Research Highlights

Are Soybeans Getting the Sulfur Needed from Virginia Soils?

By Laura Temple

Soybeans use 20 to 25 pounds of sulfur per acre each season, according to Mark Reiter, professor and director of the Eastern Shore Agricultural Research and Extension Center for Virginia Tech. He says when the crop yields 70 bushels per acre, the harvested soybeans themselves contain about 13 pounds of sulfur per acre.

Sulfur tends to stay in the soil better and move through it more slowly than nitrates. Over time, several factors have impacted how much sulfur is deposited in soils. However, routine soil tests don’t reveal sulfur needs.

“Soil fertility is critically important because crops grow based on what they can access,” Reiter says. “We are starting to see sulfur deficiencies in corn and wheat. We want to understand how to manage the potential for similar problems in soybeans.”

He is leading a multi-year research project funded by the Virginia Soybean Board to evaluate the response of soybeans to sulfur applications. The project is investigating the impact of sulfur on soybean yields, tissue content and seed quality, as well as the interaction between sulfur and nitrogen.

“Current sulfur recommendations for Virginia were developed more than 30 years ago,” Reiter explains. “At that time, many practices and factors deposited sulfur in our soils without the need for farmers to supplement those levels. Many factors have changed since then, and our recommendations should be updated to reflect current practices, conditions and higher-yielding production systems.”

Changes in Sulfur Availability and Needs

In the past, industrial air emissions deposited roughly 20 to 25 pounds of sulfur per acre into soils. Changing air emission standards have virtually eliminated sulfur-containing compounds, improving air quality. Reiter notes that one unintended cost of that improvement is the loss of a constant source of sulfur deposition on soils.

Virginia farmers also produce fewer peanuts than in the past. Peanut production included the application of gypsum or land plaster, which contains calcium sulfate. With fewer peanuts in crop rotations, less sulfur is being added to fields.

“Constant improvement in soybean breeding also means that varieties planted today may have different needs than varieties planted when our sulfur guidelines were developed,” Reiter adds. “Current varieties are generally higher yielding, removing more sulfur per acre with the oilseed.”

Today, he says one common source of supplemental sulfur in Virginia is ammonium sulfate, or AMS, which also supplies nitrogen. 

“In our research, we are watching the nitrogen component of fertility, as well,” he says. “Does the additional nitrogen help or hinder modern soybean varieties? How does the sulfur to nitrogen ratio impact the plant and soil system and soybean nodulation? We are trying to answer questions about what is happening in soybean plants so new recommendations allow farmers to supply what is most beneficial for the crop.”

Variable Responses to Sulfur Applications

Reiter has seen clear responses to supplemental sulfur applications in corn and wheat. However, soybeans have been responding differently.

Field trials at two or three locations throughout Virginia each season have monitored responses to four different sulfur application rates at planting. According to Reiter and Keren Brooks, a Ph.D. graduate student working on this project, soybean yield and quality responses have varied.

“To date, 40 percent of sites have shown a yield increase when sulfur has been applied at the rate of 10 pounds per acre,” Brooks says. “However, in some cases, a 30-pound-per-acre rate of sulfur actually decreases yield. We encourage farmers to be intentional about what they apply to soybeans. Some inputs can be helpful in some soils, while others can actually be harmful.”

Reiter and Brooks have noted that soil saturation, texture and organic matter all appear to be factors that influence soybean responses to sulfur. They do not yet have enough information to find clear patterns that can inform guideline updates.

“We have also found that sulfur has the potential to change soybean seed quality,” Brooks continues. “So far, it appears sulfur may increase oil percentage of soybean composition.”

She has also noticed visual differences, with plots receiving sulfur being greener. Tissue samples are helping them gather initial information to further study sulfur and nitrogen interaction within the plant, and how that influences both yield and quality.

To date, inconsistent results show that many soybeans are getting the sulfur they need from the soil. When soil sampling, Reiter and Brooks recommend testing subsoil, as well. That is likely where soybeans are currently pulling sulfur from, provided their roots can reach that level. However, over time those sulfur reserves will be depleted, so they expect sulfur deficiency to increase in the coming years. 

Reiter plans to continue this work to gather enough data to see patterns and make reliable updates to sulfur guidelines in soybeans. 

“We appreciate the support of the Virginia Soybean Board for applied research,” he says. “Without their help, it wouldn’t be possible to continue research like this that can directly improve practices for soybean production.”

Published: Aug 23, 2021