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 continuing interest in N fertilizer application to soybean [Glycine max (L.) Merr.]. Field experiments were conducted at seven locations 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 ('Stonewall' or 'Sharkey') and N rate/timing treatments in a split plot design. Nitrogen rate/timing treatments,were: (i) no N, (ii) 30 lb N/acre at planting, (iii) 50 lb N/acre at first bloom (R1), and (iv) 50 lb N/acre at early pod fill (R5). Plant samples were collected at R1 and R5 for dry matter yield and N determination. Grain yields were determined and grain samples were collected at harvest for protein and oil analyses. In general, Stonewall exhibited highest grain yields and seed oil concentrations, 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 concentration at planting. Nitrogen applied at R5 was the most reliable application time for increasing grain yields, however, yield decreases from N applied at R5 were also observed for both cultivars. Nitrogen fertilization affected seed oil and protein concentrations at only one location. Results of this work suggest that fertilizer-N application to soybean is, at best, a risky proposition.