Fertilizer Nitrogen and Residual Nitrate-Nitrogen Effects on Irrigated Corn Yield

Multirate nitrogen studies were conducted for a 6-yr period, on an irrigated clay loam soil, to determine the influence of applied N and residual soil N on the grain yield of corn (Zea mays L.). Soil samples were taken prior to fertilizer application each year in depth increments of 0 to 0.15, 0.15 to 0.30, 0.30 to 0.60 and 0.60 to 0.90 m and analyzed for nitrate-N (NO⁻₃-N). Applied N and residual soil NO⁻₃-N were found to significantly influence grain yields. Regression analyses of the data showed highly significant relationships between (i) quantities of NO⁻₃-N measured in the upper portions and those measured in the lower portions of the soil profile and (ii) grain yield and applied N and residual NO⁻₃-N. Highest coefficients of determination were obtained when residual NO⁻₃-N was included as a separate independent variable in the regression equation. Results indicated that residual NO⁻₃-N measured to 0.15 m would be sufficient for evaluation of residual N effects on irrigated corn grain yield on this soil. Fertilizer N requirements for several combinations of grain yield and residual soil NO⁻₃-N were calculated using the N requirement index (NRI) and a power function, and simple and multiple linear response equations generated by regression analysis. A range of values was obtained, with NRI most frequently predicting the highest N requirement. The marginal rate of substitution of residual soil NO⁻₃-N for applied fertilizer N was variable and influenced by (i) amount of residual NO⁻₃-N, (ii) depth of measurement of residual NO⁻₃-N and (iii) maximum grain yield. Fertilizer use efficiency (FUE) was influenced by grain yield, fertilizer N rate, and amount of residual soil NO⁻₃-N. The greatest reduction in FUE resulted from residual soil NO⁻₃-N. In order to maximize FUE, it is necessary to apply the amount of fertilizer N to achieve a given yield level and simultaneously leave as little as possible in the soil to carry over to the next crop year.