The trend Shaun Casteel observed in sulfur trials from 2018 to 2022 on heavy, Drummer soil near West Lafayette, Ind., pointed toward a significant response for applying sulfur before early-planted soybeans. Due to weather constraints, “early” planting in all those years was early May. What if he could plant in April?

The Purdue Extension soybean specialist found out in 2023. The first planting was April 18. “We had yield differences over 20 bushels per acre for 20 pounds of sulfur per acre applied before planting versus no sulfur,” Casteel says. “The visual effect by late summer was amazing — sulfur plots were taller and greener.”

Yield advantage for the May 12 planting for sulfur vs. the control was about 10 to 14 bushels per acre, more like what Casteel had observed previously. For the June 7 planting, as before, yield advantage was not significantly higher than the control.

Keep reading to see products and rates. “We applied near planting, and that can mean an extra trip,” he says. “If you apply up to six weeks before planting, consider rates of 20 to 25 pounds per acre in case of loss due to leaching.”

Why early planting response?

Casteel suggests that the significant yield response to sulfur ahead of early-planted soybeans is tied to less sulfur available in the soil early in the spring. It’s all about how much sulfur can mineralize in the soil and be available for plant use.

“Less sulfur is mineralized when soil temperatures are cooler,” Casteel says. “As soils warm up, more sulfur mineralizes and is available. That’s why sulfur applied ahead of planting soybeans in June has little to no benefit in yield.”

Plus, in no-till situations, sulfur can be tied up in soybean stubble, and even more so in corn stover early in the season. Breakdown and release occurs as temperatures warm up.

“Sulfur aids nodulation, and some effects may carry through into the reproductive phase,” Casteel concludes. “Our work so far indicates it is crucial to have ample sulfur available when soybeans emerge.”

Sulfur products, rates in early planting trials

The objective since 2018 in sulfur trials with soybeans at West Lafayette, Ind., was to apply 20 pounds per acre for plots receiving sulfur. No sulfur was applied on control plots.

The products compared in the 2023 trial were ammonium sulfate; ammonium thiosulfate; calcium sulfate, which is pelletized gypsum; and pelletized gypsum plus urea. In each case, application delivered 20 pounds of actual sulfur per acre.

How much actual product did it take to apply 20 pounds of sulfur? Casteel notes that application rates were 83 pounds of ammonium sulfate, 6.9 gallons of ammonium thiosulfate, 117 pounds of pelletized gypsum, and 117 pounds of pelletized gypsum plus 87 pounds of urea for the fourth treatment.

He opted for pelletized gypsum because it handles and blends more easily. If using regular bulk gypsum, it would require 500 to 1,000 pounds or more to get enough product to flow and apply evenly, supplying at least 20 pounds of sulfur, Casteel says.

No significant differences in yield were attributed to nitrogen in these trials.