INCREASING T H E E F F I C I E N C Y O F U T I L I Z A T I O N O F SOIL A N D F E R T I L I Z E R P H O S P H O R U S IN T H E S U B T R O P I C A L A N D T R O P I C A L A G R I C U L T U R A L
SYSTEMS C *
E. M A L A V O L T A * * A . M . L . N E P T U N E * * *
RESUMO
A falta de P no solo é o fator que com mais f r e q ü ê n c i a limita a p r o d u -ção nas c o n d i ç õ e s b r a s i l e i r a s , parti¬ c u l a r m e n t e a das c u l t u r a s a n u a i s . A e f i c i ê n c i a da a d u b a ç ã o f o s f a t a d a pode ser a u m e n t a d a pode d i v e r s a s m a n e i ras que às vezes são e m p r e g a d a s c o n -j u n t a m e n t e , a s a b e r : 1) C a l a g e m - au¬ m e n t a o f o r n e c i m e n t o de P do solo e diminui a c o n v e r s ã o do P do a d u b o em formas de menor a p r o v e i t a m e n t o ; 2)
* R e c e b i d o para p u b l i c a ç ã o em 2 0 / 1 2 / 8 3 .
Parte e x p e r i m e n t a l c o m a j u d a da C N E N , F A P E S P , Ara¬ fertil e B N D E . C o n f e r ê n c i a plenária no C o n g r e s s o Mundial de F o s t a d o s , B r u x e l a s , o u t . 1 9 8 3 .
** D e p a r t a m e n t o de Q u í m i c a , E S A L Q / U S P e C E N A / U S P .
L o c a l i z a ç ã o d i m i n u i n d o a p r o p o r ç ã o do f ó s f o r o fixado a u m e n t a a absorção; 3) T i p o de a d u b o - o a p r o v e i t a m e n t o do P2O5 pode variar e m f u n ç ã o do a d u ¬ bo u s a d o ; 4) E s p é c i e s e v a r i e d a d e s -o a p r -o v e i t a m e n t -o d-o f ó s f -o r -o na f-orma¬ ç ã o da c o l h e i t a é i n f l u e n c i a d o pela c a p a c i d a d e da p l a n t a ; 5) A n á l i s e do solo - as q u a n t i d a d e s de a d u b o a u¬ sar são função do teor de P do solo e da relação e n t r e c u s t o do f e r t i l i -z a n t e e valor do p r o d u t o a g r í c o l a ; 6) M a g n é s i o - a a b s o r ç ã o do P do a d u ¬ bo d e p e n d e do teor de Mg no solo po-d e n po-d o por isso ser a u m e n t a po-d a pelo f o r n e c i m e n t o de c a l c á r i o d o l o m í t i c o ou de sais s o l ú v e i s de m a g n é s i o .
INTRODUCTION
The low P level, very o f t e n a s s o c i a t e d w i t h high a c i d i t y , is the factor w h i c h more frequently limits agricultural p r o d u c t i o n in the tropical r e g i o n s , and to a lesser e x t e n t , in the subtropical c o n d i t i o n s as w e l1 (ROCHE et a l . , 1 9 8 0 ) .
The two major soil types, namely o x i s o l s and u l t i s o l s , are highly w e a t h e r e d and leached, a c i d i c , p r e s e n t i n g a high s e x q u i o x i d e c o n t e n t .
A c c o r d i n g to SANCHEZ & SALINAS (1 9 8 1) p h o s p h o r u s deficiency is to be found in 82¾ of the area of tropical A m e r i c a ; the area c o v e r e d by a c i d soils is even larger
in Brazil c o u l d be c l a s s i f i e d as low in a v a i l a b l e P. F r o m the s t a n d point of fertility the results showing in table 1 - 1 c o u l d be c o n s i d e r e d as typical of the s a v a n n a o r c e r r a d o e c o s y s t e m s w h i c h are present both in Latin A m e r i c a a n d in A f r i c a : o n the a v e r a g e a v a i l a b l e p h o s p h o
rus content is e x c e e d i n g l y low, < 1 p p m , w h i c h c o r r e s -ponds to o n l y 3 kg per h e c t a r e , not s u f f i c i e n t for the g r o w t h of any c r o p s . This e x p l a i n s w h y , u n d e r these c o n d i t i o n s , P f e r t i l i z a t i o n represents the d i f f e r e n c e b e t w e e n h a v i n g or not h a v i n g a h a r v e s t .
