SOYBEAN YIELD AND QUALITY RESPONSE TO FLUID STARTER SULFUR FERTILIZER

Sulfur (S) demand has increased as atmospheric deposition of S decreased and soybean (Glycine max (L.) Merr.) production has increased. Soybean growers have invested into agronomic practices to maximize production and alleviate potential S shortfalls including the use of starter fertilizer. For this reason, this study was designed to quantify and qualify the effects of fluid starter S fertilizer on soybean yield. The objectives were to determine an optimum source, rate, and placement of fluid starter fertilizer. A split-plot design of S source-rate and placement was used in 2019 and 2020 at West Lafayette and Wanatah, Indiana. Three starter S fertilizers were used: ammonium thiosulfate (ATS, 12-0-0-26S, Hydrite Chemical), potassium thiosulfate (KTS, 0-0-25-17S, Hydrite Chemical), and K-Fuse (derived from potassium acetate, ammonium thiosulfate and urea 6-0-12-12S, NACHURS) as well as broadcast granular ammonium sulfate (AMS, 21-0-0-24S), and an untreated control. Starter S products were applied at four S rates: 5.6, 11.2, 16.8, and 22.4 kg S ha^-1 to determine optimal S rate and in two placements (single: 0x5x1-cm; dual: 0x5x2-cm). AMS was broadcast at 22.4 kg S ha^-1

Placement did not affect a majority of the factors analyzed and was largely factored out when not significant. Leaf concentrations of essential macro-nutrients, including S, were above critical levels and were not affected by starter fertilizer at any site-year. ATS increased manganese (Mn) in 2019 and 2020 and Wanatah. In West Lafayette 2020 (timely planting), all three starter sulfur fertilizers increased yield and protein, while broadcast AMS did not. Yield and protein did not change with starter S fertilizer in the remaining site-years, which was likely due to plantings later than recommended.

To evaluate and quantify the effects of fluid starter fertilizer across early and late planting dates, a split-plot design was used with an earlier (May 13, 2020) and late (June 8, 2020) planting dates at West Lafayette, IN, as well as early (P24A80X) and late (P35A33X) maturing soybean varieties at Wanatah, IN. These were crossed with six fertility treatments: ammonium thiosulfate (ATS, 12-0-0-26S), potassium thiosulfate (KTS, 0-0-25-17S), K-Fuse (6-0-12-12S, NACHURS), 28% urea ammonium nitrate (UAN, 28-0-0), ammonium sulfate (AMS, 21-0-0-24S), and an untreated control. Starter S fertilizers were applied at 16.8 kg S ha^-1 and 28% UAN was applied at a 7.9 kg Nitrogen (N) ha^-1 rate, all in a single (0x5x1-cm) placement.

The earlier planting had greater stand and yield than the later planting. Starter fertilizers did not impact yield, protein or oil compared to untreated control. Earlier-planted soybean with KTS had higher S concentration in the leaves than UTC and other fertility treatments. Variety impacted leaf nutrient and seed protein concentration. Leaf nutrient concentrations was generally higher in the 3.5 variety compared to the 2.4 variety. Protein was higher in the 2.4 variety compared to the 3.5 variety. However, yield was not affected by variety, fertilizer, or a variety x fertilizer interaction. There was also no fertilizer effect on any essential nutrient concentration.

Soybean positive response to starter S fertilizer aligned with timely plantings rather than later plantings. Earlier plantings were cool and wet field conditions, which limited mineralization of soil organic matter and the supply of N and S. The highest yield was 4308 kg ha^-1 with KTS in West Lafayette 2020, applied at a rate of 7.5 kg S ha^-1, followed by K-Fuse and ATS, respectively. Given the minimal response to different placements, it can be concluded that the difference between single and dual placements on soybean growth and yield is negligible.