New sulfur trials find answers, questions

CHAMPAIGN, Ill. — Initial results of the Illinois Soybean Association On-Farm Trial Network’s first sulfur trials on 17 locations across the state were revealed at the recent Field Advisor Forum.

Sulfur is one of 18 essential nutrients that are required for plant growth. Soybeans need 17 pounds of sulfur per acre to produce 60 bushels.

It drives chlorophyll synthesis and energy for photosynthesis, supports nitrogen fixation and enhances biomass accumulation. In addition, sulfur and nitrogen contribute to the protein levels in soybeans.

Soybeans take up about 85% of their sulfur during the reproductive stages, making in-season availability especially critical.

The ISA team developed protocols and made field observations to help farmers answer key questions regarding sulfur fertilization.

The 2025 Sulfur Action Trial plots were a minimum of 40 acres, flexible by equipment and field shape.

They included four replications each of control and sulfur treatments — ammonium thiosulfate or ammonium sulfate — that total eight plots at each site. Plots were greater or equal to 60-feet wide.

Protocols

“Beginning in the spring, we collected baseline soil sampling data and baseline soil sulfur sampling. We tried to do that before sulfur was applied, but we had a little issue with Mother Nature this year and sometimes it was sampled after the sulfur was applied. We also did soil health sampling,” said Stephanie Porter, ISA outreach agronomist.

“Growers in the program were asked to apply 30 pounds of sulfur. This is a much higher rate than what’s normally recommended. Normal recommendations would be 15 to 20 pounds. There was also a control plot with no sulfur applied.”

Porter added that some of the controls did not have an equal amount of nitrogen on them, since sulfur products contain varying amounts of nitrogen.

Each participating farmer contributed four years of management data. ISA collected soil samples, tissue tests at the R1 the R2 reproductive growth stages, field insights and yield data.

Yield Response

Plots in Bureau, Champaign, Knox and Vermilion counties delivered the clearest sulfur responses in 2025, with yield bumps ranging from roughly 2 to 7 bushels per acre, depending on sulfur form, soil type or texture, moisture, residue, cover crop and soybean variety.

Bureau County stood out with a plus 7.4 bushels per acre response attributed to sandy soils and lower organic matter.

Champaign County also showed a positive 3.4 bushels per acre return with cover crops, higher residue and low rainfall, but with a high organic matter.

The Vermilion County trials had a 3.4 bushels per acre bump, and Knox was 1.5.

“Interestingly, planting dates and crop demand did not appear to play a role in sulfur response in 2025,” Porter noted.

All the plots, with the exception of the one in Perry County, were soybeans after corn. The Perry plot has been soybeans-on-soybeans for a long time.

Most sites showed little or no advantage with the addition of sulfur. These fields consisted of an average to very high application during 2024 and most of their 2025 spring soil tests showed adequate to high sulfur levels, according to Porter.

Why Responses Varied

Porter noted in her Field Advisor work what factors impacted the soybean response.

• Sulfur form — AMS contains 24% sulfur in the sulfate which is immediately plant available. ATS consists of half sulfate and half elemental sulfur that needs to be oxidized to be available.

• Soil type — Coarser soils such as sand tend to leach sulfate more quickly, but fine textured soils such as loam and clay typically contain more organic matter that holds more organic sulfur that must be microbially mineralized to sulfate.

• Cover crops — Cereal rye can scavenge sulfate, but sulfur could be returned to the soil from residue.

• Residue, or carbon-to-nitrogen ratio — High residue can temporarily tie up sulfur during mineralization, and nitrogen and sulfur might be needed to feed microbes to aid mineralization.

• Mineralization — Only 3 to 5 pounds of sulfur per year are released for each 1% organic matter, and cool, dry soils slow mineralization. Water-logging can cause anaerobic conditions and some sulfur can be lost as gas.

Challenges

As with any growing season, the weather played a role in the first year of data.

“There were also areas that we were not able to collect data on thanks to Mother Nature. We had heavy amount of rain. Southern Illinois was not able to plant early, and we had some that weren’t able to get the sulfur on in the spring. There were replants on some plots, another plot was planted extremely late,” Porter said.

The trials also featured a wide variety of planting and tillage practices.

“Nobody farms exactly the same. Everything is different. Row spacing varied from 10 to 15 to 20 to 30 inches. Tillage timing and practice varied. Populations ranged from 110,000 to 180,000,” Porter said, referencing cation-exchange capacity.

“Yields ranged from 90 to 40 bushels an acre, and that had a lot to do with, not necessarily with soils, but was Mother Nature. I attribute a lot to planting date, as well. Organic matter and CECs also varied across the trials.”

Looking Ahead

ISA’s On-Farm Trial Network will continue to explore sulfur’s impact in Illinois this year.

For now, sulfur isn’t a guaranteed yield booster, but with the right sulfur form, in certain soil management conditions, and with the right soybean variety and weather, it can provide a production bump.

ISA’s On-Farm Trial Network is a program for Illinois farmers and organizations interested in participating in field-scale crop trials. The program functions as the bridge between Illinois farms and research generated by universities and industry experts.

On-farm trials are an integral step in translating how small-plot or greenhouse research can apply to cropping environments. They allow for testing across a wider range of weather, soils and locations across Illinois.