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A 2-year study was conducted in three rainfall zones of Zimbabwe to explore opportunities for harnessing biological nitrogen fixation of non-cultivated herbaceous legumes, which hitherto have been regarded simply as weeds, in order to improve soil productivity on smallholder farms. The rainfall zones used ranged from sub-humid (800 mm annually) to semi-arid (< 650 mm). Nitrogen is the single most important crop nutrient under the predominantly leached sandy soils of southern Africa, and alternative N sources are lacking for smallholder farmers. Diversity and abundance of indigenous legume species were determined under different land uses using farmer participatory research techniques among other methods. Over 30 different legume species, mainly of the genera Crotalaria, Indigofera and Tephrosia, were identified across the three agro-regions. With the participation of farmers, a simple technique, termed the gwezu smell technique, was developed which enabled participating farmers to distinguish between legumes and non-leguminous plants. Diversity was higher in areas with less than 20 years of cropping compared with old farming areas with over 70 years of cultivation, suggesting that prevailing management practices have over time rendered soil environments unfavourable for productivity and regeneration of these legumes.

Depending on rainfall conditions, indigenous legumes contributed as little as 3-17% of the total shoot biomass on nutrient-depleted soils with < 10% clay, <= 0.5% organic C, pH of 3.8-4.6 (CaCl2) and < 5 mg kg(-1) available P. The fields had been abandoned by farmers due to poor soil fertility. Legume contribution, however, increased to 40% of overall biomass following application of 10 kg P ha(-1), and up to 70% when legume populations were also increased through deliberate seeding. Based on this study, there is an opportunity to increase N inputs by indigenous N-2-fixing legumes from the present 5 kg N ha(-1) to > 50 kg N ha(-1). While such relatively small amounts of N input may not result in dramatic yield increases for subsequent crops, they may make a critical contribution towards reversing the highly negative nutrient balances prevailing on the fields of resource-poor farmers. Overall plant biomass yields were < 3000 kg ha(-1), and this was attributed to the exceptionally poor soil fertility status of the fields used. Self-regenerating N-2-fixing indigenous herbaceous legumes that are adapted to specific agro-ecological regions could therefore be manipulated to improve productivity of natural fallows and contribute to the N economy of agro-ecosystems under smallholder farming conditions. We advance the concept of indigenous legume fallows as a promising initial step in meeting the challenge to integrate this under-utilized resource into existing farming systems. (c) 2005 Elsevier B.V. All rights reserved.