ATOLL RESEARCH BULLETIN NO. 339
PACIFIC ATOLL SOILS:
CHEMISTRY, MINERALOGY AND CLASSIFICATION BY
R. J. MORRISON
ISSUED BY
NATIONAL MUSEUM OF NATURAL HISTORY SMITHSONIAN INSTITUTION
WASHINGTON, D.C., U S A . SEPTEMBER 1990
PACIFIC ATOLL SOILS:
CHEMISTRY, MINERALOGY AND CLASSIFICATION BY
R J. MORRISON
A t o l l s a r e e s s e n t i a l l y r e e f s o f v a r i a b l e t h i c k n e s s b u i l t up by c o r a l s ( a n d o t h e r o r g a n i s m s ) r e s t i n g on a v o l c a n i c base; t h e y a r e u n i q u e t o t r o p i c a l and c e r t a i n s u b t r o p i c a l oceans s i n c e t h e r e e f - b u i l d i n g o r g a n i s m s r e q u i r e w a t e r t e m p e r a t u r e s i n e x c e s s o f 22OC. A t o l l s a r e w i d e s p r e a d i n t h e S o u t h P a c i f i c o c c u r r i n g f r o m Oeno and D u c i e e a s t o f F r e n c h P o l y n e s i a t o Papua New Guinea i n t h e w e s t , f r o m t h e N o r t h e r n M a r s h a l l I s l a n d s i n t h e n o r t h t o New C a l e d o n i a i n t h e s o u t h . Some c o u n t r i e s c o n s i s t e n t i r e l y o f a t 0 1 1s e . g . K i r i b a t i
,
T u v a l u , T o k e l a u , o t h e r s c o n t a i n a t o l l groups e . g . t h e Tuamotu A r c h i p e l a g o i n F r e n c h P o l y n e s i a ; and some c o u n t r i e s c o n s i s t o f m a i n l y v o l c a n i c i s l a n d s w i t h a few i s o l a t e d a t o l l s e . g . Ontong Java i n t h e Solomon I s l a n d s .The c l a s s i c a l f o r m o f an a t o l l i s a more o r l e s s c o n t i n u o u s emerged o r s l i g h t l y emerged c a l c a r e o u s r e e f s u r r o u n d i n g a l a g o o n b u t Cumberland ( 1 9 5 6 ) has i d e n t i f i e d s i x t y p e s o f i s l a n d i n t h e P a c i f i c p a r t l y o r w h o l l y a s s o c i a t e d w i t h c o r a l r e e f s . I n t h i s paper we s h a l l c o n s i d e r a t o l l s as b e i n g o f t w o m a j o r t y p e s ;
( a ) low a t o l l s where t h e maximum h e i g h t o f t h e emerged p o r t i o n ( u s u a l l y l e s s t h a n 5 m) i s made up o f a c c u m u l a t i o n s o f b r o k e n r e e f m a t e r i a l d e p o s i t e d by s t o r m s , e . g . Takapoto i n F r e n c h P o l y n e s i a , Tarawa i n K i r i b a t i , and
( b ) r a i s e d a t o l l s where t h e whole i s l a n d has been t e c t o n i c a l l y up1 i f t e d and t h e a t 0 1 1 morphology l a r g e l y
* P r o f e s s o r o f C h e m i s t r y , U n i v e r s i t y o f t h e S o u t h P a c i f i c , P 0 Box 1 1 6 8 , Suva. F I J I .
Manuscript received 4 December 1989; revised 20 August 1990
f o s s i l i z e d (new f r i n g i n g r e e f s may have d e v e l o p e d a r o u n d them) e . g . L i f o u i n t h e L o y a l t y I s l a n d s (New C a l e d o n i a ) , Nauru and N i u e . T h i s l a t t e r g r o u p i s a l s o o f t e n r e f e r r e d t o g e o l o g i c a l l y as r a i s e d c o r a l p l a t f o r m s .
The d e p o s i t i o n and a c c u m u l a t i o n o f v o l c a n i c ash a n d / o r pumice has o c c u r r e d on many a t o l l s , b u t t h e e x t e n t o f t h i s o c c u r r e n c e d e c r e a s e s on moving f r o m t h e t e c t o n i c a l l y a c t i v e zone o f r e c e n t v o l c a n i s m (10-25OS) i n t h e s o u t h - w e s t P a c i f i c t o w a r d s t h e e q u a t o r .
The w i d e l y v a r i e d n a t u r e o f a t o l l s makes i t d i f f i c u l t t o g e n e r a l i s e a b o u t them b u t many have some f e a t u r e s i n common. They u s u a l l y have l i m i t e d l a n d a r e a and few n a t u r a l r e s o u r c e s . Low a t o l l s , p a r t i c u l a r l y i n t h e e a s t e r n P a c i f i c have l i m i t e d s u p p l i e s o f f r e s h w a t e r and many a r e s u b j e c t t o p r o l o n g e d d r o u g h t s . The g r o u n d w a t e r i s o f t e n b r a c k i s h ( s l i g h t l y s a l t y ) . T h i s p e c u l i a r e n v i r o n m e n t has r e s u l t e d i n t h e d e v e l o p m e n t o f a s p e c i a l i s e d f l o r a - a p l a n t community a d a p t e d t o s a l i n e , a l k a l i n e s o i l s , s u b j e c t t o w a t e r s t r e s s and s a l t s p r a y . The n a t u r a l v e g e t a t i o n i s m o s t l y s t r a n d s p e c i e s r e c r u i t e d f r o m t h e I n d o - P a c i f i c s t r a n d f l o r a o f t h e s h o r e s o f i s l a n d s o f a l l k i n d s i n t h e r e g i o n . The a g r i c u l t u r e i s a l s o r a t h e r s p e c i a l i s e d , o f t e n b e i n g r e s t r i c t e d t o c o c o n u t s , pandanus, b r e a d f r u i t and such r o o t c r o p s as C o l o c a s i a and C y r t o s p e r m a grown i n p i t s dug down t o t h e g r o u n d w a t e r t a b 1 e .
The r a i s e d a t 0 1 1s f r e q u e n t l y have s i g n i f i c a n t 1 y g r e a t e r l a n d a r e a s and u s u a l l y have r e c e i v e d a d d i t i o n s o f v o l c a n i c o r o t h e r n o n - c a l c a r e o u s m a t e r i a l s . The g r e a t e r l a n d a r e a u s u a l l y has a l a r g e r a s s o c i a t e d g r o u n d w a t e r l e n s t h a t can be e x p l o i t e d f o r human c o n s u m p t i o n o r i r r i g a t i o n b u t t h e r a i n f a l l i s s t i l l g e n e r a l l y r e l a t e d t o l o c a t i o n and i n t h e c e n t r a l and e a s t e r n P a c i f i c p r o l o n g e d d r o u g h t s a r e common. On t h e r a i s e d a t o l l s t h e s o i l s a r e u s u a l l y o l d e r and b e t t e r d e v e l o p e d as i s t h e n a t i v e v e g e t a t i o n . S u b s t a n t i a l f o r e s t s o f C a l o ~ h y l l u m and r e l a t e d s p e c i e s have d e v e l o p e d on many o f t h e s e r a i s e d i s l a n d s .
A more d e t a i l e d d i s c u s s i o n o f t h e c h e m i s t r y and m i n e r a l o g y o f t h e s o i l s f r o m a number o f i s l a n d s i s
i n c l u d e d l a t e r i n t h i s p a p e r , b u t some g e n e r a l p o i n t s a b o u t a t o l l s o i l s can be made a t t h i s p o i n t . S o i l p r o p e r t i e s a r e t o a l a r g e e x t e n t d o m i n a t e d by t h e c a l c a r e o u s n a t u r e o f t h e p a r e n t m a t e r i a l , w h e t h e r o r n o t t h i s i s c o v e r e d w i t h v o l c a n i c a s h o r o t h e r m a t e r i a l s . The s o i l s t e n d t o be s h a l l o w , a l k a l i n e , c o a r s e t e x t u r e d , h a v i n g c a r b o n a t i c o r ( w h e r e n o n - c a l c a r e o u s d e p o s i t s have accumu 1 a t e d and w e a t h e r e d e . g . L i f o u , N i u e ) o x i d i c m i n e r a l o g y e x c e p t where t h e r e have been r e l a t i v e l y r e c e n t
a d d i t i o n s o f v o l c a n i c ash. The s o i l s a r e g e n e r a l l y o f v e r y low s i l i c a c o n t e n t . The f e r t i l i t y i s h i g h l y dependent on t h e o r g a n i c m a t t e r c o n t e n t . O r g a n i c m a t t e r can be h i g h i n u n d i s t u r b e d s o i l s under n a t u r a l v e g e t a t i o n , b u t can d e c r e a s e d r a m a t i c a l l y a s a r e s u l t o f i n a p p r o p r i a t e c u l t i v a t i o n t e c h n i q u e s e . g . l a n d c l e a r a n c e and weed c o n t r o l by f i r e .
