Effects of Previous Cropping Systems on Soil Nitrogen and Grain Sorghum Yield

Producers who grow soybean [Glycine max (L.) Merr.] in 3 to and 4-yr rotations with grain sorghum [Sorghum bicolor (L.) Moenchl or other grain crops lack information about the duration of grain yield and soil mineral N benefits of soybean in crop rotations. To determine the 1-, 2-, and 3-yr effects of soybean in crop rotations, an experiment with 8 yr of continuous soybean and grain sorghum, and soybean-grain sorghum and grain sorghum-soybean rotations combined with fertility treatments of control, N (45 kg ha⁻¹ on soybean and 90 kg ha⁻¹ on grain sorghum) and manure (16 Mg ha⁻¹ dry-matter containing 160 to 250 kg available N ha⁻¹) was terminated in 1987. In 1988 and 1989 grain sorghum was grown on all plots without fertilizer to determine the residual effects of previous cropping system and fertilizer regime on soil mineral N, sorghum grain, and stover yield. The experiment was conducted near Mead, NE on a Sharpsburg silty clay loam soil (fine montomorillinitic, mesic, Typic Arqiudoll). Early in the 1988 season plots with soybean as the previous crop had 44 to 50 kg ha⁻¹ more NO₃ -N in the 150-cm soil profile than did plots with continous grain sorghum. Early in the 1989 season, plots where soybean had been grown 2 yr previously had 17 to 23 kg ha⁻¹ more soil NO₃-N than did continous grain sorghum plots, while plots 3 yr after soybean had only 3 to 8 kg ha⁻¹ more soil NO₃-N. The yield of grain sorghum in the first, second, and third year following soybean was 2 to 3, 0.4 to 1.4, and 0.1 Mg ha⁻¹, respectively, greater than the yield of continous grain sorghum. This study indicated that soybean in a crop rotation can contribute to soil NO₃-N and consequently increase sorghum grain yield for 2 yrs if fertilizer N is limiting. Nebraska Agric. Res. Div. Journal Article no. 9434. Research partially funded by USAID Grant no. DAN 1254-G-00-0021 through INTSORMIL, the International Sorghum and Millet CRSP.