maintenance of yields and soil fertility in nonmechanized cropping systems, bolivia

Maintenance of Yields and Soil Fertility in Nonmechanized Cropping Systems, BoliviaR. G. Barber* and O. Diaz

ABSTRACTSlash and burn farmers in the tropical rain forests of eastern Bolivia

are abandoning land after one rice (Oryza saliva L.) crop because ofdeclining yields. A trial was conducted on a Typic Paleudult for41 mo to investigate whether alternative low-input nonmechanizedcropping systems could prolong soil fertility and yield maintenance,and whether soil fertility or weeds were responsible for decliningyields. Twelve cropping systems were investigated in a factorial design,with three summer-winter crop sequences: rice-peanut (Arachis hypo-gaea L.), corn (Zea mays L.)-bean (Phaseolus vulgaris L.) later substi-tuted by cowpea [Vigna unguiculata (L.) Walp.], and rice-fallow(control); two weed control treatments: minimal and optimal; and twofertilizer treatments: with and without 60 kg N ha"1 and 17.5 or 35kg P ha -'. Crop sequences significantly increased exchangeable acidity;the rice sequences significantly reduced exchangeable Ca, and corn-bean/cowpea and rice-peanut significantly reduced exchangeable Mg.Fertilization significantly increased soil P but decreased Ca. Foliaranalysis revealed N, Mg, and Zn deficiencies in all cropping systems.Rice yields, unlike corn, were significantly increased by optimal weed-ing. Corn yields were dominated by fertilization, whereas rice yieldswere mainly influenced by fertilization in the first and fourth years,and by weeds in the intervening years. Without fertilizers, rice-fallowwas not sustainable, and only corn-bean/cowpea was sustainable for3 yr. With fertilization, rice-fallow plus optimal weed control andcorn-bean/cowpea with minimal or optimal weeding were sustainablefor 3 yr. Additional fertilization and future liming would be necessaryfor more prolonged sustainability.

MANY FARMERS are practicing shifting cultivation inthe tropical rain forests of the sub-Andean foothills

of Bolivia. The soils are of low fertility (Cochrane,1973), and land is generally cropped for only 1 yr beforebeing abandoned to avoid declining yields (Thiele, 1991).Soil fertility depletion has been suggested as the probablecause of declining yields for soils of low base saturation,and weed infestation for soils of high base saturation(Sanchez, 1976). However, the relative importance ofthese two factors will probably vary with soil type, landmanagement, and even from season to season.

At Yurimaguas, Peru, on very infertile acid soils, ahigh-input mechanized system of continuous cultivationafter manual clearing, which included the application oflime and fertilizers, gave sustainable yields and waseconomically viable (Bandy and Sanchez, 1986). How-ever, given the high cost and unavailability of mostfertilizers in Bolivia, it is unlikely this system would beeconomical or widely adopted in Bolivia in the foresee-able future.

A low-input nonmechanized system of continuouscropping using rice-cowpea rotations, zero tillage, nofertilizers, and returning crop residues to the soil has alsobeen investigated at Yurimaguas (Sanchez and Benites,

R.G. Barber, British Tropical Agricultural Mission, Santa Cruz, Bolivia,c/o F.C.O. (La Paz), King Charles Street, London SW1A 2AH, UK; andO. Diaz, Centro de Investigaci6n Agricola Tropical (CIAT), Casilla 247,Santa Cruz, Bolivia. Received 22 Apr. 1993. Corresponding author.

Published in Soil Sci. Soc. Am. J. 58:858-866 (1994).

1987). These practices have enabled yields to be main-tained for 3 yr, by which time P and K deficiencies devel-oped, and more importantly, it became no longer econom-ically feasible to control grass weeds. The grass weedproblem was especially acute in rice (Mt. Pleasant etal., 1990), and was undoubtedly aggravated by lack oftillage and absence of burning (Sanchez and Benites,1987). The sowing of a 1-yr kudzu [Pueraria phaseo-loides (Roxb.) Benth.] fallow at the end of the third yearreduced weed infestation and increased yields, but onlyfor 1 yr (Alegre et al., 1991). Thus, at Yurimaguas, thelow-input system is considered a transition from shiftingcultivation to a more sustainable option such as mecha-nized cropping with high inputs, grass-legume pastures,or agroforestry (Sanchez et al., 1987).

Yapacani, in the department of Santa Cruz, Bolivia,where the present trial was established, has a slightlylower annual rainfall than Yurimaguas (1900 vs. 2100mm), and slightly different soil fertility characteristics.The soils are lower in P and organic matter but higherin exchangeable bases and pH than the soils at Yurima-guas. A trial was designed to study whether variouslow-input nonmechanized cropping systems could besustained for >1 yr, which is the normal duration ofcropping on these soils. Twelve cropping systems wereinvestigated in a factorial design, with three crop se-quences, two of which included winter crops, two fertil-izer treatments (with and without modest N and P applica-tions), and two weed control methods (minimal andoptimal control). The effects of the 12 cropping systemson soil fertility and yield maintenance, and the relativeinfluence of fertilization and weed control on crop yieldswere evaluated during seven seasons from 1986 to 1990.

