SPRING SOIL TEST N FOR IRRIGATED WINTER WHEAT

Early spring as compared to fall preplant measured residual N has potential for improving estimates of available N for irrigated winter wheat in coarse textured soils. Fall preplant or spring soil test N (NO3-N and NH4-N) was combined with fertilizer N and related to the irrigated winter wheat relative yield response at several sites receiving factorial combinations of fall (0 or 100 lb N acre-1) and spring N (0, 50, 100, 150, or 200 lb N acre-1). Fall residual N was generally greater than spring residual N, except where manuring increased fall N mineralization. Spring soil test N accounted for as much of the relative yield variability (R2=.73) as fall soil test N (R2=.71). Coefficients from a quadratic best fit of relative yield to available N did not differ when fall or spring test N was included, but including the spring test N resulted in a higher intercept (50.1 vs 40.3% relative yield). A higher intercept may reflect N already in the plant at the time of the spring sampling. Spring soil test N serves as well as fall preplant measured residual N for predicting irrigated winter wheat N requirements. In cases where significant leaching or N mineralization occurs after the fall measurement the spring test may provide a more accurate measure of N available for the remainder of the season.