Organic Matter Management as an Underlying Cause for Soil Fertility Gradients on Smallholder Farms in Zimbabwe |
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Authors: | F Mtambanengwe P Mapfumo |
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Affiliation: | (1) Department of Soil Science and Agricultural Engineering, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe |
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Abstract: | Management of spatial and temporal variability of soil fertility within fields and farms is one major challenge for increasing
farm-level crop productivity in smallholder agriculture. A study was conducted across 120 on-farm field sites in three agro-ecological
regions of Zimbabwe to identify management factors influencing the formation of within-field/farm soil fertility gradients.
Using farmer participatory research approaches, host farmers were put into three classes according to resource endowment,
namely, Resource-endowed, Intermediate and Resource-constrained farmers. Each host farmer identified the most (rich) and least
(poor) productive field or field section, which were then studied over two years. Farmer criteria for defining soil fertility
ranged from colour through elements of soil structure to crop response following external nutrient inputs. The fertility ranking
of fields consistently matched with laboratory indices, with rich fields containing significantly more soil organic carbon
(SOC) and nutrients than the corresponding poor fields. Fertility gradients were evident within and across farms belonging
to different farmer classes. The mean SOC content for rich fields were >6.0 g kg−1 compared with <4.6 g kg−1 for the designated poor fields. Rich fields belonging to Resource-endowed farmers had 16–28% more SOC than those belonging
to their resource-constrained counterparts, suggesting differences in organic matter management. Differences in SOC and fertility
status between rich and poor fields were wider in two of the study areas which had more than 70 years of cultivation in contrast
to the third site which had been under smallholder farming for only 20 years, suggesting that the observed fertility gradients
are a cumulative effect of years of differential management practices by different farmer classes. Analysis of potential benefits
from in situ organic biomass inputs suggested that the processes of organic matter capture and utilization discriminated against Resource-constrained
farmers. About 50% of in situ biomass, preferentially maize stover, was lost in three dry season months, and up to 72% of potentially recyclable N is lost
from poor fields managed by Resource-constrained farmers. In contrast, Resource-endowed farmers incorporated more than 1.5 t C,
25 kg N and 5 kg P ha−1 season−1 because of their access to draught power during the early dry season. Such inputs could make a difference on these nutrient-depleted
soils. Intermediate farmers represented a diverse transitional group whose size and variability could be indicative of the
dynamism of technology usage. It was concluded that management of soil fertility gradients to increase crop productivity on
smallholder farms hinges on increasing the capacity and efficiency with which organic matter is generated and utilized by
different farmer weaclth groups across temporal scales. |
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Keywords: | Farmer class In situ biomass Organic matter management Soil fertility gradients Soil organic carbon |
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