i s d e c o m p o s e s e x t r e m e l y slowly and w h e r e p o o r d r a i n a g e conditions p r e v a i l thick p e a t y s u r f a c e horizons a r e f o r m e d . M o s t feeding roots of the ground v e g e t a t i o n a r e in this h o r i z o n which is v i r t u a l l y the only one containing any plant food. The A l - h o r i z o n is u s u a l l y w e l l d e v e l o p e d and is of a d a r k brownish g r e y colour. The total c a r b o n content is high. T h e m i n e r a l p a r t of this horizon is f o r m e d mainly by c o a r s e sand g r a i n s with v e r y l i t t l e c o h e s i a n with the o r g a n i c m a t t e r . A f t e r c l e a r i n g and r e m o v a l of the O - h o r i z o n by burning, the o r g a n i c m a t t e r in the top p a r t of this h o r i z o n o x i d i z e s r a p i d l y . A v e r y loose, white c o l o u r e d s u r f a c e h o r i z o n 1-2 inches in t h i c k n e s s is f o r m e d which o v e r l i e s a m o r e n o r m a l l y c o l o u r e d A l - h o r i z o n . The l o w e r b o u n d a r y of t h e A l - h o r i z o n u s u a l l y f o r m s the lower l i m i t of the feeding r o o t s , and this r e f l e c t s the g e n e r a l a b s e n c e of b a s e s beyond that depth. The e l u v i a l A 2 - h o r i z o n is v e r y well d e v e l o p e d and if v e r y p o r o u s s a n d f o r m s the p a r e n t m a t e r i a l it can be a s thick a s 9 ft. Such h o r i z o n s a r e found in m a r i n e t e r r a c e s on h o m o g e n e o u s s a n d d e p o s i t s . M o r e commonly the A 2 h o r i z o n is of a m e d i u m s a n d t e x t u r e and 6-18 i n c h e s thick. An i n d u r a t e d h o r i z o n is u s u a l l y found in the l o w e r p a r t of the A 2 - h o r i z o n , the c o n s i s t e n c y of which is then v e r y h a r d when dry: such h o r i z o n s a r e difficult to p e n e t r a t e by a u g e r . T h e o c c u r r e n c e of such pans is u s u a l l y r e l a t e d to a high content of q u a r t z flour in the A 2 - h o r i z o n which is thought to be f o r m e d by d e s i n t e g r a t i o n of l a r g e r q u a r t z p a r t i c l e s . A c t u a l c e m e n t a t i o n does not s e e m to o c c u r and in wet condition the d e n s e l y p a c k e d h o r i z o n flows out of the p r o f i l e face when e x p o s e d in a pit. Under n a t u r a l v e g e t a t i o n this h o r i z o n is u s u a l l y w e a k l y m o t t l e d brown due to humus s t a i n i n g of the s a n d p a r t i c l e s ; under s e c o n d a r y jungle, w i t h a thin O - h o r i z o n , not much s t a i n i n g is found and the h o r i z o n is u s u a l l y p i n k i s h white in c o l o u r .

Geoderma, 2 (1968/1969)

209

The boundary with the underlying illuvial humus horizon is commonly very abrupt but frequently a I inch thick transitional horizon is present. This presumably indicates the flushing down of humic colloids over an uninterrupted front through the A2 and their subsequent deposition on top of the illuvial humus horizon which is thus growing upwards. The nature of the illuvial humus-horizon varies considerably and its condition is related to stage in profile development, parent material, topography and drainage. It can be soft to very hard and cemented (a duripan). Because of the extreme hardness of many such pans their thickness cannot be easily assessed, since only with explosives they can be broken up. The maximum reported depth is 2 ft. 6 inches. The horizon is usually more strongly developed (i.e., is thicker and harder) on flat-lying terrace summits than on sloping land such as found in terrain underlain by Tertiary consolidated sediments. The highest points of the terraces are usually places where the most strongly cemented pans form. If layers with a strong texture contrast occur in the profile (or at least where they have been seen within a depth of 5 ft.) the horizon usually forms in a coarse-textured layer overlying a finer-textured layer. In extreme cases pans form in gravel and boulder beds where they overlie dense clays. In deep homogeneous coarsetextured material, on the other hand, the lower boundary of the horizon frequently coincides with the highest watertable level or slightly above it. Because of the very low watertable levels in old marine terraces, where such conditions exist, the illuvial humus horizon is in many cases beyond the normal profile depth and a proper study of it is difficult. Secondary illuvial humus horizons may form if certain conditions are met. This is illustrated in Fig.2 which shows a situation in which groundwater rich in humus colloids is apparently flowing off laterally on top of the humus hardpan, the latter formed on the terrace summit. This water, finding its way through pervious coarse-textured horizontally bedded strata, brings down humic materials to lower slope areas. In dry periods when these layers dry out the humic colloids precipitate and coat the soil particles. (A film of humic materials develops around boulders and gravel.) A normal illuvial humus horizon formed by vertical transportation of humus colloids from the surface horizon and subsequent lodging in the illuvial horizon is found above the secondary one. The horizons are usually separated by a B2-horizon which is more clayey than either the overlying A2-horizon or the underlying secondary illuvial humus horizon. Horizons below the illuvial humus horizon are difficult to study if hardpans are present and can only be done in either road cuttings or other excavations. Information on these horizons is therefore mainly available from podzols with soft illuvial humus horizons. The latter soils are dominantly of a residual nature. The lower part of the B-horizon is usually of a pale yellow colour with a dense network of cracks and old root channels filled with humic materials which have filtered through from the Bh. Illuvial clay coatings are sometimes present but frequently this part of the B-horizon is formed by sandy clays in which it is difficult to distinguish cutans in the field owing to the dense packing of the material. Mottles of iron oxides are, if present at all, weakly expressed because the iron content in the parent materials is very low. They tend to form in and around old root channels. Because of the near-absence of iron the for-

210

Geoderma, 2 (1968/1969}

mation of an illuvial Bir-horizon is not detectable either in the field or in analyses. The transition from B - t o C - h o r i z o n is normally indistinct and usually m a r k e d only by a change in colour from pale yellow to almost white. In the residual podzols it is e a s i e r to r e c o g n i s e since the original structure of the underlying sandstone is often w e l l - p r e s e r v e d in the weathered material. It is not uncommon to find on flatbedded sandstone podzols in which the rock structure is still p r e s e r v e d in the illuvial humus horizon. ANALYTICAL STUDIES Detailed analytical investigations w e r e c a r r i e d out on two selected profiles: a podzol with a soft humus B-horizon on t e r r a c e material anti a podzol with a medium hardpan on T e r t i a r y consolidated sediments. A third profile description is added for c o m p a r i s o n , this being typical g r e y - w h i t e podzolic soils (Soil Survey Staff, Sarawak, 1966) derived from carbonaceous shale. The parent material of this soil type is as poor in bases, iron and weatherable minerals as the parent m a t e r i a l s of the podzols. The basic difference is texture, the podzolic soil having a much higher clay content than the podzols. P r o f i l e 1: H u m u s podzol on old t e r r a c e d e p o s i t s humus horizon

