SOYBEAN YIELD AND QUALITY RESPONSES TO NITROGEN AND SULFUR MANAGEMENT
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Reductions in atmospheric deposition of sulfur (S) coupled with increases in yields of Glycine Max (L.) Merr. (soybean) has led to S deficiencies in Indiana. Poor nodulation due to limited S, and thus a decrease in nitrogen (N) supply, restricts the yield and quality of soybean grain (i.e., protein). Sulfur is a key component of methionine and cysteine, which are important amino acids in the nutrition of foodstuffs. The objective of the first study is to improve yield and composition of soybean through various applications of N and S. Ten N+S fertility treatments were factored by 2 planting dates (early vs. late) at West Lafayette, IN in 2018 and 2019. The same 10 N+S fertility treatments were factored by 2 varieties (Asgrow 24x7 and 34x6) at Wanatah, IN in 2018 and 2019. Soybean yield increases among the N+S fertility treatments of the May 11th planting (early) were 380 to 1006 kg ha-1 over the untreated control, with no difference within the June 5th planting (late) in 2018. Cool and wet conditions that limited mineralization of N and S from the early planting are likely the source of yield improvements. Protein concentrations were maintained and even increased with N and S treatments that were coupled with yield improvements. The Wanatah location showed that protein levels were increased with the ATS and R4+ NS treatments, while the UAN Direct treatment had the lowest protein in both varieties, suggesting that having no source of S could limit protein development. Although variety did not affect yield, fertility improved yields with the V4R3 NS, Plant NS, R3 NS, R4+ NS, and V4 NS treatments. The yield improvements that developed with these treatments is interesting because each treatment contained a source of N equaling at least 44.8 kg N ha-1.
Secondly, the optimal rate and timing of foliar S applications were determined at a S-deficient location (La Crosse, IN) in 2018 and 2019. Three target application timings; V4, R3, and V4 + R3, were crossed with 4 rates of foliar S at 1.12, 2.24, 4.49, 6.73 kg S ha-1 with each application. Therefore, the sequential application (V4 + R3) received a total of 2.24, 4.49, 8.96, and 13.44 kg S ha-1. The optimal rate with 2018 yields was 4.5 kg S ha-1 at V4 or R3; whereas, the optimal rate was 7.9 kg S ha-1 with the sequential V4 + R3 treatment in 2019. Leaf tissue concentrations of S were nearly deficient (0.25%) post-V4 and post-R3. Higher rates of S had greater S concentrations in the leaf; furthermore, most cases resulted in a linear increase of S concentration with the rate of S applied. Foliar applications of S also reduced N:S ratio. Protein levels in 2018 increased at an equal rate for both the V4 and the R3 timings. In 2019, at a 6 lb ac-1 rate of S the protein levels were 39.5 and 39.8% for V4 and R3 timings, respectively. Foliar S applications at V4 vs. R3 timings had little variation in yield or protein levels, thereby resulting in flexibility for application timing for growers.