As f o r a l l t r o p i c a l s o i l s , o r g a n i c m a t t e r i n a t o l l s o i l s p e r f o r m s an i m p o r t a n t r o l e i n t h e c o n c e n t r a t i o n and c y c l i n g o f p l a n t n u t r i e n t s . I n a t o l l s o i l s , however, a second r o l e - t h a t o f m o i s t u r e r e t e n t i o n - i s e q u a l l y i m p o r t a n t . S i n c e a t o l l s o i l s a r e f r e q u e n t l y sandy and e x c e s s i v e l y we1 1 d r a i n e d , t h e m o i s t u r e r e t e n t i o n i n t h e absence o f o r g a n i c m a t t e r i s v e r y l o w ; t h e t o t a l amount o f w a t e r r e t a i n e d o f t e n r e m a i n s low and p l a n t s a r e s u b j e c t t o w a t e r s t r e s s u n l e s s t h e r a i n f a l l i s h i g h and r e l a t i v e l y c o n s t a n t o r t h e y can t a p t h e f r e s h w a t e r l e n s .
Guano d e p o s i t i o n i s common on a t o l l s , g i v i n g r e a s o n a b l e s u p p l i e s o f phosphorus. P o t a s s i u m l e v e l s , on t h e o t h e r hand, a r e f r e q u e n t l y e x t r e m e l y l o w , ' and t h e n e u t r a l t o a l k a l i n e pH o f t h e s e s o i l s can make s e v e r a l o f t h e t r a c e e l e m e n t s , p a r t i c u l a r l y i r o n , manganese and z i n c , u n a v a i l a b l e t o p l a n t s . The c o a r s e t e x t u r e , h i g h pH and c a l c i u m d o m i n a t i o n o f t h e exchange complex o f many a t o l l s o i l s would t e n d t o i n d i c a t e a low c a p a c i t y t o r e t a i n s u l p h u r , b u t some s u l p h u r i s made a v a i l a b l e c o n t i n u o u s l y by s o l u t i o n o f t h e c o r a l l i n e m a t e r i a l s w h i c h t y p i c a l l y c o n t a i n 0 . 2 - 0 . 3 % s u l p h u r p l u s a t m o s p h e r i c s u l p h u r d e r i v e d f r o m s e a - s p r a y .
F o r t h e p u r p o s e s o f t h i s d i s c u s s i o n , a t o l l s o i l s w i l l be c o n s i d e r e d i n 2 g r o u p s - low a t o l l s and r a i s e d a t o l l s . For t h e low a t o l l s i n f o r m a t i o n f r o m Tarawa, T u v a l u , Cook I s l a n d s and t h e F r e n c h P o l y n e s i a n a t o l l s o f T a k a p o t o and T i k e h a u w i l l be c o n s i d e r e d , w h i l e d i s c u s s i o n o f r a i s e d a t o l l s w i l l be r e s t r i c t e d t o L i f o u i n New C a l e d o n i a , Nauru and N i u e .
CHEMISTRY OF LOW ATOLL SOILS
Tarawa i n K i r i b a t i (1730E, 1 ° 2 0 ' b 4 ) i s a t y p i c a l example o f a low a t o l l . I t i s r o u g h l y t r i a n g u l a r i n shape w i t h t h e s o u t h e r n arm b e i n g one of t h e most d e n s e l y p o p u l a t e d a r e a s i n t h e P a c i f i c i s l a n d s . R e c e n t l y t h e s o i l s o f t w o a r e a s have been s t u d i e d i n d e t a i l ( S e r u and M o r r i s o n , 1985; M o r r i s o n and S e r u , 1 9 8 6 ) . The c h e m i c a l d a t a o b t a i n e d a r e t y p i c a l of t h o s e f o r s o i l s o f l o w a t o l l s .
Many o f
the
p r o p e r t i e s a r e r e l a t e d t o t h e o r g a n i cm a t t e r c o n t e n t o f t h e t o p s o i l . O r g a n i c c a r b o n v a l u e s v a r y f r o m a b o u t 2-20% d e p e n d i n g on t h e age o f t h e s o i l , t h e v e g e t a t i o n and s o i l management. I n t h e s u b s o i l s o r g a n i c c a r b o n v a l u e s a r e a l w a y s low ( < 0 . 5 % ) u n l e s s t h e r e has been c o n s i d e r a b l e s o i l d i s t u r b a n c e , e . g . , due t o t h e d i g g i n g o f C y r t o s p e r m a p i t s . N i t r o g e n v a l u e s f o l l o w t h e o r g a n i c c o n t e n t s c l o s e l y as C : N r a t i o s u s u a l l y r a n g e f r o m 9-12 f o r t o p s o i l s and 8-12 f o r s u b s o i l s . Water r e t e n t i o n a g a i n s t 15 b a r p r e s s u r e i s c l o s e l y c o r r e l a t e d w i t h o r g a n i c m a t t e r c o n t e n t ; v a l u e s o f 5-25% have been o b t a i n e d f o r t o p s o i l s , w h i l e f o r s u b s o i l s t h e v a l u e s a r e a l w a y s low ( 2 - 4 % ) . C a t i o n exchange c a p a c i t y i s a l s o c l o s e l y r e l a t e d t o o r g a n i c m a t t e r f o r t o p s o i l s w i t h v a l u e s i n t h e r a n g e 6-60 cmol/kg w h i l e t h e v a l u e s f o r t h e sandy c a l c a r e o u s s u b s o i l s a r e u s u a l l y l e s s t h a n 2 c m o l / k g . Exchangeable magnesium v a l u e s a r e g e n e r a l l y a r o u n d 4-6 c m o l / k g , sodium c o n t e n t s a r e a b o u t 0 . 2 c m o l / k g b u t p o t a s s i u m v a l u e s a r e a l w a y s low r i s i n g a b o u t 0 . 1 c m o l / k g o n l y i n t o p s o i l s w i t h s u b s t a n t i a l o r g a n i c m a t t e r c o n t e n t s .
The c a l c i u m c a r b o n a t e c o n t e n t i s a l w a y s h i g h r a n g i n g f r o m 55-90% f o r t o p s o i l s and b e i n g g r e a t e r t h a n 90% f o r s u b s o i l s . T h i s dominance o f t h e e n v i r o m e n t by c a r b o n a t e l e a d s t o h i g h pH v a l u e s ; pH ( w a t e r ) v a l u e s f o r t o p s o i l s were i n t h e r a n g e 7 . 5 - 8 . 5 and f o r s u b s o i l s 8 . 5 - 9 . 1 ; pH
(CaC12, 0 . 0 1 m o l / L ) v a l u e s were i n t h e range 7 . 1 - 7 . 6 f o r t o p s o i l s and 7 . 5 - 8 . 0 f o r s u b s o i l s .
E x t r a c t a b l e phosphorus ( O l s e n p r o c e d u r e ) v a l u e s a r e g e n e r a l l y low (5-15 mg/kg) f o r t o p s o i l s and v e r y low ( ( 1 mg/kg) f o r s u b s o i l s . The t o t a l phosphorus c o n t e n t s v a r y c o n s i d e r a b l y r a n g i n g f r o m 500-30,000 mg/kg f o r t o p s o i l s and f r o m 300-5000 mg/kg f o r s u b s o i l s . Phosphate e x t r a c t a b l e s u l p h a t e v a l u e s r a n g e f r o m 20-50 mg/kg f o r t o p s o i l s b u t a r e g e n e r a l l y low ( < 2 0 mg/kg) i n s u b s o i l s . T o t a l s u l p h u r v a l u e s a r e f a i r l y c o n s t a n t a t around 4000 mg/kg f o r a l l s o i l s .
T u v a l u c o n s i s t s o f 9 i s l a n d s l y i n g between 5O and
l o 0
S l a t i t u d e and 175O and 180° E l o n g t i t u d e . The i s l a n d s a r e a l l low a t o l l s , f r e e o f any m a j o r d e p o s i t s o f v o l c a n i c m a t e r i a l s a l t h o u g h p h o s p h a t i c m a t e r i a l s a r e c o n c e n t r a t e d i n s m a l l a r e a s . As p a r t o f a programme t o d e v e l o p some i n t e n s i f i e d a g r i c u l t u r e ( S . C a i g e r , p e r s o n a l c o m m u n i c a t i o n ) a number o f s o i l samples have been a n a l y s e d a t t h e Land Resources Development C e n t r e L a b o r a t o r y (UK) and a t t h e U n i v e r s i t y o f t h e S o u t h P a c i f i c I n s t i t u t e o f N a t u r a l Resources L a b o r a t o r y i n Suva. E x a m i n a t i o n o f t h e s e d a t a ( T a b l e 1 ) shows t h a t t h e r e a r e many s i m i l a r i t i e s w i t h t h a t o b t a i n e d on Tarawa. One m a j o r d i f f e r e n c e i s t h e presence i n T u v a l u o f a r e a s o f known p h o s p h a t e a c c u m u l a t i o n . These may be r e l a t e d t o b i r d n e s t i n g a c t i v i t i e s a s s o c i a t e d w i t h
TABLE 1 . Chemical D a t a f o r Some T u v a l u S o i l s
SUBS01 LS Range
Determi n a t i o n Range
pH 1 : 5 H20
E.C.
ms/cm 1:5 H20 CaC03 %Exchangeable Na
C a t i o n s K
T o t a l N % Org C % O l s e n P ppm
H o t H20 s o l B ppm
s p e c i f i c v e g e t a t i o n t y p e s i n t h e p a s t . These a r e a s g i v e v e r y h i g h t o t a l p h o s p h o r u s v a l u e s ( 2 - 5 % P ) and h i g h O l s e n e x t r a c t a b l e p h o s p h o r u s v a l u e s ( > 50 m g / k g ) .
