Profitability of climate-smart soil fertility investment varies widely across sub-Saharan Africa

Soil fertility investments in sub-Saharan Africa, where budgetary resources are scarce, must be well targeted. Using a causal forest algorithm and an experimental maize trial dataset matched with geocoded rainfall, temperature and soils data, we modelled site-specific, ex ante distributions of yield response and economic returns to fertilizer use. Yield response to fertilizer use was found to vary with growing season temperature and precipitation and soil conditions. Fertilizer use profitability-defined as clearing a 30% internal rate of return in at least 70% of the years-was robust to growing season climate and the fertilizer-to-maize price ratio in several locations but not in roughly a quarter of the analysed area. The resulting profitability-assessment tool can support decision makers when climate conditions at planting are unknown and sheds light on the profitability determinants of different regions, which is key for effective smallholder farm productivity-enhancing strategies. Planning investments in agricultural inputs can be challenging when economic returns vary widely over time and locations. Applying a random forest algorithm to a large experimental maize trial dataset of 21,400 observation points, this study generates yield response curves and estimates economic return to fertilizers in 25 sub-Saharan African countries.