Evaluating Soybean Response to Sulfur in Kentucky Chad Lee, Carrie Knott

Soybean has high demands for nitrogen (N) and sulfur (S) since they are essential in making proteins that accumulate in the seed. While the symbiotic relationship between soybean and Bradyrhizobium japonicum often supplies enough nitrogen for the soybean plant, the soybean crop relies on available sulfur in the soil for plant needs. Organic matter contains reservoir sulfur not available to plants. However, as organic matter decomposes from microbial activity some sulfur is converted into sulfate, which is available to the plants. Sulfate behaves in the soil much like nitrate and is subject to similar loss mechanisms as nitrate. Soil organic matter and its decomposition is the primary source of sulfur for soybean.

Farmers are interested in applying sulfur to soybean for higher yields. This study was conducted to determine the effect of sulfur fertilizer at planting on soybean yield. Field sites were established at Princeton, KY on a Crider silt loam soil and at Lexington, KY on a Bluegrass Maury silt loam soil. Sulfur was applied at planting in the forms of ammonium sulfate (21-0-0-24S), calcium sulfate (gypsum, 0-0-0-17S), and ammonium thiosulfate (12-0-0-26S) at Princeton. Urea ammonium nitrate was included to bring all treatments to the same N rate. At Lexington, ammonium sulfate and gypsum were applied. Urea ammonium nitrate was used as a separate set of treatments to serve as a check against the ammonium sulfate treatments.

At all locations, sulfur rates were 10, 20 and 30 lb S/ acre. At Princeton, one soybean variety was used. At Lexington, two soybean varieties were used. At both locations, all other soil parameters such as pH, phosphorus and potassium were kept constant across the study. Weed control was employed aggressively to prevent weed interference. Princeton was not irrigated. Lexington was irrigated as needed, and irrigation events occurred in July and August. The study collected tissue samples and soil samples during the study.

This report will focus only on soybean yield. At Princeton, yields ranged from 60 to 71 bushels per acre. However, all yields were within the margin of error (p=0.2614), meaning that no fertilizer treatment resulted in a statistically significant yield. Note: In this trial, a p value below 0.1000 is required for significant differences to occur. Since p=0.2614 is greater than 0.1000, none of these treatments resulted in a significant difference in yield. Remembering that all yields are within the margin of error for this trial, the four lowest yields were for the 0 Check (no S fertilizer), Ammoinum Sulfate at the 30 lb S/acre rate, Ammonium Thiosulfate at the 20 lb S/acre rate and Gypsum at the 20 lb S/acre rate.

At Lexington, the two highest yields were the Urea at 9 lb N/acre and the 0 Check (no fertilizer added). Two of the lowest yields occurred for the Gypsum at 20 lb S/acre and Urea at 26 lb N/acre. At Princeton, tissue samples taken at R2-R3 growth stages did not identify any significant differences in N concentration in the plants. However, Gypsum at the 30 lb S/acre rate resulted in the highest %S in the trial and the 0 Check (no fertilizer) and Ammonium Sulfate at 10 lb S/acre resulted in the least %S in the trial. The fertilizer sulfur was applied at planting, yet elevated sulfur concentrations were detected when soybeans were at full bloom and beginning pod stages. However, these elevated sulfur values did not result in significant yield increases in 2019. The irrigated soybeans at Lexington yielded greater than the rainfed soybeans at Princeton. Perhaps the irrigation events allowed for more soil organic release of sulfur at Lexington. At Princeton, all sulfur fertilizer treatments resulted in numerical yields greater than the zero fertilizer check.

These results make it tempting to assume that sulfur caused greater yields. However, the statistics indicate that these observations are not repeatable or predictable. We intend to repeat this study at least one more season and compare it with similar trials in other states. We hope that the multitude of locations and treatments will help us gain a better understanding when sulfur fertilizer on soybean is repeatable and predictable.