A1

0-7 inches

A1.2

7-9 inches

A2

9-12 inches

A3

12-15 inches

BI.1 15-18 inches

B1.2 18-24 inches

Geoderma,

- n o n - i n d u r a t e d ilIuvial

5YR 3/1, very dark grey, loamy medium to fine sand (finer fraction is mainly organic), crumbly, friable to s m e a r y , moist. P o r o u s . Many roots - approximately 50% of bulk material, c l e a r r e g u l a r boundary to 5YR 4/1, dark grey, loamy medium sand with inclusions of pinkish grey, s t r u c t u r e l e s s , porous, wet. Few small roots, c l e a r r e g u l a r boundary to 1OYR 7/2, pinkish grey, loamy medium to fine sand with dark grey medium sand washed in from above in old root channels (many lateral). Very moist, very few living roots. S t r u c t u r e l e s s . P o r o u s . Weak staining (1OYR 6/2) pale brownish grey. Clear, r e g u l a r boundary to 7.5YR 5/2 dark brown, loamy fine to medium sand with 1OYR 6/2 (light brownish grey) - 50/50%. Dark colour is confined to filled in old root channels. Structureless, porous, soft, moist. Some large c r a c k s show a film of humic m a t e r i a l of s m e a r y consistency. Clear, r e g u l a r boundary to 7.5YR 3/2 dark brown, and 7.5YR 8/2, pinkish white loamy fine sand - 50/50%, dark m a t e r i a l confined to filled in old root channels, s t r u c t u r e l e s s , few living roots. Slightly moist. Clear, wavy boundary to 5YR 3/3 and 3/2, dark reddish brown silty to fine sandy loam. D a r k e r coloured m a t e r i a l in old root channels, s t r u c t u r e l e s s , weakly cemented. Slightly moist. Clear, wavy boundary to

2 (1968/1969)

211

B2

B3

C D

24-28 inches 1OYR 7/3, very pale brown, medium to fine sandy loam, compact. Structureless. Dark humic material has leached down through old large root channels, this material is wet and smeary. Few living roots. Few faint mottles (10YR 7/8). Gradual boundary to 28-50 inches 1OYR 8/2, white, medium to fine sandy loam, structureless. If dried out (in exposed places) this horizon displays strong columnar structure. Many vertical old root channels filled in with clayey material of dark brown colour. Many small rootlets. Moist. Gradual change to 50-80 inches 1OYR 8/l, white, sandy clay loam, plastic, compact, moist. Gradual change to 80-108 inches 1OYR 8/l, white, sandy clay, very compact, plastic, non-sticky. Few distinct 1OYR 7/8 mottles. Moist. Watertable at location I3 ft. Below 50 inches material probably bisequent.

Remark:

Prqfile

2: Hunts

podzol

0

O-2 inches

Al

2-5 inches

Al.2

5-9 inches

A2

9-13 inches

B1.h

13-18 inches

Bl.2

18-22 inches

B2

22-33 inches

B3

33-44inches

212

011 qacartzitic

sandstorze

(strongly

developed)

Partly decomposed, 5YR 2/2, dark reddish brown, organic matter with few sand grains, mixed with dense rootmat of fine roots mainly, some large roots, slightly moist. Clear over 5YR 3/2, dark reddish brown, sand, with much organic matter, friable, crumbly, moist. Individual sand grains are white in colour and clear. Abrupt, wavy boundary to 5YR 5/2, reddish grey, medium sand (humus stained), with few roots, moist, single grain, firm,clear but wavy change to 1OYR 7/l, light grey, medium sand with reddish grey staininginplaces(75% light grey - 25% reddish grey). Single grain, firm. Some veins of humic material run through this horizon without any apparent direction. No roots, abrupt over 5YR 2/2 and 3/2, dark reddish brown loamy medium sand, weakly cemented. Some fine roots at boundary with horizon above. Irregular but clear change to IOYR 6/4, light yellowish brown, fine sandy loam, slightly wet. Many old decomposed roots. Small pockets of 5YR 2/2 colour where material is cemented. Platy structure with humus accumulation between structure elements; distinct change to 1OYR 7/3, very pale brown, loamy sand - sandy loam, compact, structureless, slightly wet. Many old root channels with organic material which also accumulates along fracture planes. Clear change to 2.5Y 8/4, pale yellow sandy clay with 10YR 6/6, brownish yellow m&tling, in some places as lateral bands in others along old root channels. Sticky and

Geoderma, 2 (1968/1969)

C

44-68 inches

plastic. Some quartz pebbles (rounded), at 44 inches becoming m o r e sandy and r e s e m b l i n g sandstone. White m e d i u m sandstone. (Deep augering c o n f i r m s o c c u r r e n c e of white clay bed at 68 - 76 inches). P e r c h e d w a t e r t a b l e at 48 inches.

Profile 3: Grey-white podzolic soil derived from carbonaceous shales A1

0-3 inches

2.5Y 7/2, light grey, sandy clay loam. Weak humus staining f r o m 2 - 3 inches, s u r f a c e gleying also p r e s e n t . Weak platy s t r u c t u r e , densely rooted, moist. C l e a r r e g u l a r boundary to A2 3-15 inches 2.5Y 7/2, light grey, sandy clay loam with faint few light g r e y (1OYR 7/1) and yellow (1OYR 7/8) m o t t l e s . Massive, c o m p a c t . L a r g e c r a c k s give r i s e to f o r m a t i o n of l a r g e p r i s m s when soil d r i e s out. In the c r a c k s dense root s y s t e m s , r e m a i n d e r of soil s p a r s e l y rooted. C l a y skins along c r a c k s . Gradual i n c r e a s e in o c c u r r e n c e of yellow s m a l l m o t t l e s , moist. Gradual change to B1 15-30 inches 2.5Y 8/0, white clay, m a x i m u m concentration of 1OYR 7/8, yellow m o t t l e s , p a r t i c u l a r l y where quartz grit is p r e s e n t . Strongly developed clayskins. Quartz grit o c c u r s in p o c k e t s and as disturbed thin stonelines (possibly f r o m quartz s t r i n g s in parent m a t e r i a l ) . Roots only p r e s e n t in extending c r a c k s f r o m s u r f a c e horizons, m a s s i v e , compact, moist. B1.2 30-60 inches 2.5Y 8/0, white clay, with pockets of q u a r t z - g r i t , weakly m o t t l e d yellow. Light g r e y colour of A horizon p e r s i s t s along c r a c k s (possibly clay illuviation). Roots mainly confined to c r a c k s . M a s s i v e , compact, moist. Gradual change to B2 60-100 inches 1OYR 7/1, light g r e y clay, m a s s i v e and compact with s m a l l common strong brown 7.5YR 5/8 mottles. I1luvial clay noticeable in l a r g e c r a c k s . No roots. P o c k e t s of q u a r t z g r i t . Moist. Abrupt but i r r e g u l a r boundary to B C 100-172 inches 1OYR 7/1, light g r e y , silty clay, m a s s i v e and v e r y compact. No c r a c k s or roots, slightly moist, abrupt i r r e g u l a r boundary to D 172-? Soft, easily cut black shale with q u a r t z - s t r i n g s . Inclusions of f o s s i l r o o t s and olive yellow coloured pyritic m a t e r i a l p a r t i c u l a r l y along f r a c t u r e planes. Remark: s u b s a m p l e d 62- 69 inches as B2.1 90- 98 inches as B2.2 124-136 inches as B3 160-172 inches as C The following analytical methods w e r e employed: cation exchange c a pacity (C.E.C.) - N. a m m o n i u m a c e t a t e pH 7 (Schollenberger and Simons, 1945; Peech, 1945); leachate f r o m C.E.C.: Na and K by Evans E l e c t r o -

