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SOYBEAN

Effect of Soybean Plant Populations in a Soybean and Maize Rotation

^a Crops Res. Inst., P.O. Box 3785, Kumasi, Ghana
^b Dep. of Agron., Univ. of Nebraska, Lincoln, NE 68583-0817

Corresponding author (mclegg1{at}unl.edu)

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 129000 plants ha^-1. Maize grain yields were highest following fallow and soybean populations <20000 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 <20000 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.

Abbreviations: DAS, days after sowing • NFRV, N fertilizer replacement value • TDM, total dry matter

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