The d a t a h e r e a l s o c l e a r l y i n d i c a t e t h e v e r y l o w l e v e l s o f c e r t a i n m i c r o n u t r i e n t s f o u n d i n t h e s o i l s o f l o w a t o l l s . I r o n , manganese, c o p p e r and z i n c a r e a l l p r e s e n t i n e x t r e m e l y l o w ( t o t a l ) amounts, s u c h t h a t t h e p l a n t a v a i l a b l e s u p p l i e s o f t h e s e e l e m e n t s m u s t be m i n i m a l i n t h e absence o f e x t e r n a l s u p p l i e s .
D a t a o b t a i n e d f o r t h e s o i l s o f Manuae and P a l m e r s t o n i n t h e Cook I s l a n d s ( B r u c e , 1 9 7 2 ) show v e r y s i m i l a r p a t t e r n s t o t h o s e d e s c r i b e d a b o v e . pH v a l u e s f r o m 7 . 1 - 8 . 3 , o r g a n i c c a r b o n c o n t e n t s o f 2-6%, C / N r a t i o s o f 11-18, t o t a l p h o s p h o r u s l e v e l s o f 100-1600 mg/kg and a v a i l a b l e p h o s p h o r u s o f 20-80 mg/kg were r e c o r d e d . CEC v a l u e s were g e n e r a l l y l o w ( 5 - 2 6 c m o l / k g ) w i t h v e r y l o w l e v e l s o f p o t a s s i u m .
T e r c i n i e r ( 1 9 6 9 ) i n h i s d i s c u s s i o n o f T a k a p o t o a l s o s t r e s s e s t h e i m p o r t a n c e o f t h e o r g a n i c m a t t e r c o n t e n t . He o b s e r v e d a r e l a t i o n s h i p between o r g a n i c m a t t e r and w a t e r r e t e n t i o n . O r g a n i c m a t t e r i n c r e a s e s i n m o v i n g f r o m t h e ocean s i d e s , bombarded by waves t o more s t a b l e a r e a s n e a r t h e l a g o o n . The c a t i o n r e t e n t i o n c a p a c i t y o f t h e s o i l s i s a l s o c l o s e l y r e l a t e d t o t h e o r g a n i c m a t t e r c o n t e n t and t h e a c c u m u l a t i o n o f n i t r o g e n , p h o s p h o r u s and p o t a s s i u m i n t h e o r g a n i c r i c h s u r f a c e l a y e r s was o b s e r v e d .
On T i k e h a u , Jamet ( 1 9 8 5 ) f o u n d t h a t t h e c a l c i u m c a r b o n a t e c o n t e n t o f t o p s o i l s was 80-90% w i t h g e n e r a l l y more t h a n 90% i n t h e s u b s o i l s . T o t a l p o t a s s i u m c o n t e n t was a l w a y s l o w ( < 0 . 0 5 % ) as was e x c h a n g e a b l e p o t a s s i u m ( < 0 . 3 c m o l ( p + ) / k g ) w h i l e t h e p h o s p h o r u s l e v e l s v a r i e d d e p e n d i n g o n w h e t h e r o r n o t s i g n i f i c a n t guano d e p o s i t i o n had o c c u r r e d . S u b s o i l pH v a l u e s u s u a l l y were between 8 and 9, b u t i n t h e t o p s o i l s w i t h r e l a t i v e l y h i g h o r g a n i c m a t t e r c o n t e n t s t h e v a l u e s r a n g e d f r o m 7 . 0 - 7 . 5 .
Thus i t c a n be seen t h a t i n t h e l o w a t o l l s t h e s o i l s a r e a l k a l i n e , w i t h m o s t o f t h e s o i l " f e r t i l i t y " r e l a t e d t o t h e a c c u m u l a t e d o r g a n i c m a t e r i a l . Under t h e s e c o n d i t i o n s n i t r i f i c a t i o n i s f a v o u r e d b u t t o x i c a c c u m u l a t i o n s o f n i t r a t e a r e u n l i k e l y u n l e s s t h e r e a r e u n u s u a l h y d r o l o g i c a l c o n d i t i o n s . V o l a t i l i z a t i o n o f n i t r o g e n a s NH3 f r o m ammonium and u r e a f e r t i l i z e r s w i l l o c c u r , w i t h p a r t i c u l a r 1 y l a r g e l o s s e s o c c u r i n g i f t h e s e m a t e r i a l s a r e n o t i n c o r p o r a t e d . The a v a i 1 a b i 1 i t y o f p h o s p h o r u s i s c o n t r o l l e d by c a l c i u m a c t i v i t y and much f e r t i l i z e r P w i l l be p r e c i p i t a t e d a s c a l c i u m p h o s p h a t e s o r a d s o r b e d o n t h e s u r f a c e s o f t h e c a r b o n a t e s . Band a p p l i c a t i o n s o f P a r e
recommended s o t h a t P w i 11 be a v a i l a b l e t o seed1 i n g s soon a f t e r emergence. K a v a i l a b i l i t y i s d e c r e a s e d by h i g h Ca a n d / o r Mg l e v e l s ; t h e l o w l e v e l s o f K i n t h e c o r a l l i m e s t o n e p a r e n t m a t e r i a l s mean t h a t t h i s e l e m e n t w i l l a l w a y s be i n s h o r t s u p p l y .
S u p p l i e s o f a v a i l a b l e Ca and Mg a r e p l e n t i f u l i n l o w a t o l l s o i l s b u t i m b a l a n c e s w i t h K and m i c r o n u t r i e n t s cause s i g n i f i c a n t p l a n t n u t r i t i o n p r o b l e m s . S u l p h u r i s u s u a l l y a v a i l a b l e i n s m a l l q u a n t i t i e s f r o m s o l u t i o n o f l i m e s t o n e and f r o m r a i n w a t e r b u t i f c r o p s w i t h l a r g e S r e q u i r e m e n t s a r e grown i n t e n s i v e l y
,
e x t e r n a l a d d i t i o n s w i 1 1 be r e q u i r e d . A v a i 1 a b l e B d e c r e a s e s m o d e r a t e 1 y w i t h i n c r e a s i n g s o i l pH, b u t few 3 d e f i c i e n c i e s have been r e p o r t e d on c a l c a r e o u s s o i l s . Mo s o l u b i l i t y and a v a i l a b i l i t y i n c r e a s e w i t h i n c r e a s i n g pH b u t low Mo c o n t e n t s i n t h e p a r e n t r o c h may l e a d t o d e f i c i e n c i e s .S o l u b i l i t i e s o f Cu, Fe, Mn, Zn d e c r e a s e w i t h i n c r e a s i n g s o i l pH. Cu d e f i c i e n c i e s a r e l e s s r e l a t e d t o s o i l pH t h a n a r e t - h o s e o f t h e o t h e r m i c r o n u t r i e n t s . Zn f o r m s r e l a t i v e l y i n s o l u b l e z i n c a t e s i n c a l c a r e o u s s o i l s and Fe u p t a k e i s r e d u c e d by h i g h b i c a r b o n a t e 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 . W i t h t h e r e l a t i v e l y low c o n t e n t s i n c o r a l l i m e s t o n e a l l o f t h e s e e l e m e n t s a r e l i k e l y t o be h i g h l y d e f i c i e n t i n s o i l s o f t h e low a t o l l s .
MTNERALOGV OF LOW ATOLL SOILS
As t h e s o i l s o f t h e low a t o l l s a r e d o m i n a t e d by c a l c i u m c a r b o n a t e t h e m i n e r a l o g y i s a l m o s t e x c l u s i v e l y c a r b o n a t e . The d o m i n a n t r n i ~ e r a l s a r e c a l c i t e and a r a g o n i t e , t h e common f o r m s o f c a l c i u m c a r b o n a t e d e p o s i t e d by r e e f f o r m i n g and r e e f l i v i n g o r g a n i s m s . C a l c i t e c o n t a i n s v a r y i n g amounts o f magnesium ( s u b s t i t u t i n g f o r c a l c i u m i n t h e m i n e r a l s t r u c t u r e ) . I f t h e magnesi um c o n t e n t i s > I % , t h e m i n e r a l i s d e s c r i b e d as h i g h magnesium c a l c i t e ; o t h e r f o r m s a r e r e f e r r e d t o a s l o w magnesium c a l c i t e . T h e r e i s c o n s i d e r a b l e v a r i a t i o n i n t h e m i n e r a l c o n t e n t o f l o w a t o l l s o i l s as i l l u s t r a t e d b e l o w .
