Crop Nutrient Response Functions for Africa: The Database and Extrapolation for Fertilizer Use Optimization.

Optimization of Fertilizer Recommendations in Africa (OFRA) is an effort involving research teams in 13 sub-Saharan Africa nations that is funded by the Alliance for a Green Revolution in Africa, managed by CABI, and technically supported by the University of Nebraska-Lincoln. Fertilizer use by smallholders commonly has severe financial constraints. Choice of the crop-nutrient-rate combinations that maximize net return on investment in fertilizer use is very important to profitability and gradually reducing the financial constraints. Determination of robust crop nutrient response functions for targeted agro-ecological zones (AEZ) are essential for these choices. A database using results from past and current research conducted in Africa has enabled the determination of >3500 geo-referenced crop nutrient response functions. Extrapolation of results across Africa is done using predictive equations and spatial information queries to select functions determined under growing conditions similar to those of the targeted AEZ. Spatial variables used include: soil pH, organic C, and sand content from the Africa Soil Information Service; the aridity, seasonality, and growing degree day layers of the Global Yield Gap Atlas; elevation; and latitude. Extrapolated results complement locally determined response information in determining the crop-nutrient response functions for the AEZ of interest. Country teams then incorporate the functions in fertilizer optimization tools, with Excel Solver, mobile phone, and paper versions. The different versions are in response to the varied capability for computer and mobile phone use of farmers and their advisors. Country extension teams are trained to enable farmers and their public and private sector advisors to use the optimization tools in decisions on fertilizer use.