MATERIALS AND METHODSThe trial was conducted in the Yapacani regional research

center (17°25'S, 63°55'W; 400 m above sea level) situatedin the sub-Andean foothills of eastern Bolivia. The averageannual precipitation of 1948 mm permits two cropping seasons,summer from October to March, with 1356-mm average rain-fall and a mean temperature of 26.1°C, and winter fromApril to September, with 592-mm average rainfall, a meantemperature of 22.2 °C, and mean minimum extreme tempera-tures of 7.1 °C in July and August.

The trial was located on a gently sloping (0-2%) site on afine-loamy isohyperthermic Typic Paleudult (Barber and Diaz,1987). Mean values of selected soil properties are given inTable 1.

The trial was conducted from October 1986 until the harvestof the seventh crop (four summer and three winter crops)in March 1990. Twelve treatments were investigated in athree-factor ( 3 x 2 x 2 ) experiment using a randomizedcomplete-block design with four replications and 10 by 10 mplots. The treatments comprised three crop sequences, rice-fallow, corn-bean/cowpea, and rice-peanut, with the winter

Abbreviations: ECEC, effective cation-exchange capacity. *Significantat the 0.05 probability level.

858

BARBER & DIAZ: YIELD AND FERTILITY MAINTENANCE IN NONMECHANIZED SYSTEMS 859

Table 1. Selected initial properties of the Typic Paleudult at Yapacani.

Depth

m0-0. 15§

0.15-0.30§0.30-0.4S§0.45-0.6010.60-0.7510.75-0.901

Sand

605554525048

Silt

262727272827

Clay

141819212225

pH (H20)(1:5)

5.605.405.124.934.854.79

Exchangeable bases

Ca

3.231.491.230.900.750.65

Mg

0.410.180.140.150.170.12

K Nai i, -i

0.39 0.030.24 0.050.17 0.040.13 0.030.12 0.060.14 0.03

Totalbases

4.071.951.581.211.100.95

ECECt

4.262.542.692.662.863.12

Basesat.}

a

957661474032

Alsat.i

fc ———-1833495563

AvailableP

mg kg-1

4.751.541.080.750.750.45

Organicmatter

%1.591.130.790.730.710.73

TotalN

0.0770.0670.0450.042*0.038#0.036*

t ECEC = effective cation-exchange capacity.t sat. = saturation.§ Mean values of 24 measurements.1 Mean values of 20 measurements.# Mean values of 12 measurements.

crops given second; two weed control treatments, minimal andoptimal; and two fertilizer treatments, without and with modestN and P applications. In the corn-bean/cowpea treatment, thefirst winter crop, bean, performed poorly and so was replacedby cowpea. The weed control treatments refer to the summercrops, all winter crops being weeded once only. Minimal weedcontrol comprised one weeding at 20 to 35 d after sowing forthe first three summer crops, but this was increased to twoweedings in the fourth summer crop because of increasing weedinfestation. Optimal weed control comprised two weedings at20 to 35 and 70 to 75 d after sowing for the first three summercrops, and then three weedings for the fourth summer cropdue to increasing weed problems. Weeds were generally con-trolled by hoeing except in very wet conditions when propanil|W-(3,4-dichlorophenyl) propionamide] and 2,4-D (2,4-dichlo-rophenoxyacetic acid) were applied in rice, and paraquat (1,1-dimethyl-4,4'-bipyridinium ion) as a directed application incorn. In the fertilized treatment, 35 kg P ha"1 as triple super-phosphate was applied at sowing with a jab planter at 2- to5-cm depth to the first summer crop. A jab planter is a planterwith two hinged handles, a combined hopper and seed chute,and planting jaws that can be thrust into the soil. On openingthe handles, the seed or fertilizer is released from the plantingjaws into the soil. After this initial P boost, the rate wasreduced to 17.5 kg P ha"1 for all subsequent summer crops.Nitrogen was applied as urea with a jab planter at 2- to 5-cmdepth at a rate of 60 kg N ha"1 in three split applications toeach summer crop, applying one-third at sowing, one-third at30 d after sowing, and one-third at panicle initiation in riceand at 10 d before inflorescence in corn. Fertilizer rates weredetermined solely on the basis of soil analyses since no fertil-izer-crop response data are available. No fertilizers were ap-plied to the winter crops.