Geoderma, 2 (1966/1969)

213

selenium Ltd (E.E.L.) Flame Photometer, Ca and Mg by ethylene diamine tetraacetic acid (E.D.T.A.), exch. AL by Yuan's (1959) method; mechanical analyses by Piper's pipette method (1950, pp.59-74), % C by WalkleyBlack's method (Jackson, 1958, p.219). Total chemical analysis by fusion with sodium carbonate, using Dobritskaya's method (I 962). Mineralogical analyses were carried out according to the method described by Mohr and van Baren (1954,pp.219-220), while the clay mineral analyses were carried out using a Philips 2KW X-ray diffractometer with CoK radiation, The results of the latter have been expressed as relative amounts using the method described in the 7th Approximation (U.S. Soil Survey Staff, 1960/67). DISCUSSION Sarawak podzols under natural vegetation are morphologically similar to the turfy-strongly podzolic soils in Russia (Rode, 1962, pp.314-334), those with disturbed natural vegetation and partial absence of the Al-horizon can be compared with Rode's podzols. There may be genetic reasons for distinguishing between such soils in Russia, but the difference in Sarawak is man-made and can be neglected for classification purposes. They can also be compared with the "humus podzols" of central and western Europe (Kubiena, 1953) developed on quartz sands and have strong similaritieswith the humic podzols of Canada where they would possibly mainly belong to the hydromorphic type (Damman, 1962). They can further be comparedwith Australianpodzols and groundwater podzols (Stephens, 1956). According to the U.S. 7th Approximation to a universal classification system all Sarawak podzols belong to the suborder of aquods (order spodosols), these under natural vegetation and poor drainage conditions belong to the subgroup of histic tropaquods while those under secondary vegetation and weakly developed O-horizons are best classified under the typic Tropaquods. Possibly duraquods occur as well. The podzols with very thick albic horizons such as occur in old deep sandy marine deposits cannot be classified as typic tropaquods and must be classified under the order ofentisols (soil group: quartzipsamments). As these spodosols and entisols are genetically the same, it appears that, from the viewpoint of Sarawak soils, the thickness of the albic horizon is over-emphasised in this system. Common to all the podzols mentioned above, regardless under what other environmental conditions they have formed, is the fact that the parent materials are dominantly of a sedimentary nature, are sandy, porous, and quartzitic and have low contents of bases and total iron. The chemical analyses (Fig.3-5) show the extreme poorness of the parent materials in Sarawak. The relative increase in total SiO2 content (Fig.3, profiles I and 2) in the upper horizons is clearly visible and according to Rode (1962) is characteristic for a podzolization process. It is accompanied by a decrease in the finer fractions, notably the clay fraction, and an increase of these in the B-horizon. Noteworthy is the increase of the silt fraction in the lower horizons of profile 3 which may indicate that this fraction is also affected by a "lessivage" (Duchafour, 1951) or illumerization process in podzolic soils (Fridland, 1958). In the absence of micromorphological studies it cannot be assessed by 214

Geoderma, 2 (1968/1969)

visual means whether the clay i n c r e a s e in the B - h o r i z o n is due only to leached clay transported in s u s p e n s i o n or whether clay destruction also takes place in the A 2 - h o r i z o n followed by leaching of their components and possibly subsequent new formation of clay in the illuvial horizon. Some conclusions may, however, be drawn from the analytical data. It is generally accepted that kaolinite is the least easy clay mineral to r e m o v e under a podzolic weathering p r o c e s s , it being in c o m p a r i s o n with doublelattice clays difficult to d i s p e r s e (Gradusov and Dyazdevich, 1961, p.752). The general relative d e c r e a s e of this m i n e r a l in the upper horizons may therefore point to break-down of this clay mineral particularly since this is accompanied by a relative i n c r e a s e of clay size quartz in these horizons. This would point to a podzolization p r o c e s s (Fridland, 1958). The total absence of m o n t m o r i l l o n i t e / v e r m i c u l i t e clays in the upper horizons may be due to their relative unstable nature under the reigning weathering p r o c e s s , and they may have been either d e c o m p o s e d or r e m o v e d by illimerization. The contrast between the podzols and the podzolic soil is most m a r k e d in the mineral composition of the clay fraction (see Table I). In the f o r m e r there is a relative d e c r e a s e of quartz in the clay fraction down the profile, while in the latter the relative amounts of quartz r e m a i n virtually constant. The composition of lattice clay m i n e r a l s in the latter is, however, v a r i e d and shows changes from illite into m o n t m o r i l l o n i t e / v e r m i c u l i t e m i n e r a l s in the top horizons, a much s i m i l a r change was found in studies s u m m a r i z e d by Gorbunov (1961). T h e r e is little destruction and r e m o v a l of kaolinite apparent in this profile. The i n c r e a s e of clay in the B - h o r i z o n (Fig.3) is not accompaniedby a relative i n c r e a s e in any one clay m i n e r a l and is therefore, probably mainly due to m e c h a n i c a l t r a n s p o r t of all c l a y - s i z e particles except for m o n t m o r i l l o n i t e / v e r m i c u l i t e and gibbsite which may either have been destroyed during the p r o c e s s or they a r e newly formed in the top h o r i z o n through destruction of the illite which may point to weak podzolization in these horizons (Gradusov, 1961). The p r o c e s s in the profile is, however, mainly one of illimerization. F r o m these differences in clay m i n e r a l c o m position one may conclude that the weathering p r o c e s s in the podzolic soil is essentially different from that in the podzols. The difference in the nature of the clay in the podzols and podzolic soils is further illustrated by Fig.4 which shows the relation between C.E.C. percent C and clay content. In the podzols, because of the high quartz content in the clay fraction, the exchange complex is mainly a function of the organic compounds while in the podzolic soils it is directly related to clay content. A study of the heavy m i n e r a l association (Fig.5) underlines the p o s s i bility that quite a n u m e r of podzols have developed in bisequent parent m a t e r i a l s . In profile 2 no field evidence indicated that within the profile the soil m a t e r i a l w a s heterogeneous, the textural contrast between A2 and B being in any case typical for podzols. Mineralogical analyses suggest, however, that the textural B may in fact be partly caused by weathering of shale m a terial found in this horizon. On the other hand, shaly m a t e r i a l may have been initially p r e s e n t throughout the profile but has been totally weathered in the upper horizons and no bisequence may be implied. P r o f i l e 4 has been added to show the impact of a podzolic weathering p r o c e s s on weatherable m i n e r a l s . The break-down of hornblende in the upper horizons is c l e a r l y indicated. The slight i n c r e a s e in the O - h o r i z o n is Geoderma, 2 (1968/1969)

