The theoretical potential for tailored fertilizer application. The case of maize in Sub-Saharan Africa

Abstract

Adjusting crop nutrient rates to local differences in soil fertility and yield potential is considered a promising way of improving fertilizer use efficiency in smallholder agriculture. Despite its obvious appeal, the benefits of this approach have not been evaluated empirically. In the absence of appropriate data, the theoretical potential of nutrient tailoring needs to be evaluated, and suitable models exist to do this. Such analysis may provide a benchmark for expectations of success and would form a valuable starting point for reflection and debate on the limitations of current data, models, and assumptions. This study presents a theoretical ex-ante assessment of the short-term economic benefits of fertilizer adjustments to differences in nutrient and water constraints, under the assumption that these are the only determinants of nutrient response. Combining two calibrations of a mathematical production function with digital maps of predicted nutrient and water availability, economically optimum macro-nutrient rates and resulting maize yields were calculated across a theoretical representation of soils across Sub-Saharan Africa. Consistent economic benefits from fertilizer application were predicted, raising theoretical yields from 19 to as much as 61 or 88% of water limited levels depending on the model used. Under the magnitude of soil fertility variation considered here, matching nutrient rates to indigenous levels was predicted to result in marginal added benefits at best, particularly at higher, more profitable, investment levels. Strong spatial heterogeneity in water limitation translated into more pronounced effects of rate adjustments to water limited yields, raising average predicted profit by up to 127 USD/ha compared to a fixed-rate recommendation. This suggests that tailoring to different yield levels can offer non-trivial benefits if good estimates of yield potential are available. Adding realistic levels of uncertainty around soil fertility and use efficiency parameters reduced predicted gain from tailoring and suggested that acquiring sufficient experimental data for improved nutrient recommendations, both general and specific, may be challenging. The implications and limitations of these theoretical results and potential improvements in validation, predictions and outcomes are discussed.