All crops were sown with a jab planter. Rice cv. CICA-8was sown at 0.30 by 0.30 m spacing with 10 to 15 seeds perplanting hole, and corn cv. SUWAN was sown in 1-m rowsat 0.50-m spacing within rows and three to five seeds perplanting hole. The corn was subsequently thinned to two plantsper planting hole. Bean cv. Carioca SEL-2 and the replacementcrop, cowpea cv. IT-83D-889, were sown at 0.50 by 0.10 mspacing. During the first winter (1987) the peanut cv. ColoradoPalmar was sown at 0.40 by 0.30 m spacing, but this wasreplaced by a higher yielding local cultivar, Perla de Saavedra,sown at 0.40 by 0.20 m spacing.

There were some variations in crop residue managementfrom year to year. After the harvest of the first summer crops,the rice and corn stubble was cut, piled, and burned followinglocal practice. Prior to sowing the second summer crop, thewinter crop residues and weeds were too wet to be burnedand so were removed from the plots to permit sowing. The

residues from the second summer crop were slashed and left asa mulch in an attempt to introduce a more conservation-orientedpractice. This practice gave no problems for cowpea emergencefrom the corn stubble, but the higher quantities of rice stubbleand cold conditions inhibited peanut germination, and 2 wkafter sowing, the rice mulch had to be piled and burned andpeanut resown. Thereafter, the residue from all plots washeaped and burned prior to sowing the next crop. Pest anddisease control was carried out according to recommendationsgiven by CIAT (1986).

Composite soil samples comprising 25 subsamples werecollected from each plot at 0- to 0.15-m depth on 27 Oct.1986, 5 d after burning the vegetation, and then on 7 Apr.and 7 Dec. 1987, 21 Sept. 1988, 20 Oct. 1989, and 16 Aug.1990. Composite samples comprising six subsamples werecollected from successive 0.15-m depths from 0.15 to 0.90 mon 7 Apr. 1987 and 16 Aug. 1990. Composite plant sampleswere collected for foliar analysis, consisting of 50 leaves ofrice either at panicle initiation or midtillering, and 20 earleaves of corn at silking.

Soil and plant samples were analyzed according to themethods of Cochrane and Barber (1993). Soil pH was measuredin a 1:5 soil/water suspension, organic C was determined bythe Walkley-Black method, and total N by the Kjeldahl method.Available P was determined by the Olsen method, exchangeablebases by 1 M NHtOAc extraction, exchangeable acidity andAl by 1 M KC1 extraction, and ECEC by the summation ofbases and exchangeable acidity. Exchangeable Ca and Mgwere assayed by atomic absorption spectrophotometry, K andNa by flame photometry, and P by absorption spectrophotom-etry.

For plant analysis, N was determined by a modification ofthe micro-Kjeldahl method, and S by Mg(NO3)2 fusion, HC1O4digestion, and a turbidimetric assay using BaCl2. The elementsCa, Mg, K, P, Fe, Mn, Cu, Zn, and B were extracted by dryashing at 470 °C and dissolving the residues in dilute HC1.The assay of Ca, Mg, Fe, Mn, Cu, and Zn was by atomicabsorption spectrophotometry, K by flame photometry, and Pand B by spectrophotometry through the formation of molybde-num blue and azomethine-H complexes, respectively.

Foliar nutrient deficiencies were identified according to datapublished by Jones (1967) for corn and Ward et al. (1973)for rice.

Statistical analyses were carried out following the proceduresof Gomez and Gomez (1984). Despite blocking and treatmentrandomization, significant differences in initial soil organicmatter contents (P > 0.05) were found between the fertilizedand unfertilized treatments. Moreover, organic matter levelswere significantly correlated with crop yields (r = 0.291*) inthe first season that treatments were applied. Thus for crop

860 SOIL SCI. SOC. AM. J., VOL. 58, MAY-JUNE 1994

yields, the soil organic matter contents determined at sowingwere used as covariates to adjust yields to the mean organicmatter contents in the analysis of variance. In this way, theconfounding effect of organic matter differences on fertilizertreatment effects was removed. Mean values were separatedusing 5% least significant differences, and treatment meansin interactions were compared by single degree of freedomorthogonal contrasts. Variations in soil parameters with timefor crop sequence and fertilizer treatments were analyzed bysimple linear regressions, and differences in trends were evalu-ated by testing for the homogeneity of regression coefficients.