215

~l.

BI

100%

20%

40%

60%

eo%

ioo%

Profile l-Humus podsol on old olluvium (well developed )

o~ ~o

iiii!iii!i!ii!!iiii.:.iiiiiiiii!iiiii!iiii!i!!i!!iiii!!!i i IU l

o~ oo

~o

"I

D.

~I.

A

° o

C

80%

Profile I-Humus podsol on old alluvium (well developed)

:':':':':':':':':':':':'.'.'.'.'.'.'-'.'.'-

B2

B3

":':':':':':':'.'-'.'.'.'.'.'.:':.:-:':':':':'"

B,2

.,., ::::::::::::::::::::::::::::::::::::::::::::::::::::::

:':'.':'.'.'.'.'.'.'.'.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:

I

60.%

A3

i

40%

"'"'":':':':':':'X'.'.'.'-'.:.:-:-:.:-:.:.:.:+..

|

20%

.2

0

%

t,o -,3

& i

o

1..

20%

:.:...:.:.:.:.:.:."

.......

..:.:.:."

60% !

80%

loo%

Underbroken

sequence of horizons

m

~

Medium sand 1 , 0 0 0 - 5 0 0 ,u

Fine sand 5 0 0 - 5 0 A J

~

Coarse sand >l,O00Au < 2 /u

Organic rnoteriol

Clay

Silt 5 0 - 2 ~u

~o% =5% 20% 2s%

%Fe=O~

O/oAir Os

°/oMgO; CoO; MnO 8= TiO 2

I •

% Loss on ignition

%SlOt

arofile 3-Grey-white podsolic sail on carbonaceous shales

5%

~00 %

Fig.3. G r a i n - s i z e distribution in humus podzols and podzolic soils (left). Total chemical analysis of fine earth < 2000/~) in humus podzols and podzolic soils (right).

"~'~

=

40%

Profile 3-Grey-white podlolic soil on carbonaceous shales

c/D 1.:.:.:-:.:-:..:.:....

12

B2'l ,'."

.,~

2 .:.:.:.:.:.:.:.:." .,l :.:.:.:.:.:."

AI

(3o

i-=

P

0 ¢D

Q

0

3

I

2

°°.°+°°**

:5

Profile I- Humus

2

4

6

5 °°oo°=°

6

on old a l l u v i u m

5

o..o°=.

podsol

4

7

7

9

• °° *° °-.

8

9 --."

(well developed)

8

"" '.~' *"~

B

'"il

Profile 2-Humu~

oodsnl

n..,,n.+.;++o

"' HIIIItlIIIIIIIIII ...

'2

~,I

. ' . + . ' . ' , ' . ° + " . ' ° * *+' . .' . *. +. ' . ' . ~ • + * - ..° °+. • • ~ . . ~ - -

lnn.~*...

( ltrnnnlv

AI~'"""'"'""'"'""""''""""'"'""""'"'"'-~"~-'" "" """ ~"+~" '-"~

0

0

II

Meq+

N'ual~=

"l ~l

I0 II M e q . • ".%2-~

I0

i

J

./

.."

+" " ' . . . . + + + .

..+.,

6.1%

,5,+q

'"

'++'+

........ .,...

"

'

"

~

'+++

.... ,,. .....

++

14.5 M e q

...."

"

+" +",

260/0

0

m

4

5

6

7

8

9

I0

Total cations mora than C.E.C -probably all extractable AI

Unsaturated, possibly H+anly

Each. AI 3÷

Exch. Na +

Exch. Ca 2+, MO2+, ~ K +

Profile 3-Grey-white podsolic soil an carbanaceous shale



I I Meq.

/'

.....................

\

/

\

//

\

/

\

% Clay

Yoc.

~. E. C.

/

( I \

/

\

)89a/o ,~.41Meq.

/

I

J

';.:.

"'*.......

:,7.6 %

.,....""

...... .,,.,

,......

",....,

Fig.4. Summation diagrams of cations in exchange complex (left). Relation between C.E.C., %C and clay content (right).

O

C/O.

C"

B3-

32.2-

B2.1"

BI.2.

BId-

A2-

0 AI-

'7

~

£0

:'C

~":'

":~

"'2. . . . .

??

oC. . . .

7

"-

-i- !

Profile I

Profile I - H u m u s podsoI on old alluvium (well developed) 0 O-

IO

20

30

40

50

60

70

80

90

100%

C

50

I00%

AI.

AI2A2-

cr

BI-

o Z

B3, C

0

Profile 2 - H u m u s podsol on quortzitic sandstone (strongly developed)

Profile 2,

0

O

I0

20

30

40 ~ _

50 I

60

70 . . l..

~-------::::::.:..

B~ "a

80 I

_=-------~--Z~_~ .'..'.'.V.'."""

90 100% . j ....

: ::Z:I:I:

:.:ii::

I

z

Profile 4.

