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152

Wadsworth

et al.

dominated by kaolini te with lesser amounts of

gibbsite and hydroxy-interlayered vermiculi te.

Subsurface horizons contain only kaolini te and

gibbsite in their clay fractions (W adswor th, 1987).

The original vegetat ion of the area, as described by

West et al. (1985), was tropical rain forest. Cur-

rently, the vegetat ion in most areas consists of open

savanna with islands of forest and no longer

resembles classical rain forest. Most large trees

have been removed through select ive logging, and

the remaining vegetat ion is bi layered: the over story

:being fairly continuous and reaching heights of

approximately 10 m, and the understory being open

herbaceous grassland or low (0.5 m) herbs, shrubs

and fugitive pineapple plants. Lianes are occasion-

al ly present , and epiphytes are common only on

isolated trees in the savanna. Tw o previous studies

in Tabasc o showed tha t as l it t le as 3 y of pineapple

cultivation resulted in a large decrease in infil-

tration rate (Cisneros-Dominguez, 1983) and sig-

nificant loss of nutrients (Mejia-Nunez

et al.,

1983).

aterials and methods

Soil sampling sites representing eight ages of

fores t fallow (Table 1) were selected with the assist-

ance of a local resident (Asbel Milla) from small-

scale, commercial slash-and-burn pineapple

Ananas comosus L.) farms. Selection was based on

profile characteristics (determined from auger

samples to 2m depth) and uniformity of soil ,

agricultural management, and present vegetat ion.

The sites were all within an area of 5 km 2, and were

in 0.5 to 1 ha fields that had been cleared b y hand,

and initially intercropped with corn

Z e a may s

L.)

and manioc

Manihot esculenta

Crantz) and

pineapple fol lowed with 3 to 4y of pineapple

Table 1 Ye a rs a f t e r c le a r ing a nd s ta ge in the c rop c yc le of the

e ight s a m pl ing loc a t ions

Ye a r s of c le a r ing S ta ge in c rop c yc le

0

5

6

10

15

20

25

50

Cle a re d 3 m o pr ior to s a m pl ing

1 y fa l low af ter 4 -5 y crop

2 y fa l low af ter 4 -5 y crop

5 y fa l low af ter 4-5 y crop

10 y fa l low af t er 4-5 y cro p

15 y fa l low a f ter 4 -5 y crop

20 y f a l low a f te r 4 -5 y c rop

50 y f al low no kn own c rop

monocrop, then abandonment. Replicate fields

were located for all ages, resulting in 16 fields for

sampling a nd de scription. Nine by 16 m grids were

randomly located in each field and 12 surface

samples (0--20cm) were collected in each grid,

giving 192 surface samples for assay. Subsurface

samples were collected from pits dug to 2 m in one

field of each age and the soils were described.

Horizon samples were collected from each pit for

chemical and physical analyses.

Surface and subsurface samples were air dried,

crushed, and their fert il i ty assayed by conv entional

procedures. The method s o f analyses, with the ex-

ceptions noted, are those presented in the

monograph ed i ted by Page et al. (1982). Organic C

(Walkley-Black), N (Kjeldahl), pH (1:1 soil:water

suspension) were determined in all samples. After

these analyses were completed, the surface sam ples

from each plot were composited to faci l i tate the

remaining analyses. On these composites available

N was es t imated from ammonium product ion

during a 7 d, 40 °C anaer obic inc ubation; available

P by a modified Bray procedure; exchangeable Ca,

Mg and K were measured in M, pH 7 NH4OAC

extracts; and A1 and H in M KCI extracts.

Nonexchangeable K was est imated using the con-

centrated H2SO4 extract ion of Hunte r and Prat t

(1957). Available S was extracted by 0.05M

Ca(H2PO4)2 and by 0.03M NaH2PO 4 in 2 M

CH3COOH and the concent ra t ion of S es t imated

by inductively coupled plasm a spectrometry.

