Relative Efficiency of Applied N and Soil Nitrate for Winter Wheat Production¹

Abstract

Abstract The fertilizer N recommendation for a given yield of dryland winter wheat ( Triticum aestivum L.) is lowered by the amount of nitrate‐nitrogen (NO 3 ‐N) measured in the upper 1.2 m soil profile. This implies that soil NO 3 ‐N substitutes for fertilizer N on an equal basis. The study was designed to compare the effectiveness of soil NO 3 ‐N in the 1.2 m depth with fertilizer N for wheat production. Data were from 19 siteyears of N, P, and K rate combinations in 23 treatments planned to fit the nested‐cube statistical design. A response function for grain yield was estimated using fertilizer N, P, and K, soil NO 3 ‐N, P, and water plus seasonal (April‐July) precipitation in a nonlinear regression model. All explanatory variables except fertilizer N and soil NO 3 ‐N were set at fixed values to derive the yield response function. Marginal rates of substitution (MRS) of soil NO 3 ‐N for fertilizer N were derived from representative yield levels. Results indicate that soil NO 3 ‐N in the 1.2 m surface depth is not as efficient (approximately one‐third) as fertilizer N for grain production. Substantial differences in economic returns exist between comparisons of fertilizer recommendations made using the response surface vs an accepted one‐to‐one substitution. Differences are greatest in the range of 75 to 150 kg of soil NO 3 ‐N/ha; the maximum difference, $44.36/ha occurs at a NO 3 ‐N level of 100 kg/ha. Additional research is needed on the N supplying ability of soil NO 3 ‐N for plant growth based on its zones of concentration within the soil profile.