~T~

Traces of epidote, garnet, corundum

Zircon Tourmaline (mainly brown)

I00%

l

t

Profile 4 - I r o n podeoI on subrecent coastal sand

~

50

N88

Stone fragments (mainly shale )

Titoniferous minerals (Rutile, Anotase, Brookite)

Hornblende (mainly green)

Alterites (mainly Leucocene)

Underbroken sequence of horizons

Fig. 5. Summation d i a g r a m s of non-opaque m i n e r a l s (left). Opaquenon-opaque ratios (right).

t~ t~ }.a

cD v

Go

b$ A

o

O o

I

tr.

xxxx

xx

xx

xx

A2

B1

B2

C

xxxx

xxx

xxx

xxxx

xxx

xxxx

xxxx

BI, I

B1.2

B2.1

B2,2

B3

C

C/D

xxxx

xxx

A2

D

xx

A1

x

x

tr.

x

x

x

tr.

x

x

x

xx

xx

xx

x

sandstone

xx x

xx

x

xx

xx

xx

xx

xx

xx

xx

xx

-

-

-

-

-

-

-

-

x

xx

shale

(strongly

xx

-

-

-

tr.

tr.

x

-

-

-

-

-

(well-developed)

vermiculite

developed)

Montmorillonite-

soil on carbonaceous

xx

xxx

xxx

xxx

xx

x

on quartzitic

podsolic

x

xx

x

xx

xx

x

podsol

tr.

xx

xx

xx

xx

tr.

x

x

on old alluvium

xxx

xx

xx

tr.

x

pedsol

3: G r e y - W h i t e

xxxx

A1.2

Profile

xxxx

A1

2: Humus

xx

B3

C

Profile

xxx

B2

xxxx

xxxx

A3

B1,2

xxxx

A2

xxxx

xx.xx

A1.2

B1.1

xxxx

Profile

AI

podzols

Kaolinite

in Sarawak

Anatase

composition

Quartz

1: H u m u s

mineral

Horizon

Clay

TABLE

tr. tr.

xx xx xx

x

xx

xx

tr.

tr.

tr.

x

tr.

x

tr°

tr. x

tr.

tr.

tr°

tr.

xx

tr.

tr.

Feldspar

tr°

tr°

Illite

tr.

Gibbsite

p r o b a b l y due to addition of w i n d - b l o w n s a n d f r o m f r e s h d e p o s i t s . Cady (1960) r e p o r t s an a l m o s t i d e n t i c a l b r e a k - d o w n of h o r n b l e n d e in podzols d e v e l o p e d over g l a c i a l deposits in the n o r t h w e s t of the U n i t e d States. T h e question of w h e t h e r b a s e s a n d s e s q u i o x i d e s a r e m o v i n g down a s h u m a t e colloids or as s o l u b l e m e t a l - o r g a n i c c o m p l e x e s ( c h e l a t e s ) does not a r i s e b e c a u s e of the p o o r n e s s of the p a r e n t m a t e r i a l s in b a s e s and s e s quioxides. It is s u g g e s t e d that the h u m u s is likely t r a n s p o r t e d as a l m o s t p u r e h u m u s colloids d u r i n g p e r i o d s of high r a i n f a l l when the o r g a n i c t o p s o i l s a r e e x t r e m e l y wet. The organic c o m p o u n d s flush t h r o u g h the p o r o u s A 2 - h o r i z o n p o s s i b l y in i n t e r m i t t e n t s t a g e s a s i n d i c a t e d by the wavy b a n d s in the l o w e r A 2 - h o r i zon. A v e r y s i m i l a r p r o c e s s is i n v o l v e d in the f o r m a t i o n of C a n a d i a n podzols (Stobbe and W r i g h t , 1959). T h e r e a s o n or r e a s o n s for the p r e c i p i t a t i o n of the h u m u s c o l l o i d s in the h u m u s B - h o r i z o n has b e e n the s u b j e c t of m u c h s p e c u l a t i o n (Stobbe a n d W r i g h t , 1959). The p r e s e n t study m a y c l a r i f y at l e a s t s o m e p o i n t s . In S a r a w a k t h e r e a r e no s i g n i f i c a n t v a r i a t i o n s in c h e m i c a l c h a r a c t e r i s t i c s within the p r o f i l e depth w h i c h c o u l d i n f l u e n c e p r e c i p i t a t i o n of the h u m u s colloids, but t h e r e a r e s o m e o u t s t a n d i n g p h y s i c a l v a r i a t i o n s which e i t h e r s i n g u l a r l y or in c o m b i n a t i o n give r i s e to the f o r m a t i o n of i l l u v i a l h u m u s horizons. In l o c a t i o n s w h e r e a s t r o n g t e x t u r e c o n t r a s t e x i s t s the h u m u s B h o r i z o n f o r m s on top of t h e l a y e r with the h i g h e r clay content. F r e q u e n t l y this is a sandy clay d e p o s i t o v e r l a i n by sand. In a n u m b e r of l o c a l i t i e s the s a n d and sandy clay d e p o s i t s a r e s e p a r a t e d by a g r a v e l and b o u l d e r l a y e r in which i n v a r i a b l y a h u m u s h a r d p a n h a s f o r m e d . In a r e a s w h e r e the u p p e r s a n d d e p o s i t is v e r y deep as is the c a s e in m a n y old m a r i n e t e r r a c e s the h u m u s B - h o r i z o n i s found at g r e a t depths ( s o m e t i m e s m o r e than 6 ft.) a n d is then a l w a y s s i t u a t e d j u s t above t h a t p a r t of the p r o f i l e which d u r i n g the w e t t e s t p a r t of the y e a r is s a t u r a t e d . It is s u g g e s t e d that i n i t i a l l y the p e r c o l a t i o n r a t e of the w a t e r r i c h in h u m u s c o l l o i d s is s l o w e d down owing to the t e x t u r a l change. Due to the a l m o s t flat t e r r a i n l a t e r a l m o v e m e n t of t h i s w a t e r is v e r y slow and a p e r c h e d w a t e r t a b l e f o r m s on top of the m o r e or l e s s i m p e r m e a b l e s u b s t r a t a . T h e s u r f a c e l a y e r s a r e a b l e to d r y out i n t e n s i v e l y d u r i n g the dry s p e l l s in the l e s s wet s e a s o n and the h u m u s c o l l o i d s p r e c i p i t a t e and coat the s a n d p a r t i c l e s . T h e d r y i n g out p r o c e s s is e s s e n t i a l l y one of e v a p o r a t i o n and not l a t e r a l d r a i n a g e . T h i s m a y be the r e a s o n why on s l o p e s of m o r e than 8 ° (and in the c a s e of v e r y c o a r s e m a t e r i a l m u c h l e s s } i l l u v i a l B - h o r i z o n s do not f o r m . L a t e r a l d r a i n a g e is s u f f i c i e n t l y fast that h u m u s colloids a r e c a r r i e d away by the g r o u n d w a t e r a n d not d e p o s i t e d following e v a p o r a t i o n . T h i s c o a t i n g p r o c e s s is n e c e s s a r i l y one of long d u r a t i o n but once the p o r e s of the u n d e r l y i n g m a t e r i a l have b e e n s e a l e d and c o n d i t i o n s for a p e r c h e d w a t e r t a b l e f o r m e d the p r o c e s s m a y a c c e l e r a t e . T h e h u m u s B - h o r i zon then g r o w s upward. S y s t e m s of c l a s s i f i c a t i o n in which the t h i c k n e s s of the A 2 - h o r i z o n is t a k e n a s a n i n d i c a t i o n to the i n t e n s i t y of the p o d z o l i z a t i o n p r o c e s s (Rode, 1 9 6 2 ) a r e u n d e r such c o n d i t i o n s without any m e a n i n g s i n c e the t h i c k n e s s of the A 2 - h o r i z o n is m e r e l y r e c i p r o c a l to the t h i c k n e s s of the B h - h o r i z o n , a n d it is the l a t t e r which s h o u l d be e m p h a s i z e d . T h i s m a y b e the r e a s o n why the t h i c k e s t a n d h a r d e s t h a r d p a n s a r e found on the s u m m i t s of t e r r a c e s w h e r e the m o s t d r y c o n d i t i o n s p r e v a i l d u r i n g the l e a s t wet 222