On the o t h e r h a n d , the d e m a n d for P is further e n h a n c e d by the h i g h f i x a t i o n power of such soils w h i c h is a c o n s e q u e n c e of low pH a n d of the high s e x q u i o x i d e c o n t e n t , two p r e v a i l i n g features a l r e a d y p o i n t e d o u t . BOYER ( 1 9 8 2 , p p . 1 8 1 - 2 ) c o l l e c t e d information w i t h respect to the p h o s p h o r u s fixing c a p a c i t y of the f e r r a
-litic soils r e p r e s e n t a t i v e of tropical a n d subtropical r e g i o n s , as shown in table 1 - 2 .
Data o n the f r a c c i o n a t i o n of soil P are c o h e r e n t w i t h those related to f i x a t i o n since a l u m i n u m and iron p h o s p h a t e s (P-Al a n d P-Fe, respectively) represent the d o m i n a n t f o r m s , in e x c e s s of c a l c i u m (Ca-P) p h o s p h a t e s of h o g h e r s o l u b i l i t y in the soil s y s t e m (see table 1 - 3 p r e p a r e d w i t h data od B O Y E R , 1 9 8 2 , pp. 1 1 7 - 1 2 0 ) . A n o t h e r
indication is d e r i v e d fron these f i g u r e s : the ratio a v a i l a b l e P/total P is usually m u c h lower than l M O .
Tropical a n d subtropical regions w h i c h allow for r a i n f e d a g r i c u l t u r e do not possess large d e p o s i t s of sof t, h i g h reactivity rock p h o s p h a t e s : p r o v e n B r a z i l i a n reser-v e s , for i n s t a n c e , will be d e p l e t e d in c i r c a 50 years of u s e . A n d since there is no p o s s i b l e replacement for p h o s p h o r u s in the life of p l a n t s , the limits of g r o w t h of these regions is set by the size of its P d e p o s i t s or by its c a p a c i t y of importing a n d p r o c e s s i n g p h o s p h a t i c
L I M I N G
ce the soils under c o n s i d e r a t i o n h e r e i n a r e a c i d i c a n d rich in Fe a n d Al h y d r o x i d e s it is not s u r p r i s i n g that the h i g h e s t c o r r e l a t i o n c o e f f i c i e n t b e t w e e n a g i v e n c h a r a c t e r i s t i c a n d f i x i n g power is found w i t h respect to
iron c o m p o u n d s : F A S S B E N D E R ( 1 9 7 5 , p. 290) has c a l c u l a t e d the r e g r e s s i o n e q u a t i o n
Y = 1 7 . 6 3 + 1 0 . 8 3 x + 0 . 8 9 Q x2
r = 0 . ^ 3 2 * x
w h e r e y = f i x i n g p o w e r , x = % F e 2 0 ^ .
It follows that a beneficial e f f e c t of liming o n the u t i l i z a t i o n of b o t h soil a n d f e r t i l i z e r P is to b e e x p e c t e d . T h e literature s h o w i n g this to be true is rather v o l u m i n o u s .
Fig. 2 - 1 shows results o b t a i n e d by ELTZ e t a l .
( 1 9 7 5 ) a n d by FERRARI e t a l . ( 1 9 7 6 ) , being very clear that liming the soil increased the e f f i c i e n c y of u t i l i z a t i o n of f e r t i l i z e r P. E a r l i e r , h o w e v e r , M C C L U N G
e t a l . ( I 9 6 0 v e r i f i e d that limestone a p p l i c a t i o n s
a c t u a l l y d e c r e a s e d the size of c o t t o n r e s p o n s e to P2O5 f e r t i l i z a t i o n . T h e s e a p p a r e n t l y c o n f l i c t i n g results e x p l a i n e d , at least in p a r t , by F i g . 2 - 2 : in the soil s e c o n d c a s e liming the soil r e l e a s e d e n o u g h of the native P to meet the r e q u i r e m e n t s o f the c o t t o n
(MALA-V O L T A e t a l . , I 9 5 5 ) . T h e beneficial e f f e c t of raising pH through liming is s o m e t i m e s o b s e r v e d e v e n w h e n rock p h o s p h a t e is the s o u r c e of P 2 O 5 . LIMA & M A L A V O L T A (1 9 8 2 ) suggest that this e f f e c t is indirect d u e to better root
invironment c r e a t e d by the limestone w h i c h h e l p s the plant to a b s o r b m o r e p h o s p h o r u s , thereby c o u n t e r a c t i n g the influence of b o t h high pH a n d h i g h C a+
2 c o n c e n t r a t i o n o n the s o l u b i l i t y of the f e r t i l i z e r .
