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 ( 21 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.