soil fertility variability in sandy soils and implications for nutrient management by smallholder...

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Soil Fertility Variability in Sandy Soils and Implicationsfor Nutrient Management by Smallholder Farmers inZimbabweT. J. Chikuvire a , S. Mpepereki b & R. Foti aa Department of Agriculture , Bindura University of Science Education , PO Box 1020,Bindura, Zimbabweb Department of Soil Science and Agricultural Engineering , University of Zimbabwe , PO BoxMP 167 Mt. Pleasant, Harare, Zimbabwe E-mail:Published online: 21 Sep 2008.

To cite this article: T. J. Chikuvire , S. Mpepereki & R. Foti (2007) Soil Fertility Variability in Sandy Soils and Implications forNutrient Management by Smallholder Farmers in Zimbabwe, Journal of Sustainable Agriculture, 30:2, 69-87, DOI: 10.1300/J064v30n02_08

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Soil Fertility Variabilityin Sandy Soils and Implications

for Nutrient Managementby Smallholder Farmers in Zimbabwe

T. J. ChikuvireS. Mpepereki

R. Foti

ABSTRACT. In-field variability of soil properties creates niches thathave been perceived by smallholder farmers, especially in hostile environ-ments, as constituting an essential part of their subsistence farming. Theycan exploit niche variability as a risk minimisation strategy for cropproduction. Smallholder farmers largely base their nutrient managementstrategies on their perception of niche fertility. The study physicallyand chemically characterised soil samples up to the 130 cm depth of pre-dominant arable niches from a representative sample of nine smallholderfarms with fields cropped for over 70 years. The niches were homesteadsurroundings, termitaria environments, areas under Parinari curatellifoliaand open sandy patches. The data were analysed using discriminant analy-sis, a statistical method that investigated niche differentiation based onsimultaneous analysis of soil nutrient variables. The analysis of variancecomplemented the discriminant analysis. Results showed that the first two

T. J. Chikuvire and R. Foti are affiliated with the Department of Agriculture, BinduraUniversity of Science Education, PO Box 1020, Bindura, Zimbabwe.

S. Mpepereki is affiliated with the Department of Soil Science and AgriculturalEngineering, University of Zimbabwe, PO Box MP 167 Mt. Pleasant, Harare, Zimba-bwe (E-mail: [email protected]).

Address correspondence to: T. J. Chikuvire at the above address (E-mail:[email protected]).

Journal of Sustainable Agriculture, Vol. 30(2) 2007Available online at http://jsa.haworthpress.com

© 2007 by The Haworth Press, Inc. All rights reserved.doi:10.1300/J064v30n02_08 69

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discriminant functions contributed significantly (p < 0.001) to niche dif-ferences and accounted for 95% of the total variation. The discriminationwas most powerful in classifying termitaria environments and aroundhomestead niches, indicated by greater than 81% aggregation of cases.Termitaria environments had the lowest significant (p < 0.05) bulk densityof 1.47 g/cm3 while the remaining niches had bulk densities greater than1.57 g/cm3. The texture of termitaria environments significantly differed(p < 0.001) from the rest of the niches and consisted of sandy loamsand loamy sands. Homestead surroundings, areas under P. curatellifoliaand open sandy patches had sandy soils with greater than 90% sand.Macronutrients generally declined with depth. Total C in the 0-20 cm wasrelatively high in under P. curatellifolia (14.8 Mg/ha) and lowest inthe open sandy patches (7.2 Mg/ha). Total N for all niches was low(<1.4 Mg/ha), indicating possible limitation to crop growth. Available Pin the active rooting zone of 0-40 cm was adequate only in homestead sur-roundings (30-50 ppm) and marginal (15-30 ppm) in termitaria environ-ments and under P. curatellifolia. However, it was deficient in the opensandy patches. Homestead surroundings and termitaria environmentswere the niches with relatively high soil pH (>6.5) and exchangeable basecontent of Ca, Mg and K. Areas under P. curatellifolia, perceived byfarmers to be more fertile, had no comparative advantage over the opensandy patches in texture and macronutrient levels. Farm managementpractices of applying less nutrient inputs to termitaria environments andareas under P. curatellifolia were misguided and perpetuated low produc-tivity in smallholder cropping environments. We recommend site-specificnutrient management of niches where limited purchased nutrients areapplied to relatively fertile niches, for example, homestead surroundingsand termitaria environments while open sandy patches are left to recoverunder natural or improved fallow. doi:10.1300/J064v30n02_08 [Article cop-ies available for a fee from The Haworth Document Delivery Service:1-800-HAWORTH. E-mail address: <[email protected]>Website: <http://www.HaworthPress.com> © 2007 by The Haworth Press, Inc.All rights reserved.]

