Nutrient Management for Grain Legume Crop Production in Africa

Soybean yield intensification in smallholder farming systems in sub-Saharan Africa has been limited by lack of proper nutrient management strategies tailored to the highly variable soil fertility conditions. On-farm experiments were established at three sites in western Kenya to assess the effect of balanced fertilizer use, and manure and lime application on N fixation potential, yield, and profitability of soybean under a range soil fertility conditions. The experiment was laid out in a randomized complete block design with three replications. The treatments comprised of: 1) Control (without innoculant and fertilizer); 2) Inoculation alone; 3) N+P+K; 4) P+K; 5) N+P; 6) N+K; 7) N+P+K+S+Ca+Mg+Zn+Mo and 8) N+P+K+S+Ca+Mg+Zn+Mo+Manure+Lime (soybean was inoculated with USDA-110 rhizobia for treatments 2 to 8). Fertilizers were applied at rates required to achieve the attainable yields in the study area as follows: 30 kg/ha P, 60 kg/ha K, 23 kg/ha S, 20 kg/ha Ca, 5 kg/ha Mg, 3 kg/ha Zn, 3 kg/ha Mo. Starter N was applied at a rate of 20 kg/ha. Manure was applied at 10 t/ha and dolomitic lime at 5 t/ha. The experiment was conducted for two seasons and located in three fields with contrasting soil fertility conditions: Masaba (low fertility), Eshirali (medium fertility) and Nyabeda (high fertility).

Inoculation alone significantly increased soybean grain yield compared to the control treatment in Nyabeda alone. NK had significantly lower nodule number, nodule dry weight and grain yield compared to NPK, NP and PK in all sites, suggesting that P was the most limiting nutrient. However, the highest values for soybean nodule score, nodule number, nodule dry weight and grain yields were achieved with N+P+K+S+Ca+Mg+Zn+Mo+Manure+Lime treatment in all sites. Soybean productivity and profitability was low on the infertile sandy soils of Masaba. Higher revenue was generated in fertilized treatments at the more-fertile Eshirali and Nyabeda sites, with the highest gross margins realized with N+P application. The results indicate a higher potential of increasing biological N fixation, soybean yields, and profits in the more-fertile clay soils than in the depleted sandy soils.

There is opportunity for smallholder farmers to profitably intensify soybean production, and this will largely depend on integrated nutrient management strategies that address multiple soil fertility constraints and tailoring fertilizer recommendation to different soil fertility conditions.