Soil fertility gradients in smallholder farms of western Kenya. Their origin, magnitude and importance.

Soil fertility depletion is the main biophysical factor affecting food production in sub-Saharan Africa. Negative nutrient balances at farm scale reveal its intensity. However, the sign and absolute magnitude of the nutrient balances are not equal for the different fields within one farm but vary substantially, creating zones of depletion and accumulation. Farmers allocate resources to several production activities and to the different fields according to socio-cultural aspects and to land quality attributes, both groups of factors operating at different spatial scales (i.e. Site-specific, Inherent properties and Wealth factors). Soil Fertility Management (SFM) options to improve food production should be designed to cope with the actual within-farm variability, referred to as soil fertility gradients (SFG). This study was conducted to understand the processes that lead to the creation of SFG, to measure their magnitude in terms of crop performance and soil properties, and to assess their impact on overall resource use efficiency.