S u m m i n g u p , it c o u l d be s t a t e d that a better u t i l i z a t i o n of P p r o m o t e d by liming c o u l d be due to several reasons w h i c h do not e x c l u d e each o t h e r :
1 . d i m i n u t i o n in P f i x a t i o n ;
2 . increased u t i l i z a t i o n of soil-P - "wakincj" up the " d o r m a n t " p h o s p h o r u s (LARSEN, 1 9 7 3 ) ;
As p o i n t e d out by RITCHEY e t a l (1981 . pp. 141-2), high Al s a t u r a t i o n restricts root g r o w t h ; low Ca+2 jn the soil s o l u t i o n , h o w e v e r , can impose an e v e n m o r e severe r e s t r a i n t . T a b l e 1-1 m a k e s it c l e a r that both c o n d i t i o n s o c c u r j o i n t l y in the c e r r a d o s o i l s . P h o s p h o g y p s u m , e i
ther by itself or w h e n a p p l i e d as o r d i n a r y s u p e r p h o s p h a -te, m o v e s d o w n the soil p r o f i l e w h i c h helps roots to d e v e l o p in deeper layer of soils due to the d i m i n u t i o n
in Al s a t u r a t i o n . A s a c o n s e q u e n c e plants do not w i l t a s m u c h as those w i t h s h a l l o w root s y s t e m imposed by o t h e r p h o s p h a t i c f e r t i l i z e r , a n d t h e r e f o r e , y i e l d m o r e .
P L A C E M E N T A N D G R A N U L A T I O N
T h e d y n a m i c s of soil a n d f e r t i l i z e r P is r e p r e s e n -ted in Fig. 3~1 w h i c h is a m o d i f i c a t i o n of that d r a w n by SADLER e STEWART (1971
*) .
A n a d e q u a t e supply of inorganic P, P i , to the plant (reactions w i t h c o n s t a n t s k5 a n d k7) will be obtai ned w h e n the level of the e l e m e n t in c o m p a r t m e n t soln P,
(solution Pi) is kept at a h e i g h t h c o m p a t i b l e to the needs for g r o w t h and d e v e l o p m e n t . T h i s in turn requires that all r e a c t i o n s w h o s e c o n s t a n t s have an e v e n s u b s cript are p r e v e n t e d or kept at a m i n i m u m w i t h the p o s s i
-bile e x c e p t i o n of those w h i c h take place w i t h i n the plant itself, that is k6 and k8. In practice the chief o b j e c t i v e is to reduce f i x a t i o n or the c o n v e r s a t i o n of Labile Pi into Stable Pi c
i n c e this represents m o s t l y u n a v a i l e b l e f o r m s .
B r o a d c a s t a n d i n c o r p o r a t i o n of the F e r t i l i z e r PT in the top 2 0 - 3 0 c m layer w o u l d a p p a r e n t l y m a x i m i z e supply to the root s y s t e m since the e l e m e n t w o u l d o c c u p y n e a r l y 100¾ of the soil v o l u m e . T h i s , h o w e v e r , w o u l d e n c o u r a g e f i x a t i o n (reactions w i t h c o n s t a n t s kk a n d k 2 ) . T h e p l a c e m e n t o f the f e r t i l i z e r in bands at o n e or two sides a n d b e l o w the seed is the usual a l t e r n a t i v e . T h i s p r a c t i c e , o n its t u r n , m a k e s that o n l y 1 - 2 ¾ o f the soil v o l u m e r e c e i v e s P. T h e r e a r e l i m i t a t i o n s o n the root c a p a c i t y to a b s o r b p h o s p h o r u s . Since o n l y a r e s t r i c t e d root s u r f a c e is a c c e s s i b l e to the P w h i c h d i f f u s e s o u t of the f e r t i l i z e r g r a n u l e or p a r t i c l e , then the p o s s i b i
-lity e x i s t s that g r o w t h a n d y i e l d m a y be limited by lack of P ( G A C H O N , 1 9 7 7 ) - T h i s led BARBER ( 1 9 7 7 ) to w o r k o u t a c o m p r o m i s e s o l u t i o n : a p p l i c a t i o n as a strip to c o n t a c t a b o u t o n e tenth of the s u r f a c e f o l l o w d by m i x i n g a n d p l o w g h i n g . W h e n c o m p a r e d w i t h b a n d i n g by row a n d b r o a d -cast a n d p l o w g h i n g , this type of positional p l a c e m e n t g a v e , r e s p e c t i v e l y , 0 . 5 a n d 1 . 0 t/ha o f c o r n m o r e , a v e r a ge of 5 y e a r s of e x p e r i m e n t s .