The r e s u l t s o f t h e m i n e r a l o g i c a l a n a l y s e s o f s a m p l e s f r o m s e v e r a l pedons on A b a t a o I s l e t , Tarawa a r e summarised i n T a b l e 2 ( M o r r i s o n and S e r u , 1 9 8 6 ) . These c o n f i r m t h e dominance o f t h e 2 p r i n c i p a l f o r m s o f c a l c i u m c a r b o n a t e - c a l c i t e and a r a g o n i t e . I n c o n t r a s t t o t h e l a g o o n s e d i m e n t m i n e r a l s a m p l e s , (Weber and Woodhead, 1 9 7 2 ) c a l c i t e i s t h e d o m i n a n t m i n e r a l i n a l l o f t h e s o i l s a m p l e s w i t h a r a g o n i t e b e i n g t h e m i n o r component. The r e l a t i v e p r o p o r t i o n o f h i g h and l o w magnesium c a l c i t e v a r i e s b u t t h e r e i s n o p a r t i c u l a r p a t t e r n t o t h i s v a r i a t i o n e x c e p t t h a t h i g h magnesium c a l c i t e t e n d s t o d o m i n a t e i n s o i l s on t h e l a g o o n
SOIL TRW 7
8 9
9 10 11 12 13 16 17 17 18
TABLE 2. ABATAO SOILS : APPROXIMATE MINERALOGICAL COMPOSITION
( < 200 MESH SAMPLES)
DEPTH (cm) ARAGONITE CALCITE (High Mg) CALCITE (Low Mg)
6 - 3 1 3 5 ( > 7 0 % ) 2
3 1 - 60 2 5 ( > 70%) 2
0 - 15 3 4 3
50 - 60 2 5 ( > 70%) 2
OTHERS
A p a t i t e 2
s i d e o f t h e i s l e t s . A p a t i t e was d e t e c t e d i n o n l y one sample.
The m i n e r a l o g y d a t a f o r t h e Tarawa s o i l s a r e i n g e n e r a l agreement w i t h t h e o b s e r v a t i o n s o f Hathaway ( 1 9 6 5 ) f o r s o i l s o f t h e N o r t h e r n M a r s h a l l I s l a n d s . Hathaway f o u n d t h a t h i g h magnesium c a l c i t e was t h e dominant m i n e r a l i n most s o i l s , e x c e p t f o r t h e c o a r s e s t f r a g m e n t s i s some p r o f i l e s where a r a g o n i t e was t h e m a j o r c o n s t i t u e n t . Low magnesium c a l c i t e was p r e s e n t i n o n l y one sample. Hammond ( 1 9 6 9 ) i n a s t u d y o f s o i l s o f C h r i s t m a s I s l a n d ( K i r i t i m a t i ) f o u n d t h a t a r a g o n i t e was t h e d o m i n a n t component, t h i s m i n e r a l b e i n g p r e s e n t i n t h e g r e a t e s t amounts i n t h e c o a r s e f r a g m e n t s . Cal c i t e was more abundant where f o r a m i n i f e r a dominated t h e d e p o s i t s . Most c a l c i t e was h i g h i n magnesium b u t low magnesium c a l c i t e was p r e s e n t m a i n l y i n t h e c o a r s e sand, v e r y f i n e sand and s i l t f r a c t i o n s .
Samples f r o m Manuae and P a l m e r s t o n showed a g e n e r a l dominance o f a r a g o n i t e o v e r c a l c i t e w i t h t h e a r a g o n i t e c o n t e n t b e i n g g r e a t e s t i n t h e c o a r s e r s o i l s ( B r u c e , 1 9 7 2 ) . S i g n i f i c a n t amounts (10-20%) o f a p a t i t e were a l s o d e t e c t e d and a s m a l l amount o f an unknown amorphous m a t e r i a l was p r e s e n t i n a l l samples.
A r a g o n i t e w h i c h f o r m s t h e h a r d p a r t s o f c o r a l s o r a1 gae and h i g h magnesi um c a l c i t e f r o m a1 g a l s k e l e t a 1 m a t e r i a l a r e more abundant i n most s h a l l o w w a t e r m a r i n e e n v i r o n m e n t s t h a n low magnesium c a l c i t e , b u t among deep sea oozes r i c h i n c a l c i t i c f o r a m i n i f e r a and c o c c o l i t h s , t h e more s t a b l e low magnesium c a l c i t e i s t h e p r e d o m i n a n t phase (Chave, 1 9 6 2 ) . I t s h o u l d be n o t e d t h a t low magnesium c a l c i t e i s t h e t h e r m o d y n a m i c a l l y s t a b l e phase under t h e c o n d i t i o n s p r e v a i l i n g a t t h e E a r t h ' s s u r f a c e .
The p r e s e n c e o f s u b s t a n t i a l q u a n t i t i e s of low magnesium c a l c i t e i n t h e s u r f a c e h o r i z o n s o f f o u r Abatao pedons i s o f c o n s i d e r a b l e i n t e r e s t . W h i l e t h i s m i n e r a l was f o u n d on C h r i s t m a s I s l a n d i t was a l w a y s a m i n o r component (Hammond, 1969) and was o n l y d e t e c t e d i n one N o r t h e r n M a r s h a l l s sample (Hathaway, 1 9 6 5 ) . The p r e s e n c e o f t h i s m i n e r a l may be caused by s u r f a c e a d d i t i o n o f a l g a l s k e l e t a l m a t e r i a l o f low magnesium c o n t e n t o r may be t h e r e s u l t o f a l t e r a t i o n o f a r a g o n i t e t o c a l c i t e , a l t h o u g h t h e l a t t e r p r o c e s s was c o n s i d e r e d t o be o f l i t t l e i m p o r t a n c e i n t h e N o r t h e r n M a r s h a l l s (Hathaway, 1 9 6 5 ) . Chave ( 1 9 6 2 ) i n a s t u d y o f c a r b o n a t e s e d i m e n t s f o u n d an i n c r e a s e i n low magnesium c a l c i t e c o n t e n t w i t h d e c r e a s e i n p a r t i c l e s i z e and a s c r i b e d t h i s t o a s e l e c t i v e l o s s o f u n s t a b l e m i n e r a l s due t o i n v e r s i o n ( t o a more s t a b l e f o r m ) o r s o l u t i o n .
CLASSIFICATION OF LOW ATOLL SOILS
One o f t h e m a j o r d i f f i c u l t i e s e n c o u n t e r e d when c o n s i d e r i n g t h e c l a s s i f i c a t i o n o f a t o l l s o i l s i s t h e m a j o r change t h a t h a s t a k e n p l a c e i n s o i l c l a s s i f i c a t i o n o v e r t h e l a s t 50 y e a r s . Much o f t h e e a r l y work on a t o l l s o i l s was c a r r i e d o u t by A m e r i c a n s c i e n t i s t s w o r k i n g i n t h e c e n t r a l and w e s t e r n P a c i f i c I s l a n d s i n t h e 1 9 4 0 ' s and e a r l y 1 9 5 0 ' s . A t t h a t t i m e s o i l c l a s s i f i c a t i o n was based on t h e s y s t e m d e v e l o p e d f o r p u b l i c a t i o n i n t h e 1938 " S o i l s and Men" monograph ( B a l d w i n e t a l . , 1 9 3 8 ) . As p o i n t e d o u t by S m i t h ( 1 9 8 3 ) t h i s s y s t e m o f c l a s s i f i c a t i o n was p r o d u c e d h u r r i e d l y t o meet t h e p u b l i c a t i o n d e a d l i n e and a s a consequence many f a c e t s were i n c o m p l e t e l y d e v e l o p e d and d e f i n e d . A l t h o u g h t h e ' 1 9 3 8 ' s y s t e m u n d e r w e n t a s e r i e s o f m o d i f i c a t i o n s up u n t i l 1959, i t was r e c o g n i z e d by 1950 t h a t m a j o r d i f f i c u l t i e s c o u l d n o t be overcome and t h a t t h e d e v e l o p m e n t o f a new s y s t e m was e s s e n t i a l i f a s a t i s f a c t o r y taxonomy was t o be o b t a i n e d . The new taxonomy was d e v e l o p e d t h r o u g h a s e r i e s o f a p p r o x i m a t i o n s and p u b l i s h e d i n 1975 a s S o i l Taxonomy ( S o i l S u r v e y S t a f f , 1 9 7 5 ) . S o i l Taxonomy i s now w i d e l y u s e d t h r o u g h o u t t h e w o r l d and i s c o n t i n u a l l y b e i n g m o d i f i e d as new i n f o r m a t i o n o r b e t t e r c r i t e r i a f o r g r o u p i n g s o i l s become a v a i l a b l e . I n t h i s p a p e r a l l s o i l s a r e c l a s s i f i e d a c c o r d i n g t o S o i l Taxonomy b u t , where a p p r o p r i a t e , r e l a t i o n s h i p s t o t h e ' 1 9 3 8 ' s y s t e m ( a n d i t s m o d i f i c a t i o n s ) a r e i n c l u d e d . P r i o r t o d i s c u s s i n g t h e d e t a i l s o f t h e c l a s s i f i c a t i o n o f i n d i v i d u a l s o i l s , some i n t r o d u c t o r y comments on t h e t w o s y s t e m s a r e p r e s e n t e d .
I n t h e ' 1 9 3 8 ' s y s t e m ( p a r t i c u l a r l y a f t e r t h e r e v i s i o n s up t o and i n c l u d i n g 1949) s o i l s were c l a s s i f i e d i n a h i e r a r c h e a l s y s t e m h a v i n g s i x c a t e g o r i e s - o r d e r , s u b o r d e r , g r e a t s o i l g r o u p , f a m i l y , s e r i e s and t y p e . T h r e e s o i l o r d e r s ( Z o n a l , I n t r a z o n a l and A z o n a l ) were d e v e l o p e d . A t o l l s o i l s were i n c l u d e d i n t h e H a l o m o r p h i c and C a l c i m o r p h i c s u b o r d e r s o f t h e I n t r a z o n a l s o i l s p l u s t h e Azonal s o i l s ( n o s u b o r d e r s were p r e p a r e d f o r t h i s o r d e r ) . The p r i n c i p a l g r e a t g r o u p s were R e n d z i n a s o i l s , L i t h o s o l s and R e g o s o l s . R e n d z i n a s were s o i l s h a v i n g d a r k c o l o u r e d b a s e - r i c h s u r f a c e h o r i z o n s f o r m e d f r o m c a l c a r e o u s p a r e n t m a t e r i a l s i n humid a r e a s m a i n l y u n d e r f o r e s t . L i t h o s o l s were s o i l s w i t h t h i n , and o f t e n i r r e g u l a r , h o r i z o n s o v e r r o c k w h i l e R e g o s o l s were d e v e l o p e d m a i n l y f r o m s o f t o r u n c o n s o l i d a t e d p a r e n t m a t e r i a l s w i t h o r w i t h o u t t h i n c o v e r i n g s o f t r u e s o i l .
