Rate and time of nitrogen application for wheat following different crops

Winter wheat (Triticum aestivum L.) is often rotated with other crops in the eastern Great Plains to diversify cropping systems. Optimum management of N fertilizer is important for maximum profit potential and for avoiding environmental concerns. This study was conducted from 1986 through 1991 in southeastern Kansas to determine the influence of previous crop (grain sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.] and spring oat [Avena sativa L.]) on the N requirement for hard red winter wheat and to determine the optimum time of N application (fall, late winter, or fall + late winter and fall + late winter + early spring). Doublecrop soybean followed wheat in a 2-yr cropping rotation. Soil type was a Parsons silt loam soil (fine, mixed, thermic, Mollic Albaqualf) with 2.8% O.M. Influence of previous crop on wheat yield, grain protein, and plant N increased with time in the rotation sequence. Differences between previous crops were small after the first cropping cycle; however, after the third cycle, all values were significantly highest following spring oat and lowest following grain sorghum. Grain yield response to applied N was highest following grain sorghum, intermediate following soybean, and lowest following spring oat. Nitrogen response differences probably are due to a larger net mineralization of soil-N and crop residue N following spring oat than soybean or grain sorghum and to immobilization of applied N following grain sorghum. Number of heads per sq ft was highly correlated (R 2 = 0.85) with grain yield response. Split N (fall + late winter) and split-spring N (fall + late winter + early spring) applications generally showed no consistent yield advantage over single N treatments (fall preplant or late winter topdress). Grain protein, however, was often higher when N was delayed until late winter or early spring. Results indicate that fertilizer N requirements for hard winter wheat are likely to be low following another small grain crop, intermediate following soybean, and high following a high residue crop, such as grain sorghum. For the climatic and soil conditions in this study, applying N fertilizer at the proper rate was more important than timing of application. Applying up to 1/4 of the total N in early spring, however, could be beneficial for increasing grain protein content in hard winter wheat areas.