The science of gypsum

By Matt Reese

Gypsum was the focus of discussion at yesterday’s Midwest Soil Improvement Symposium sponsored by GYPSOIL. “Research and practical insights into using gypsum in Ada on the campus of Ohio Northern University. Everyone agrees that, first, it is important to note that gypsum is not a solution to every water quality or soil structure challenge out there, but the complex science behind gypsum use could be a very useful tool for some situations.

“In an optimum balance of soil, there is 50% pore space for air and water and 50% minerals,” said Joe Nester, with Nester Ag consulting in Williams County. “Water — too much or too little — is the No. 1 thing that affects yield in the field. Yield potential is minimizing stress to the plant.”

Clint Nester talks about the benefits of balancing Mg and Ca with gypsum at the Midwest Soil Improvement Symposium.

With this in mind, a two-inch rain can be a stress or a benefit depending on the soil structure. In heavy clay soils that are prevalent in many parts of the state, calcium and magnesium levels are important in developing good soil structure to minimize the stress and maximize the benefits of that rain. Nester wants to see 70% calcium and 12% to 15% magnesium in heavy clay soils.

“Calcium and magnesium react differently with clay. Calcium is a much larger particle and it can flocculate clays and make building pieces that are a good part of soil structure. Magnesium tends to disperse soil particles around it and peptize the soil. Together, they really affect how water can move through your clay soils,” Nester said. “Gypsum is calcium sulfate and we use that on high magnesium soils to leach that small magnesium particle out of the soil. The sulfate is a double negative repelled by the soil and, as it moves down through the soil, it pulls the double positive magnesium with it.”

This helps to get the calcium/magnesium ratio to the balance that Nester wants to see. But to be effective, other factors need to be present as well.

“You have to have conditions that promote that leaching. It is not a silver bullet — you have to have other conditions for it to work,” Nester said. “You also want to be careful with gypsum on sands because it can pull too much magnesium out. You need a good soil test data base to do this as well.”

In addition, the calcium portion of gypsum can lightly bond with phosphorus and hold it in the field and, when combined with the improved soil structure, improve water quality.

The ideal application rate of gypsum depends on the cation exchange capacity of the soil.

“In high magnesium fields, we use a ton per acre on our 15 to 20 cation exchange soils and 1,000 or 1,500 pounds on lower exchange soils,” Nester said. “With gypsum we want to improve water infiltration, minimize stress on the crop, enhance biological and physical properties in the soil. This pairs really well with cover crops. We also can improve nutrient efficiency with bigger root systems that can retrieve more nutrients in the soil. Good management techniques can really improve the productivity of those heavy clay soils.”

It should be noted that Ohio State University Extension research on gypsum has yielded mixed results through the years without showing consistent yield benefits for corn, soybeans or wheat and many in Extension are skeptical about the suggested benefits. The Extension “Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa” bulletin has this to say about sulfur:

“Sulfur is taken up as sulfate by plants. Sulfate sulfur is supplied primarily by microbial decomposition of soil organic matter. Sulfate is a negative ion and easily leaches in soils. Most soils in Indiana, Michigan and Ohio will adequately supply needed sulfur for plant growth. Sandy soils low in organic matter that are subject to excessive leaching may not supply adequate sulfur. Crops such as wheat and alfalfa that grow rapidly at cool temperatures when mineralization of S is slow are most likely to be S deficient. If elemental sulfur is used, it should be applied at least 2 months before the crop is planted. This would allow time for the S to be converted to the plant —available sulfate form by the soil bacteria. Sulfur should be added in the sulfate form if added less than 2 months before plant uptake.”

Here is more from Nester in a video shown at the Midwest Soil Improvement Symposium.

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