I n S o i l Taxonomy t h e r e a r e s i x c a t e g o r i e s i n t h e h i e r a r c h e a l s y s t e m . The b a s i s f o r t h e g r o u p i n g s i n e a c h c a t e g o r y i s summarised i n T a b l e 3 . Low a t o l l s o i l s w i l l
u s u a l l y be f o u n d i n t h e o r d e r s o f E n t i s o l s ( i . e . , r e c e n t s o i l s , h a v i n g no m a j o r d i a g n o s t i c f e a t u r e s ) , I n c e p t i s o l s ( s o i l s n e a r t o p o i n t o f i n c e p t i o n , showing e v i d e n c e o f p r o f i l e development beyond t h a t o f t h e E n t i s o l s ) o r M o l l i s o l s ( b a s e - r i c h s o i l s h a v i n g r e l a t i v e l y deep d a r k c o l o u r e d , o r g a n i c r i c h , w e l l - s t r u c t u r e d s u r f a c e h o r i z o n s - t h e m o l l i c e p i p e d o n ) .
TABLE 3 The D i f f e r e n t i a t i n g C h a r a c t e r i s t i c s o f t h e C a t e g o r i e s i n S o i l Taxonomy ( a f t e r Buol e t a l . ,
1980)
- -- - - -
C a t e g o r y N a t u r e o f D i f f e r e n t i a t i n g C h a r a c t e r i s t i c s
Order S o i l - f o r m i n g p r o c e s s e s as i n d i c a t e d by p r e s e n c e o r absence o f m a j o r d i a g n o s t i c h o r i zons.
S ~ b o r d e r G e n e t i c h o m o g e n e i t y . S u b d i v i s i o n o f o r d e r s a c c o r d i n g t o p r e s e n c e o r absence o f
p r o p e r t i e s a s s o c i a t e d w i t h w e t n e s s , s o i l
m o i s t u r e r e g i m e s , m a j o r p a r e n t m a t e r i a l , and v e g e t a t i o n a l e f f e c t s as i n d i c a t e d by key
p r o p e r t i e s ; o r g a n i c f i b r e d e c o m p o s i t i o n s t a g e i n H i s t o s o l s .
G r e a t group S u b d i v i s i o n o f s u b o r d e r s a c c o r d i n g t o s i m i l a r k i n d , a r r a n g e m e n t , and d e g r e e of e x p r e s s i o n o f h o r i z o n s , w i t h emphasis on upper sequum;
base s t a t u s ; s o i l t e m p e r a t u r e and m o i s t u r e r e g i m e s ; p r e s e n c e o r absence o f d i a g n o s t i c l a y e r s ( p l i n t h i t e , f r a g i p a n , d u r i p a n ) . Subgroup C e n t r a l c o n c e p t t a x a f o r g r e a t group and
p r o p e r t i e s i n d i c a t i n g i n t e r g r a d a t i o n s t o o t h e r g r e a t g r o u p s , s u b o r d e r s , and o r d e r s ; e x t r a g r a d a t i o n t o " n o t s o i l . "
S e r i e s
Broad s o i 1 t e x t u r a l c l asses averaged o v e r c o n t r o l s e c t i o n o r s o l um; m i n e r a l o g i c a l c l a s s e s f o r d o m i n a n t m i n e r a l o g y o f solum;
s o i l t e m p e r a t u r e c l a s s e s "based on mean annual s o i l t e m p e r a t u r e a t 50 cm ( 2 0 i n . ) d e p t h s
K i n d and arrangement o f h o r i z o n s ; c o l o u r , t e x t u r e , s t r u c t u r e , c o n s i s t e n c e , and r e a c t i o n o f h o r i z o n s ; 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 h o r i z o n s .
A m a j o r d i f f e r e n c e between t h e ' 1 9 3 8 ' s y s t e m and Soi 1 Taxonomy r e l a t e s t o t h e g r o u p i n g o f s o i 1 s e r i e s i n t o f a m i l i e s . The ' 1 9 3 8 ' system i n t r o d u c e d s o i l f a m i l i e s as a c a t e g o r y between t h e g r e a t s o i l g r o u p s and s o i l s e r i e s , b u t c o n s i d e r a b l e d i f f i c u l t y was e n c o u n t e r e d i n d e t e r m i n i n g what c h a r a c t e r i s t i c s t o use f o r such g r o u p i n g s . As l a t e as 1949 R i e c k e n and S m i t h commented " A t p r e s e n t t h e r e i s l i t t l e p u b l i s h e d m a t e r i a l f o r g u i d a n c e i n t h e g r o u p i n g o f s e r i e s i n t o f a m i l i e s " . I n c o n t r a s t , S o i l Taxonomy has s o i l f a m i l i e s d e f i n e d as g r o u p i n g s o f s o i l s e r i e s " h a v i n g s i m i l a r p h y s i c a l and c h e m i c a l p r o p e r t i e s t h a t a f f e c t t h e i r management and m a n i p u l a t i o n f o r u s e " .
The s o i l s o f t h e l o w a t o l l s f r e q u e n t l y c o n s i s t of a c c u m u l a t i o n s o f o r g a n i c m a t t e r , guano, pumice o r o t h e r t r a n s p o r t e d m a t e r i a l on t o p o f a c a l c a r e o u s sand ( c a l c a r e o u s a l g a e , f o r a m s , s h e l l s , e t c . ) o r l i m e s t o n e s u b s t r a t u m . L i t t l e p r o f i l e development has o c c u r r e d and t h e development o f any s i g n i f i c a n t B h o r i z o n s i s n o t e x p e c t e d because o f t h e n a t u r e o f t h e p a r e n t m a t e r i a l ( c a l c i u m c a r b o n a t e ) , t h e r e l a t i v e y o u t h o f many low a t o l l s ( < 5 , 0 0 0 y e a r s ) and t h e g e o g r a p h i c a l i s o l a t i o n w h i c h has r e s t r i c t e d ( t o g e t h e r w i t h t h e e n v i r o n m e n t ) t h e development o f t h e f l o r a . Repeated removal o f A h o r i z o n s has o c c u r r e d on some a t o l l s as a r e s u l t o f c y c l o n e s , r e q u i r i n g s o i l f o r m a t i o n t o recommence p e r i o d i c a l l y .
As a r e s u l t t h e v a s t m a j o r i t y o f s o i l s o f t h e low a t o l l s a r e E n t i s o l s ( S o i l Survey S t a f f , 1975) c o n s i s t i n g o f t h e a c c u m u l a t i o n o f a t h i n l a y e r o f o r g a n i c r i c h c o r a l sand o v e r t h e c o r a l l i n e s u b s t r a t u m . A t t h e s u b o r d e r l e v e l t h e E n t i s c l s a r e s u b d i v i d e d on t h e b a s i s o f t h e s o i l m o i s t u r e r e g i m e o r t h e p a r t i c l e s i z e d i s t r i b u t i o n i n t h e c o n t r o l s e c t i o n ( 2 5 cm - 1 m , 25 cm t o a l i t h i c c o n t a c t i f one i s p r e s e n t w i t h i n a d e p t h o f 1 m ) . S o i l s h a v i n g t h e g r o u n d w a t e r t a b l e n e a r t h e s u r f a c e have an a q u i c s o i l m o i s t u r e r e g i m e and { a r e c l a s s i f i e d as Aquents.Many a t o l l s o i l s a r e sandy i . e . t h e t e x t u r e o f t h e f i n e e a r t h i s sand o r loamy sand t h a t c o n t a i n s l e s s t h a n 50 p e r c e n t v e r y f i n e sand and r o c k f r a g m e n t s make up l e s s t h a n 35 p e r c e n t by volume ( a n d do n o t have an a q u i c s o i l m o i s t u r e r e g i m e ) and q u a l i f y as Psamments ( R e g o s o l s i n t h e ' 1 9 3 8 ' s y s t e m ) a t t h e s u b o r d e r l e v e l . S o i l s h a v i n g more t h a n 35 p e r c e n t c o a r s e f r a g m e n t s such as c o r a l g r a v e l , s t o n e , and r o c k s q u a l i f y as O r t h e n t s ( L i t h i c R e g o s o l s o r L i t h o s o l s i n t h e ' 1 9 3 8 ' s y s t e m ) . I n b o t h t h e Psamments and t h e O r t h e n t s t h e g r e a t g r o u p s a r e s e p a r a t e d ( w i t h t h e e x c e p t i o n o f t h e Q u a r t i z i p s a m m e n t s ) on t h e b a s i s o f t h e s o i l m o i s t u r e and t e m p e r a t u r e r e g i m e s . The t e m p e r a t u r e r e g i m e f o r most low a t o l l s i s i s o h y p e r t h e r m i c e . , t h e average annual t e m p e r a t u r e a t 50 cm d e p t h i s >22OC1 w i t h t h e v a r i a t i o n f r o m summer t o w i n t e r <5OC), and t h e s o i l m o i s t u r e r e g i m e s
i n t h e absence o f g r o u n d w a t e r i n f l u e n c e a r e u s t i c ( i . e . , t h e s o i l p r o f i l e d r i e s o u t f o r e x t e n d e d p e r i o d s i n most y e a r s ) o r u d i c ( i . e . , t h e s o i l p r o f i l e d r i e s o u t f o r o n l y s h o r t p e r i o d s i n most y e a r s ) . Thus t h e g r e a t g r o u p c l a s s i f i c a t i o n s f o r t h e Psamments o r O r t h e n t s o f t h e low a t o l l s a r e Ustipsamments o r Tropopsamments and U s t o r t h e n t s and T r o p o r t h e n t s . The common subgroups a r e T y p i c , L i t h i c o r A q u i c .
