Previous Crop Influence on Fertilizer Nitrogen Requirements for Double‐Cropped Wheat1

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Abstract
Few studies have addressed factors which affect wheat (Triticum aestivum L.) production in double‐cropping systems. This study was conducted for 4 years at two locations in Georgia (Limestone Valley and Coastal Plain regions) to determine the influence of the previous crop on fertilizer N requirements for double‐cropped wheat. The N fertilizer treatments were fall (0, 11, 22, 33, and 44 kg N ha−1 as ammonium nitrate) and spring (0, 22, 44, 66, and 88 kg N ha−1 as ammonium nitrate) N rates applied to wheat following soybeans (Glycine mar L. Merr.) or grain sorghum [Sorghum bicolor (L.) Moench]. The soil types were Cedarbluff silt loam (fine‐loamy, siliceous, thermic Fragiaquic Paleudult) in the Limestone Valley region and Greenville sandy clay loam (clayey, kaolinitic, thermic Rhodic Paleudult) in the Coastal Plain region. Plant samples were obtained each year at the time of the spring N application, and the N content of plant tissue was determined. In addition, whole plant and flag leaf N concentrations were determined on samples obtained during the boot and flowering stages in 1981. Grain yields and the N content of mature grain were also determined each year. The N concentration in plant tissue at the time of spring application was consistently greater for wheat following soybeans than for wheat following grain sorghum at equal fall N rates. In the Limestone Valley region, maximum yield was obtained with a total of 85 kg N ha−1 for wheat following soybeans and 105 kg N ha−1 for wheat following grain sorghum. In the Coastal Plain region, maximum yield was obtained with a total of 103 and 137 kg N ha−1 for wheat following soybeans and wheat following grain sorghum, respectively. Extractable soil N was not related to plant N status or grain yield. The N concentration in the flag leaf at flowering was significantly correlated with grain yields and may provide a useful diagnostic criterion for the N status of soft red winter wheat. Regardless of previous crop, location or N rate, maximum yields were obtained when flag leaf N concentration was 35 to 40 g kg−1. Results from this study indicate that wheat growth, N status, and grain yield are significantly influenced by the previous crop and are important to the management of double‐cropping systems.