Expert Advice

Understanding Gypsum

Authored by:

Allen Pyle

Gypsum is calcium sulfate dihydrate (CaSO4 2H2O). It is a natural mineral that occurs in certain types of sedimentary rocks. Gypsum forms when water evaporates in mineral-rich marine soil environments. Over long periods of time, evaporation brings more minerals to the soil surface, eventually forming a solid deposit.

Gypsum can also be produced by capturing (“scrubbing”) sulfur from the emissions of factories and power plants to reduce pollution levels in exhaust gasses. This type of gypsum is called “flue gas desulfurization” (FGD) gypsum. Chemically, FGD gypsum is the same as mined gypsum. In the process that scrubs sulfur dioxide from emissions, limestone (calcium carbonate) reacts with sulfur dioxide to form calcium sulfate dihydrate from exhaust gas emissions.

Gypsum as a Soil Amendment

Gypsum has several beneficial effects on soil structure and soil chemistry. It is also a source of plant available calcium and sulfur.

It is important to understand that gypsum does not change soil pH, even though it contains sulfur. The sulfur in gypsum is in a soluble sulfate form and is readily available to plants and soil microbes. Elemental sulfur, which is not soluble, is used to reduce the pH of alkaline soils.

Gypsum is soluble and dissolves and moves into the soil rapidly. It dissolves more quickly than limestone after surface application.

Gypsum improves water infiltration in soils, reducing runoff and erosion potential. It helps to reduce soil crusting after rainfall or irrigation. Crusted soil can reduce seed emergence and seedling establishment.

Particularly in clay soils, gypsum helps soil particles to create stable aggregates, improving soil structure and ensuring there is good air and water content in the soil. Good soil structure promotes strong growth of plant roots and soil microbes.

Combining gypsum with our proprietary and patented Nutri-Bond Technology™ enhances gypsum’s soil-improving benefits. The combination provides enhanced water infiltration and increased soil stabilization.

Best Practices for Gypsum

Gypsum is soluble and can be applied at any time in spring through fall to lawns, gardens, or landscape plantings. Up to two applications can be made in a year, and applications are usually made in spring and in fall if a second application is needed. Gypsum can be surface applied or tilled into the soil.

For general lawn maintenance, apply 25 pounds of gypsum per 5,000 square feet to established lawns. Vegetable, herb, and flower gardens can benefit from 1 pound of gypsum for every 100 square feet of bed. For shrubs and small trees, apply 1 cup surround the base of the plant. Larger trees can receive 1 cup per inch of diameter. For container plants, gypsum can be applied at 1 teaspoon for every 6 inches of pot diameter

Fun Fact

White Sands National Park has the world’s largest gypsum dunefield, an area of 275 square miles that contains an estimated 4.5 billion tons of gypsum. Only in extremely dry environments will soluble gypsum persist in sand-like dunes.