A t t h e f a m i l y l e v e l a l l t h e low a t o l l E n t i s o l s have an i s o h y p e r t h e r m i c s o i l t e m p e r a t u r e r e g i m e and c a r b o n a t i c m i n e r a l o g y . The Psamments do n o t r e q u i r e a s e p a r a t e s t a t e m e n t o f p a r t i c l e s i z e c l a s s as t h i s i s a l r e a d y d e s i g n a t e d a t t h e s u b o r d e r l e v e l by t h e Psamm- p r e f i x ; t h e O r t h e n t s have s a n d y - s k e l e t a l p a r t i c l e s i ze c l a s s i n d i c a t i n g t h a t r o c k f r a g m e n t s g r e a t e r t h a n 2 mm make up 3 5 % o r more o f t h e volume and t h e r e i s enough sand t o f i l l
i n t e r s t i c i e s l a r g e r t h a n 1 mm. The Aquents u s u a l l y have a sandy o r s a n d y - s k e l e t a l p a r t i c l e s i z e c l a s s .
Where t h e s o i l s have been r e l a t j v e l y u n d i s t u r b e d o r have had a good v e g e t a t i o n c o v e r f o r a r e a s o n a b l e t i m e p e r i o d M o l l i s o l s ( S o i l Survey S t a f f , 1975) may be f o u n d . These a r e so c l a s s i f i e d because o f t h e p r e s e n c e o f m o l l i c e p i p e d o n and t h e f a c t t h a t t h i s r e s t s on c o r a l sand where t h e base s a t u r a t i o n w i l l o b v i o u s l y be g r e a t e r t h a n 50%. A m o l l i c e p i p e d o n i s c h a r a c t e r i z e d by d e p t h , d a r k c o l , o u r s , h i g h o r g a n i c m a t t e r c o n t e n t , h i g h base s a t u r a t i o n , good s t r u c t u r e and t h e p r e s e n c e o f s u f f i c i e n t m o i s t u r e f o r 3 months o r more t o f a c i l i t a t e p l a n t g r o w t h . The s t r u c t u r e o f t h e s u r f a c e h o r i z o n s o f many low a t o l l s o i l s i s n o t s t r o n g enough t o meet t h e r e q u i r e m e n t s o f t h e m o l l i c e p i p e d o n b u t l i m i t e d a r e a s o f M o l l i s o l s have been
i d e n t i f i e d .
The s u b o r d e r s i d e n t i f i e d on low a t o l l s a r e U s t o l l s and R e n d o l l s ( R e n d z i n a s i n t h e ' 1 9 3 8 ' s y s t e m ) d e p e n d i n g on t h e s o i l m o i s t u r e r e g i m e , w h i l e t h e H a p l - g r e a t g r o u p and E n t i c subgroup would be e x p e c t e d t o be t h e most common.
The f a m i l y c l a s s e s a r e u s u a l l y sandy o r s a n d y - s k e l e t a l , c a r b o n a t i c , i s o h y p e r t h e r m i c .
C a l c a r e o u s s o i l s o f low a t o l l s have been s t u d i e d i n a number o f l o c a t i o n s . The q u a l i t y and d e t a i l o f t h e work v a r i e s c o n s i d e r a b l y and i t has been d i f f i c u l t i n some cases t o compare t h e s o i l s o f t h e d i f f e r e n t a t o l l s .
1 . Tarawa
About 20 pedons i n S o u t h Tarawa have been c l a s s i f i e d a t t h e f a m i l y l e v e l i n S o i l Taxonomy ( S e r u and M o r r i s o n ,
1985; M o r r i s o n and Seru 1 9 8 6 ) . The s o i l s g e n e r a l l y have
an u s t i c s o i l m o i s t u r e r e g i m e , e x c e p t where t h e r e i s a marked g r o u n d w a t e r i n f l u e n c e . The s u b g r o u p s i d e n t i f i e d were T y p i c Ustipsamments, A q u i c Ustipsamments, L i t h i c Ustipsamments ( a l l R e g o s o l s ) , T y p i c T r o p o r t h e n t s , L i t h i c T r o p o r t h e n t s ( L i t h o s o l s ) , E n t i c H a p l u s t o l l s and T y p i c T r o p a q u e n t s ( w e t R e g o s o l s ) . I n one a r e a ( B i k e n i b e u ) t h r e e p r o f i l e s were f o u n d t h a t d i f f e r e d s i g n i f i c a n t l y f r o m t h e o t h e r s because o f t h e i n c o r p o r a t i o n o f o r g a n i c m a t t e r t o a c o n s i d e r a b l e d e p t h ( f o u n d as p o c k e t s i n t h e s u b s u r f a c e m a t e r i a l ) and t h e a s s o c i a t e d p o c k e t s o f m a t e r i a l e x h i b i t i n g s t r u c t u r e . The s t r u c t u r a l f e a t u r e s and o r g a n i c m a t t e r i n c o r p o r a t i o n a r e o b v i o u s l y t h e r e s u l t o f human a c t i v i t i e s . One p r o f i l e showed a f a i r l y u n i f o r m i n c o r p o r a t i o n o f o r g a n i c m a t t e r ( w i t h t h e a s s o c i a t e d d a r k e r c o l o u r s ) t o a d e p t h o f a b o u t 1 m e t r e . These s o i l s have o b v i o u s l y been s u b j e c t e d t o c o n s i d e r a b l e d i s t u r b a n c e as t h e y o c c u r i n an a r e a o f h i g h p o p u l a t i o n and consequent s u b s i s t e n c e a g r i c u l t u r a l a c t i v i t y . T h i s i s e n t i r e l y e x p e c t e d as t h e B i k e n i b e u a r e a i s one o f t h e most d e n s e l y p o p u l a t e d i n t h e South P a c i f i c .
The c l a s s i f i c a t i o n s o f 2 o f t h e s e p r o f i l e s a s T y p i c Ustipsamments ( R e g o s o l s ) , however, g i v e s no i n d i c a t i o n o f t h e i n f l u e n c e o f human a c t i v i t y w i t h t h e i n c o r p o r a t i o n o f o r g a n i c m a t t e r , n u t r i e n t s and s t r u c t u r e t o c o n s i d e r a b l e d e p t h s i n t h e p r o f i l e ; t h e t h i r d i s c l a s s i f i e d a s an A q u i c Ustipsamment. These l a t t e r f e a t u r e s may have c o n s i d e r a b l e agronomic i m p ~ r t a n c e i n t h e s e l o w a t o l l s o i l s w h i c h o t h e r w i s e a r e f e a t u r e l e s s below t h e A h o r i z o n . The i n c o r p o r a t i o n o f o r g a n i c m a t t e r w i l l i n c r e a s e t h e o t h e r w i s e low m o i s t u r e r e t e n t i o n c a p a c i t y a t d e p t h . Thus
i t may be n e c e s s a r y t o p r o v i d e a s e p a r a t e subgroup ( A n t h r o p i c ) o f t h e Ustipsamments f o r s e p a r a t i o n o f s o i l s w h i c h as a r e s u l t o f human a c t i v i t y , have o r g a n i c m a t e r i a l s and a s s o c i a t e d p r o p e r t i e s i n c o r p o r a t e d t o d e p t h
j n t h e p r o f i l e . 2 . F r e n c h P o l y n e s i a
The s o i l s o f a number o f low a t o l l s i n F r e n c h P o l y n e s i a have been d e s c r i b e d by T e r c i n i e r ( 1 9 5 6 , 1969) and Jamet ( 1 9 8 5 ) . Some o f t h e a t o l l s o i l s o f F r e n c h P o l y n e s i a a r e c o n s i d e r e d t o have an u d i c s o i l m o i s t u r e regime ( C . G a r n i e r , p e r s o n a l c o m m u n i c a t i o n ) and a r e t h e r e f o r e Tropopsamments ( R e g o s o l s ) and T r o p o r t h e n t s ( L i t h o s o l s ) w h i l e o t h e r s have r e c e n t l y been shown t o have an u s t i c s o i l m o i s t u r e r e g i m e ( S e r v i c e de l a M e t e o r o l o g i e , F r e n c h P o l y n e s i a , p e r s o n a l c o m m u n i c a t i o n ) and a r e t h e r e f o r e , Ustipsamments, U s t o r t h e n t s w i t h p o s s i b l y l i m i t e d a r e a s o f H a p l u s t o l l s .