As a rule in the a c i d , low in P soils of the s u b -tropical a n d -tropical r e g i o n s , d u e to the m o d e s t rates of
In the c a s e of p e r e n n i a l s , c o f f e e , a surface appl_i^ c a t i o n in the d r i p p i n g line a l l o w s for a higher u p t a k e of Fertlizet P than d i s t r i b u t i o n in the b o t t o m of c i r c u
-lar f u r r o w s as shown in T a b l e 3
-2 taken from M A L A V O L T A 6 N E P T U N E ( 1 9 7 7 ) ; foliar a p p l i c a t i o n s , h o w e v e r , a r e m u c h m o r e e f f i c i e n t .
In soils w i t h higher L a b i l e Pi and less f i x a t i o n power the effe c t of broadcast a p p l i c a t i o n s can be increa sed simply by broadcast in the fertilizer b e f o r e
zone to be f o r m e d in d e e p e r layers of the soil profile w h i c h helps the e s t a b l i s h e d c r o p to stand better dry spells w h i c h may o c c u r a l o n g the growing s e a s o n . It is self e v i d e n t that d e e p p l a c e m e n t of the fertilizer band w o u l d h a v e similar e f f e c t w h e n this m e t h o d of a p p l i c a t i o n
has to be u s e d ( A N O N Y M O U S , 1 9 8 0 ) .
G r a n u l a t i o n can p r e v e n t , to some e x t e n t , the f i x a -tion of F e r t i l i z e r P p r o v i d e d it is s o l u b l e . The effect o n rock p h o s p h a t e w o u l d , h o w e v e r , be the o p p o s i t e since
its a v a i l a b i l i t y d e p e n d s upon s o l u b i l i z a t i o n in the soil w h i c h is h e l p e d by the increase in the surface of exposi tion. Fig. 3-5 p r e p a r e d from data of BARRETO e t . a l .
(1978) gives support to this s t a t e m e n t .
M I C R O O R G A N I S M S
M i c r o o r g a n i s m s play at least there roles c o n c e r n i n g u t i l i z a t i o n of soil a n d F e r t i l i z e r P.
As shown in Fig. 3~1, the reaction w i t h constant kl 1 c o n s i s t s in the m i n e r a l i s a t i o n of P o r g . T h i s source of p h o s p h o r u s may be a very important o n e as in the c a se of s o m e B r a z i l i a n a n d West A f r i c a n soils in w h i c h c o c o a
is g r o w n . T h e f o l l o w i n g figures are r e p r e s e n t a t i v e : P o r g . = 1,300 k g / h a , S t a b l e Pi = 900 k g / h a , L a b i l e Pi + Soln = 10 k g / h a .