Geoderma, 2 (1968/1969)

s e a s o n . T h i s possibly i n d i c a t e s that it is the i n t e n s i t y of the d r y i n g out p r e c e s s which controls the f o r m a t i o n of h a r d p a n s . In conditions where c o n t r a s t i n g t e x t u r e s do not c o n t r o l and check the p e r c o l a t i o n of groundwater the b a s e of the humus B - h o r i z o n is found a t t h e depth to which the d r y i n g - o u t p r o c e s s extends which depends again on the t e x t u r e and the depth of the g r o u n d w a t e r . C a p i l l a r y m o v e m e n t m a y be exp e c t e d in m a t e r i a l containing s o m e clay, in the c a s e of p u r e sand this is a l m o s t nil and the humus B is f o r m e d a l m o s t d i r e c t l y above the ground watertable. In otherwise uniform e n v i r o n m e n t a l conditions podzols do not f o r m in m a t e r i a l s which have a t e x t u r e h e a v i e r than sandy l o a m , and this fact d e s e r v e s s o m e attention. Two e x p l a n a t i o n s a r e s u g g e s t e d . F i r s t l y , it is p o s s i b l e that through c a p i l l a r y a c t i o n in a m a t e r i a l h e a v i e r than sandy loam m o s t w a t e r is e v a p o r a t e d at the s u r f a c e and that any humus c o l l o i d s it contains a r e again added to the s u r f a c e h o r i z o n . Secondly, it has been n o t i c e d that although the soil p a r e n t m a t e r i a l s a r e c h e m i c a l l y as poor as those on which podzols form, the f o r e s t is a l w a y s of a b e t t e r quality. T h e s e s o i l s have a higher clay content and t h e r e f o r e h i g h e r exchange c a p a c i t y . It is t h e r e f o r e a l s o p o s s i b l e that m o s t plant n u t r i e n t s r e m a i n in the s o i l - v e g e t a t i o n - l i t t e r - s o i l cycle, and l i t t l e is l o s t through leaching, while in a p u r e sand any n u t r i e n t s not i m m e d i a t e l y taken up by the plant r o o t s a f t e r d e c o m p o s i t i o n of the plant l i t t e r a r e l o s t and c a r r i e d away by the g r o u n d w a t e r . While both light and heavy t e x t u r a l s o i l s a r e poor t h e r e f o r e t h e r e is no tendency in the l a t t e r for n u t r i e n t s to be d e p l e t e d even f u r t h e r to the point w h e r e changes occur in the v e g e t a t i o n and c h a r a c t e r of the l i t t e r which might induce a podzol p r o c e s s . Fig.1 i l l u s t r a t e s the o c c u r r e n c e of t e x t u r e c o n t r a s t s in the p r o f i l e s f o r m e d on T e r t i a r y s e d i m e n t s . T h e a b s e n c e of p o d z o l s on m a t e r i a l s with t e x t u r e s h e a v i e r than sandy l o a m in the s u r f a c e h o r i z o n s is worth noting.

CONCLUSIONS P o d z o l s o c c u r r i n g in the t r o p i c a l lowlands of S a r a w a k a r e m o r p h o l o g i c a l l y s i m i l a r to m o s t humus p o d z o l s found in r e g i o n s with a t e m p e r a t e c l i m a t e . The cause for t h e i r d e v e l o p m e n t i s , however, not n e c e s s a r i l y i d e n tical. It a p p e a r s that e n v i r o n m e n t a l conditions m u s t be s p e c i f i c . The n a t u r e of the p a r e n t m a t e r i a l is r e g a r d e d as of o v e r a l l i m p o r t a n c e . It m u s t be e x t r e m e l y poor in plant n u t r i e n t s to induce the growing of plant s p e c i e s which p r o d u c e l i t t e r with a v e r y low b a s e and an e x t r e m e l y high C / N quotient. T h e p a r e n t m a t e r i a l s a r e in t h i s p l a y i n g an equivalent r o l e of that of c l i m a t e in t e m p e r a t e r e g i o n p o d z o l s . I m p o v e r i s h m e n t of s o i l m a t e r i a l does not i n i t i a t e the p r o c e s s of p o d z o l i z a t i o n in S a r a w a k , or only in the v e r y e a r l y s t a g e s and then it is of s h o r t d u r a t i o n . T h e c h e m i c a l p r o c e s s e s involved a r e a l m o s t f r o m the onset of s o i l d e v e l o p m e n t e n t i r e l y c o n t r o l l e d by the dominance of humus compounds and SiO 2. One could r e g a r d the whole p r o f i l e depth as being i n i t i a l l y a deep A2h o r i z o n over which, through p h y s i c a l p r o c e s s e s humus colloids a r e d i s t r i b uted and d e p o s i t e d giving a s the end r e s u l t a podzol morphology. A p a r t f r o m being c h e m i c a l l y a l m o s t i n e r t the p a r e n t m a t e r i a l s m u s t a l s o be highly Geoderma, 2 (1968/1969)

223

0o

~D

oe~

Horizon

D e p t h (in i n c h e s )

A1 A1.2 A2 A3 BI.1 B1.2 B2 B3 C

0-7 7-9 9-12 12-15 15-18 18-24 24-28 28-50 60-67

5.07 1.49 0.69 0.10 2,40 9.61 8.04 3.59 0.76

O A1 A1.2 A2 B1 B1.2 B2 B3 C CII

0-2 2-5 5-9 9-13 13-18 18-22 22-33 33-44 44-68 68-76

44.68 3.97 0.83 0.25 3.35 5.65 2.30 1.08 0.28 1.02

$4298 $4299 $4300 $4301 $4302 $4303 $4304 $4305 $4306 $4307

A1 A2 BI.1 B1.2 B2.1 B2.2 B3 C C/D D

0-3 3-15 15-30 30-60 62--69 90--98 124-136 160-172 at 180 at 10 ft.