RESULTS AND DISCUSSIONMaintenance of Soil Fertility

Simple linear regression analysis of soil fertilitychanges with time in the 0- to 0.15-m horizon for thethree crop sequence treatments showed that exchangeable

acidity, base saturation, exchangeable Ca, available P,and ECEC all gave statistically significant trends (Fig.1). All three sequences caused significant increases inexchangeable acidity and significant decreases in basesaturation, but these changes were not sufficient to givesignificant changes in soil pH. Rice-peanut registered asignificantly faster increase in exchangeable acidity anddecrease in base saturation, as indicated by significantlyhigher regression coefficients, compared with the othercrop sequences. Rice-fallow and rice-peanut both gavesignificant decreases in exchangeable Ca, which is consis-tent with the greater removals of Ca by rice and peanutthan by corn (Nye and Greenland, 1960). Changes inexchangeable Mg over 46 mo were not significant, dueto a large increase at the last sample date, presumablydue to random sampling errors. When this last valuewas ignored, however, a significant negative correlation

0.60.50.40.30.2O.I

4.0

3.0

2.0

1.0

8.07.06.05.04.03.0

EXCHANGE ACIDITY (cmoLkg'1)- A Rf i h ° 75t> b=0.0045b

-^Cb/cir|o.82«U=O.Oq4lbo

EXCHANGEABLE Co(cmolckg~')

-A- Rf -, £0.68*• Cb/c;r*=0.5l

-A- Rp j r*=0.69*

AVAILABLE P (mg kg'1)A A

AA

*_.A._RfCb/c' r2= 0.10Rp ; r = 0 . 2 6

0 6 14 23M O N T H S

36

100

90

80

0.5

0.4

0.3

0.2

5.0

4.0

3.0

BASE SATURATION (V.)

_A-Rf , £0.73*1 b=-O.I2l°—•-- Cb/fc jr^O.89** b=-O.I37b

-A-Rp ir2=0.90*%=-0.272a

EXCHANGEABLE Mg(cmolckg*')A Rf

«A

Regression for first 36 months only

E C E C ( cmol kg'1)

i46

I6 14 23

M O N T H S36 46

Fig. 1. Simple linear correlations between soil fertility properties at 0- to 0.15-m depth and time after forest burning for rice-fallow (Rf), corn-bean/cowpea (Cb/c), and rice-peanut (Rp) crop sequences. Regression lines are only given for statistically significant correlations, and valuesof the regression coefficient (b) are only given where there are significant diflerences between regressions. Values of b with the same lettersubscript are not significantly different at the 0.05 level; r = correlation coefficient. *, ** Statistically significant at 0.05 and 0.01 probabilitylevels, respectively.


was found for Mg up until the time the last crop wassown for corn-bean/cowpea and rice-peanut. Magne-sium concentrations decreased from 0.46 and 0.44 cmolckg"1 in October 1986 for the rice-peanut and corn-bean/cowpea, respectively, to very low values of 0.26 cmolckg"1 at the beginning of the last cropping season inOctober 1989. These findings are substantiated by foliaranalysis (Table 2), which shows the existence of Mgdeficiency hi corn from at least the 1987-1988 seasononward, and in rice for the last season, 1989-1990.

A significant negative correlation was found betweenthe ECEC and time for rice-peanut, but not for theother crop sequences. This is difficult to explain as nosignificant trends were found for soil organic matter.The only significant correlation between available P andtime was a positive trend for rice-fallow, which canprobably be attributed to a lower utilization of P fertilizerby this sequence than by the more nutrient demandingsequences that included winter crops. This is consistentwith foliar analysis data for 1988-1989 (Table 2), whenin the absence of fertilizer, rice-peanut was deficient inP whereas rice-fallow was not.

The fertilizer treatment in which 17.5 or 35 kg P ha"1

as triple superphosphate and 60 kg N ha"1 as urea wereapplied to each summer crop gave significant trends ofincreasing available P, and decreasing exchangeable Ca

(Fig. 2). The significant decrease in Ca with time canprobably be attributed to fertilizer-enhanced crop growthintensifying Ca uptake, and to the acidifying effects ofurea application. Foliar analysis data (Table 3) revealedsignificant increases in the uptake of N, K, P, Ca, Mg,Mn, Zn, Cu, and Fe with the application of N and Pfertilizers. Although these increases, with the exceptionof Ca, were not of sufficient magnitude to be manifestin detectable changes in soil nutrient levels, the increasinguptake and depletion of these nutrients due to N and Pfertilization would be expected to result in lower sus-tainability unless additional nutrients are supplied.

The soils were not analyzed for micronutrients, butfoliar analyses (Table 2) showed the existence of Zndeficiencies in both fertilized and unfertilized treatmentsfor both rice and corn from 1987-1988 onward, and apossible Cu deficiency in rice in 1989-1990.

Despite apply ing the urea in three split applications,most of the corn and rice crops were deficient hi Naccording to foliar analysis, and the severity of N defi-ciency increased with time (Table 2). The critical foliarN concentrations used to denote N deficiency were 2.76 %for corn at silking (Jones, 1967), 3.81% forriceatmidtiller-ing, and 2.85% for rice at panicle initiation (Ward etal., 1973). The low efficiency of the N fertilizer isconsistent with the zero NOs retention capacity measured

Table 2. Foliar analysis of rice and corn crops with and without fertilizer indicating deficient and near-deficient values in 1987-1988,1988-1989, and 1989-1990.