KEYWORDS. Niche, nutrient management, sandy soils, smallholderfarmer, soil fertility, variability

INTRODUCTION

Inherent soil fertility is a major determinant factor in the productivityof agricultural soils in Zimbabwe. While clays soils derived from mafic

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minerals cover much of the commercial farming areas (Nyamapfene,1989), sandy soils derived from granite are predominant in most small-scale communal areas where over 70% of the population ekes out aliving from agriculture. The sandy soils have low organic matter (<1%)and are inherently deficient in N, P and S (Grant, 1981). Moreover, soilsin communal areas are not always heterogeneous. Land micro-variabil-ity is widespread in semi arid regions of Africa and has a major effect oncrop yields in areas of marginal rain and low soil fertility (Brinnand Black, 1993). Physical and chemical soil properties may vary overshort distances, attributable to both pedogenic and anthropogenic fac-tors. Soil fertility levels may vary due to parent material, clay contentand differential organic matter inputs. Lateral changes in texture, pro-file, depth and chemical properties of the topsoil are strongly reflectedin annual crop growth and productivity. Household litter inadvertentlystrewn around homestead surroundings, old pen sites or old homesteadsites within fields also contribute to micro-variability (Carter and Murwira,1995). Spatial and temporal variability is most important for risk reduc-tion in subsistence agriculture, unlike in high external input agriculture.

In recent years, there has been an upsurge of interest in micro-ecologicalvariability at a spatial scale (niches) that coincides with the scale atwhich farmers make management choices. For example, the generalobservation in literature is that soils modified by termites usually havehigher concentrations of exchangeable bases (Arshad, 1981; Trapnellet al., 1976; Watson, 1976; Wild, 1952). Crop growth variability fromsuch niches are being recognised as constituting an essential part of sub-sistence agriculture by smallholder farmers in marginal areas (Brouweret al, 1993). Spatial variability in field soils could be exploited to benefitsmallholder farmers and it is not helpful for researchers and extensionworkers to assume homogeneity of fields. According to Brouwer andBouma (1996) blanket recommendations of nutrient inputs are inappro-priate in circumstances where variability (1) is widespread in farmers’fields, (2) may reduce production risk and increase yield stability, and(3) affects water and nutrient use efficiency. While the occurrence ofdifferent niches in smallholder farmers’ fields is recognised, informa-tion on the nature, extent of the variation and management of the nicheshas largely been unavailable. Such information could assist in makingsoil fertility recommendations that could boost crop yields leading towidespread adoption by smallholder farmers in semi-arid environmentscharacterised by soil spatial heterogeneity.

The objective of this study was to physically and chemically assesssoil fertility variability in predominant niches occurring in smallholder

Research, Reviews, Practices, Policy and Technology 71

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farmers’ fields in Zimbabwe and suggest appropriate recommendationsfor nutrient management.

MATERIALS AND METHODS

Study Area Description

The study was conducted in Mutoko small-scale communal areas inMashonaland East province in the North Eastern area of Zimbabwe.Much of the communal area is in agro-ecological region 4 with a portionin the southwest occurring in agro-ecological region 3. The communalarea is suitable for semi-intensive and semi-extensive crop-livestockagriculture (Brinn, 1986). Annual rainfall ranges from 650-700 mmwith a coefficient of variation of about 30% over 20 years. Much ofthe study area is underlain by granitic rocks of the Basement Complexwith scattered and localised intrusions of dolerite (Stagman, 1978). Thesoils, cultivated for over 70 years, are mainly granite-derived coarsegrained sands of low inherent fertility that are mainly used for drylandcropping of maize. Other crops grown are pearl millet, finger millet,groundnuts and to a small extent, sunflower and cotton. Vegetables andmangoes (Mangifera indica L.) contribute significantly to the localeconomy (Ashworth, 1990). The smallholder farmers rear cattle, goats,sheep and pigs.

