Made up of calcium sulfate dihydrate, gypsum is a non-toxic mineral that's naturally occurring and contains high levels of calcium and sulfur.
It's sold commercially in a granular, powdered, or pellet form for use in home gardens. Touted as being a great option for breaking up heavily compacted clay soils, it's also argued that the calcium and sulfur content in gypsum can help to promote healthy plant growth.
Some horticulturists believe, however, that there are more efficient and scientifically-proven solutions that are more beneficial.
What Is Gypsum Used For Globally?
Gypsum is the most commonly utilized sulfate mineral mined across the world. Amongst other things, it's used in the building trade to make up drywall, plaster, and building blocks. It's even used to produce writing chalk, as a food additive, and, in its fine-grained alabaster variety, it's made into ornamental sculptures.
Another common usage for gypsum is for soil conditioning and fertilizing on an industrial or large-scale agricultural level.
Why Is Gypsum Used in Gardening?
The granular or powdered form of gypsum specifically marketed towards garden use has a variety of different applications.
Primarily, gypsum is used for helping to break up heavily compacted clay soils making them more porous and able to absorb moisture. This is particularly helpful in areas prone to drought. The addition of gypsum can promote better drainage and air circulation, which can aid root development and nutrient absorption.
Gypsum changes the soil composition through a process called flocculation. The gypsum enables the small and dense clay particles to join together to form bigger particles, more closely resembling loose sand.
Another occasion when gypsum is often introduced in a garden setting is if the soil in your garden lacks calcium. The addition of gypsum can help to restore the balance. Soils lacking in calcium can lead to slow and poor root development.
Most soils in North America are unlikely to be suffering from calcium deficiencies. When they are, adding lime is often recommended. Unlike limestone, gypsum is more soluble and effective at migrating deep into the soil. This can be an advantage when trying to balance out acidic soils and access deep plant roots. Plus, the gypsum won't change the pH level of the soil like lime will.
It can also be helpful if you have a coastal garden that's subjected to strong salty winds. The gypsum can reduce the sodium levels in the soil.
How Much Gypsum Should Be Used?
Applying too much gypsum to your garden soil can be problematic. To minimize any problems, you should first establish if your soil will benefit from any addition, and you should carefully follow any pack instructions.
Doing a soil analysis will establish how much calcium and sulfur are already present.
Why Gypsum Might Not Be the Best Solution
Over-application of gypsum can strip essential nutrients from your soil, and this can harm plant growth. It can also strip out too much sodium from soils that are already low in salt.
You also need to apply gypsum regularly to ensure it has a continued benefit. It isn't enough to do one treatment and think this will be a permanent solution. After a few months, the effects will begin to wear off and the soil composition will revert to its original state. Applying a treatment at least annually will be required.
Despite claims by some manufacturers, there isn't any strong evidence to suggest that gypsum is a worthwhile addition in terms of fertility.
What Are the Other Alternatives?
There have been studies that have suggested that providing your soil has at least 10% organic matter, there will be no major benefit from adding gypsum.
Taking the time to enrich your soil with organic matter is not only cheaper, but it will benefit your soil in terms of nutrients for promoting healthy growth. It can also improve movement and water drainage in heavy clay soils.
If you are impacted by strong coastal winds, you could always look into plants that are known for being salt-tolerant.
Unless you have a severely compacted soil, and you want to lessen the physical work you'll have in loosening it, then there isn't any strong reason to do a gypsum application.
Gypsum: An Old Product With a New Use. Iowa State University
“Gypsum as a Soil Additive: Use It or Lose It?” MSU Extension,
Calcium Deficiency. Penn State College of Agricultural Sciences