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

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 N2-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 N2-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.