It is well e s t a b l i s h e d , that many soil m i c r o o r g a -n i s m s , o -n the o t h e r h a -n d , help to drive reactio-n S t a b l e Pi •*• L a b i l e Pi T h i s is due in part to the fact that such o r g a n i s m s {Pcnicil'Hum, Aspergillus, Rhizopus, Can-dida , Oidiodendvi >n, Psc udogymnoascms, Paeudomonas) a b o u n d
(KATZNELSON et a l . , 1 9 6 2 ; F E R N A N D E S , 1 9 6 0- a , b ) . Seed inoculation w i t h P s o l u b ' l i z i n g m i c r o o r g a n i s m s {lia^illun megatherium var. phonphaLiaum) increased s i g n i f i c a n t l y
the u t i l i z a t i o n of a d d e d p h o s p h a t i c f e r t i l i z e r s (super-p h o s (super-p h a t e a n d h y d r o x y a (super-p a t i t e ) by oat s e e d l i n g s (BAJPAI,
Soln P u p t a k e is increased by mycrorrhiza1 fungi due p r i m a r i l y to the fact the h y p h a e e x p a n d root s u r f a c e B e s i d e s this direct e f f e c t , a n o t h e r o n has to be c o n s i -d e r e -d . S i n c e P soln is -d e p l e t e -d f a s t e r , transfer of L a b i l e Pi to the soil s o l u t i o n , a n d p e r h a p s , c o n v e r s i o n of S t a b l e Pi into L a b i l e Pi is e n h a n c e d ; in o t h e r w o r d s : s o l u b i l i z a t i o n a n d d i f f u s i o n a r e i n c r e a s e d . For details o n the role of r h i z o s p h e r e m i c r o o r g a n i s m s (not o n l y m y c o r r h i z a l ) see T I N K E R (1980, p p . 617-65*0 a n d LOPES 6 S I Q U E I R A ( 1 9 8 1 , pp. 2 2 5 - 2 4 2 ) .
It s h o u l d be kept in m i n d , h o w e v e r , that m y c o r r h i -zae u s u a l l y help o n l y in s i t u a t i o n s w h e r e i n the actual transfer of L a b i l e Pi to the root via Soln Pi is the rate limiting step in the overall s y s t e m d e p i c t e d in Fig. 3 " 1 - T h i s is, of c o u r s e , the c a s e found in soils w h e r e a v a i l a b l e p h o s p h o r u s is low.
S i n c e this p r e s e n t s a c o n d i t i o n for an effect of the m y z o r r h i z a l a s s o c i a t i o n to be r e l e v a n t , it becomes clear t h a t : (I) little i n f l u e n c e , if a n y , is to be e x -pected w h e n e v e r the soil has an a m p l e s u p p l y of a v a i l a b l e P; ( 2 ) h i g h y i e l d s are hardly to be o n t a i n e d w i t h o u t the use of Fert i1i zer P i .
The e f f e c t s of m y c o r r h i z a e , e i t h e r n a t i v e or inocu l a t e d , are c o m m o n l y d r a m a t i c in pot e x p e r i m e n t . Under field c o n d i t i o n s , h o w e v e r , the benefit t h e r e f o r e may be n e g l i g i b l e as shown in Fig. 4- 1 p r e p a r e d w i t h data pre-s e n t e d by A N O N Y M O U S ( 1 9 8 1 ) . T h i s does not m e a n h o w e v e r , that the role p l a y e d by m i c o r r h i z a e w i t h i n a natural or man m o d i f i e d e c o s y s t e m c o u l d be i g n o r e d ; a l t h o u g h not o v e r e m p h a s i z e d , it c o u l d be s i g n i f i c a n t w h e n e v e r natural, a c t i v e p o p u l a t i o n s do not exist or do not thrive for
GENET I CAL A S P E C T S
U p t a k e , long d i s t a n c e t r a n s p o r t , r e d i s t r i b u t i o n and u t i l i z a t i o n of n u t r i e n t s for y i e l d f o r m a t i o n repre-sent p r o c e s s e s w h i c h are under genetical control (EPSTEIN,
1975, p p . 2 6 9 - 2 8 5 ) .
On the w h o l e P e f f i c i e n t species or v a r i e t i e s s h o u l d posses a h i g h c a p a b i l i t y of c o n v e r t i n g the e l e m e n t in the final p r o d u c t , as d e f i n e d by the simple e q u a t i o n _^ _^
Ep = y i e l d . P a b s o r b e d . cycle ;
in o t h e r w o r d s , high Ep v a r i e t i e s s h o u l d be a b l e to prod u c e m o r e w i t h less P taken u p , t r a n s p o r t e prod a n prod a s s i m i
-lated.
It is not s u f f i c i e n t t h e r e f o r e that a given species or c u l t i v a r is m e r e l y a b l e to a b s o r b m o r e p h o s p h o r u s per unit of root length or root w e i g h t due to a lower km. In this c o n t e x t d i f f e r e n c e s a m o n g species or c u l t i v a r s are known to e x i s t , as shown in Fig. 5
-1 a n d in T a b l e 5~ -1 . T h e reason for the s t a t e m e n t being the f a c t , t h a t , o t h e r external factors remaining c o n s t a n t , the v e l o c i t y of transfer of P soln to P root may very a c c o r d i n g to the size of root s y s t e m , root m o r p h o l o g y and d i s t r i b u t i o n in the soil v o l u m e (BARBER, 1 9 8 2 ) .