3 . Cook I s l a n d s .
The s o i l s o f 2 a t o l l s i n t h e N o r t h e r n Cooks were s t u d i e d by B r u c e ( 1 9 7 2 ) . The M u r i s e r i e s was d o m i n a n t ; t h i s i s c o n s i d e r e d t o have an u d i c s o i l m o i s t u r e r e g i m e and i s c l a s s i f i e d as T y p i c Tropopsamment ( R e g o s o l ) .
4 . Arno A t o l l and t h e N o r t h e r n M a r s h a l l s
The s o i l s o f Arno a t o l l were s t u d i e d i n some d e t a i l by S t o n e ( 1 9 5 1 ) who i d e n t i f i e d f o u r m a j o r s o i l s e r i e s . The l i m i t e d m e t e o r o l o g i c a l d a t a i n d i c a t e t h a t Arno has an e s t i m a t e d annual r a i n f a l l >2500 mm and t h e s o i 1s o f Arno a t 0 1 1 t h e r e f o r e a r e c o n s i d e r e d t o have an u d i c s o i 1 m o i s t u r e r e g i m e and t h e Arno s e r i e s i s t h u s a T y p i c Rend011 ( R e n d z i n a ) and t h e S h i o y a s e r i e s a T y p i c Tropopsamment ( R e g o s o l ) .
S o i 1s showing s u r f a c e a c c u m u l a t i o n s o f p h o s p h a t i c m a t e r i a l as f o u n d i n t h e Jemo s e r i e s ( F o s b e r g , 1956) were n o t i d e n t i f i e d on Tarawa, F r e n c h P o l y n e s i a o r i n t h e Cook I s l a n d s . However, t h e N o r t h e r n M a r s h a l l s a r e c o n s i d e r a b l y d r i e r t h a n t h e s o u t h e r n group ( e s t i m a t e d annual r a i n f a l l 1000-2000 mm) and t h e r e i s a marked d r y season November- A p r i l l e a d i n g t o t h e c o n c l u s i o n t h a t t h e y would have an u s t i c m o i s t u r e r e g i m e . The s o i l s examined i n t h e N o r t h e r n M a r s h a l l s were c o r r e l a t e d w i t h t h o s e o f Arno, ( F o s b e r g , 1956) b u t i n t e r m s o f S o i l Taxonomy t h i s would now appear i n a p p r o p r i a t e . A c o m p a r i s o n w i t h s o i l s o f Tarawa o r C h r i s t m a s I s 1 and ( K i r i t i m a t i ) w o u l d appear more r e l e v a n t and t h e p r e s e n c e o f T y p i c Ustipsamments, T y p i c U s t o r t h e n t s and E n t i c H a p l u s t o l l s i s i n d i c a t e d by t h e o r i g i n a l r e p o r t
( F o s b e r g , 1 9 5 6 ) .
5 . C h r i s t m a s I s l a n d ( K i r i t i m a t i ) , K i r i b a t i
Hammond ( 1 9 6 9 ) produced a d e t a i l e d r e p o r t on t h e s o i l s o f C h r i s t m a s I s l a n d . He a t t e m p t e d a c o r r e l a t i o n w i t h t h e s o i l s o f t h e M a r s h a l l I s l a n d s and w i t h t h e c o a s t a l sandy s o i l s o f H a w a i i . S i n c e C h r i s t m a s I s l a n d has r e l a t i v e l y low r a i n f a l l ( a p p r o x i m a t e l y 800 mrn a n n u a l l y on a v e r a g e ; T a y l o r , 1973) t h e s o i l m o i s t u r e regime i s u s t i c
i n t h e absence o f any g r o u n d w a t e r i n f l u e n c e . Thus comparison w i t h t h e u d i c s o i l s o f Arno i s a g a i n i n a p p r o p r i a t e . The s o i l s o f C h r i s t m a s I s l a n d a r e dominated by T y p i c Ustipsamments ( d r y R e g o s o l s ) b u t t h e y g e n e r a l l y do n o t have a s w e l l d e v e l o p e d A h o r i z o n s a s t h e s o i l s o f Tarawa and s h o u l d t h e r e f o r e be c o n s i d e r e d a s s e p a r a t e s e r i e s . A n a l y s i s o f Hammond's r e p o r t w o u l d a l s o i n d i c a t e t h a t a r e a s o f T y p i c U s t o r t h e n t s ( L i t h o s o l s ) a r e a l s o p r e s e n t on C h r i s t m a s w h i c h w o u l d c o r r e l a t e w i t h t h e B o n r i k i s e r i e s on Tarawa.
6 . T u v a l u
The s o i l s o f T u v a l u have r e c e i v e d l i m i t e d s t u d y as p a r t o f a Land Resources S u r v e y (UNDP, p e r s o n a l c o m m u n i c a t i o n ) . The s o i l s have been c l a s s i f i e d a c c o r d i n g t o t h e FAO/UNESCO ( 1 9 7 4 ) Legend m a i n l y as C a l c a r i c R e g o s o l s ; d a t a a v a i l a b l e i n some c a s e s i s i n s u f f i c i e n t t o f u l l y c l a s s i f y t h e s o i l s by S o i l Taxonomy. T u v a l u s o i l s have u d i c s o i l m o i s t u r e r e g i m e s i n t h e absence o f g r o u n d w a t e r i n f l u e n c e and most w i l l t h e r e f o r e , be Tropopsamments ( R e g o s o l s ) o r T r o p o r t h e n t s ( L i t h o s o l s ) . I n s u f f i c i e n t d e t a i l on c o l o u r and s t r u c t u r e p r e v e n t s t h e c o n f i r m a t i o n o f m o l l i c e p i p e d o n s and hence t h e p r e s e n c e o f M o l l i s o l s .
7. Ontong J a v a , Solomon I s l a n d s
T h i s i s o l a t e d low a t o l l , n o r t h o f t h e main Solomon I s l a ~ d s g r o u p a l s ' o has s o i l s t h a t show s i m i l a r i t i e s i n morphology t o t h o s e on Tarawa ( W a l l and H a n s e l l , 1 9 7 6 ) . However, t h e c l i m a t e on Ontong J a v a i s s u f f i c i e n t l y wet t o o b t a i n an u d i c s o i l m o i s t u r e r e g i m e and t h e d o m i n a n t s o i l s a r e c o n s e q u e n t l y T y p i c Tropopsamments ( R e g o s o l s ) and T y p i c T r o p o r t h e n t s ( L i t h o s o l s ) .
8 . Hawai i
The Jaucas s e r i e s , mapped i n c o a s t a l sands on s e v e r a l o f t h e H a w a i i a n i s l a n d s , i s d e r i v e d f r o m c o r a l l i n e sand and i s c l a s s i f i e d as a T y p i c Ustipsammment, c a r b o n a t i c , i s o h y p e r t h e r m i c ( F o o t e e t a 1 . , 1 9 7 2 ) . E x a m i n a t i o n o f t h e Jaucas d a t a i n d i c a t e s t h a t t h i s s o i l has a l e s s w e l l d e v e l o p e d A h o r i z o n t h a n t h e Abatao s e r i e s on Tarawa as e v i d e n c e d by h i g h e r v a l u e s and chromas and l o w e r o r g a n i c c a r b o n c o n t e n t s . T h e r e w o u l d appear t o be g r e a t e r s i m i l a t i t y between t h e Jaucas s e r i e s and t h e s o i l s o f C h r i s t m a s I s l a n d a s s i g n e d t o t h e S h i o y a s e r i e s by Hammond
( 1 9 6 9 ) .
9. M i c r o n e s i a o t h e r t h a n t h e M a r s h a l l I s l a n d s
F i v e s o i l s u r v e y s c o n d u c t e d i n t h e F e d e r a t e d S t a t e s o f M i c r o n e s i a ( L a i r d , 1982; L a i r d , 1983 a , b ; S m i t h , 1983 a , b ) i n d i c a t e t h a t s u b s t a n t i a l a r e a s o f s o i l s d e r i v e d f r o m c o r a l l i n e m a t e r i a l s a r e f o u n d . These s o i l s e r i e s , e . g . , Ngedebus, I n s a k , D u b l o n have an u d i c o r p e r u d i c ( i . e . , s o i l p r o f i l e n e v e r d r i e s o u t t o any e x t e n t ) s o i l m o i s t u r e
r e g i m e as t h e r a i n f a l l i n t h e a r e a r a n g e s from 3000-5000 mm a n n u a l l y w i t h no marked d r y season. The c o r a l l i n e s o i l s have t h e r e f o r e been c l a s s i f i e d i n t h e Tropo- g r e a t g r o u p s o f E n t i s o l s r a t h e r t h a n i n U s t i - g r e a t g r o u p s .
Thus i t can be seen t h a t t h e dominant s o i l s o f t h e low a t o l l s a r e E n t i s o l s , b e i n g m a i n l y c l a s s i f i e d i n t h e Psamments o r O r t h e n t s s u b o r d e r . Trop- and U s t - g r e a t groups o c c u r depending on t h e s o i l m o i s t u r e r e g i m e . L i m i t e d a r e a s o f Aquents and M o l l i s o l s may a l s o be f o u n d .
