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 i n N fertilizer application to soybean [Glycine max (L.) Merr.]. Field experiments were conducted at seven locations from 1990 to 1991 in Ala bama to determine soybean response to N fertilization at various growt h stages. Treatments included a factorial arrangement of soybean culti var ('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 an d oil analyses. In general, Stonewall exhibited highest grain yields a nd seed oil concentrations, while Sharkey had highest protein concentr ations. A positive grain yield response to N fertilizer was observed a t five of seven locations. Yield responses, however, were inconsistent among those locations with respect to N rate/timing treatments and in teraction with soybean cultivar. Grain yield response to N fertilizati on appeared to be dependent on soil nitrate-N concentration at plantin g. Nitrogen applied at R5 was the most reliable application time for i ncreasing 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.