As more farmers push to plant soybeans early, one nutrient is emerging as a valuable difference-maker in the crop: sulfur. The macronutrient is helping deliver some of the largest yield responses Shaun Casteel says he has seen in recent field trials.
“Never would you think you’d see double-digit results, let alone 20-bushel numbers in soybean yield from one treatment,” says Casteel, Purdue University agronomist and Extension soybean specialist.
Yet that’s exactly what he has documented in some Indiana fields where supplemental sulfur was applied, especially in early planted soybean fields.
Why Sulfur Matters More Now
Sulfur is required by all crops, but Casteel says soybean needs are unique compared with grass crops like corn. In soybeans, sulfur is critical as a co-factor for nodulation, the biological process that allows soybean plants to use atmospheric nitrogen (N).
“If we don’t have good sulfur supply, we don’t have good nodulation and fixation,” Casteel explains. “If you’re sold short on nitrogen in soybeans, you’re sold short on yield in a major way.”
Historically, sulfur came “free” from the atmosphere and also from mineralization of organic matter in the soil. Cleaner air regulations have reduced atmospheric deposition, and Casteel says many farmers are starting to see sulfur shortages that weren’t obvious just as recently as a decade ago.
The classic high-response situations for sulfur — coarse-textured, sandy soils with less than 2% organic matter — still stand out. But Casteel’s work is showing the story for sulfur doesn’t end there.
“I also have fields that are flat and black as a table, with 4% organic matter, where we’re getting sizable yield differences,” he says.
Early Planting Amplifies Sulfur Response
Casteel links some of the most dramatic sulfur responses to a broader trend across the country: earlier soybean planting.
In Indiana, planting patterns have shifted sharply in recent years. Soybeans that once went in the ground two weeks after corn are now being planted within a day or two of corn — and in many cases, are planted first.
Early planting improves yield potential by giving soybeans more time to develop nodes and reproductive branches. But it can also expose a weakness in the natural sulfur supply.
Casteel points out that mineralization of sulfur from soil organic matter depends on microbial activity and warm temperatures. When soybeans are planted in late April or early May, Indiana soils – as week as soils in other states – are often too cool for the microbes to release much sulfur.
“In those cooler conditions, that mineralization really isn’t occurring,” he says.
Across multiple studies where planting date was combined with sulfur use, Casteel has seen consistently stronger responses in early-planted soybeans.
“We’ve got years that we’re averaging an 8- to 11-bushel response on prairie soil,” he says. In these trials, sulfur was (e.g., ammonium sulfate, pelletized gypsum, ammonium thiosulfate) applied pre at 20 pounds per acre during a 5-year period.
Beyond Fertility: A Surprising Disease Connection
Sulfur’s role may extend beyond delivering nutrition and helping fix nitrogen in soybeans. Casteel and his research team are seeing signs that sulfur helps reduce the severity of sudden death syndrome (SDS) in soybeans.
In a 2023 soybean trial, as Casteel began rating symptoms of SDS, he noticed a clear difference between sulfur-treated and untreated strips.
“We had good conditions for SDS development — cool, wet conditions during early vegetative growth. We had a marked, substantial reduction in SDS in those areas that had the sulfur treatment,” he recalls.
The unexpected result prompted a deeper look in 2024, when Casteel worked with Plant Pathologist Darcy Telenko on trials that combined planting dates, sulfur rates and SDS inoculation. Early data from those studies pointed in the same direction: soybeans receiving sulfur showed reduced disease expression.
“Beyond the fertility effect, beyond the fixation-boosting capacity that comes with this, there is evidence that we have some disease control or suppression,” Casteel says, cautioning that the results are still based on only a few years of data.
“If you really think about it, the first fungicides on the market 100 years ago were sulfur-based, so it’s not too surprising that we might be seeing something here,” he adds.
Big Upsides Where Sulfur Use Fills The Gap
Casteel is careful to note that the sulfur response in soybeans is often site-specific. Classic sandy soils and low-organic-matter fields are prime candidates for the nutrient. But his work suggests that even high-organic-matter fields can show strong gains when sulfur is limiting.
That variability doesn’t dampen his enthusiasm. Instead, he sees sulfur as a high-upside tool for intensive soybean managers who already have the basics — variety selection, disease packages, and timely planting — under control.
“It’s fun to have treatments out there that are providing hope and promise,” Casteel says. “We’re seeing numbers with sulfur that really move the needle.”
With earlier planting becoming the norm and biological sulfur supply under pressure, Casteel expects interest in using Sulfur to keep growing. For those growers chasing 100-bushel soybeans, especially, he believes sulfur deserves more consideration as they develop fertility plans.
“If you have not explored sulfur on your soybean crops, I suggest applying strips of S fertilizer that is soluble (e.g., ammonium sulfate, pelletized gypsum, ammonium thiosulfate) between 15- to 25-pounds of S per acre to determine if you have fields or production practices that are responsive to boosting nodulation and N fixation,” he recommends. “Applications can be applied mid-March through planting with higher rates the earlier you apply the S fertilizer.”
More information on Casteel’s research results with sulfur in soybeans is available here.
