Tapping indigenous herbaceous legumes for soil fertility management by resource-poor farmers in Zimbabwe

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.