Non-Responsiveness of Maize and Soybean Crops to Inorganic Fertilizers: Assessing Probabilities and Understanding Biophysical Causes Across African Agro-Ecosystems.

Many initiatives are being undertaken in sub-Saharan Africa to increase the rate and scale of inorganic fertilizer use by smallholder farmers. Numerous times it has been found that inorganic fertilizers don’t generate satisfactory responses in yield and/or profit. We carried out experiments with maize and soybean crops on 92 farmer’s fields in four smallholder farming systems in DRCongo, Kenya, Nigeria and Tanzania over two growing seasons. Experimental sites included gradients in soil properties as well as rainfall conditions to investigate underlying factors determining risks and causes of fertilizer non-responsiveness in maize and soybean crops across these tropical agro-ecosystems.

Maize grain yields did not increase more than 1 ton ha^-1 for 25% of fields on Acrisols in Kenya, 58% on Ferralsols in DRCongo, 51% on Cambisols in Tanzania, and 79% on Lixisols in Nigeria. On the other hand, PK fertilizers did increase soybean yields by more than 0.3 ton ha^-1 for 98% of fields in DRCongo, 90% in Kenya and Tanzania, and 73% in Nigeria. Fertilizer responses of maize crops within individual study areas showed moderate relationships with soil CEC, pH, and extractable phosphorus, magnesium and potassium, next to the amount and/or distribution of rainfall. Responses of soybean demonstrated weak relationships with soil CEC, sum of bases, pH, land slope and extractable phosphorus, magnesium and calcium, as well as the amount and/or distribution of rainfall in DRCongo and Tanzania. The covariate analysis shows that many factors are curtailing fertilizer responses of maize and soybean crops. Soil mineralogy points out that differences in fractions of 2:1 clays, 1:1 clays and sesquioxides may contribute to explaining differences in fertilizer responses between the sites. Comprehensive information about the causes of non-responsiveness, gained from studies like this, will help in targeting effective fertilizer and soil management recommendations to achieve an African Green Revolution.