Research Highlights
Impact of Sulfur Fertilization on Soybean Seed Protein and Amino Acids and Yield

Soybean 14 days after the fourth leaf growth stage on a medium sulfur soil in Memphis, TN in 2019.

By Sangeeta Bansal, Xinhua Yin and Angela McClure, The University of Tennessee

Sulfur is a component of the amino acids methionine and cysteine, and is required for protein synthesis. Protein is one of the most valuable components of soybean seeds, but in recent years the protein content of U.S. soybeans has been declining, significantly devaluing the soybean products. In addition, a decrease in sulfur content has been recently noted in crops in Tennessee and nationwide, likely due to the increased use of sulfur-free fertilizers, less sulfur deposition from the atmosphere to the soil, greater sulfur removal from soil by crops and decreased use of sulfur-containing pesticides. However, there is little information available about the impact of sulfur fertilization on soybean seed protein and amino acid content.

In 2018, two soybean trials were conducted on low-sulfur fertility fields at Jackson and Milan, Tennessee, with the support of the Tennessee Soybean Promotion Board. In 2019, two trials were carried out on low- and medium-sulfur soils at the Agricenter International in Memphis, Tennessee, which were funded by the United Soybean Board at the $33,961 level.

All trials had the following treatments:

  1. 0 lb. sulfur/acre (control)
  2. 10 lb. sulfur/acre soil applied at planting by soil application
  3. 20 lb. sulfur/acre soil applied at planting by soil application
  4. 30 lb. sulfur/acre soil applied at planting by soil application
  5. 20 lb. sulfur/acre applied with sprinkler irrigation (fertigation) at the fourth leaf growth stage (V4)
  6. 20 lb. sulfur/acre applied with sprinkler irrigation (fertigation) at the beginning of the pod development stage (R3).

The 2018 results generally showed that irrigation in the early growing season had a negative effect on leaf color and plant growth, possibly due to lower soil temperatures caused by irrigated water. Delaying application to the pod development stage caused light leaf color during the early growing season, but increased the use efficiency of the applied sulfur fertilizer. Seed yields were lower under the fertigated sulfur treatments (treatments 5 and 6). All five test conditions resulted in increased seed sulfur concentration on fields with low sulfur fertility. Residual soil sulfur level was higher under higher sulfur application rates (20 and 30 lb./acre) relative to zero sulfur after harvest.

In terms of seed quality, 20 pounds sulfur/acre applied via fertigation at R3 (treatment 6) resulted in higher protein levels than other treatments at Milan, which suggests that only sulfur application in late growing season is beneficial for protein. At Jackson, protein levels did not change.

The differential responses between locations could be due to the lower initial soil sulfur content at Milan. Application of 20 lb. sulfur/acre via fertigation at R3 resulted in the lowest linolenic but highest oleic level at Milan in 2018, which suggests that only sulfur application at late growing season is beneficial for oleic.

The results of the 2019 tests revealed that all five test conditions caused small, statistically insignificant increases in leaf chlorophyll content, plant growth, plant sulfur nutrition and seed yield on both low-sulfur and medium-sulfur soils. The seed samples from these trials are currently undergoing quality analyses.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.