Results

This study evaluated changes in the chemical

characteristics of soils with time after slash-and-

burn from analyses of samples from 16 carefully

selected fields representing 8 ages, 0 to 50y, of

forest fallow. The si tes sampled were from an area

of uniform soils, agricultural managem ent, and

sequence of vegetat ion. Stat ist ical analyses of the

data were made assuming variance homogeneity

and are presented to pr ovide est imates of variabil-

i ty. The study differed from those o f Sanchez et al.

(1983), Aweto (1981) and Aina (1979) which fol-

lowed changes at specific sites, in that a much

longer rotat ion was evaluated, only two repli-

cations w ere possible, and site variability was poss-

ibly greater. The chemical characteristics evaluated

8/20/2019 Wadsworth 1990


ORGANIC C AND N

%

6'0 I I~ ' CARBON

.of

4.0

3.0L~ ~ ~ , J

0 251 ~ -

0.20 , ; , b '

TIME AFTER CLEARING, y

Fig 1 Surface soi l con tent o f C and N w ith t ime af ter clear ing.

Dashed line i s 3 point running average of N content .

were those of primary concern to plant nutri t ion

and included est imates of organic C, total and

available soil N, available P, Ca, Mg, and S, pH

and exchangeable and reserve K.

Organic matter

The changes in mean surface soil organic C and

Kjeldahl N con tents with t ime are show n in Figure

1. Soil content of both elements declined rapidly

after clearing from their initially high levels to

min imum levels, appr oxim ately 20 lower, at

10y. After this decline, a fairly steady increase

occurred and at 50 y their contents were not dif-

ferent fro m those a t 0 y. Similar rapid declines in

orest fallow and fertility of Mexican ultisol

153

the organic matter content are kno wn to occur with

drast ic changes in management (Jenny and

Raychaudhuri , 1960) and part icularly in tropical

environments (Nye and Greenland, 1964). The

equally rapid recovery is at tributed to reestablish-

ment of the natural vegetat ion from seeds and

sprouts not ki l led during the cropping cycle

(Lambert and Arnason , 1986) . The C :N ra t io

varied little from the me an valu e of 14.6 during this

period.

As es t imated from the smoothed curve of Figure

(three-point running average) over 1 000k gha -~

of N was lost from these soils in the first 5 to 10y

of the rotat ion. This loss of C and N is a most

serious consequence of this rotat ion for not only

does i t entail a loss of a valuable resource of organic

matter and available N, b ut also the loss of nutrient

cat ions. The pineapple crop produced during the

cult ivated period of this rotat ion was undoubtedly

low yielding and remo ved less than 100 kgh a -~ of

N in the harvested po rtio n (Sanchez, 1976). Assu m-

ing dentrification losses of approxima tely on e-half

of the remaining N (Grimm e and Juo, 1985) leaves

400 kg ha- ~ ikely leached with Ca, Mg and K. Th e

accompanying loss of cat ions would be equivalent

to 1.4 Mg ha ~of lime. But more importantly, since

cations are lost in relat ion to their equil ibrium

concentrat ions in the soil solut ion, the loss of K, a

nutrient already at poverty levels (Vilela and

Ritchey, 1985), would be proport ionately greater.

As expected in high organic matter soils, avail-

able N m easured by anaerob ic incubation (Table 2)

was relatively high (Powers, 1980) and increased

with age of secondary forest . The ready availabil i ty

of N in these soils supports the loss data measure d

by total N differences. Organic consti tuents were

Table 2 Avai l able N, P and S , exchangeable H, a nd ex changeable and r eserve K o f t he 0 to 20cm soil l ayer wi th t ime af t er c l earing

Time af t er Avai l able pH Exchangeable Reserve

clear ing

N P S H K K

(y) (mg kg- a ( cm ol (+) kg- i ) (mg kg- t )

0 47b 0.3a 24a 5.4a 0.32bc 0.18a 97a

5 47b 0.4a 23a 5.5a 0.15d 0.08a 42b

6 45b 0.2a 25a 5.2a 0.40bc 0.18a 93a

l0 52b 0.6a 16a 5.1a 0.30bcd 0.07a 37b

15 105a 0.3a 26a 5.2a 0.23cd 0.09 54b

20 78a 0.4a 16a 5.6a 0.60a 0.17a 60b

25 82a 0.2a 22a 5.4a 0.43b 0.08a 40b

50 79a 0.5a 23a 5.3a 0.39bc 0.06a 31b

Nu mb ers fol low ed by the same let ter do n ot dif fer s ignif icantly at P < .05 by Dunc an s mult iple range test .

