When Illinois Soybean Association (ISA) Outreach Agronomist Stephanie Porter launched the group’s first on-farm sulfur (S) trial network, she wanted to answer a question farmers across the state were asking:
If I’m already applying sulfur to corn, do my soybeans need it, too?
After a season of trial work across Illinois in 2025, the answer is sometimes — but only under certain conditions. Porter addressed the research during the 2026 Field Advisor Forum.
What the ISA Trials Found
ISA used fairly large plots to conduct the research rather than small research strips. Each trial had:
- At least 40 acres
- Four replications of a sulfur treatment and four untreated strips
- Strips at least 60 feet wide, or twice the width of the combine head
- A target sulfur rate of 30 lb. S/acre, spring applied
Porter says the 30-lb. rate was intentionally high for research purposes. It was designed to give sulfur every opportunity to produce a response, not to establish a farmer recommendation.
Growers used ATS or AMS, with some gypsum included in the broader research effort.
“[ATS] is easier, like kind of a liquid product for them to apply. The one key thing I want you to note about that is that ATS is half sulfate, half elemental sulfur. So that sulfate is the readily available form for that plant to take up, but you still have the elemental sulfur that’s not readily available,” Porter says.
The Illinois research team collected soil and tissue samples, plant measurements, yield data and four years of historical field-management history. All protocols can be found on FieldAdvisor.org.
Only Four Fields Showed a Clear Yield Response
Across the statewide trials, only four fields had a clear yield response to sulfur. One other field had a numerical increase that was not statistically significant.
The responsive fields did not all look alike. However, Porter says several common factors stood out.
One Bureau County field produced a 7.4-bu./acre increase even though the grower applied only about half the planned rate — roughly 15 lb. S/acre.
That field had relatively sandy soil, about 3% organic matter and medium CEC. Those conditions can increase the chance of sulfur loss through leaching and may limit the amount of sulfur supplied by organic matter, Porter notes.
Other responsive fields had more organic matter — around 4.5% — but were still relatively sandy.
The lesson is important: no single soil characteristic predicted a sulfur response. Soil texture, organic matter, CEC and field history all played a role.
Start With the Soil
Porter recommends farmers evaluate their soils before considering the fertilizer product.
Sandy soils are more likely to lose sulfate through leaching, which can increase the chance of a sulfur response.
Organic matter also supplies sulfur. As a general estimate, each percentage point of organic matter can mineralize roughly 3 lb. to 5 lb. of sulfur per year. Fields with less organic matter may have less sulfur available to the crop.
CEC and soil type can provide additional clues. Higher-CEC soils often have more organic matter and a greater ability to supply sulfur. But some higher-CEC fields still responded in the trials.
A key point is that some Illinois soils can supply enough sulfur on their own. Applying sulfur without considering that supply can add cost without adding yield, Porter says.
Management Matters, Too
Soil quality was only part of the story. Field management also helped explain the differences in response among sites.
Early planting:
Early-planted soybeans came up repeatedly in the discussions about sulfur. Porter says the trial results support the idea that early-planted soybeans may be more likely to benefit from sulfur, but early planting alone does not guarantee a response.
Residue and tillage:
Several responsive fields had heavy residue, including no-till and fall-till fields with substantial corn residue remaining on the soil surface.
Residue can temporarily tie up nutrients as microbes break it down. The carbon-to-nitrogen ratio of the residue affects this process, and sulfur can also be involved, Porter explains.
She adds that one particularly interesting Knox County field compared AMS and ATS. The AMS treatment produced the better response. One possible reason is that AMS supplies sulfur entirely as sulfate, making it almost immediately available.
Cover Crops May Change the Picture
The use of cover crops was another interesting part of the 2025 trials.
One strong sulfur response occurred in a plot where cover crops had been grown. That fits with some university research in Illinois and Indiana showing greater potential for sulfur response in fields with cover crops, Porter says.
Cover crops do more than tie up nutrients. They can capture and hold nutrients such as sulfur, potentially changing when that sulfur becomes available to the next crop.
Porter says that raises an important question: Are cover crops helping keep sulfur in the system, or are they delaying its availability to soybeans?
Previous Sulfur Applications Matter
One of the clearest differences between responsive and non-responsive fields came from the four-year management histories collected by Porter and her colleague Darby Danzl, a regional technical agronomist for the Illinois Soybean Association.
The team rated previous sulfur use from none to very high. Fields that responded in 2025 generally had no or very little sulfur applied in 2024.
Many fields that did not respond had received sulfur regularly, usually as part of their corn fertility program.
That suggests soybean sulfur decisions should not be made one year at a time.
If sulfur has been applied regularly to corn, soybeans may already be benefiting from that fertility. This is one reason farmers should take their whole rotation into consideration, and not just the current soybean crop, notes Porter.
Soil and Tissue Tests Can Help
Soil sulfur tests are useful, but Porter cautions that they are only snapshots of what is happening in the field.
Two responsive sites in Champaign and Vermilion counties, east-central Illinois, had relatively low spring soil sulfur levels—24 ppm or less on the scale used in the trials. At R1 to R2, tissue tests also showed higher sulfur concentrations in the treated strips.
The opposite occurred in a Menard County, Ill., field. It had the highest soil sulfur test in the trial, at about 31 ppm. Sulfur treatment did not produce a meaningful difference in tissue sulfur or yield.
Porter’s perspective is soil and tissue tests can help identify fields that may be short on sulfur, but neither test should be used as the only reason to apply it.
Don’t Chase Yield With 30 Pounds of Sulfur
The 30-lb. rate used in the ISA trials was for research — not a recommendation.
“I do not want you to go out and apply 30 pounds of sulfur; that is not economical,” Porter told farmers.
Danzl’s economic analysis of the roughly 2.5-bu./acre response in Vermilion County shows why.
At an ATS price of $320/ton, the treatment barely broke even. At a more typical $400/ton price from 2024, it would have lost money.
For fields where soil, management and field history suggest a possible sulfur shortage, Porter said between 15 and 20 lb. S/acre is a
more realistic rate to consider.
What Farmers Should Watch For
The 2025 trials reinforced what Porter would tell farmers to keep in mind: Sulfur should not be used as a blanket application for soybeans.
Instead, in trying to decide where sulfur use would make financial sense, she says to look for fields where a portion or all of these eight risk factors come together:
- Sandy or relatively coarse-textured soil
- Lower organic matter
- Heavy corn residue
- No-till or other systems with substantial surface residue
- Cover crops
- Early planting
- Little or no recent sulfur application
- Low soil sulfur test results
More Questions for 2026
The first year of ISA’s on-farm network answered some questions about sulfur but raised others. Porter says Illinois researchers still want to know:
- Is fall or spring application better for soybeans?
- How do cover crops affect the timing of sulfur availability?
- Can sulfur improve soybean protein or grain quality enough to have economic value?
- Do different soybean varieties respond differently to sulfur?
- How much sulfur applied to corn carries over to the following soybean crop?
For now, she says the best approach farmers can take is to look at the soil, consider the rotation, recent fertilizer history and risk factors, and test before treating the whole field.
