Spatial variability of indigenous supplies for N, P and K and its impact on fertilizer strategies for irrigated rice in West Africa

Present nutrient management recommendations for irrigated rice in West Africa are typically uniform for large regions. Even with optimal crop management, spatial variability of indigenous nutrient supplies may cause low fertilizer efficiency, low productivity of expensive inputs and high losses to the environment. Substantial efficiency increases were achieved with site- and season-specific nutrient management approaches, but the relative importance of different components (site or season) or of the precision level used (field, scheme, or region) remained unclear. We conducted a field trial in the Senegal River valley to investigate short-range variability of indigenous nutrient supplies of N (INS), P (IPS), and K (IKS) on a three hectare farm, and subsequently used the field data and simulation tools to study the agro-economic effects of fertilizer management options with different precision levels. Spatial variability of soil characteristics and of indigenous nutrient supplies (IS) at field level was high and covered a large part of the variability reported in regional studies. INS ranged from 19 to 78 kg N ha(-1), IPS ranged from 11 to 39 kg P ha(-1), and IKS ranged from 70 to 150 kg K ha(-1). Rice yield ranged from 2.2 to 6.0 Mg ha(-1) in N omission plots, from 4.1 to 9.8 Mg ha(-1) in P omission plots, and from 5.3 to 9.6 Mg ha(-1) in K omission plots. The highest yield in the fully fertilized treatment was 11.6 Mg ha(-1). Simulated potential yield was 11.8 Mg ha(-1). Field-specific fertilizer management and an economically optimal target yield resulted in an average yield of 9.6 Mg ha(-1) compared to 7.5 Mg ha(-1) for the existing uniform recommendation. Net benefit from fertilizer use dropped by 19% as a result of reduced precision. Non-season-specific recommendations accounted for 12% of net benefit loss, whereas lower spatial precision contributed 7% to the net benefit loss. We concluded that uniform domain-specific recommendations within agro-ecological zones (i.e. adjusted to the seasonal yield potential) modified by crop diagnostics offer the best opportunities to optimize fertilizer efficiency and net benefits of fertilizer use for intensive irrigated rice-based systems in West Africa.