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Research Highlights

Research Highlights
Adding Sulfur to Soybean Fields Proves Beneficial in Michigan’s Sandy Soils

In-season sulfur application in Michigan soybeans. Photo: B&M Crop Consulting

By Carol Brown

Agricultural areas in the United States, particularly the Northeast, are seeing sulfur declines in fields and crops are showing sulfur deficiencies. Pennsylvania, Indiana, Ohio, and areas of New York have experienced the highest declines over the last two decades. And the issue is creeping into Michigan.

Missy Bauer, with B&M Crop Consulting, is studying sulfur deficiencies in soybeans through a research project funded by the Michigan Soybean Committee. Two years of the three-year study are complete, and she is finding that sulfur fertilizers are needed in specific areas of the state.

“The locations where we are testing have sandy loam soils, in which sulfur tends to leach out,” says Bauer. “Sulfur is mobile in most soils, and sandy soils makes it a little easier for sulfur to move.”

Atmospheric sulfur has been declining since the 1990 amendment to the federal Clean Air Act, which requires industry such as coal power plants to add sulfur scrubbers to clean their air emissions. While this is good for humans, it also decreases sulfur availability for plants that need it. Sulfur is a component of two essential amino acids found in soybeans. Higher sulfur levels in soybeans improves bean quality.

Bauer is exploring the addition of sulfur fertilizer in three locations in south central Michigan. Her goal is to determine if sulfur should be applied to soybean fields to increase yield. 

“We have been testing both dry and liquid application of sulfur and timing of application. Some plots receive it prior to planting and others have sulfur added at the V3 growth stage,” Bauer says. “We are measuring soil sulfur levels, taking leaf tissue samples two times during the season, and finally, yield data.”

Additionally, Bauer and her team are measuring sulfur response in management zones to see if variable rate application would prove the most successful for the plant as well as the most economical for the farmer.

“Over the two years, we have found that soybean yield increased with all sulfur treatments for both seasons and in all locations, averaging 4.5 bushels per acre increase compared to the control plots,” she says. “The in-season application of dry ammonium sulfate (AMS) at V3 growth stage had the best response overall. The next best was the pre-plant AMS application with a 3.7 bu/acre average increase.”

Table 1. 2019-2020 Multi-Location Net Dollars Increase or Decrease vs. Control

Economically, the same application of in-season AMS had the highest net return of $29.31/acre over the two years of the study, compared to the control.

Soil tests were taken in April, prior to planting or sulfur in-season application, depending on the location. The potential for deficiency is greater when there are low amounts of organic matter and sulfur levels are below 10 ppm. Overall, sulfur levels are lower compared to a decade ago, when Bauer saw soil tests around 15-16 ppm; now she sees levels in the single digits. But sometimes the soil test isn’t the most accurate because sulfur mobility. 

B&M Crop Consulting intern Carson Rose pulls leaf samples for sulfur level evaluation.

Soybean plants exhibit signs of low sulfur levels through leaves that are more yellow, but this could also indicate a nitrogen deficiency. Plant tissue sampling is the best way to detect particular nutrient deficiencies. Bauer’s tissue samples showed small sulfur level increases in all the tests where sulfur was applied, which were in line with yield gains. 

When farmers run their soil tests every two or three years, Bauer recommends including a sulfur test. Results could show variability of sulfur levels within a field; areas with low sulfur levels should respond well to a sulfur application.

For those who farm in sandy loam soil, Bauer’s research reflects that sulfur applied at soybean V3 stage is the most economical and will help to improve yield. The final year of this research project will help to confirm these recommendations.

To find research related to this Research Highlight, please visit the National Soybean Checkoff Research Database.