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Annual legumes are widely grown by smallholder farmers in Zimbabwe. In addition to providing food, these crops are widely recognised to help maintain soil fertility. In this review we highlight their soil fertility benefits and shortcomings and suggest areas for future work. Most of our knowledge about the soil fertility benefits from annual legumes has come from research stations where they have been demonstrated to input large amounts of N and organic matter. But we still know surprisingly little about the magnitude and dependability of those gains on smallholder farms in Zimbabwe where acidic sandy soils, rainfall extremes and pests often place severe limits on legume growth. Recent work suggests that benefits under those conditions with traditional annual legumes such as groundnuts are much smaller and take longer to achieve than on research stations. We need to screen new legumes for local adaptation to see if there are new species or accessions that do better on smallholder farms. Some green manures, especially velvet beans, have given a reasonable biomass on extremely nutrient depleted and fairly acidic soils, and have potential to help rehabilitate degraded fields when coupled with lime and P. Green manuring will be difficult for smallholders to adopt unless large benefits can be obtained quickly. More work with velvet beans is required on farms to establish the size and speed of yield gains for following maize crops. Also, more participatory work amongst farmers is needed to integrate green manures into existing farming systems. Smallholder farmers already rotate grain legumes with maize. In many cases the rotation does help maintain productivity and soil fertility although at levels much reduced from those found on station. Further long-term on-farm work with the farmers being closely involved is needed to measure benefits from the rotations, and to examine the technology and management options to help farmers grow better crops of existing and new (such as promiscuous soybeans) grain legumes both for their direct benefit as food and the subsequent benefit to succeeding cereal crops. These options would include use of fresh seed, lime and perhaps P. The intercropping of grain legumes with maize can often raise the productivity of land, except in dry areas. Although farmers are rarely going to intercrop annual legumes primarily to improve soil fertility, spin-off benefits on soil fertility may still be worthwhile and have recently been achieved on station in Zimbabwe. These have yet to be demonstrated and quantified on smallholder fields through longer-term trials. In conclusion, although benefits will often not be large, annual legumes can help to maintain soil fertility on smallholder farms in Zimbabwe. The farmers will need more help to access seed of a range of annual legumes that they can integrate into their farming system. A bigger effort by research, extension and seed houses to produce and distribute seed at prices affordable to smallholder farmers is necessary, along with more appropriate guidance on crop management.
Transactions of the Zimbabwe Scientific Association Volume 72 (Supplement) 1998, pp. 15-26