3.43 1.43 0.48 0.85 0.98 0.80 0.64 1 .O6 0.84 0.72

P r o f i l e 3: G r e y - W h i t e p o d z o l i c s o i l on c a r b o n a c e o u s

S4369 $4370 $4371 $4372 $4373 $4374 $4375 $4376 $4377 $4378

P r o f i l e 2: H u m u s p o d z o l on q u a r t z i t i c s a n d s t o n e

$4288 $4289 $4290 $4291 $4292 $4293 $4294 $4295 $4296

shale 1.47 1.73 3.02 1.21 0.14 0,69 0.17 0.15 0.17 0.19

2.70 5.33 6.69 6.43 5.06 5.43 4.43 3.25 12.72 9.05

0.40 4.12 2.44 2.66 3.04 1.99 1.89 3.05 0.87

24.05 21.32 14.46 11.47 4.66 7,37 6.31 23.32 27.49 11.28

37.08 76.58 76.69 74.19 65.73 59.21 59.70 60.35 58.99 32.07

55.48 67.00 61.49 60.77 61.29 53.04 49.96 50.76 46.48

21.97 21.59 12.65 9.74 8.88 8.45 31.49 12.50 4.37 19.47

2.40 4.49 8,06 10.07 7.95 6.85 8.28 7.36 6.51 5.82

16.49 7.00 11.41 13.61 9.36 11.89 11.44 7.88 6.94

P e r c e n t s a n d in m i c r o n 1,000 1 , 0 0 0 - 5 0 0 500-53

podzolic soil

L o s s on ignition(%)

P r o f i l e 1: H u m u s p o d z o l on old a l l u v i u m

Lab.No.

M e c h a n i c a l a n a l y s e s of S a r a w a k p o d z o l s a n d g r e y - w h i t e

TABLE II

24.15 28.45 25.17 31.75 44.33 46.86 35.11 32.45 39.20 43.96

9.50 5.53 4.66 8.63 11.55 9.55 10.48 12.06 14.35 19.46

17.56 13.35 19.63 18.72 16.83 16.24 16.78 17.06 15.38

Percent silt 50-2 micron

27.60 26.90 47.07 46.80 42.13 37.51 27.12 31.90 29.68 25.96

6.10 4.73 4.80 3.01 8.31 15.70 17.20 18.76 9.56 35.10

5.78 5.62 3.72 6.10 7.63 10.00 12.48 18.68 31.23

Percent clay 2 micron

102.67 101.41 102.85 101.82 101.12 101.69 100.86 101.38 101,76 101.59

102.46 100.63 101.73 102.58 101.95 102.39 102.39 102.76 102.41 102.52

100.78 99.18 99.39 101.96 100.55 102.77 100.59 101.02 101.66

Total

porous, at least in the top l a y e r s , to allow the flushing through of water rich in humus colloids. These a r e deposited through a drying out p r o c e s s e s s e n tially involving evaporation of the groundwater to a c e r t a i n depth in the profile where the humus colloids precipitate. The topography must be near flat to prevent quick lateral drainage through which the humus colloids might be c a r r i e d away before they can be deposited. T e x t u r e s h e a v i e r than sandy loam in the top horizon may r e s u l t in drawing up this water through capillary action and the humus colloids a r e r e t u r n e d to the surface. The oxidation of s u r f a c e litter which a c c o r d i n g to orthodox pedological theories would prevent podzolization in the t r o p i c s a p p e a r s to be strongly r e t a r d e d by the wet, s t e r i l e conditions in the highly acid organic m a t t e r . Such conditions are very s i m i l a r to those giving r i s e to the development of peat deposits in tropical lowlands with a high watertable. The podzolization p r o c e s s in Sarawak is thus essentially the same as that occurring outside tropical a r e a s , the difference being only that the initiation p r o c e s s (Stobbe and Wright, 1959) is absent, this in the t e m p e r a t e regions being a factor of the vegetation induced by a t m o s p h e r i c climatical factors. Removal of b a s e s and sesquioxides from s u r f a c e horizons, although only present in minor quantities, does take place; clay destruction happens in the A2-horizon and r e m o v a l of both clay p a r t i c l e s and clay p a r t i c l e constituents occurs. Possibly b r e a k - d o w n of clay p a r t i c l e s is dominant in the double-lattice clay m i n e r a l s while kaolinitic m i n e r a l s a r e m o r e stable. In contrast with this in the podzoUc soil (profile 3) illimerization is dominant for most clay size material, while break-down of clay m i n e r a l s s e e m s to be absent or only weakly o c c u r r i n g in the surface horizon and then mainly of the double-lattice clays. P r e c i p i t a t i o n of r e m o v e d m a t e r i a l s takes place in the illuvial horizon but b e c a u s e of the generally low clay content which is related to the lack of w e a t h e r a b l e m i n e r a l s in the parent m a t e r i a l s , p r o nounced textural B - h o r i z o n s can never form. T h e s e are, t h e r e f o r e , of little significance in Sarawak podzols. The position of the illuvial humus horizon in the profile is mainly either r e l a t e d to abrupt textural changes in the profile which in many c a s e s a r e a result of the bisequent nature of the parent m a t e r i a l s (the texture c o n t r a s t m a y be amplified by deposition of some leached clay from the s u r f a c e horizons) or to the depth of the ground w a t e r table. The latter is true only for deep homogeneous sandy m a t e r i a l s in which t h e r e is no physical hindrance to percolating surplus rainwater. In textures somewhat heavier than sand capillary action may play a role in the f o r m a tion of the illuvial humus horizon slightly above the watertable. P e r i o d i c drying out of the s u r f a c e horizons is essential to allow the humus colloids to precipitate. This is consistent with the ideas of Duchaufour (1956, p.203). It is suggested that not only in Sarawak podzols but also in poazols in and outside other t r o p i c a l regions the bisequent nature of the parent m a t e r i a l s is playing an important role in the development and position of the illuvial humus horizon. The podzols in Sarawak a r e t y p i c a l l y intrazonal and it is perhaps i r o n ical that the podzol profile orginally taken as being a typical example of zonality in soils has p r o v e d to be as intrazonal as many other soils. In fact, poazols as o c c u r r i n g in Sarawak could be used equally well to p r o v e the overall i m p o r t a n c e of p a r e n t m a t e r i a l on soil development; one could perhaps classify the l a t t e r as edaphic podzols while those o c c u r r i n g in t e m p e r a t e regions could be best r e g a r d e d as climate poazols. Geoderma, 2 (1968/1969)

225

0 C~ (~

Horizon

Depth (inches)

A1 AI.2 A2 A3 BI.1 BI.2 B2 B3 C

0-7 7-9 9-12 12-15 15-18 1824 24-28 28-50 60-67

4.2 4.2 5.0 4.7 4.5 4.8 5.1 5.1 5.4

0.03 0.04 0.01 0.03 0.06 0.07 0.06 0.03 0.02

0 A1 A1.2 A2 B1 B1.2 B2 B3 C CII

0-2 2-5 5-9 9-13 13-18 18-22 22-33 33-44 44-68 68-76

3.0 3.3 4.2 4.7 3.6 3.9 4.3 4.3 4.1 4.2

1.02 0.16 0.02 0.004 0.05 0.006 0.01 0.003 0.003 0.01

25.82 4.78 0.46 0.14 4.52 2.64 0.57 0.18 0.05 0.10

$4298 $4299 $4300 $4301 $4302 $4303 $4304 $4305 $4306 $4307

A1 A2 BI.1 B1.2 B2.1 B2.2 B3 C C/D D

0-3 3-15 15-30 30-260 62--69 90-98 124-136 160-172 at 180 a t 10 ft.

