Clay soil is composed of tightly compacted soil particles. Wet or dry clay soil doesn't allow the free flow of water and air through it, presenting problems for plants that require good soil drainage. Gypsum, which is calcium sulfate, is a common soil additive used since colonial times to improve the structure of clay soil. It does this by binding the soil particles into larger bunches, creating more air space within the soil.
Features
Gypsum isn't considered a fertilizer, although it adds calcium and sulfur to the soil. Gypsum in the soil causes it to leach iron and manganese which might lead to deficiencies in plants. A bonus, though, is that the gypsum displaces any sodium in the soil, which is leached out during irrigation and rainfall.
Facts
Gypsum doesn't begin working immediately, according to Ed Hume of Ed Hume Seeds. He suggests that for effects to be noticed you must apply annual applications for three years. Washington State University's Linda Chalker-Scott says that gypsum's effects only last "a matter of months," adding that an organic mulch reduces soil compaction more economically and is more sustainable. However, organic mulch doesn't remove sodium or supply the soil with the calcium that gypsum does.
Quantity and Application
Apply 30 to 40 lbs. of gypsum per 1,000 square feet when preparing new beds. Apply the gypsum to dry soil and combine it well. Use a broadcast or rotary spreader to apply the product to established beds and lawns. Use 40 lbs. per 1,000 square feet and apply half of it in north-to-south strips and the other half in east-to-west strips. The criss-cross pattern provides more even coverage. Water to a depth of 10 inches after gypsum application to soak it into the soil.
Considerations
Gypsum does not change the soil's pH, so it is safe to use in beds with acid-loving plants, such as hydrangeas that you want to remain pink or blue, azaleas and blueberries. Gypsum doesn't work on sandy soils and shouldn't be applied to soils amended with 10 percent or more of organic matter. Wait at least two weeks after the application of gypsum to apply lime to your lawn.
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