Crop Yield as Affected by Rotation and Nitrogen Rate. I. Soybean

Crop rotation is reported to increase seed yield of soybean [ Glycine max (L.) Merr.], but rotation effects are less pronounced than for cereal crops. This study compares yield of soybean grown in continuous monoculture with that of soybean grown (i) in a 2‐yr rotation with corn [ Zea mays L.]; (ii) in a 2‐yr rotation with grain sorghum [ Sorghum bicolor (L.) Moench]; (iii) in a 4‐yr grain sorghum‐oat+clover [ Avena sativa (L.) + 80% Melilotus officinalis (L.) Lam., 20% Trifolium pratense ]‐corn‐ soybean rotation; and (iv) in a 4‐yr corn‐oat+clover‐grain sorghum‐ soybean rotation. Interactions between crop rotation and fertilizer N rate were also determined. The study was conducted for 4 yr on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll) near Mead, NE. Continuous soybean produced less seed (2.4 Mg ha −1 ) than soybean in rotation (2.7 Mg ha −1 average). Rotations in which soybean followed sorghum in rotation produced higher seed yield (2.8 Mg ha −1 ) than soybean following corn (2.6 Mg ha −1 ), mainly because soybean following sorghum responded positively to N applications, while soybean following corn did not.