W h a t really m a t t e r s , t h e n , is the plant a b i l i t y of t r a n s f o r m i n g P taken u p , t r a n s p o r t e d or r e d i s t r i b u t e d iji to teh p r o d u c t w h i c h is the goal at h a r v e s t : ç e a f , s t a l k , tuber or r o o t , fruit or g r a i n . It is not e n o u g h that p o s i t i v e r e l a t i o n s h i p s are f o u n d w i t h total dry m a t t e r w h e n most of the m a t t e r p r o d u c e d is of little or no v a
-lue for cornsuption or s a l e .
T h e r e are s t r i k i n g d i f f e r e n c e s a m o n g c u l t i v a r s in this r e s p e c t . A few e x a m p l e s are g i v e n :
u p l a n d v a r i e t i e s of r i c e , a l l o w y i e l d i n g utilize for the p r o d u c t i o n of 1 t of rice in c o m p a r i s o n w i t h IR 8, high y i e l d i n g v a r i e t y ;
2) F i g . 5
-2 indicates that low y i e l d i n g v a r i e t i e s export relatively less P than IR 8 ;
3) T a b l e 5
2 p r e s e n t s the Ep values for the c u l t i -vars under c o n s i d e r a t i o n .
For d e t a i l s see M A L A V O L T A & FORNASIERI ( 1 9 8 3 , p p .
Figure 5-2. P e x p o r t e d as % of P a b s o r b e d by rice var i et i e s .
E f f i c i e n t P u t i l i z a t i o n is a c h a r a c t e r i s t i c s o m e -times a s s o c i a t e d w i t h t o l e r a n c e to e x c e s s Al in the s u b s t r a t e as Fig. 5-1» indicates (MALAVOLTA e t a l . ,
I 9 8 I ) : the b e n e f i t d e r i v e d w o u l d , t h e r e f o r e be d o u b l e
--lower needs of P 2 O 5 f e r t i l i z a t i o n a n d less lime to c o r r e c t soil a c i d i t y .
S O U R C E S AND RATES
sub-tropical a n d sub-tropical regions show that c i t r i c acid a n d c i t r a t e s o l u b l e s o u r c e s of P 2 O 5 a r e , as a r u l e , as e f f i
-cient as the w a t e r s o l u b l e p r o d u c t s . This c o u l d be e x p l a i n e d to a large e x t e n t by the chief soil c h a r a c t e
-ristics - h i g h a c i d i t y a n d h i g h P fixing p o w e r .
T a b l e 6 - 1 gives the results of an e x p e r i m e n t w i t h sugar c a n e (ALVAREZ e t a l . , 1 9 6 5 ) s h o w i n g that m a g n e s i u m t h e r m o p h o s p h a t e (Mg TP) g a ve h i g h e r y i e l d s than o r d i n a r y s u p e r p h o s p h a t e (OSP) a f i n d i n g w h i c h is c o n f i r m e d by o t h e r trials w i t h d i f f e r e n t s p e c i e s . A l t o u g h the unit cost o f P 2 O 5 in M g T P is h i g h e r than in O S P , the two s o u r c e s h a v e the s a me r e l a t i v e e c o n o m i c e f f i c i e n c y (Ta-ble 6 - 2 ) . T h i s type of results is due to several rea-sons b e s i d e s the soil c h a r a c t e r i s t i c s , o n e of them being the Mg c o n t e n t . A s d e m o n s t r a t e d by L 0 U R E N Ç 0 e t a l .
