Sorghum-soybean rotation and fertilizer effects on selected soil properties and crop productivity

In efforts to identify alternative production systems satisfying economical and environmental needs, the potential of crop rotations merits re-evaluation. A crop rotation experiment was initiated in 1980 on a Sharpsburg silty clay loam (fine montmorillinitic, mesic. Typic Argiudoll) near Mead, NE with the objectives of quantifying yield reductions from continuous soybean (Glycine max (L.) Merr.) cultivation and to determine cropping effects on soil-plant interactions. The cropping sequences included continuous soybean, continuous grain sorghum (Sorghum bicolor (L.) Moench), and soybean-grain sorghum or grain sorghum-soybean rotations. Fertilizer treatments consisted of control (no fertilizer) nitrogen (45 kg ha$\sp{-1}$ on soybean plots and 90 kg ha$\sp{-1}$ on sorghum plots), and manure applied at 15.8 Mg dry matter ha$\sp{-1}$ yr$\sp{-1}$ (160-250 kg N ha$\sp{-1}$). Manure increased total soil nitrogen and organic matter contents in the upper 15 cm of soil but had little or no effect at lower soil depths. Amounts of nitrate recovered in the 0-120 cm soil zone in June (averaged over 1986 and 1987) were 97 kg ha$\sp{-1}$ with soybean as the previous crop and 49 kg ha$\sp{-1}$ with sorghum as the previous crop. Microbial biomass was generally increased with manure application and sorghum as present or previous crop. Water content in the upper 30 cm was generally highest with continuous sorghum and lowest with continuous soybean. Averaged for 1986 and 1987, root length was reduced by 34% for soybean and by 24% for sorghum if the previous crop was soybean. Grain yield of soybean was increased with rotation, but was not affected by fertilizer N treatment. Grain yields of unfertilized sorghum increased with rotation by 17 to 60%. Manure and fertilizer N increased sorghum grain yields. Soybean yield increases as a result of rotation during the period 1981-1987 were negatively related to combined April-May rainfall (r$\sp2$ = 0.71, P = 0.02). The findings of this study confirm that continuous soybean cultivation leads to yield depression and indicates changes in soil water status as one possible contributor to this effect