Site Selection, Soil Sampling and Analysis

The study was restricted to three villages Chapfika, Chigaba andSamatanda of Charewa Ward in Mutoko district. A two-stage clustersampling technique was used to randomly select 86 farms. Farmersparticipated in the identification of niches that showed differences in cropgrowth and development. Four dominant niches were selected on thebasis of their prevalence in fields within a farm and on different farms.A sub-sample of 63 farms in which predominant nichesoccurred, wasestablished. From this sample, nine farms were randomly selected. Sketchmaps of all farmers’ fields were made and the predominant niches wereidentified as homestead surroundings, termitaria environments, areasunder Parinari curatellifolia and open sandy patches.


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On each farm, a total of three random sampling points for soil collec-tion were marked in each type of niche. Soil bulk density was deter-mined using the method of Blake and Hartge (1982). Composite soilsamples with three replicates for textural analysis (Gee and Bauder1986) were taken on each type of niche on the nine farms. These sam-ples were collected from the 0-20 cm and 20-40 cm depths.

Composite soil samples for chemical characterisation were collectedfrom each niche type in the nine farms from five depths namely,0-20 cm, 20-40 cm, 40-70 cm, 70-100 cm and 100-130 cm using anauger. The composite soil samples were air dried and passed througha 2 mm sieve prior to chemical analysis. Samples were analysed fortotal organic C by a wet oxidation method of Baker (1976), totalN by first digesting in concentrated sulphuric acid and then colori-metrically (Anderson and Ingram, 1993). Available P was determinedcolorimetrically using the method of Watanabe and Olsen (1965).Exchangeable K, Ca and Mg were determined after extraction with1M ammonium acetate solution, adjusted to pH 7 with acetic acid andaqueous ammonia (Anderson and Ingram, 1993). Soil pH was measuredin 0.01M CaCl2 in a 1:10 soil solution.

Data Analysis

Discriminant analysis (Norusis, 1992) was used as an exploratorytool to investigate niche differentiation based on simultaneous analysisof soil nutrient variables. Linear combinations of independent variablesare formed to serve as the basis for classifying cases into groups orniches. The discriminant function statistics include the eigen values (theratio of between-group to within-group sum of squares) and the canoni-cal correlation (a measure of the degree of association between thediscriminant score and the groups). The square of the canonical correla-tion represents the proportion of the total variance attributable to differ-ences among the groups. A “good” discriminant function is one that hashigher “between-group variability” when compared with “within-groupvariability.” Good functions are associated with large eigen values. Thepercentage of cases classified correctly, based on the statistical formu-lae of the software package, was taken as an index of the effectivenessof the dicriminant analysis.

Analysis of variance (ANOVA) with a randomised complete blockdesign for bulk densities, texture, and soil chemical properties wasused. A block was considered a farm with four different niche types astreatments. Graphs were plotted to show trends of nutrient levels.


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Termitaria environments contributed high variability across niches inexchangeable Ca and Mg and these were treated separately.

RESULTS

Discriminant Analysis

The first two discriminant functions contributed significantly (p <0.001) to niche differences and they had highest eigen and canonicalcorrelation values (Table 1). The two functions accounted for 95% ofthe total variation. The discrimination was most powerful in classifyingtermitaria environments and around homestead niches indicated bygreater than 81% aggregation of cases. It was less powerful (76% aggre-gation) for niches under P. curatellifolia (Table 2). An ordination plotdiscriminated termitaria environments and around homesteads whichexhibited distinct dissimilarities (Figure 1). Niches under P. curatellifoliaand open sandy patches showed almost the same diversity in discrimi-nating soil nutrient variables so that they ended up similar with almostequal group centroids. The discriminant analysis was effective, as itcorrectly classified 83% of grouped cases.


TABLE 1. Canonical discriminant functions.

Function Eigen value Percentageof variation

Cumulativevariation

Canonicalcorrelation

1 3.88 78.71 78.71 0.892 0.79 16.11 94.81 0.673 0.26 5.19 100.00 0.45

TABLE 2. Classification results.