Foliar analysis}:Crop sequence! N Ca Mg Fe Mn Cu Zn

-gkg" • mg kg-With fertilizer

1987-1988Rice§ (fallow)Cornl (bean/cowpea)Rice§ (peanut)


1989-1990Ricetl (fallow)Cornf (bean/cowpea)Riceit (peanut)

1987-1988Rice§ (fallow)Cornf (bean/cowpea)Rice§ (peanut)


1989-1990Ricett (fallow)Cornl (bean/cowpea)Riceft (peanut)

38.9 2.82l.1# 2.538.8 2.8

2.6 35.6 1.61.1 21.9 3.02.1 36.5 1.4

19.6 2.4— 17.Q 3.0- 22.4 2.1

36.5 3.1- 18.1 2.4- 33.9 3.2

2.5 32.9 1.41.3 17.6 2.62.2 33.1 1.2

- 17.5 2.4— 12.5 2.6

1M !•'

17.916.2ft18TT

24.220.124.4

16.923.017.5

Without

15.216.919.8

23.819.522.8

16.719.715.9

2.12.92.1

4.24.14.5

5.78.56.2

fertilizer

1.82.82.4

3.73.63.6

4.76.84.3

1.5U1.5

2.31.32.5

1.2i SLI

1.11.41.4

1.81.21.8

0.91.52J

1519094

888693

3910350

1328775

627569

387541

15029

149

17249

200

22050

274

722991

8146

108

9634

159

869

979

596

767

77

10

fI

191021

24Ifi25

1116M

M1121

M_

U14JJ

——-

95

10

91110

_--

93

10

141012

t Winter crops given in parentheses.$ Mean values of eight measurements.$ Sampled at midtfflering.1 Sample of ear leaf at silking.# Deficient values underlined with double lines.ft Values approaching deficiency levels underlined with single lines.tt Sampled at panicle initiation.


10.09.08.07.06.05.04.03.0

AVAILABLE P (mg kg-1)

EJ -Fvr^O-IAiP—O-O•-•»• F; r=0.7ltWot$975

0 14 23MONTHS

0Q

4.0

3.0

2.0

EXCHANGEABLE Co(cmolckg')

Q -F ;r2=0.59r2= 0.70*

36 46 0 6 14 23MONTHS

36 46

Fig. 2. Simple linear correlations between soil fertility properties at 0- to 0.15-m depth and time after forest burning for fertilized ( + F) andunfertilized (- F) treatments. Regression lines are only given for statistically significant correlations.

for this soil (Wong et al., 1990), and the high rainfallenvironment. The predominantly kaolinitic mineralogyof the soil rules out the possibility of NRt fixation. Nosignificant effects of weed control treatments on soilfertility were found.

Treatment Effects on Crop YieldsYields of the winter crops were 522 kg ha"1 for bean

in 1987, followed by 897 and 651 kg ha'1 for cowpeain 1988 and 1989, respectively. Peanut yields were 1104and 616 kg ha~! for 1987 and 1988, respectively; no yielddata was recorded for 1989. The subsequent discussion isconcerned with yields of the summer crops.

All yield data were adjusted using soil organic mattercontent as the covariate as described above. Table 4shows that, with the exception of the 1987-1988 season,rice yields were not significantly affected by the presenceor absence of a winter crop. This is despite the signifi-cantly greater rate of increase in exchangeable acidityand the significant decrease in Mg levels with rice-peanut compared with rice-fallow.

Significantly higher yields were recorded for the opti-mal weed control treatment than for minimal control for

all but the 1986-1987 season (Table 4). This is explainedby the significantly higher weed biomass, measured pre-harvest, of 5.85 Mg ha"1 in 1987-1988 and 6.11 Mg ha"1

in 1989-1990 for the minimal weed control treatment,compared with 1.12 Mg ha~l in 1987-1988 and 3.37Mg ha"1 hi 1989-1990, for the optimal weed controltreatment. Weed biomass was not measured in 1986-1987, but no significant differences in weed density werefound (Gonzales et al., 1991, unpublished data).

An interaction effect between crop sequence and weedcontrol was observed hi two of the four seasons and fortotal yield (Table 4). Thus, optimal weed control gavea significantly greater yield than minimal control forrice, but not for corn. This is in agreement with theresults of Mt. Pleasant et al. (1990). In contrast, therewas no significant difference between the effects of weedcontrol treatments on rice yields whether the sequenceincluded fallow or peanut as the winter crop. No interac-tion was found between weed control and fertilizationtreatments.

