World’s Soil Resources

The restoration of productivity on degraded soils can be complex insofar as soils may have been degraded
to the point where they cannot readily respond to fertility-improving management techniques. These
complex interactions among inherent soil properties, management history and the response to inputs is
well illustrated in the work of Rusinamhodzi et al. (2013) on maize production intensification on smallholder
farms in Zimbabwe. In this region two major controls of productivity exist – significant differences in yield
between sandy and clay soils (e.g. inherent soil properties); and pronounced fertility gradients between more
productive fields close to the homestead and more degraded soils in outfields further from the homestead
(a management-induced fertility gradient common in many areas of Africa). The sandy soils required longterm
additions of manure to restore soil functions before the benefit of the mineral fertilizer additions could
begin to be realized; however even after nine years of substantial organic inputs, the highly degraded sandy
outfields did not recover their productivity. The authors speculate that the initial soil organic carbon levels
in the sandy outfields were too low for yields to recover. Moreover at the village scale, the overall amount of
manure produced is insufficient to apply the required amounts of manure in all fields.
Figure 7.4 The food wedge and the effect of soil change on the area of the wedge. Source: Keating et al., 2014.
The relative sizes of the effects of soil change on the food wedge are not drawn to scale.
Status of the World’s Soil Resources | Main Report The impact of soil change on ecosystem services
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