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

The fertilizer N recommendation for a given yield of dryland winter wheat (Triticum aestivum L.) is lowered by the amount of nitrate-nitrogen (NO₃-N) measured in the upper 1.2 m soil profile. This implies that soil NO₃-N substitutes for fertilizer N on an equal basis. The study was designed to compare the effectiveness of soil NO₃-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 NO₃-N, P, and water plus seasonal (April-July) precipitation in a nonlinear regression model. All explanatory variables except fertilizer N and soil NO₃-N were set at fixed values to derive the yield response function. Marginal rates of substitution (MRS) of soil NO₃-N for fertilizer N were derived from representative yield levels. Results indicate that soil NO₃-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 NO₃-N/ha; the maximum difference, $44.36/ha occurs at a NO₃-N level of 100 kg/ha. Additional research is needed on the N supplying ability of soil NO₃-N for plant growth based on its zones of concentration within the soil profile.

Key Words: Triticum aestivum L. • Marginal rate of substitution • Yield response • Economic N recommendations • Nested-cube statistical design

² Former associate professor, Soils, Southern Agric. Res. Ctr., Rte. 1, Box 131, Huntley, MT 59037; presently associate professor, Texas A&M Univ., Agric. Res. and Ext. Center, P.O. Drawer E, Overton, TX 75684; former research assistant in agricultural economics; professor agricultural economics; associate professor, statistics; and extension soil specialist, Montana State Univ., Bozeman, MT 59717.

Received for publication January 5, 1982.