Managing legume cover crops and their residues to enhance productivity of degraded soils in the humid tropics: a case study in Bukoba District, Tanzania

In degraded soils, establishment of soil-improving legumes can be problematic and requires investment of labour and other resources. We investigated various aspects of managing herbaceous legumes in farmers¿ fields in Bukoba District, Tanzania. Biomass and N accumulation by Crotalaria grahamiana was 1.1 Mg ha¿1 and 34 kg N ha¿1 when established without farmyard manure (FYM) and 3.0 Mg ha¿1 and 95 kg N ha¿1 when established with 2 Mg FYM ha¿1, and incorporation of the biomass gave an increment of 700 kg ha¿1 of grain in the subsequent maize crop. Maize grain yield at different application rates of Tephrosia candida residues ranged from 1.4 to 3.3 Mg ha¿1 and from 2.0 to 2.8 Mg ha¿1 in the high and low rainfall zone, respectively. Application of tephrosia biomass at a rate of 2 Mg ha¿1 had no significant effect on maize yield whereas rates of 4, 6 and 8 Mg ha¿1 gave comparable yields. Apparent N recovery efficiencies at all rates of tephrosia residues were maximally 27 and 13% for the high and low rainfall zones, respectively. Mulching with Mucuna pruriens suppressed weeds by 49 and 68% and increased maize yield by 57 and 103% compared with the weedy fallow in the respective zones. Incorporated residues had a weaker effect on suppressing weeds and poor labour productivity (2 l and 36 kg grain person-day¿1) compared with mulched residues (32 and 52 kg grain person-day¿1) in the high and low rainfall zone, respectively. These results indicate that if well managed, legume residues have the potential to increase yields of subsequent maize crops on degraded soils