The application of modest amounts of N and P signifi-cantly increased both rice and com yields (Table 5), givinga 46% increase in total yield and a 63% rate of returnon fertilizer cost during the 4 yr. Only in the first summer

Table 3. Influence of fertilization on foliar analysis of rice and corn combined for 1987-1988,1988-1989, and 1989-1990 seasons.Foliar analyses!

Fertilizer (F) treatment

1987-1988-F+ FStatistical significance



S

_—-

2.01.9NS

__-

N

29.533.1***

27.931.3#**

16.219.7***

P

g kg-1O O

2.92.7NS

1.72.0***

2.32.5*

K

17.317.4NS

22.122.9

*

17.419.2**

Ca

2.32.4NS

3.74.3***

5.36.8*+*

Mg

1.31.4NS

1.62.0***

1.11.5***

Fe

98112NS

6989

***

5164*

Mn

64109***

78140***

96181***

Cu. _,— mg Kg

6.77.8**

7.98.4NS

4.66.6***

Zn

1716NS

1822***

1315NS

B

_—-

7.48.2NS

12.310.1NS

*, **, *** Significant at 0.05, 0.01, and 0.001 probability levels, respectively. NS = not significant.t Mean values of 24 measurements.


Table 4. Crop sequence, weed control, and interaction eflects on adjusted rice and corn yields.

Cropsequencet

Rice (fallow)

Yields*Weed control

MinimalOptimal

1986-87

28683006

Mean§

2937

1987-88

18942794

Mean§

2344b1

1988-89

——— kgha-17623323

Mean§i2S42b

1989-90

18262603

Mean§

2214b

Total

835011726

Mean§

10037b

Corn (bean/cowpea)

Rice (peanut)

MinimalOptimalMinimalOptimal

Orthogonal comparisonsCrop sequence (C)Minimal vs.

optimal weeding (W)C x W(Rice vs. com) x W[Rice (fallow) vs. rice

(peanut)] x W

2969288229513227

NSNSNS

NS

2925

3089

NS

2468244824843046

******»

NS

2458b

2765a

2965328715823742

********

NS

3126a

2662ab

3105329117392394

NSNS

NS

3198a 11707a1150711908

2067b 8 756 10 583b12409

*******

NS*, **, *** Significant at 0.05, 0.01, and 0.001 probability levels, respectively. NSt Winter crops given in parentheses.| Each value is the mean of eight measurements.§ Mean values for the crop sequence.1 Values in the same column with the same letter are not significantly different.

season (1986-1987) was there a crop sequence-fertiliza-tion interaction, when corn gave a significantly greaterresponse to fertilizer than rice.

Relative Importance of Fertilization and WeedControl on Crop Yields

Data from Tables 4 and 5 have been used to calculatethe percentage yield increases due to fertilization andoptimal weed control for each crop sequence at eachharvest (Table 6). Corn yields were dominated by fertil-ization throughout, whereas for rice, fertilization wasdominant in the first season after burning (1986-1987)when weed problems were slight. Thereafter, as weedinfestation increased, weed control became the dominantfactor for the second and third rice harvests, but wassuperseded by fertilization in the fourth rice harvestwhen nutrient deficiencies, N, Mg, Zn, and possiblyCu, became more acute (Table 2). The change in therelative importance of fertilization compared with weed

• not significant.

control in the fourth rice crop may be due to the increasein the number of weedings in the minimal and optimalweed control treatments in this season, and/or to themarked deterioration in soil fertility, which promotedweed infestation, making it necessary to increase thenumber of weedings in both weed control treatments.We have observed a marked correlation between deterio-rating soil fertility and increasing weed problems in otherparts of Santa Cruz.

Sustainability of Crop YieldsWhen considering yield sustainability, the seasonal

summer rainfall (October-March) was plentiful in allfour years, 1314, 733, 1126, and 986 mm for the 1986-1987, 1987-1988, 1988-1989, and 1989-1990 seasons,respectively. Thus, rainfall cannot be considered as alimiting factor, except in 1987-1988 when there wasexcessive rainfall at the very beginning of the season,which in combination with late planting resulted in poor

Table 5. Fertilization, crop sequence, and interaction effects on adjusted rice and corn yields.

Crop Fertilizer (F)sequencet treatment

Rice (fallow)

Corn (bean/cowpea)

Rice (peanut)

Orthogonal comparisons-Fvs. +F(Crop sequence) ( - F vs. + F)(Rice vs. corn) ( - F vs. + F)[(Rice (fallow) vs. rice (peanut)][-Fvs. +F]

-F+ F-F+ F-F+ F

1986-87

269831772287356428393338

********

NS

1987-88

198027082007290926312898

***NSNS

NS

Yield*

1988-89

——— kg ha-' ————206830172543370923812943

***NSNS

NS

1989-90

144929792172422512362898

***NSNS

NS

Total

8195118819009

144079087

12077

***NSNS

NS

*, **, *** Significant at 0.05, 0.01, and 0.001 probability levels, respectively. NS = not significant.t Winter crops given in parentheses.t Mean values of eight measurements.


