As little as 25 lbs per acre of sulfur can increase first- and second-cutting alfalfa yields by as much as 20%, according to University of Wisconsin studies.

Soil scientist Keith Kelling says these yield increases are happening on heavier soils that don’t fit the typical profile for sulfur fertilization.

“Historically, our sulfur recommendations were limited almost exclusively to sandier soil types — the ones with low organic matter levels,” says Kelling. “But now we’re seeing yield increases from sulfur on silt loam soils where the organic matter ranges from three to four percent.”

Similar trends have been noted in the Northeast. Although heavy soils are common there, agronomist Ev Thomas with New York’s Miner Institute says sulfur levels are declining in tissue tests from corn and hay silage.

“We're also seeing a trend toward higher yields when we use ammonium sulfate instead of ammonium nitrate to fertilize corn silage, reed canarygrass, timothy and orchardgrass,” says Thomas.

Kelling and Thomas say the growing need for sulfur is largely due to clean-air legislation. Power plants have cut back on sulfur dioxide emissions, so crops don’t get as much sulfur from the atmosphere.

“A Wisconsin Department of Natural Resources estimate shows that Wisconsin’s sulfur emissions have declined by 40% from the mid-1980s to the mid-1990s,” reports Kelling.

Nationally, sulfur dioxide emissions have declined by 31% since 1980, with the greatest dcline occurring since 1991. That coincides with early reports of sulfur deficiencies in Wisconsin’s silt loam soils.

“By 1996, we had documented yield increases of up to 17% from sulfur,” says Kelling.

Increases like these are common in northern Wisconsin, where sandy soils are naturally low in sulfur. But Kelling says this was the first documented yield increase from sulfur fertilizer on the silt loam soils of southern Wisconsin.

In follow-up studies from 1997 to 2001, Kelling recorded yield increases of up to 20% from sulfur applications on silt loams.

“These data strongly suggest that the potential for sulfur response is higher in southern and eastern Wisconsin than it was just a few years ago,” he says.

To correct sulfur deficiencies in the year of application, Kelling recommends using sulfate forms like ammonium sulfate (21-0-0-24S).

“Sulfate is immediately available to crop roots,” he explains. “But elemental sulfur is not available until it is converted to sulfate, and this conversion can take four to six weeks.”