SOILS OF THE RAISED ATOLLS
As t h e s o i l s o f t h e r a i s e d a t o l l s show c o n s i d e r a b l y g r e a t e r v a r i a t i o n t h a n i s f o u n d i n t h e low a t o l l s , t h e i n d i v i d u a l i s l a n d s w i l l be c o n s i d e r e d s e p a r a t e l y .
L i f o u , i n t h e L o y a l t y I s l a n d s o f New C a l e d o n i a i s a r a i s e d a t 0 1 1 o f a r e a 1 ,149 km2
.
The p r o x i m i t y o f t h e v o l c a n i c zones o f Vanuatu has l e d t o t h e d e p o s i t i o n o f s u b s t a n t i a l q u a n t i t i e s o f p y r o c l a s t i c m a t e r i a l s on t h e l i m e s t o n e . These p y r o c l a s t i c m a t e r i a l s have weathered t o g i v e p r o d u c t s o f t h e a l l o p h a n e f a m i l y i n t h e youngest d e p o s i t s and b a u x i t i c m a t e r i a l s i n t h e o l d e s t d e p o s i t s . Three m a j o r s o i 1 e n v i ronments have been i d e n t i f i e d(Latham, 1 9 8 1 ) .
On t h e w e s t c o a s t , s t e e p c l i f f s o f a r a m p a r t overhang a narrow c o r a l sandy beach. On t h e r a m p a r t a k a r s t i c m i c r o r e l i e f i s f o u n d ; o r g a n i c m a t t e r accumulates g i v i n g s o i l s up t o 1 m deep i n t h e deepest p o c k e t s . W i t h i n t h e f o r m e r l a g o o n two s o i l s e r i e s a r e f o u n d . One i s a s h a l l o w c a l c a r e o u s s o i l w i t h an o r g a n i c r i c h s u r f a c e l a y e r b u t o f v a r i a b l e d e p t h ( L i t h i c T r o p o r t h e n t o r L i th o s o l ) . The o t h e r i s a red-brown o x i d e r i c h s o i l o f 30-80 cm d e p t h r e s t i n g on t h e l i m e s t o n e . On t h e e a s t ( w i n d w a r d ) c o a s t t h e r e i s a a c o a s t a l p l a i n 100-200 m w i d e w i t h sandy o r g r a v e l l y 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 pumice.
They a r e r i c h i n o r g a n i c m a t t e r w h i c h accumulates t o a d e p t h o f 30-50 cm. These a r e f e r t i l e s o i l s and many c r o p s t h r i v e on them i f s h e l t e r e d f r o m t h e p r e v a i l i n g w i n d s . On t h e c o a s t a l p l a i n s , s i l i c a t e
-
c o n t a i n i n g breakdown p r o d u c t s o f t h e a l l o p h a n e f a m i l y have been i d e n t i f i e d , b u t on t h e r a m p a r t and i n t h e o l d l a g o o n t h e n o n - c a l c a r e o u s m a t e r i a l s a r e g e o l o g i c a l l y o l d e r and have l o s t a l l t h e s i l i c a t o g i v e s o i l s dominated by g i b b s i t e , b o e h m i t e , h a e m a t i t e and g o e t h i t e .These o x i d i c s o i l s a r e o f v a r i a b l e d e p t h , b u t c o v e r a major p a r t o f t h e s u r f a c e o f t h e i s l a n d . They have pH v a l u e s f r o m 6 - 7 . 5 and have h i g h l e v e l s o f o r g a n i c m a t t e r
( o f t e n
> l o %
f o r t o p s o i l s ) under n a t u r a l v e g e t a t i o n . I n t h e absence o f s i l i c a t e c l a y m i n e r a l s , a d s o r p t i o n and a v a i l a b i l i t y o f n u t r i e n t s i s dependent on t h e o r g a n i c m a t t e r c o n t e n t . CEC v a l u e s a r e c o r r e l a t e d t o o r g a n i c m a t t e r l e v e l s . Exchangeable base c o n t e n t s a r e r e a s o n a b l e i n t h e t o p s o i l s , b u t a r e o f t e n v e r y l o w i n s u b s o i l s (sumo f
bases
C1.5 c m o l / k g ) and p o t a s s i u m l e v e l s a r e f r e q u e n t l y p a r t i c u l a r l y low ( < 0 . 1 c m o l / k g ) . Water r e t e n t i o n i s g r e a t e r t h a n on t h e l o w a t o l l s as t h e o x i d e s c o n t r i b u t e a l o n g w i t h t h e o r g a n i c m a t t e r t o t h i s p r o p e r t y . Z i n c , manganese and b o r o n d e f i c i e n c i e s have been o b s e r v e d and s i l i c o n d e f i c i e n c y i s e x p e c t e d f o r c r o p s l i k e c e r e a l s t h a t r e q u i r e s i g n i f i c a n t q u a n t i t i e s o f t h i s e l e m e n t .The c l a s s i f i c a t i o n o f t h e s e s o i l s p r e s e n t s p r o b l e m s . S e v e r a l have a m o l l i c e p i p e d o n o v e r l y i n g an o x i d e r i c h s u b s o i l where t h e CEC i s n e a r t h e b o r d e r l i n e f o r t h e o x i c h o r i z o n . I n t h e s u b s o i l s t h e base s a t u r a t i o n i s f r e q u e n t l y low so t h o s e s o i l s t h a t do n o t q u a l i f y as O x i s o l s ( T y p i c A c r u s t o x - L a t e r i t e s o i l s i n ' 1 9 3 8 ' s y s t e m ) a r e o f t e n O x i c H u m i t r o p e p t s ( L a t o s o l s i n ' 1 9 3 8 ' s y s t e m ) . Latham ( 1 9 8 2 ) has a r g u e d t h a t t h e T y p i c A c r u s t o x c l a s s i f i c a t i o n does n o t g i v e any i n d i c a t i o n o f t h e p r e s e n c e o f s u b s t a n t i a l q u a n t i t i t e s o f o r g a n i c m a t t e r .
Nauru i s a r a i s e d a t o l l ( 0 0 3 2 ' S , 167003'W) o f 2 2 . 6 km2 w i t h i t s h i g h e s t p o i n t a p p r o x i m a t e l y 70 m above mean sea l e v e l . The l i m e s t o n e has been c o v e r e d w i t h a s u b s t a n t i a l d e p o s i t o f p h o s p h a t i c m a t e r i a l , t h e o r i g i n o f w h i c h has n o t y e t been f u l l y e x p l a i n e d . The s o i l s o f Nauru a r e b e i n g r a p j d l y removed by p h o s p h a t e m i n i n g o p e r a t i o n s . A s t o c k p i l e o f t o p s o i 1 i s b e i n g a c c u m u l a t e d and, a f t e r t h e c o m p l e t i o n o f m i n i n g a c t i v i t i e s a r o u n d 1995 A . D . , may be s p r e a d o v e r a l i m i t e d a r e a and used f o r s m a l l s c a l e a g r i c u l t u r a l p r o d u c t i o n t o s u p p o r t t h e r e m a i n i n g p o p u l a t i o n . The t e r r a i n r e m a i n i n g a f t e r m i n i n g i s so rugged t h a t an e x t e n s i v e a g r i c u l t u r a l development would be p r o h i b i t i v e l y e x p e n s i v e .
A r e c e n t s t u d y has been made o f t h e s o i l s i n a r e a s r e m a i n i n g u n d i s t u r b e d (Manner and M o r r i s o n , u n p u b l i s h e d d a t a ) . T h r e e s e p a r a t e s o i l s e r i e s o c c u p y i n g s i g n i f i c a n t a r e a s have been i d e n t i f i e d . The f i r s t s e r i e s has a d a r k e p i p e d o n o v e r l y i n g a l a y e r o f m i x e d s u r f a c e m a t e r i a l and b r o k e n l i m e s t o n e , w h i c h i n t u r n l i e s on l i m e s t o n e r o c k . T h i s p r o f i l e has been c l a s s i f i e d as a L i t h i c H a p l u s t o l l
( R e n d z i n a i n ' 1 9 3 8 ' s y s t e m ) . The second s e r i e s has a t h i n ( < I 0 cm) d a r k e p i p e d o n d i r e c t l y o v e r l y i n g t h e l i m e s t o n e s u b s t r a t u m and i s c l a s s i f i e d as L i t h i c U s t o r t h e n t ( L i t h o s o l ) . The t h i r d s e r i e s o c c u r s i n a r e a s where t h e p h o s p h a t i c m a t e r i a1 has a c c u m u l a t e d between 1 im e s t o n e p i n n a c l e s . The p h o s p h a t i c m a t e r i a l i s f r e q u e n t l y r e d d i s h y e l l o w ( 7 . 5 YR 7 / 4 ) c o l o u r e d between 25 cm and 75 cm d e p t h , c h a n g i n g t o p i n k i s h g r a y ( 5 YR 7 / 2 ) c o l o u r a t g r e a t e r d e p t h s . The Bw h o r i z o n i s o c c a s i o n a l l y s t o n y o r b o u l d e r y w i t h lumps o f l i m e s t o n e o c c u r r i n g t h r o u g h o u t . S o i l s o f t h i s t h i r d s e r i e s have e p i p e d o n s t h a t meet t h e c r i t e r i a f o r m o l l i c , and have t h e r e f o r e , been c l a s s i f i e d
