Nutrient Management for Grain Legume Crop Production in Africa

There is growing interest to increase soybean yields in smallholder farming systems in western Kenya, but productivity has remained very poor due to poor soil fertility conditions and limited understanding of effective nutrient management strategies for soybean production intensification. On-farm experiments were established at three sites in western Kenya to assess the effect of fertilizer, manure and lime applications on yield and profitability of soybean under variable soil fertility conditions. Treatments included: a) control (without Rhizobia innoculant or fertilizer), b) inoculation alone, c) N+P+K, d) P+K, e) N+P, f) N+K, g) N+P+K+S+Ca+Mg+Zn+Mo, and h) N+P+K+S+Ca+Mg+Zn+Mo+Manure+Lime. Treatments b) to h) also had seed inoculation done with USDA-110. Fertilizers were applied at rates required to achieve the attainable yields in the study area as follows: 20 kg/ha N, 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. Manure was applied at 10 t/ha and dolomitic lime at 5 t/ha. The experiment was conducted for two seasons in three fields with contrasting soil fertility conditions: Masaba (low fertility), Eshirali (medium fertility) and Nyabeda (high fertility).

Inoculation alone significantly increased grain yield compared to control in Nyabeda. The treatment NK had significantly lower grain yield compared to NPK, NP and PK at all sites, suggesting that P was the most limiting nutrient at the experimental sites. However, the highest yields were achieved with N+P+K+S+Ca+Mg+Zn+Mo+Manure+Lime treatment at 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. The study also highlighted that while the highest gross margins were realized with N+P application, achieving maximum yields would require the combination of mineral fertilizers along with manure and lime, with greater yield potential existing in the fertile clay soils. Thus, although there is a good scope for smallholder farmers to profitably intensify soybean production, this will largely depend on nutrient management strategies that address multiple soil fertility constraints and tailoring fertilizer recommendations to different soil fertility conditions.