Gypsum is often sold as a soil additive to increase soil pH and break up heavy clay soils. In most cases, the addition of gypsum is not an effective way to correct either of these soil problems.
Correcting pH
Gypsum (calcium sulfate) is sometimes sold as a remedy for alkaline soils; however, gypsum has little or no effect on most alkaline soils. Much of the confusion comes from the misunderstanding of the terms alkaline soil and alkali soil. An alkaline soil is a soil with a high pH. An alkali soil has an excess of sodium.
Sodic Soil
A sodic or alkali soil has a high concentration of sodium. The addition of gypsum followed by flushing the soil with large quantities of water reclaims sodic soils. Always get a soil test before adding gypsum to make sure the problem is sodic soil. Gypsum has little or no effect on other alkaline soils, and sodic soils are rare in most parts of the U.S.
Improving Structure
Gypsum is also sold as a soil additive to break up heavy clay. Although gypsum helps reduce soil crusting and improves moisture retention, the best way to improve clay soil is to add large amounts of organic material.
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