Actual group No. ofcases

Predicted group membership

Homesteadsurroundings

Termitariaenvironments

Under Parinaricuratellifolia

Open sandypatches

Homestead surroundings 44 36 (81.8%) 0 (0%) 3 (6.8%) 5 (11.4%)Termitaria environments 45 3 (6.0%) 42 (93.3%) 0 (0%) 0 (0%)Under Parinaricuratellifolia

45 2 (4.4%) 1 (2.2%) 34 (75.6%) 8 (17.8%)

Open sandy patches 45 2 (4.4%) 0 (0%) 7 (15.6%) 36 (80.0%)

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Physical Analysis

Bulk Densities and Mechanical Analysis

The open sandy patches had the highest mean bulk density (1.68 g/cm3)and termitaria environments had the least bulk densities (1.47 g/cm3).From analysis of variance, bulk densities of niche types were highly sig-nificant (p < 0.001). Least significant difference (LSD) showed thattermitaria niches were the only ones that differed significantly from theother three niches (Table 3).

There was no significant difference in texture between the 0-20 cmand the 20-40 cm depths for each niche type. Comparison of sand,silt and clay in the active rooting zone (0-40 cm) across niches washighly significant (p < 0.001). However, only termitaria environmentsdiffered significantly in the sand, silt and clay content from the otherthree niches (Table 3).


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Function 1

Group Centroids

NICHE

Open sandy patches

Under P. curatellifolia

Fun

ctio

n 2

Termitaria

H/stead surroundings−4

FIGURE 1. Ordination plot of soil descriptors loaded on the two canonicaldiscriminant functions.

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Chemical Analysis

Generally, the proportion of P, K, Ca and Mg (Figures 4-7) in theactive rooting zone (0-40 cm) to that of the entire 130 cm depth was inthe order: Homestead surroundings > under P. curatellifolia > opensandy patches > termitaria environments. Significant niche by depthinteractions occurred with C, Ca and Mg and indicated that the influ-ence of depth on different niches was variable.

Total Carbon and Total Nitrogen

In the 0-20 cm depth, the total carbon content was in the order fromhighest: Under P. curatellifolia > termitaria environments > homesteadsurroundings > open sandy patches (Figure 2). The order changed withsoil depth and generally the percentage carbon decreased for all niches.Termitaria environments had the highest total carbon in the entire soilprofile depth of 0-130 cm followed by under P. curatellifolia niches.The open sandy patches had the least carbon content. The same trend ofdecrease of total carbon content as in the 0-130 cm depth was observed inthe active rooting zone of 0-40 cm. Under P. curatellifolia niches showedthe biggest drop in total carbon from the 0-20 cm to the 20-40 cm depths.

The effect of termitaria environments on total nitrogen was signifi-cantly (p < 0.001) different from that of other niches. Total nitrogencontent in termitaria environments was relatively high in the entire 130cm depth but low in the rest of the niches that had almost the same levelsof about 0.6 Mg/ha (Figure 3).

Available Phosphorus

Available phosphorus was high in all niches in the 0-20 cm soildepth, except in termitaria environments. Homestead surroundings had


TABLE 3. Bulk density, percent sand, silt and clay of the upper 40 cm of soil indifferent niches.

Niche type Bulk density (g/cm3) % Sand % Silt % Clay

Around homesteads 1.64 b 92.0 b 4.4 b 3.3 bTermitaria environments 1.47 a 77.4 a 10.6 a 12.1 aUnder Parinari curatellifolia 1.58 b 90.6 b 5.3 b 4.0 bOpen sandy patches 1.68 b 92.0 b 4.7 b 3.3 b

Means followed by the same letter in a column are not significantly different (p < 0.05) as assessed by LSD0.05Significance of original means for texture is after comparison using arcsine transformed data.

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the highest P content (Figure 4). The available phosphorus decreasedwith depth in all niches except in termitaria environments where itsteadily increased with depth.

The effect of depth and that of niche type on available phosphoruswere each highly significant (p < 0.001). Available phosphorus intermitaria environments was significantly (p < 0.05) different from thatin homestead surroundings and open sandy patches whilst that in nichesunder P. curatellifolia differed significantly from the phosphorus inhomestead surroundings.

