Response of Dryland Soybeans to Nitrogen in Full‐Season and Doublecrop Systems

The addition of N fertilizer to soybeans Glycine max (L.) Merr. has been adopted by many producers in Virginia without knowing the potential yield benefit or loss. The objective of this study was to evaluate the response of soybeans (seed yield, maturity, plant height, lodging, seed weight, seed quality, and seed protein and oil content) to N applied near planting or at flowering in dryland full‐season and doublecrop soybeans in Virginia's Coastal Plain soils. At Suffolk and Warsaw in 1988 and 1989, N treatments (0 and 25 lb N/acre of 30% urea NH4NO3 or UAN) were the whole plots and six maturity group (MG) V soybean cultivars were the subplots in a randomized complete block (RCB) experimental design. Nitrogen was applied with burndown herbicides to kill a winter wheat (Triticum aestivum L.) cover crop 21 to 42 d prior to planting full‐season, no‐till soybeans. In full‐season studies in 1990, N (0 or 50 lb/acre) was applied to the soil surface at flowering to seven MG V cultivars in a RCB experimental design at Painter, Suffolk, and Warsaw. In doublecrop studies in 1990, N (control, 50 lb/acre at planting, or 50 lb/acre at flowering) was applied to the soil surface to seven MG V cultivars in a RCB experimental design at Painter, Suffolk, and Warsaw. Nitrogen significantly (P < 0.05) increased seed yield in only one of 10 enviroments, in which N (25 lb/acre) was applied before planting in a full‐season system at Warsaw (a dry, low‐yielding environment). In the doublecrop study at Suffolk, N significantly decreased seed yield when applied at flowering. Averaged over cultivars and environments, however, N did not significantly affect seed yield, maturity, lodging, plant height, seed quality, or seed oil content. Nitrogen (25 lb/acre applied before planting) significantly increased seed weight by 91 seeds/lb in the 1988 and 1989 studies, but did not affect seed weight in other tests. In the 1990 full‐season tests, N applied at flowering significantly reduced seed protein content, but protein content was unchanged in the doublecrop tests. The cultivar × N, N × environment, and cultivar × N × environment interaction effects were not significant for any of the traits measured. These results suggest that a yield response to the addition of N (50 lb or less of N/acre) applied near planting or at flowering is unlikely in dryland soybeans in Coastal Plain soils of the southeast USA.