Gypsum is a soft mineral, calcium sulfate, bonded to two molecules of water. Benjamin Franklin was a big proponent of using gypsum to improve plant growth, showing the effects of gypsum on grass by writing, "This land has been plastered" with gypsum. The taller grass spelled out his message to passersby. Scientists don't question the usefulness of gypsum in some situations, but gypsum is not called for in all circumstances.
Reasons to Use Gypsum
Suppliers tout using gypsum to change particle size, improve drainage, increase soil pH, desalinate soils, add needed minerals, detoxify soil aluminum and treat soil compaction. Changing the particle sizes of your soil is desirable if you have heavy clay soils. Increasing the size of the soil particles also increases the drainage of the soil. If your soil is too acidic, many agricultural crops will not grow well.
Gypsum and Soil pH
Low soil pH is often caused by evergreen needles or other organic materials decomposing, but it can also be caused by overfarming and soil leaching. Low pH soils are acidic, meaning there are too many free hydrogen atoms in the soil. Adding gypsum won't modify the soil's pH because it doesn't take hydrogen atoms out of solution. To increase soil pH, add lime, which is calcium carbonate. The carbonate ion will take up the excess hydrogen.
Soil erosion is an ongoing problem on slopes especially in areas where roads have been cut into the landscape. Experts now believe that gypsum may help solve this problem by making soils more water permeable. The application of gypsum to soils releases electrolytes that in turn encourage clay particles, which are highly charged, to adhere to each other. When the fine clay particles clump together they leave openings in the soil that water can penetrate.
What Gypsum Helps
If your soil is poor in calcium or sulfate, using gypsum will help if you have neutral or alkaline clay-based soils. On sandy or acidic soils, adding gypsum can tie up phosphorous, copper, zinc, iron, potassium and magnesium. Gypsum can also desalinate soils. Take your native soil into account before deciding to use gypsum. In many cases, adding organic matter, even as a mulch, will be more effective on correcting soil imbalances.