Table 6. Crop sequences and rice and corn yield increases due to fertilization and optimal weed control treatments, 1986 through 1990,evaluated from data in Tables 4 and 5.

Cropping sequence!

Yield increase

1986-1987 1987-1988 1988-1989Fertilizer): Weeding§ Fertilizer Weeding Fertilizer Weeding

t Winter crops given in parentheses.| Increase in yield due to with fertilizer compared with without-fertilizer treatment.§ Increase in yield due to optimal weeding compared with minimal weeding treatment.

1989-1990Fertilizer Weeding

Rice (fallow)Corn (bean/cowpea)Rice (peanut)

185617

509

374510

47023

464624

8811136

10594134

42637

initial growth and a marked yellowing of the crops inthe unfertilized plots. Pest and disease problems wereadequately controlled throughout, and so with the excep-tion of 1987-1988, any yield decline can largely beattributed to weed or soil fertility factors.

Figure 3a shows that in the absence of fertilizersand with only minimal weed control, none of the crop

i.e.

A.O-

-o Rice (fallow).min. weed cont.- Rice(fallow),opt. weed cont.-A Rice (peanuts), min. weed cont.4 Rice (peanuts), opt. weed cont.

Maize (beans/cowpeas). min. weed contr.Maize(beans/cowpeas), opt. weed contr.

0.61(5) 2(18) 3(29) MA1)

HARVEST NUMBER (MONTHS SINCE FOREST BURNING)Fig. 3. Changes in adjusted rice and corn yields of crop sequence-

weed control treatments with time after forest burning for (a)without fertilizers, and (b) with fertilizers.

sequences sustained yields, although corn-bean/cowpeaonly gave an 18% decline in yield through the fourcropping seasons. Rice-fallow, which is similar to thetraditional system, except that the same parcel of land iscontinuously cultivated, showed a very dramatic declinefrom 2.51 Mg ha"1 in the first year to 1.46 Mg ha"1 inthe second year. This can be explained by the almostthreefold greater weed biomass at preharvest in thiscropping sequence than in rice-peanut (Gonzales et al.,1991, unpublished data). Thereafter, yields declinedslowly, whereas for rice-peanut, yields decreased almostlinearly with time from 2.74 to 0.91 Mg ha"1 in thefourth year, at which time farmers would be expectedto abandon the land.

In contrast to the rice-based cropping sequences, theyields of corn-bean/cowpea declined much more slowlyfrom 2.47 Mg ha"1 in 1986-1987 to 2.02 Mg ha'1 in1989-1990. This may be attributed to various factors,e.g., lower nutrient depletion compared with rice-pea-nut, the use of the corn stubble as a surface mulch inthe 1988 winter season, which by not being burnedconserved nutrients and enhanced weed control, and thelower vulnerability of corn to grass weeds comparedwith rice (Gonzales et al., 1991, unpublished data).

The highly significant effect of optimal weed controlon yields (Table 4), especially with rice, is reflected inthe greater sustainability of the rice-fallow and rice-peanut yields with optimal weed control than with mini-mal control (Fig. 3a). However, yield maintenance,which was slightly superior for rice-peanut than rice-fallow, did not extend beyond the third harvest. Riceyields then declined dramatically from the third to thefourth harvest, from 2.64 to 1.66 Mg ha"1 for rice-fallow and from 3.19 to 1.56 Mg ha"1 for rice-peanut.The final yields were similar to those with minimal weedcontrol. This pronounced yield decline may be attributedto the onset of severe Mg and possible Cu deficiencies,as indicated by foliar analysis for the 1989-1990 ricecrop, which were not evident in the 1988-1989 season(Table 2). Moreover the density of broad-leaved andgrass weeds was lower in the 1989-1990 season than inthe 1988-1989 season (Gonzales et al., 1991, unpub-lished data), which strengthens the argument that soilfertility was mainly responsible for yield decline in thefourth year. The presence of a winter crop, i.e., fallowor peanut, did not influence the sustainability of the tworice sequences.


Table 7. Yield sustainability for crop sequence, fertilizer, and weed control treatment combinations for the period 1986 through 1990.

