-1 Managing legume cover crops and their residues to enhance productivity of degraded soils in the humid tropics: a case study in Bukoba District, Tanzania F.P. Baijukya 1,2 , N. de Ridder 1 and K.E. Giller 1, * 1 Plant Production Systems, Wageningen University, 6700 AK, Wageningen, The Netherlands; 2 Lake Zone Agricultural Research and Development Institute-Maruku, P.O. Box 127, Bukoba, Tanzania; *Author for correspondence (e-mail: Ken.Giller@wur.n1) Received 19 March 2004; accepted in revised form 6 May 2005 Key words: Labour productivity, Nutrient depleted soils, Short fallow legumes, Smallholder farms Abstract 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 l and 95 kg N ha 1 when established with 2 Mg FYM ha 1 , and incorporation of the biomass gave an incre- ment 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 (2l 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. Introduction Improved fallows with leguminous cover crops are a promising option to improve crop productivity in the low-income agricultural systems of the hu- mid and sub-humid tropics (Hartemink et al. 2000; Sanchez 2002). The benefits of this technology, however, are likely to be smaller on poor soils where the biomass and nitrogen accumulation by legumes may be limited (Uexku¨ ll and Mutert 1994; Houngnandan et al. 2001). Prospects for improv- ing the productivity of legumes on infertile soils have been extensively discussed by Giller (2001) and Hungria and Vargas (2000). Most literature suggests the use of external inputs such as lime (in acidic soils), phosphate fertilisers and starter doses of nitrogenous fertilisers (Uexku¨ll and Mutert 1994; Houngnandan et al. 2001). Although min- eral fertilisers are by far the most effective means of providing deficient nutrients in soils, organic resources, such as farmyard manure (FYM) can have additional benefits such as helping to raise the pH of acidic soils. Moreover, organic resources can increase the availability of nutrients like P, K, Mg and micronutrients that are essential for legume growth and N 2 -fixation (Ridder and Keulen 1990; Nutrient Cycling in Agroecosystems (2005) 73:75–87 Ó Springer 2005 DOI 10.1007/s10705-005-7262-0