Exchangeable Potassium

ANOVA that excluded termitaria environments showed significantdifferences in exchangeable potassium between the three niche types (p <0.001) and soil depths (p < 0.05). Generally, exchangeable potassiumdecreased with depth around homesteads and under P. curatellifoliawhilst it remained constantly low in open sandy patches (Figure 5). In


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Car

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FIGURE 2. Variation of total carbon with depth across niches.

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termitaria environments, exchangeable potassium was quite variableand increased with depth.

Exchangeable Calcium and Magnesium

Exchangeable calcium decreased with depth (Figure 6) exceptin termitaria environments where it increased with depth (Figure 7).The termitaria environments had the highest overall calcium levels andthe open sandy patches the least. Relatively, high levels were within the0-70 cm depth for all niches except termitaria environments.

In soils from homestead surroundings and under P. curatellifolia,exchangeable magnesium decreased with depth up to the 40-70 cmdepth thereafter levelling off (Figure 8). Magnesium levels in opensandy patches showed little variation with depth. Like Ca, most exchange-able magnesium was in the 0-70 cm depth in all niches except termitaria


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Homestead surroundingsUnder P. curatellifolia

Termitaria environmentsOpen sandy patches

Depth

Nit

rog

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/ha)

100-

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70-1

00 cm

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0-20

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FIGURE 3. Variation of total nitrogen with depth across niches.

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environments. In termitaria environments exchangeable magnesiumincreased with depth (Figure 9).

Soil pH

Homestead surroundings and termitaria environments had the high-est overall soil pH (Figure 10) and these were significantly different(p < 0.01) from each other. The effect of depth on soil pH was signifi-cantly (p < 0.05) different between open sandy patches and underP. curatellifolia.

DISCUSSION

The discriminant analysis confirmed spatial variability in macro-nutrients in arable niches and grouped the open sandy patches and areas


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FIGURE 4. Variation of available phosphorus with depth across niches.

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under P. curatellifolia niches as similar whilst homestead surroundingsand termitaria environments were distinct. The ANOVA showed thathomestead surroundings and termitaria environments were relativelyrich in macronutrients while open sandy patches were poor.

Discriminant analysis differentiated niches on the basis of macro-nutrient levels and soil pH. Though macronutrient levels alone do notalways indicate niche variability and productivity they provide a basisfor niche differentiation when discriminant analytical tools were used.It generally hinted at the need for site or niche specific managementstrategies that promote nutrient availability to plants and reduce the riskof nutrient mining. Open sandy patches and under P. curatellifolia gen-erally required similar nutrient management strategies.

Bulk density and textural differences were not statistically significantin all niches except termitaria environments. According to Brady (1974)slight differences in bulk densities can have important implicationson crop growth though soil texture may be the same. Relatively highbulk densities could be attributed to high percentages of sand in granite


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Homestead environments Termitaria environmentsOpen sandy patchesUnder P. curatellifolia

Depth

Po

tass

ium

(cm

ol/k

g)

0-20 cm

20-40 cm

40-70 cm

70-100 cm

100-130 cm

0.25

FIGURE 5. Variation of exchangeable potassium with depth across niches.

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derived soil and also low amounts of organic matter resulting from con-tinuous cultivation exceeding 70 years in the communal area. Themean bulk density of termitaria environments was slightly lower thanthe 1.49 g/cm3 measured by Watson (1977) in the adjacent MurewaCommunal Area.

Generally, the carbon content down the soil profile was very low inall the niches. It was lowest in the active rooting zone of open sandypatches. Direct addition of organic matter as litter fall under treecanopies and household litter strewn around homestead surroundingsbenefited these niches.

The total nitrogen content in the soil for all niches was extremelylow. The low nitrogen supplying capacity of soils in niches is partlyexplained by the quality and quantity of organic matter (Brady, 1974;Davies et al, 1993; Foth, 1990). This implies low mineralisable nitrogenthat is inadequate for plant requirements. Substantial amounts of organicmatter of high quality or nitrogen fertilizers are required for improvedgrowth of most crops in the active rooting zone of all niches. Planting of


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Depth

Cal

ciu

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)

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Homestead surroundings Under P. curatellifolia Open sandy patches

0-20

cm

20-4

0 cm

40-7

0 cm

70-1

00 cm

100-

130

cm

FIGURE 6. Variation of exchangeable calcium with soil depth across niches.

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deep-rooted legume crops that fix nitrogen effectively may also be prof-itable in improving both the carbon and nitrogen status of the soil indifferent niches.