Crop sequencet

Rice (fallow)Rice (fallow)Rice (peanut)Rice (peanut)Corn (bean/cowpea)Coin (bean/cowpea)Rice (fallow)Rice (fallow)Rice (peanut)Rice (peanut)Corn (bean/cowpea)Corn (bean/cowpea)

Weedcontrol

MinimalOptimalMinimalOptimalMinimalOptimalMinimalOptimalMinimalOptimalMinimalOptimal

Fertilizer Initial (1986-1987)application yield (Y,)

WithoutWithoutWithoutWithoutWithoutWithout

WithWithWithWithWithWith

kgha-1

250928872742293724692104322831263160351634693659

Final (1989-1990)yield

kgha-'12361661910

156220172326241535442569322741934257

85% of initialyield

kgha-1

213324542331249620991788274426572686298929493110

Yield a; 0.85 y*

y020223030333

Average annualyield change

kgha'1-424-409-611-458-151+ 74-271+ 139-197-96+ 241+ 199

t Winter crops given in parentheses.t Evaluated from number of years when yield was within 85% of initial yield (Yi).

In contrast, corn with optimal weeding in the absenceof fertilizers gave a slight increase in yield from 2.10Mg ha"1 in 1986-1987 to 2.33 Mg ha'1 in 1989-1990.The reasons for the greater sustainability are the sameas for corn-bean/cowpea with minimal weed control.The weed control treatment did not significantly affectcorn yields except in the fourth harvest, when optimalweed control gave a 6% yield increase (Table 4).

In the presence of modest fertilizer applications andoptimal weed control (Fig. 3b), yields were significantlyand notably higher by about 46% than without fertilizers(Table 5). However, the two rice sequences gave declin-ing yields between the third and fourth harvests, whereascorn-bean/cowpea showed steadily increasing yieldsfrom the second to the fourth harvests. The decliningrice yields may be due to the onset in 1989-1990 ofmore serious Zn deficiencies than occurred in corn.Foliar analysis gave Zn concentrations of 14 mg kg"1

for rice, compared with a critical value of 33 mg kg"1,whereas the Zn concentration in corn was 16 mg kg"1

compared with a critical value of 20 mg kg"1.Yield sustainability was evaluated using two indices

for each crop sequence-fertilizer-weed control treatmentcombination. The first index is the number of years after1986-1987 when yields did not decline by more than15% of the initial value. Although yield trends with timewere generally not linear (Fig. 3a and 3b), the averageannual yield decrease or increase has been used as asecond index of yield sustainability. These indices donot take into account other factors such as labor require-ments or the economics of the different treatments. Table7 shows that rice-fallow with minimal weed control andno fertilization was not sustainable, and gave an averageannual yield decrease of 424 kg ha"1. However, thiswas superior to the more acidifying rice-peanut se-quence. Only corn-bean/cowpea gave average annualyield increases without fertilizer application. When Nand P fertilizers were applied, rice-fallow with optimalweeding and corn-bean/cowpea with minimal or optimalweed control all gave 3 yr of sustainable yields andaverage annual yield increases. In all three of thesecropping systems, however, there was evidence of N,Mg, and Zn deficiencies (Table 4) and increasing soilacidity (Fig. 1). This suggests that Mg, Zn, higher N

application rates and future liming would be needed toensure sustainability for a more prolonged period than3yr.

CONCLUSIONSAll three cropping systems were soil degrading, but

rice-fallow was less demanding on P and Mg than rice-peanut or corn-bean/cowpea. Triple superphosphate andurea applications increased available P, but decreasedexchangeable Ca and were unable to prevent N deficien-cies, probably due to the high NOs-leaching susceptibilityof the soil. Fertilizer application generally enhancednutrient uptake, and foliar analysis revealed serious Zn,Mg, and possible Cu deficiencies. Nevertheless in theabsence of fertilizers, corn-bean/cowpea with optimalweeding maintained yields for 3 yr, far longer thanthe 1-yr cropping period hi the traditional rice-fallowsystem. When fertilizers were applied, rice-fallow withoptimal weeding, and corn-bean/cowpea with minimalor optimal weed control, gave increasing yields through-out the subsequent 3 yr. Weed control was the dominantfactor influencing rice yields until the time that nutrientdeficiencies became acute, whereas corn yields weregoverned by soil fertility. Evidence from soil and foliaranalyses suggests that higher N application rates, Mgand Zn fertilizers, and future liming will be needed tosustain yields for a more prolonged period than 3 yr.

ACKNOWLEDGMENTSGregorio Gonzales, who was in charge of the experiment,

is thanked for providing yield and weed data, and Jos6 LufsEsc6bar for his help in the day-to-day management of the trial.The help of Morag Webb in the collection and analysis ofweed data and David Ruiz for assistance in the soils dataanalysis is also gratefully acknowledged. The referees and Dr.T.T. Cochrane are thanked for their helpful comments on thedraft manuscript. The director of the Centre de InvestigacionAgricola Tropical, Santa Cruz, Bolivia, and the OverseasDevelopment Administration, London, are acknowledged forgiving permission to publish this article.

866 SOIL SCI. SOC. AM. I., VOL. 58, MAY-JUNE 1994

maintenance of yields and soil fertility in nonmechanized cropping systems, bolivia

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