Nitrogen Fertilizer Effects on Soybean Growth, Yield, and Seed Composition

While previous research has been contradictory, potential grain yield responses and seed protein increases have led to con­ tinuing interest in N fertilizer application to soybean (Glycine max (L.) Merr.]. Field experiments were conducted at seven lo­ cations from 1990 to 1991 in Alabama to determine soybean response to N fertilization at various growth stages. Treatments included a factorial arrangement of soybean cultivar ('Stone­ wall' or 'Sharkey') and N rate/timing treatments in a split plot design. Nitrogen rate/timing treatments were: (i) no N, (ii) 30 Ib N/acre at planting, (iii) SO Ib N/acre at first bloom (RI), and (iv) SO Ib N/acre at early pod fill (RS). Plant samples were collected at RI and RS for dry matter yield and N determina­ tion. Grain yields were determined and grain samples were col­ lected at harvest for protein and oil analyses. In general. Stonewall exhibited highest grain yields and seed oil concen­ trations, while Sharkey had highest protein concentrations. A positive grain yield response to N fertilizer was observed at five of seven locations. Yield responses, however, were inconsistent among those locations with respect to N rate/timing treatments and interaction with soybean cultivar. Grain yield response to N fertilization appeared to be dependent on soil nitrate-N con­ centration at planting. Nitrogen applied at US was the most reli­ able application time for increasing grain yields, however, yield decreases from N applied at RS were also observed for both cultivars. Nitrogen fertilization affected seed oil and protein con­ centrations at only one location. Results of this work' suggest that fertilizer-N application to soybeau is, at best, a risky propo­ sition.