Gypsum Applications: Proper Use of Gypsum for Soil

Gypsum (from the Greek word “gypsos”) is a sulfate mineral that gardeners use as a fertilizer or that manufacturers powder and add to plasters and concrete. Gypsum minerals come in many colors, including white, gray, pink, brown, green, and others, depending on the specific impurities and other inclusions. For gardening purposes, retailers usually sell gypsum in a granular form or as pellets, which are usually white, gray, or brown.

The chemical formula for gypsum is CaSO4·2H2O. Gypsum consists of calcium sulfate dihydrate (or bassanite), but the term can refer to a variety of gypsum sulfate minerals or gypsum crystals with different physical properties. Some of these minerals and crystals include:

• Alabaster: a fine-grained, massive form white stone that is a frequent sculpting medium
• Satin spar: a fibrous variety
• Selenite: a transparent and colorless gypsum crystal

Gypsum forms as an evaporite mineral—a byproduct when a liquid, usually water, evaporates. Gypsum can result when the mineral anhydrite (or calcium sulfate) comes into contact with water, as well as when sulfides come into contact with oxygen. Mineralogists often find deposits of gypsum in saline lakes, sedimentary rock formations, caves, hot springs, and dolomite limestone. The unique conditions that lead to gypsum’s formation make it rich in both sulfur and calcium, which plants need to thrive.

People have worked with gypsum since ancient times, using it for plasters and as sculpture materials. Societies still utilize it today—for example, it has many applications in both construction and agriculture. Here are four common uses of gypsum:

1. 1. To fertilize crops: Farmers have been using gypsum as a fertilizer since at least the eighteenth century, when they discovered and applied Nova Scotia gypsum—from deposits in Ohio, Iowa, and New York—to their fields as a soil conditioner for better crop yields.
2. 2. To form drywall materials: Gypsum is a material ingredient in drywall, along with other additives for preventing mildew, reducing moisture, and making the product inflammable. Other names for drywall building materials include gypsum board, plasterboard, or wallboard.
3. 3. To serve as plaster: Gypsum plaster is a calcium sulfate hemihydrate that is a white cement material. Manufacturers make it by first dehydrating the mineral so workers can apply it to surfaces. Once the workers rehydrate the product with water, it hardens. Another name for this product is plaster of Paris.
4. 4. To slow the hardening of cement: Manufacturers add gypsum to Portland cement—a common, general-use concrete—to prevent the cement from setting too quickly.

Gypsum can serve as a soil amendment, providing a source of calcium, sulfur, and plant nutrients to your gardens or crops. It can also loosens dense soil structures like heavy clay soils and sandy soils by binding small clay particles in a process called flocculation to improve soil aeration and water movement. Additionally, gypsum improves soil aggregation, or the quantities of organic material or organic matter in the soil, which promotes root growth and can prevent blossom end rot. To utilize gypsum in your garden, try following these steps:

1. 1. Test your soil. Gypsum works well in acidic soils, as it can correct an abundance of saline and decrease the toxicity levels of aluminum in the subsoil. It’s therefore a good idea to first perform a soil test or soil analysis to determine your soil acidity. This will impact whether it’s safe for your plants to use gypsum in your soil, as well as how much. For example, if your soil pH levels are below 5.0, adding gypsum can damage your plants. Follow the instructions on the product’s packaging or consult the manufacturer.
2. 2. Spread evenly. To move forward with adding gypsum to your soil, apply roughly one-half pound of gypsum per square foot of soil in your garden using a lawn spreader. Walk back and forth with the tool to achieve an even spread. Avoid mixing gypsum directly into soil mixes, as it can result in an over-tilled garden and possibly even compaction—similar to the effects of overworking a wet clay soil.
3. 3. Reapply as needed. You should apply gypsum only once per year, as it works very slowly in the soil. Always retest your soil before reapplying gypsum to see how your soil reacted to the previous application.

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