Exploring socio-ecological niches for legumes in western Kenya smallholder farming systems

Keywords: adaptability, agro-ecosystems, biophysical and socio-economic heterogeneity, economic benefits, N2-fixation, productivity.This thesis explores the potential of using herbaceous and grain legume species to improve soil fertility and farm productivity in the heterogeneous smallholder farming systems of westernKenya. Poor soil fertility is responsible for the limited productivity of the westernKenyasmallholder farming systems. Although legumes have the potential for improving productivity, their sustainable use is impeded by the high degree of biophysical and socio-economic heterogeneity that characterizes the farming systems. The socio-ecological niche concept was proposed as a framework for facilitating the identification and integrated assessment of biophysical and socio-economic factors with potential influence on the choice of sustainable legume technologies for smallholder farmers.The utility of the socio-ecological niche concept was tested through on-farm experiments and socio-economic surveys in westernKenya. The on-farm experiments were conducted across three major agro-ecological zones (AEZ), and under different soil fertility conditions, to assess legume emergence, survival, nodulation, diseases tolerance, grain yield, biomass production, atmospheric N ₂ -fixation and net N contribution to soil N fertility. In addition, the economic benefits of growing grain and green manure legumes in rotation with maize were assessed to determine how they are influenced by agro-ecological conditions and within farm soil fertility heterogeneity. Socio-economic surveys characterized farmer legume production objectives, as well as socio-cultural, economic and institutional factors with potential impact on the use of legume technologies by westernKenyasmallholder farmers. The biophysical and socio-economic factors were integrated and analysed to identify legume species for different farmer resource endowment groups, agro-ecological conditions and field typologies. Analysis of the alternative legume production scenario was undertaken, to test the utility of the socio-ecological niche concept. Legume grain yield, total dry matter production (TDM) and atmospheric N ₂ -fixation increased with rainfall and soil fertility status. TDM ranged from 0.1 Mg ha ^-1 to 13.9 Mg ha ^-1 , and was generally less for the grain legumes, compared with the green manure and forage legumes. However, soyabean and groundnut showed greater potential among the grain legumes, producing up to 4.6 Mg ha ^-1 TDM. While the legume species and varieties showed capacity to form viable nodules with naturally occurring rhizobia, application of P was essential for good nodulation. Generally, the species fixed 23-90% of their N requirements in AEZ 1 (Museno) and AEZ 2 (Majengo), compared to 7-77% of their N requirements in AEZ 3 (Ndori). However, N ₂ -fixation by the green manure species (29-232 kg N ha ^-1 ) was greater than that by grain legume species (3-172 kg N ha ^-1 ). Net N input by the grain legumes was negatively correlated with grain yield, and legume grain yields of above 1 Mg ha ^-1 resulted in negative net N inputs. Economic benefits of fitting legumes into the smallholder cropping systems in rotation with maize varied with rainfall, soil fertility and legume species. Yearly maize productivity of these rotations (short and long rains crops) decreased by 47%, from AEZ 1 to AEZ 3, and by 33%, from fertile fields to least fertile fields. Although continuous maize fertilized with both N and P had the largest total maize productivity, returns to land and labour were greatest with grain legume-maize cropping systems. In AEZ 2, where moisture was not limiting during the experimentation period, mean returns to land for grain legume-maize cropping systems were US$ 879 ha ^-1 , compared with US$ 533 for green manure-maize, and US$ 459 for continuous maize with N and P.Rainfall, soil fertility, land, labour, and livestock ownership were identified as the most important factors influencing the choice of appropriate legumes for the smallholder socio-ecological niches. The analysis of the current legume production situation showed that the medium and the low resource endowed farmers were food insecure due to a combination of land and labour scarcity. However, when alternative legumes species selected according to the socio-ecological niche concept were used, maize self-sufficiency increased by 21-48%. This study demonstrated the utility of the socio-ecological niche concept as a useful tool for facilitating the integration of legumes into the westernKenyasmallholder farming systems to improve soil fertility and farm productivity.