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 (Rl), 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. Research Question Although most of the N required by soybean is provided through symbiotic N 2 fixation, supplemental N fertilizer may boost soybean grain yields. In addition to potential grain yield benefits, supplemental N applications to soybean may improve seed protein or oil concentrations. The potential to incrase soybean seed protein or oil concentration is of interest, because developing soybean marketing strategies hinge on these quality considerations. The objective of this research was to determine the effect of applying supplemental N to soybean at various growth stages on soybean early growth, grain yield, and seed composition. Literature Summary Research on fertilizing soybean with N has been contradictory. Reports of both increased and decreased soybean yields from supplemental N applications can be found in the agronomic literature. Yield decreases from supplemental N applications generally have been attributed to reduced nodule formation and decreased symbiotic N 2 fixation. Soybean yield and seed protein increases from N fertilizer appear to depend on N application time, available soil‐N at planting, and perhaps cultivar selection. Few studies, however, have determined grain yield and seed composition responses of differing cultivars to N applied at different growth stages. Study Description Nitrogen was applied at various growth stages to two soybean cultivars at seven locations in Alabama. Locations: Crossville (2 yr; SMS90, SMS91), Decatur (TVS91), Headland (2 yr; WGS90, WGS91), Shorter (2 yr; EVS90, EVS91) Cultivars: ‘Sharkey’ and ‘Stonewall’ Treatments: No N, Starter (30 lb N/acre), N at first flower (N at Rl; 50 lb N/acre), and N at early pod fill (N at R5; 50 lb N/acre) Application: Banded 2 to 4 in. from row N Source: Ammonium‐nitrate Applied Question Does applying N to soybean increase grain yields and seed protein and oil concentrations? Supplemental N applications to soybean increased grain yields at five of seven locations in Alabama (Table ). At two locations (EVS91 and WGS91), high soil nitrate‐N levels at planting apparently prevented soybean response to N fertilizer. Although grain yield responses to supplemental N were found at most locations in this study, the appropriate application time was impossible to predict (Table ). Response to N fertilizer differed among locations and between cultivars. Nitrogen application at R5 provided the most consistent grain yield incrases, but yield losses were also observed with that treatment. Economic evaluation, based on N fertilizer price and return on soybean grain, indicated that applying supplemental N to soybean is, at best, a risky proposition. Soybean protein and oil concentrations were affected by fertilizer N at only one location (TVS91). We feel that selecting cultivars with the desired oil and protein concentrations would be a more desirable strategy than N fertilization to produce soybean for a discount/premium price advantage based on seed composition. Recommendation Because of the inconsistency of grain yield response and the lack of improvement in seed protein or oil concentrations, we do not recommend supplemental N applications to soybean. Soybean grain yield response to supplemental N at five locations in Alabama.