( 1 9 6 8 ) m a g n e s i u m ions s t i m u l a t e P u p t a k e (Fig. 6 - 1 )
a p p a r e n t l y by lowering kms in the two isotherms w h i c h d e s c r i b e the k i n e t i c s o f a b s o r p t i o n (Table 6 - 3 ) - W o r k i n g w i t h soil M A D R I D e t a l . ( 1 9 7 7 ) v e r i f i e d that m a g n e s i u m s u l f a t e increased the f e r t i l i z i n g value of triple s u p e r -p h o s -p h a t e , d i c a l c i u m -p h o s -p h a t e a n d o c t a c a l c i u m -p h o s -p h a t e ;
T h e r e is a good deal of i n t e r e s t , a n d a c o r r e s p o n -d i n g l y large number of p u b l i c a t i o n s , o n the -direct a p p
c a t i o n of rock u p t a k e (RP) (BRAGA, 1970; K H A S A W N E H & D O L L , 1978; LEON & F E N S T E R , 1979; O L I V E I R A et a l . , 1982).
these p r o d u c t s can c o m p e t e e f f e c t i v e l y w i t h a c i d u l a t e d or fused p h o s p h a t e s . T h e / may have a p l a c e , h o w e v e r , in
the so c a l l e d c o r r e c t i v e f e r t i l i z a t i o n in w h i c h h i g h rates a r e b r o a d c a s t and p l o u g h e d in a n d no short term results a r e e x p e c t e d .
P a r t i a l l y a c i d u l a t e d roch p h o s p h a t e (PARF) shows a great promise as c o u r c e s of P2O5 as d e m o n s t r a t e d in pot and field e x p e r i m e n t s w i t h d i f f e r e n t crops (see Fig.
6-2 ) . T h e r e is an o b v i o u s - and not so new interest (COOKE, I956) in this type of product in c o u n t r i e s w h e r e
sulfur as a new m a t e r i a l , has to be imported. T h i s " m o -d i f i e -d O S P " c o n t a i n s S w h i c h represents an a -d -d e -d agricul
d e s e r v e s c o n s i d e r a t i o n .
R e c o m m e n d a t i o n s for P f e r t i l i z a t i o n In the c a se of non perencial c r o p s is u s u a l l y based o n soil chemical a n a l y s e s . For the e x t r a c t i o n of a v a i l a b l e P (k
Labile Pi + Soln PI + f r a c t i o n of S t a b l e PI) m a n y e x t r a c t i n g s o l u t i o n s (and m o r e recently e x c h a n g e resins) are used
do in the f i e l d . W h a t e v e r the e x t r a c t i n g p r o c e d u r e em-p l o y e d , it is c o m em-p u l s o r y that they show a h i g h em-power of r e s o l u t i o n a m o n g soils w i t h d i f f e r e n t levels of a v a i l a -ble P. In o t h e r w o r d s : the c l a s s e s w h i c h d e f i n e the p h o s p h o r u s s t a t u s of the soil s h o u l d be s u f f i c i e n t l y narrow to a l l o w for m o r e Drecise r e c o m m e n d a t i o n s : o t h e r -w i s e the rates to a p p l y -w o u l d be either i n s u f f i c i e n t or e x c e s s i v e . In the f i r s t c a s e the y i e l d s w o u l d not s h o w
the d e s i r a b l e i n c r e a s e , a n d in the s e c o n d c a s e , the results c o u l d be e i t h e r d e t r i m e n t a l o r less e c o n o m i c a l .
Increasing the e f f i c i e n c y of u t i l i z a t i o n of F e r t i -lizer P is a p r o p o s i t i o n to be f i n a l l y e x a m i n e d w i t h i n the overall e c o n o m i c s of f e r t i l i z a t i o n . In B r a z i l , for i n s t a n c e , f e r t i l i z e r s a n d their d i s t r i b u t i o n in the field r e p r e s e n t f r o m 5 (sugar cane) to 15¾ (upland rice) of the cost of p r o d u c t i o n ; in o t h e r c o u n t r i e s , w h e r e i n most or all f e r t i l i z e r m u s t be i m p o r t e d , the f i g u r e s c o u l d be e v e n h i g h e r . T h i s m e a n s that r e c o m m e n d a t i o n s for P f e r t i l i z a t i o n s h o u l d take into a c c o u n t the ratio w/t b e t w e e n unit v a l u e of the p r o d u c t (w) a n d unit cost of P2O5 ( t ) , b e i n g a d j u s t e d w h e n e v e r the cost of the
f e r t i l i z e r or the v a l u e of the a g r i c u l t u r a l p r o d u c t , or b o t h , are c h a n g e d . A n e x a m p l e is g i v e n in T a b l e 6-4
Last but not least: a l t h o u g h lack of a v a i l a b l e P is the m o s t f r e q u e n t limiting f a c t o r , o t h e r n u t r i e n t s (N, K, S, s o m e t i m e s m i c r o n u t r i e n t s such as B, C u , a n d Zn) are a l s o very o f t e n in short s u p p l y . It they are not a p p l i e d in the n e e d e d a m o u n t s a n d in the right p r o p o r -tions relative to P 2 O 5 , the u t i l i z a t i o n of both soil and f e r t i l i z e r p h o s p h o r u s w o u l d not reach the d e s i r a b l e d e g r e e of e f f i c i e n c y (WAGNER, 1 9 7 9 ) .
