N2Africa Putting nitrogen fixation to work for smallholder farmers in Africa

Grain legumes are a key source of nitrogen-rich edible seeds, providing a wide variety of high-protein products and constituting the major source of dietary protein in the diets of the poor in most parts of sub-Saharan Africa. Largely grown as subsistence food crops, they are predominantly crops grown by women and used within the family, with an annual per capita consumption of about 9 kg and providing 88 kcal/capita/day. Legumes such as groundnut and soybean are also major sources of edible oil and other industrial by-products.

The ability to fix atmospheric nitrogen makes legumes excellent components within the various farming systems because they provide residual nitrogen and reduce the needs for mineral nitrogen fertilizers by associated non-legumes. Intensification of low-input agricultural production has led to a rapid increase in soil degradation and nutrient depletion in many parts of sub-Saharan Africa, constituting serious threats to food production and food security. Nitrogen depletion in maize-based systems in some farmers’ fields in West African savanna is estimated to be 36-80 kg N ha^-1 per year (Sanginga et al., 2001) and it has been obvious since the mid-1990s that fertilizer use is necessary if sustainable agricultural production in smallholder farms is to be raised to levels that can sustain the growing population. Assuming that only seeds are harvested, net soil nitrogen accrual from the incorporation of grain legume residue can be as much as 140 kg N ha^-1 depending on the legume variety (Giller, 2001). This N tends to be released quickly when legume residues are incorporated into the soil and can contribute to substantial improvements in yield of subsequent crops. This easily surpasses the 50 kg nutrient ha^-1 fertilizer use across sub-Saharan Africa recommended by African heads of states at the Fertilizer Summit held in 2006 in Abuja, Nigeria.

Residues of grain legumes as well as herbaceous and fodder tree legumes provide an excellent source of high quality feed to livestock especially during dry seasons when animal feeds are in short supply. In the sub-humid savanna zone of West Africa, natural pastures are improved by under-sowing with legumes to provide high quality fodder. Increase in productivity of animals fed from these ‘fodder banks’ could be over 20% above those that are grazed on natural pastures (Tarawali and Ogunbile, 1995). In central highlands of Kenya, where there is a thriving smallholder dairy industry, fodder legume trees are used to supplement livestock feeds, resulting in enhanced milk yields and farmer incomes. Thus, legumes represent a major direct source of food for man and livestock and, therefore, make a critical contribution to increased food security of subsistence farmers, reduced cost of food for poor consumers and enhanced rural incomes. The opportunity exists to improve yields of legumes in sub-Saharan Africa since current yields are only a fraction of their potential (Table 1).

Table 1. Average yields (t ha^-1)^a of some economically important grain legumes in Africa

^aFAOSTAT (last updated 11 june 2008);^b PROTA, 2006