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154 W a d s w o r t h et al.

maximum in all pedons in the surface horizon.

Their contents decreased rapidly with depth to a

minimum at approximately 100cm and remained

stable thereafter (Wadsworth, 1987).

E x c h a n g e a b l e c a t io n s a n d p H

Trends with time in exchangeable Ca, Mg, K and

A1 are shown in Figure 2, and the pH, exchangeable

H and nonexchangeable K data are in Table 2. Soil

levels of NH4OAC extractable Ca and Mg did not

differ significantly with time. It is, however, readily

apparent that their levels were much lower in the 0

to 10 y samples than in the 15 to 50 y samples. They

did not vary with time as did those of Nye and

Greenland (1960) and Sanchez (1976) were higher

immediately after burning than their levels in the

old secondary forest. It may be attributed to the

relatively smaller amounts of residues and lower

temperatures reached in burning these as compared

to classical rain forest sites, and to decomposition

of organic matter and leaching losses during the

3 mo period between the burn and sampling of the

0 y sites. The exchangeable A1 data, although

somewhat variable, are uniform with time as would

be expected with a very slowly leached cation in

similar but differently managed soils. The

exchangeable K level of the soils of this area is very

low, as is the level of reserve K (Table 2), and

accounts for the uniformity of these characteristics

among the sites. Supply of available K is identified

as second to P as a growth limiting factor and the

most difficult fertility management problem (Vilela

and Ritchey, 1985). The exchangeable H and pH

data show little variation with time after slash-and-

burn. Neither A1 nor Mn toxicities would be

anticipated at the pH levels observed here

(Kamprath, 1984).

P h o s p h o r u s a n d s u l f u r

EXCHANGEABLE CATIONS

cmol (+) kg-

2 •

Ca I ~ o

0 i i i

t

g I . o o - J - t • I I •

• l

0 • i

0

t q • •

t | •

A J I | -

• •

' ' 5'o

0 I 0 3 0

T I M E A F T E R C L E A R I N G y

Fig. 2.

ExchangeableCa, Mg, K and A1 of0--20 cm soil samples

with time after clearing.

N u t r i e n t s o i l d e p t h r e l a t i o n s h i p s

The distribution of nutrients with depth in these

profiles was measured (Wadsworth, 1987).

Nutrient contents were highest in the surface

horizons, due largely to biocycling and organic

associations (Wadsworth, 1987; Wadsworth e t a l .

1988). There was a general decrease in available

nutrients starting in the upper Bt horizon (50---

100cm) and continuing to 2 m. Variability in sub-

surface characteristics was low demons trating that

changes occurring in the surface were due to treat-

ment rather than natural variability. Magnesium

content differed from that of the other nutrients in

that it increased in the lower Bt horizons. In three

of the pedons there was a slight increase in Ca with

depth. Depths at which cation increases were ob-

served corresponded roughly to the depths at which

low chroma mottles appeared, (Wadsworth e t a l .

1988) suggesting transport as nitrate salts and

deposition with nitrate reduction in association

with water table fluctuations.

The available P and S levels of these soils are

given in Table 2. The extractable P level of these

soils is very low (Thomas and Peaslee, 1973), did

not change appreciably with time, and is identified

as the principle limiting factor to crop production

in these soils. In contrast, extractable S values in-

dicate adequate levels of this nutrient (Reisenauer

e t a l . 1973) for a range of crops.

