SDSU Extension Soils Specialist Jim Gerwing said SDSU researchers added gypsum to soil at six locations, from Brown County to Clay County. Rates of gypsum ranged from 140 lbs. to 1,500 lbs./acre, depending on the site. Crops tested were corn, soybeans and wheat.
"There was no hint of significant yield increases to applied gypsum at any of the six sites," Gerwing said.
The two corn sites in Brown County averaged 193 bu./acre without gypsum, and 187 bu. with 300 lbs./acre of gypsum. Wheat yields in Brown and Brookings counties averaged 71 bu. without gypsum, and 66 bu. with gypsum. In Turner County, soybean yields were 39 bu. without gypsum and 35 bu. where 1,500 lbs./acre gypsum was added. Corn on a salt-affected soil in Turner County yielded 108 bu. without gypsum and 98 with 1,500 lbs. of added gypsum.
Gerwing said one factor driving the research is that many claims about the benefits of gypsum have been made in the last few years. It's been claimed that adding gypsum can reduce compaction, improve soil tilth, improve water infiltration, raise soil pH, lower soil pH, correct salt problems and improve the calcium-to-magnesium ratio.
"Some of the claims are justified under certain conditions but most do not hold up to scientific scrutiny," Gerwing said. "For all practical purposes, the only real benefit to applying gypsum, other than supplying sulfur if needed, comes when it is applied to sodium-affected soils. In South Dakota and surrounding states many people confuse sodium-affected soil with salt-affected soil."
South Dakota does have salt-affected soils in low ground, along drainage ways and side hill seeps, often appearing as white areas on the surface when soils dry. But Gerwing said the problem in those areas nearly always is caused by salts other than sodium, such as potassium, chloride, calcium and magnesium carbonates and gypsum.
"In many cases, much of the 'white stuff' seen on the surface in these areas is gypsum. Adding more gypsum to these areas is not going to help them," Gerwing said.
In cases where a sodium problem exists, gypsum, which is calcium sulfate, supplies a soluble source of calcium that will displace some of the sodium. The sodium can then be leached deeper into the soil profile as the soluble salt sodium sulfate. If soils are sodium-affected, it often takes 1,500-3,000 lbs. of gypsum per acre to effectively remove enough sodium to improve soil structure.
"If soils are not sodium-affected, gypsum will not improve poor soil structure," Gerwing said.
Calcium and magnesium ratio "correction" has also been touted as a use for gypsum. But Gerwing noted that research at universities in Wisconsin, Minnesota, Ohio and Nebraska has shown that varying this ratio widely has no affect on crop yield.