Adapting to change in banana-based farming systems of northwest Tanzania: the potential role of herbaceous legumes

Keywords: Land use changes; Herbaceous legumes; Adoptability; N 2 -fixation; Residual effect; Legume management; Exploration of options, Nutrient depleted soils.The banana-based farming system inBukoba District,Tanzania, has been in existence for over 300 years. At present, banana productivity in homegardens is declining largely due to the decline in soil fertility, which in many years was counteracted by the availability of manure. The grazing land is being converted to crop fields and other uses. Crop fields have in part, assumed the role of grazing land of providing for fodder to the few cattle kept today, which is not sustainable in many years. This thesisexplores opportunities for integrating herbaceous legumes in the farming system, to act as a engine to keep maintain the farming system by providing fodder to the cattle (hence manure for use in the home gardens) and improvement the fertility of soils of annual crop fields as improved fallows.The area of grasslands was shown to have decreased over 50 years by 40% whereas the area of annual crop fields increased by 225%. Encroach on grasslandreduced the ability of farmers to restore the fertility of their soils as possibilities to keep livestock, thus the supply of manure diminished.This had a consequence on nutrient balances where by the home gardens receiving manure, had positive balances of N, P and K whereas the home gardens receiving no manure had negative nutrient balances. Nutrient balances of annual crops were negative particularly with maize, indicating that they are vulnerability to impoverishment.Field experiments showed that the biomass, N accumulation and N 2 -fixation varied among the legume species. The performance of legumes was regulated more by the soil N and the soil pH. The non-forage legumes species Tephrosia candida , Crotalaria grahamiana and the forage species Mucuna pruriens and Macrotyloma axillare performed better among the tested legumes, and were selected by farmers on the basis of biomass yield, weed suppression and tolerance to pest and diseases. Laboratory experiments showed that the rate of N release from decomposing legume residues depended on the quality [(polyphenols + lignin)-to-N ratio, lignin-to-N ratio and lignin content] of residues, whereby residues with low (polyphenols + lignin)-to-N ratio, lignin-to-N ratio or lignin contents decomposed faster. Maize yield doubled or tripled when legume residues were applied though the yield response to legume residues was limited when compared with the application of the recommended rate of mineral N fertilisers (50 kg N ha -1 ). It was further observed that, in short term, application of large quantities of legume residue (above 2 Mg ha -1 ) does not add to significant higher maize production. In the degraded soils, the biomass yield and N accumulation of legume species increased by 100% when established with farmyard and had higher residual effect (80%) on the yield of the subsequent maize. Mulching with legume residues was the best option to apply legume residues as it suppressed weeds in the maize crop and had higher labour productivity. Field experiment with farmers showed that the growing legumes as improved fallows increase maize production and maintained positive N balance. However, growing legumes for fodder was in conflict with maize production and N balance of annual crop fields. The model experiments with a multiple goal liner programming (MGLP) model showed that legumes can act asaengine to keep maintain the farming system by providing fodder to the cattle, hence manure to the homegardens and improvement the fertility of soils of annual crop fields as improved fallows. Farmers have different preferences on legumes and choice of legumes to be introduced in the farming system should be based on farmer production objectives. The main policy implication of the findings is that promotion of legumes is best approached by taking the socio-economic systems into account. These include securing other farm inputs and marketing of farm produce by reliable and attractive markets.