Gypsum is a soil additive, but the advertised benefits are not well understood. It is a mineral found in everyday items such as toothpaste, shampoo, cement, drywall and molds for dental impressions. Gypsum is often marketed as a soil improver and conditioner, for decompaction, and for improving soil tilth.

While gypsum has some very specific benefits to some soils, not all soils have identical structure or chemical composition. As said in Shakespeare's 1596 play “The Merchant of Venice,” “All that glisters is not gold.”

Properties of gypsum

It is best to understand the soil before determining if gypsum is a product that will benefit the soil and be worth the investment. Here are some things to know:

Gypsum is not a liming agent. Gypsum or calcium sulfate (CaSO₄2H₂O) is not a liming agent in its pure form. For soil pH adjustments, a liming material must contain carbonate (CO₃^-) to neutralize soil acidity by reacting with the hydrogen ions. While some synthetic sources of gypsum have been modified to include some liming materials, calcium sulfate is not a liming agent.

Gypsum relieves aluminum toxicity. It can relieve aluminum toxicity as a replacement ion on soil particles, which increases aluminum absorption from the soil solution. Aluminum toxicity to plant roots typically occurs in soils lower than 4.5 pH where the Al³⁺ becomes soluble in the soil solution.

Gypsum can add calcium and sulfur to the soil. This mineral can add calcium to the soil more quickly because it is more soluble than a lime material. Gypsum as the sulfur source tends to be less soluble than other sources such as ammonium sulfate. Due to the reduction in sulfur dioxide emissions from coal plants since 1990, there has been increased discussion that sulfur may be a yield-limiting factor in production agriculture. The University of Missouri Strip Trial Program is conducting statewide research using sulfur to determine its effectiveness.

Gypsum improves saline or sodic soils. It does this by removing sodium on the soil exchange site and replacing it with calcium. This desalinization of sodium rich soils can result in improvements in clay soil structure, and the soil is less prone to compaction. Adding gypsum to sandy or non-sodic soils is not necessary.

Regions that benefit from gypsum

With the exception of arid and coastal regions where salts are high and in areas of the southeastern U.S. where highly weathered heavy clay soils are common, gypsum amendments are typically not necessary.

To reduce compaction or any of the other stated benefits — a soil improver and conditioner, for decompaction, and for improving soil tilth — in nearly any landscape, application of an organic material is more economically and environmentally sustainable.

Understanding the soil is very important before the use of gypsum as a soil amendment. The best way to start understanding the soil is through collecting a representative soil sample. Generally, soil sampling probes and soil sampling guides are available through any MU Extension center.

Lorenz is a University of Missouri Extension agronomy specialist. He writes from Booneville, Mo.