5.0 4.6 4.7 4.5 4.6 4.7 4.6 4.5 4.3 3.3

0,09 0.04 0.07 0.09 0.11 0.10 0.07 0.08 0.12 0.13

0.89 0.11 0.05 0.03 0,05 0.05 0.03 0.05 0.08 0.63

4.41 2.94 5.99 7.77 7.67 6.62 4.41 4.41 5.57 4.20

14.50 6.4 3.0 0.5 4.7 7.5 1.0 0.5 0.5 4.3

6.51 1.47 0.63 1.89 4.52 18.59 11.24 3.78 7.25

0.34 tr 0.18 0.06 0.06 0.12 0.12 0.06 tr 0.30

0.54 0.42 0.30 0.42 0.19 0.12 0.19 0.19 0.42 0.19

0.63 0.40 0.38 0.34 0.38 0.68 0.18 0.37 0.67

tr. 0.24 0.18 0.24 0.06 tr. 0.06 0.12 0.30 0.18

0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0,01 0.01

1.55 0.97 0.79 0.73 0.77 0.73 1.03 0.78 1.14

0.11 0.19 0.11 0.14 0.16 0.16 0.16 0.18 0.19 0.20

0.25 0.17 0.05 0.05 0.06 0.06 0.06 0.06 0.07 0.24

0.10 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.08

0.02 0.10 0.01 0.01 0.01 0.01 0,02 0.07 0.02 0.04

0.40 0.39 0.33 0.33 0.36 0.37 0.33 0.33 0.33 0.41

0.06 0.02 0.04 0.27 0.27 0.38 0.38 0.27 0.04

Mequiv. p e r 100 g (oven dry) C . E . C . Ca Mg K Na

P r o f i l e 3: G r e y - W h i t e podzolic s o i l on c a r b o n a c e o u s s h a l e

$4369 $4370 $4371 $4372 $4373 $4374 $4375 $4376 $4377 $4378

Profile 2: Humus podzol on quartzitic sandstone

S4288 $4289 $4290 S4291 $4292 $4293 $4294 $4295 $4296

2.50 0.47 0.16 0.65 1.38 3.36 1.61 0.44 0.08

pH P e r c e n t oven d r y (1/2.5 t o t a l N o r g . C H20)

Profile i: Humus podzol on old alluvium

Lab.No.

4.0 4.4 6.0 7.15 8.85 6.85 4.65 4.40 5.50 4.31

0.62 0.61 0.15 0.10 3.82 4.53 2.41 2.55 1.70 2.84

1.31 0.22 0.11 1.30 2.10 5.80 4.92 1.51 6.90

A1

Chemical analysis of the fine earth fraction of Sarawak podzols and grey-white podzolic soil

TABLE III

0.20 0.20 0.10 0.60 0.60 0.80 1.20 2.10 0,80 1.10

0.3 0.2 0.2 0.2 0.25 0.30 0.55 0.65 1.20

83.72 0.99 83.371.03 69.151.60 67.25 1.45 63.46 1.40 65.78 1.50 68.91 1.00 69.64 0.84 66.14 1.50 66.66 1.60

60.77 92.77 97.25 92.73 94.42 83.83 88.15 85.57 92.47 77.91

98.2 92.9 98.2 96.7 93.3 93.2 86.5 86.5 81.6

9.67 10.99 20.07 21.92 23.84 22.38 20.13 20,33 21.85 18.54

0 0.26 0 0 2.65 5.56 7.15 7.95 5.56 16,49

1.06 0.40 0.66 1.32 2.12 3.58 5.70 7.68 13.51

0.96 1.13 0.92 0.81 0.81 0.75 0.42 0.65 0.69 0.75

0.08 0.21 0.27 0.16 0.42 0.31 0.27 0.42 0.21 0.42

0.38 0.27 0.54 0.69 0.83 0.60 0.65 0.69 0.60

0.07 0.06 0.02 0.06 0.04 0.06 0.02 0.04 0.09 0.09

0.11 0.17 0.15 0.09 0.13 0.09 0.11 0.24 0.11 0.13

0.06 0 0 0.18 0.13 0.01 0.05 0.05 0.01

0.22 0.25 0.59 0.62 0.74 0.71 0.52 0.51 0.75 0.64

0.06 0.02 0.06 0.06 0.11 0.11 0.11 0.06 0.08 0.06

0.04 0.01 0.01 0.07 0.14 0.04 0.07 0.12 0.23

Total c h e m i c a l a n a l y s i s p e r c e n t oven dry SiO 2 F e 2 0 3 A1203 T i O 2 CaO MgO

ACKNOWLEDGEMENTS T h i s p a p e r is p u b l i s h e d with p e r m i s s i o n of the D i r e c t o r of A g r i c u l t u r e , S a r a w a k . M o s t of the m a t e r i a l u s e d in t h i s study w a s c o l l e c t e d d u r i n g s o i l s u r v e y w o r k for this D e p a r t m e n t . Some data r e l a t e d to s o i l s u r v e y s c a r r i e d out by c o l l e a g u e ' s J.R.D. W a l l a n d I.M. Scott h a s b e e n m a d e u s e of a n d t h e i r work i s g r a t e f u l l y acknowledged. Most a n a l y t i c a l data w e r e c a r r i e d out by the C h e m i s t r y D i v i s i o n of the D e p a r t m e n t of A g r i c u l t u r e with the e x c e p t i o n of the m i n e r a l o g i c a l a n a l y s e s for w h i c h I have to t h a n k W . L . P . J . M o u t h a a n of the Soils I n s t i t u t e , U n i v e r s i t y of U t r e c h t for his k i n d a s s i s t a n c e , a n d P r o f . Dr. F . A . v a n B a r e n for a l l o w i n g m e to m a k e u s e of the f a c i l i t i e s i n the U n i v e r s i t y l a b o r a t o r i e s . S p e c i a l t h a n k s a r e due to Dr. H.J. van M. H a r m s e who c a r r i e d out the c l a y m i n e r a l a n a l y s e s a l s o in U t r e c h t . REFERENCES

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