Available phosphorus in the active rooting zone (0-40 cm), for allniches was marginal (15-30 mg/kg) according to levels defined byMashiringwani (1983) except homestead surroundings where it wasadequate (30-50 mg/kg). The indication is that P deficiencies are likelyto become acute in the three niches and may manifest in the long term ifno substantial amounts are added.

Low amounts of exchangeable bases in the soil are generally inducedby low clay and organic matter content and poor nutrient management.The marginal (0.05-0.1 cmol/kg) potassium levels in open sandy patchescould be a result of many years of nutrient mining, inadequate or nonexistent fertilizer dressings and leaching. Relatively high K, Ca and Mglevels were in homestead surroundings and termitaria environments.


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FIGURE 7. Variation of Ca with soil depth in termitaria environments.

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The high levels of exchangeable bases together with phosphorus inhomestead surroundings were due to preferential crop and soil fertilityapplications done by farmers. Stromgaard (1991) observed markedincrease in exchangeable K, Ca and Mg in the soil after addition of ash.In termitaria environments, high levels of these bases could be ascribedto relatively high clay content and enrichment due to termite activity.

Generally, the relatively high levels of exchangeable bases contrib-uted to high soil pH in termitaria environments and around homesteadniches. For niches under P. curatellifolia, soil pH increased slightlywith depth. This trend agreed with findings by Hatton and Smart (1984)and Dunham (1991) that trees were associated with increased soil pHdown the profile on some sandy soils due to decrease in organic matterand organic acids with depth. The soil pH values in all niches are suit-able for growth of most crops and there is no lime requirement. Sincethe study area has been in cultivation for over 70 years, the soil pH


0.9

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Homestead surroundings Under P. curatellifolia Open sandy patches

0.1

Depth

Mag

nes

ium

(cm

ol/k

g)

0

0-20

cm

20-4

0 cm

40-7

0 cm

70-1

00 cm

100-

130

cm

FIGURE 8. Variation of exchangeable magnesium with soil depth acrossniches.

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values contradict findings by Nyamangara and Mpofu (1996) that 30%of communal area soils in Zimbabwe become acidic after a period of 10years.

Differences in texture, nutrient content and bulk densities of nichesnot only affect soil fertility but soil moisture regimes. These can lead todisparate crop development within the same field and thus to differ-ences in sensitivity to drought stress at a particular time. It was observedin preliminary surveys that farmers were aware of variability in soilproperties and reduced or exploited such variability by managementbased on their perception of soil fertility. It can be concluded from thestudy that blanket recommendations for low external input agricultureare likely to be ineffective. The problem may be that of applying inap-propriate nutrient inputs.

There is need to develop soil fertility management options through par-ticipatory research that could enable the farmers to successfully manage the


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FIGURE 9. Variation of Mg with soil depth in termitaria environments.

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productivity of different niches. Such options could include exploitationof nutrients from organic sources, for example, manure and leaf litterwith modest inputs of inorganic fertilizers in different niches. Inade-quate information is available on the optimum use of mineral fertilisersto supplement nutrients from locally available organic resources (Palmet al., 1997).

CONCLUSION

The study confirmed the existence of niches with different texture,bulk densities and macronutrient levels in smallholder farmers’ fields.Soil productivity is directly dependent on the inherent fertility in theseniches. A clear understanding of the extent of soil variability in farmers’fields has an important bearing on management of niches such as type of


8

7.5

7

6.5

6

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Depth

So

il p

H

0-20

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20-4

0 cm

40-7

0 cm

70-1

00 cm

100-

130

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FIGURE 10. Variation of soil pH with depth across niches.

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crops to grow and appropriate use of organic or inorganic fertilizers.The results from the study show that soil fertility is lowest in open sandypatches and under P. curatellifolia. These niches are best left to recoverunder natural or improved legume fallows. Smallholder farmers arelikely to improve crop yields by concentrating purchased inputs on rela-tively fertile niches like the homestead surroundings and termitariaenvironments.

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RECEIVED: 06/20/05REVISED: 02/23/06

ACCEPTED: 03/13/06

doi:10.1300/J064v30n02_08


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soil fertility variability in sandy soils and implications for nutrient management by smallholder...

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