Gardeners in Oklahoma all have probably had experience trying to deal with the clayey soils that prevail in many parts of the state. Probably many have even heard the common myth among homeowners that if gypsum is added to clay-like soil, it can improve its physical condition. But in reality, there is no scientific basis or research that supports the claim that gypsum softens hard, clayey soils.
Gypsum is the common name for calcium sulfate, a very water-soluble form of calcium. This makes it a good source of plant-available calcium and sulfur. In most soils, calcium is primarily responsible for helping to hold clay particles together into clumps or clods, thus ultimately improving soil structure. But just because gypsum contains both calcium and sulfur, doesn't mean that it can improve soils. Gypsum can be used in the landscape as an amendment for correcting sodic of alkali soils, but as of yet, there is no indication that it corrects clayey soils.
We're not sure how the gypsum myth ever got started. Perhaps a gardener heard that gypsum was a good source of sulfur and since the amendment is fairly inexpensive, decided to spread the off-white colored material throughout the garden. After spreading, the gardener would then have had to rototill it several times before the color was back to normal and the gypsum was mixed thoroughly. Maybe even neighbors had noticed all the off-white substance spread over the garden and, when the gardener had an above average harvest, these same neighbors might have noticed and asked the gardener what he or she had done differently this year. But, in truth, it was more likely that the garden produced so much because of the deep tillage that broke up hard pans of compacted layers of soil that form after several years production. All the gypsum probably did was allow the gardener to know when the soil had been completely worked and when complete and thorough tillage had been completed.
Soils can become clayey because, over time, excess sodium causes clay in the soil to become dispersed. When clay disperses, the individual clay particles are no longer held together, thus releasing them to move through the soil and concentrate in a single dense layer. Frequently, this layer of dispersed clay is so dense that the movement of water and oxygen is severely limited.
All soils, especially clayey ones, can benefit from tillage before planting. Sometimes, too, soil tests may show that a soil has too much nitrogen or other substances for optimum growing. Studies have shown that what can really help improve soil over the years are high levels of organic matter. But as of yet, no case for gypsum as a regular amendment for clay-like soils has been found.
Ray Ridlen is an agriculture/horticulture educator for the Oklahoma County Extension Service. His column addresses frequently asked horticulture questions. For more information, call 713-1125.