Optimizing Soybean (<i>Glycine max L.</i>) Yield with Nitrogen and Sulfur Applications

Although soybean (Glycine max L.) yields have increased over the past decade, even greater improvement is in demand. Nontraditional practices such as applying nitrogen (N) to soybean for yield optimization, might be one way to meet this demand. Also, sulfur (S) is becoming a more important limiting nutrient in production due to higher yielding crops, lower S containing production inputs, and reduced supply from the environment. The N study involved two environments, irrigated and dryland, in Milan (35.9198° N, 88.7589° W) and Jackson (35.6145° N, 88.8139° W), TN in 2015 and 2016. Urea fertilizer treatments were 34, 67, and 101 kg N ha^-1 (per hectare). Soybean height, nodes per plant, total biomass weight, biomass N concentration, total nodules per plant, total active nodules per plant, total adolescent nodules per plant, total nodule weight per plant, 100 seed weight, and yield were collected to evaluate treatment effects. Data analysis concluded that N applications significantly increased plant height, plant nodes, and plant biomass of soybean. However, N treatments significantly reduced active and adolescent nodule production. Soybean yield was not significantly increased by the N applications. The irrigated sites yielded at or below the dryland comparison, probably due to lodging, which may have compromised yield potential. The S study included soybean and corn (Zea mays L.) experiments, which were conducted in an S deficient soil in Milan, TN in 2015 and 2016. Ammonium sulfate treatments were 11, 23, and 34 kg S ha^-1 (per hectare). Plant height, leaf S concentrations, seed S and N concentrations, 100 seed weight, and yield were collected to evaluate treatment effects. Data analysis concluded that S application significantly increased leaf S concentrations in corn but not soybean. Soybean and corn seed S was significantly increased but not seed N. Soybean yield was not increased, but corn yield was significantly increased 16% across all S rates, with no significant differences detected among S rates. Overall, results indicate N affected soybean growth and nodule development while the impact on yield was not demonstrated. In deficient soils, S fertilizer may improve corn yields, but may not be economical for soybean.