Maximizing Net Returns to Finance-Constrained Fertilizer Use.

Financial constraints commonly limit fertilizer use by smallholder farmers and it is important that they maximize net returns on their investment. Fifteen crop-nutrient response functions, including six crops, were derived from results of 84 trials conducted in Uganda. The benefit:cost ratio was greatest for a small amount of N applied to dry bean (Phaseolus vulgaris L.) followed by N applied to rice (Oryza sativa L.). Next most profitable is P applied to groundnut (Arachis hypogaea L.) and soybean (Glycine max L.), followed by N applied to maize (Zea mays L.)and sorghum (Sorghum bicolor L.), with lower or no benefit:cost for the remaining nine response functions. The Fertilizer Optimization Tool was developed for Uganda to maximize net returns to fertilizer use for finance limited crop management in consideration of the area of crops to be planted, fertilizer costs, expected grain value, and money available for investment. It optimizes across the 15 response functions to provide the crop-nutrient-rate combinations expected to maximize net returns. The tool does not account for soil test information, expected yield, and previous crop as these did not account for significant variation in yield. In an example with 1 ha each of the above six crops and US$135 available for fertilizer use, the optimized benefit:cost ratio was 17.4 compared to 4.3 and 6.0 for N applied to maize and rice at rates to maximize net returns per ha. The solution commonly determines low or zero rates for nutrients applied to some crop but the greatly improved returns on investment create the potential for gradually investing more in fertilizer for future crops. This approach to fertilizer use of maximizing net returns on investment has potential to gradually enable much increased fertilizer use because of the relatively high returns on investment compared to traditional fertilizer use recommendations.