D i s c u s s i o n

This study was limited by our inability to dis-

tinguish natural variability from differences result-

ing from management. A reasonable degree of site

uniformity is suggested by the uniform levels of

exchangeable Ai and pH of these sites. Variability

8/20/2019 Wadsworth 1990


o f s u b s u r f a c e c h a r a c t e r i s t i c s w a s a l s o r e l a t i v e l y

l o w , f u r t h e r s u p p o r t i n g t r e a t m e n t r a t h e r t h a n

n a t u r a l v a r i a b i l i t y a s a p r i n c i p a l s o u r c e o f t h e d i f -

f e r e n c e s o b s e r v e d . T h e a p p a r e n t u n i f o r m i t y o f

a v a i l a b l e P a n d K l e v e ls w a s a t t r i b u t e d t o t h e i r lo w

l e v e l s a n d a n a l y t i c a l i n s e n s i t i v i t y .

D e c r e a s e s i n o r g a n i c C , N , a n d m i n e r a l n u t r i e n t

l e v e l s w i t h c l e a r i n g a n d t h e i r s u b s e q u e n t i n c r e a s e s

w i t h f o r e s t r e g r o w t h h a v e b e e n o b s e r v e d b y s e v e r a l

w o r k e r s J e n n y a n d R a y c h a u d h u r i , 1 96 0; N y e a n d

G r e e n l a n d , 1 96 0; R a m a k r i s h n a n a n d T o k a y , 1 9 81 ;

S a n c h e z , 1 9 7 6 ; S a n c h e z

et al.

1 98 2 ; S a n c h e z

et al.

1 98 3) . T h e i n i t i a l e f f e ct s d e p e n d o n t h e f u e l s u p p l y

a n d b u r n i n t e n s it y , a n d t h e r e c o v e r y d u r i n g f o r e s t

r e g r o w t h t h e f a l l o w p e r i o d ) on r e e s t a b l i s h m e n t o f

t h e n a t u r a l d e e p - r o o t e d v e g e t a t i o n f r o m s p r o u t s

n o t k i ll e d d u r i n g t h e c r o p p i n g c y cl e L a m b e r t a n d

A r n a s o n , 1 9 86 ). I n t e n s i v e l y w e a t h e r e d s o i l s , s u c h

a s t h e P a l e u d u l t s o f t h i s s t u d y , a r e d e p l e t e d o f

p r i m a r y m i n e r a l s t h a t p r o v i d e n u t r i e n t b u f f e r i n g

a n d r e c o v e r v e r y s l o w l y f r o m n u t r i e n t l o s s e s i n -

c u r r e d d u r i n g a p e r i o d o f c u l t iv a t i o n . I n t h i s s tu d y ,

r e t u r n t o t h e i n i t i a l lo w f e r t i l i t y s t a t u s w a s e s s e n -

t i a l l y c o m p l e t e a f t e r 4 5 y o f f a l l o w .

A g r i c u l t u r a l i n t e n s i f i c a t i o n c o u l d p r o b a b l y b e

u n d e r t a k e n i n t h i s e c o s y s t e m b u t n o t w i t h o u t

c h a n g e s i n a g r i c u l t u r a l t e c h n o l o g i e s . T h e s e w o u l d

i n c l u d e d i f f e r e n t c r o p p i n g a n d w e e d c o n t r o l p r a c -

t ic e s , a p p l i c a t i o n s o f P a n d K f e r t il i z e r s , l i m i n g i f

o t h e r t h a n a c i d t o l e r a n t c r o p s w e r e t o b e g r o w n ,

a n d p e r h a p s m i c r o n u t r i e n t f e r t il i z a t io n a s so i l

c h a r a c t e r i s t i c s w e r e c h a n g e d a n d p r o d u c t i o n l e ve l s

i n c r e a s e d. R e s e a r c h t o d e v e l o p c r o p p i n g s y s t e m s

l i k e t h o s e d e s c r i b e d b y S a n c h e z a n d B e n i t e s 1 9 8 7 )

t h a t a r e a d a p t e d t o t h e s o c i o e c o n o m i c c o n d i t i o n s

o f t h e a r e a a r e e s s e n t i a l t o th e w e l f a r e o f t h e s e

p e o p l e .

cknowledgement

W e t h a n k A r m a n d o M e j i a - N u n e z , Ju l io

C a m a r a - C o r d o v a , t h e fa r m e r s o f F r a n c i s c o R u e d a ,

a n d t h e s t a t e o f T a b a s c o f o r s u p p o r t i n t h e f i e ld .

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