Effect of Soybean Plant Populations in a Soybean and Maize Rotation

Plant population of soybean [Glycine max (L.) Merr.] may influence the residual N contribution to a cropping system and yield benefits to following cereals. Field studies were conducted from 1994 to 1996 on a N‐depleted Sharpsburg silty clay loam soil at Mead, NE to: (i) determine soybean yield at different plant populations; (ii) investigate residual N, chlorophyll–N–yield relations, and yield benefits from these different soybean populations to a following maize (Zea mays L.) crop; (iii) and compare N credits from soybean assessed with fallow and cereal plots as references. Eight soybean populations from14 000 to 544 000 plants ha−1 in narrow 50‐cm rows, a fallow plot, and a maize plot were followed by maize in a rotation study. Soybean yield was highest at populations of ≥129 000 plants ha−1. Maize grain yields were highest following fallow and soybean populations <20 000 plants ha−1, intermediate following higher soybean populations, and least in continuous maize. This is most likely due to N uptake as indicated by chlorophyll and N accumulation of maize. Nitrogen credits to maize were 16 to 46 kg N ha−1 when calculated as Nitrogen Fertilizer Replacement Values (NFRVs). This is probably overestimating the potential N contribution from soybean because N credits from soybean populations assessed with fallow instead of maize as references were negative. A net positive N balance due to soybean reached a maximum of 17 kg N ha−1, but soil N was depleted at populations <20 000 plants ha−1. We conclude that yield increases of maize in rotation with soybean may be due to N from reduced N immobilization, N added to the soil from N2 fixation, and possibly from non‐N rotation effects such as water use efficiency.