A C K N O W L E D G E M E N T S
T h e e x p e r i m e n t a l part was s u p p o r t e d by the follow i n g i n s t i t u t i o n s : F A P E S P , B N D E S , C N P q , CNEN a n d A R A F E R -TI L .
SUMMARY
A s a r u l e , soils of the subtropical a n d tropical r e g i o n s , in w h i c h rainfall is not limiting, are a c i d i c , and low in p h o s p h o r u s , a n d , to a less e x t e n t , in o t h e r m a c r o a n d m i c r o n u t r i e n t s as w e l l , such a s u l f u r , boron and z i n c .
T h e e s t a b l i s h m e n t of a p e r m a n e n t a g r i c u l t u r a l prac. tice t h e r e f o r e , d e m a n d s relatively high usage of liming and p h o s p h a t i c f e r t i l i z a t i o n , to b e g i n w i t h .
T h e use of liming m a t e r i a l s b r i n g i n g up pH to 6 . 0 6 . 5 c a u s e s the c o n v e r s i o n of iron a n d a l u m i n u m p h o s p h a -tes to m o r e a v a i l a b l e c a l c i u m p h o s p h a t e s ; o n the o t h e r h a n d , by raising c a l c i u m s a t u r a t i o n in the e x c h a n g e c o m -p l e x , it im-proves the d e v e l o -p m e n t a n d o -p e r a t i o n if the root s y s t e m w h i c h a l l o w s c
o r a higher u t i l i z a t i o n of all soil n u t r i e n t s , including p h o s p h o r u s , a n d helps of s t a n d w a t e r d e f i c i t s w h i c h may o c c u r .
T h e role of m y c o r r h i z a l fungi should be c o n s i d e r e d as a w a y of increasing soil a n d f e r t i l i z e r P utilization, as well as the limitations t h e r e o f .
S c r e e n i n g of a n d b r e e d i n g for v a r i e t i e s w i t h higher e f f i c i e n c y of u p t a k e a n d u t i l i z a t i o n of soil a n d f e r t i l i z e r p h o s p h o r u s leads to a r e d u c t i o n in cost of
inputs a n d to h i g h e r b e n e f i t / c o s t r a t i o s .
C o r r e c t i v e f e r t i l i z a t i o n using g r o u n d rock p h o s -phate helps to s a t u r a t e the f i x a t i o n power of the soil thereby r e d u c i n g , as a c o n s e q u e n c e , the need for p h o s -phorus in the m a i n t e n a n c e f e r t i l i z a t i o n .
M a i n t e n a n c e f e r t i l i z a t i o n , in w h i c h s o l u b l e p h o s -phatic sources a r e u s e d , c o u l d be improved by several means w h o s e p e r f o r m a n c e has b e e n p r o v e d : limimg, g r a n u l a
t i o n , p l a c e m e n t , use of m a g n e s i u m s a l t s .
L a s t , cost of p h o s p h a t e f e r t i l i z a t i o n c o u l d be further r e d u c e d , w i t h o u t impairing y i e l d s , t h r o u g h im-pairing y i e l d s , through c h a n g e s in technology d e s i g n e d to o b t a i n p r o d u c t s better a d a p t e d to local c o n d i t i o n s a n d to the a v a i l a b i l i t y or raw m a t e r i a l s and e n e r g y s o u r c e s .
R E F E R E N C E S
ç ã o d a c a n a d e a ç ú c a r . X I . E x p e r i ê n c i a s c o m d i v e r -sos f o s f a t o s ( 1 9 6 1 a 1 9 6 3 ) . B r a g a n t i a 2 4 : 9 7 - 1 0 7 .
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6 5
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