Abstract
In subsistence agriculture of semiarid regions, soil fertility is largely managed through organically held nutrients because fertilizer use is restricted by climatic production constraints that make investment in fertilizer risky. Traditionally organic matter levels were regenerated with lenghthy fallow periods but increasingly, these are replaced by short fallows and organic matter transfers. In a companion paper (Silveira et al., this volume), we presented models for the organic matter and nutrient transfers with which farmers maintain soil fertility in selected areas. To what extent such transfers are able to compensate for the organic matter and fertility losses incurred during cultivation has not been documented. In order to measure the impact of organic matter and nutrient transfers on subsistence farming, we analysed a grid of samples from a small-holder farm in Zimbabwe, that had been under the same continuous cropping and management system for over 20 years. There were significant clustered differences in total N (and therefore organic matter), available and total P levels across the homestead. Available P was increased near the hut, but total N was not, indicating that organic matter added in refuse and residues was largely mineralised. Other differences were attributable to natural phenomena such as erosion and termitaria. Much of the impact of on-farm redistributions of organic matter and soil, while improving year to year crop production, left little measurable effect in the soils.
Published Version
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