Research Highlights
Field Conditions Influence Sulfur Impact on Protein Quality Yield

Photo: Kansas State University

By Laura Temple

Sulfur deficiency has been reported in soybeans throughout the United States. That finding has sparked many questions. Does fertilizing with sulfur improve soybean seed quality? Can sulfur impact yield? Do other factors influence how soybeans respond to sulfur?

To find answers, the national soy checkoff funded a meta-analysis of the impact on sulfur fertilization on soybean composition and yield. The analysis encompassed three years of field trials across 18 locations in eight states.

“We found some general trends in the impact sulfur fertilization has on soybeans,” reports Dr. Ignacio Ciampitti, professor in the Department of Agronomy at Kansas State University. He led the meta-analysis, together with his team, including post-doctoral researcher Dr. deBorja Reis and student Moro Rosso. They collaborated with researchers from other states. “We found that sulfur does improve protein quality, but its impact depends on many factors. One recipe or recommendation will never fit all locations.”

Ciampitti notes that there is much to learn about the interaction between inputs and environmental conditions. The analysis of data from a broad range of locations and conditions attempted to better understand these interactions. The findings have recently completed per review and been published in an academic agronomy journal. 

Researchers collaborated to analyze the impact of sulfur fertilization on soybeans from field trials across 18 locations in eight states. Photo: Kansas State University

“We found that while quality responses to sulfur fertilization tended to be consistent, yield responses were very site-specific,” he says. “As we analyzed trial data by a variety of factors, a few themes emerged that help us better understand sulfur’s role in the complex interactions between soil, environmental conditions and soybeans.”

Quality Questions

Sulfur is linked to protein quality, specifically the amino acids cysteine and methionine. These two essential amino acids are often low in soybean meal. Livestock diets relying on soybean meal for protein often have to add supplements of these amino acids, especially for monogastrics like pigs and poultry.

Ciampitti says more companies have been asking about soybean quality. He meets with nutritionists who are interested in work being done to improve soybean meal protein content and quality.

“The quality of soybean components matters to end users,” he says. “Managing soybeans for quality is different than managing them for yield. Farmers should start thinking about managing for quality as it becomes more important to customers.”

The field trials indicated that applying sulfur at planting increased overall protein content in soybeans an average of 0.3 percent, which is notable. However, the general protein responses varied by location. 

Of even greater significance, field trials showed that fertilizing with sulfur, regardless of application timing, consistently increased the concentration of sulfur amino acids — cysteine and methionine — by 1 percent. Topdressing with sulfur in vegetative or reproductive stages did not improve yield or protein concentration, but it did increase levels of those sulfur amino acids.

Yield responses from sulfur applications were limited to site-specific hot spots, but in those locations, yield increases were significant enough that the entire meta-analysis showed an average yield gain. However, as mentioned before, yield responses were very site-specific in a reduced number of locations. A deeper look at the data showed key factors impacting soybean response to sulfur.

Figure 1. These charts summarize the average results of studies on management for seed quality focusing on soybean yield, protein and oil content, and specific amino acid concentration. Source: Kansas State University

Soil Organic Matter

“One trend that emerged was that soils with low organic matter content, sandy soils, fields with a long history of production agriculture or fields in continuous soybeans for several years were more likely to show responses to sulfur applications,” Ciampitti says.

Soil organic matter was one factor that contributed to great variation in soybean responses. The overall plant demand for sulfur increased with soybean yields. 

However, soybean quality response was different. In soils with intermediate levels of organic matter, sulfur amino acid concentration increased by 1.2 percent.

Stress Limits Plant Response

Ciampitti explained that plants under another growth-limiting stress, such as drought stress, did not respond to additional inputs. For example, under high drought stress, sulfur fertilization did not impact yield or quality.

“If plants are not growing due to droughty conditions, then the overall nutrient supply is not the most limiting factor,” he says. “In analyzing field trial data based on levels of water stress, additional inputs like sulfur do not pay off in terms of yield or quality.”

Under intermediate drought stress, researchers observed both quality and yield responses. Environments presenting low drought-stress levels showed only a response in sulfur amino acids concentration, with no response to overall protein levels.

In-Field Implications

According to Ciampitti, analyzing data by environmental factors that caused variability in results helped reveal underlying causes that influence soybean response to sulfur fertilization. Understanding these factors will help farmers make informed decisions about sulfur fertilization.

“We saw more quality responses in soils with low sulfur levels, but no response when sulfur levels are high,” he says. “Soil testing is a key tool for farmers to use to determine the need for sulfur, but soil type and field history should also be considered when making this decision.”

As soybean quality becomes more valuable to end users, farmers should grow in their understanding of managing for quality. Sulfur fertilization is one example of that.

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.