Gypsum is a mineral sometimes used as a soil amendment.
Gypsum is a naturally occurring mineral that for generations has colloquially been known to improve the quality of soil. As with many gardening and farming traditions that are handed down from generation to generation, however, the value of this colloquialism lies somewhere between urban legend and fact. Luckily, by learning a bit about gypsum and analyzing scientific research, you can easily determine if adding gypsum to your lawn soil would be advantageous or deleterious.
Uses of Gypsum
The traditional uses of gypsum as a soil amendment vary somewhat depending on who you ask. According to Washington State University Extension Horticulturist Linda Chalker-Scott, gypsum is said to improve "problem soils" by "changing the particle size and loosening compaction" as well as by increasing drainage, decreasing acidity and eliminating soil salts. Iowa State University horticulturalist Richard Jauron also notes that gypsum has been used to improve excessively saline or "sodic" soil by changing its pH value.
Scientific Orientations
Gypsum is calcium sulfate, and over time it can change the structure and the fertility of heavy clay soils. When soil has too much salt to support healthy plant growth ("sodic" or "saline" soils), gypsum leaches sodium from the soil and replaces it with calcium, thereby offsetting the excessive salinity of the soil. Gypsum also increases leaching of aluminum, iron and manganese. All of these minerals have varying effects on the soil, and examination of these effects is key to understanding whether gypsum would be beneficial or harmful to your soil.
Actual Effects on Soil
According to Chalker-Scott's research, "one can see improvement in clay soil," and sodium-rich soils can be desalinated to the degree that they will support healthy plant growth. But in general, her conclusion is that "[w]ith the exception of arid and coastal regions ... and the southeastern United
States ... gypsum amendment is just not necessary in non-agricultural areas." Gypsum does not appear to have any effect on soil acidity. Jauron echoes this view, noting that adding gypsum to soil is "of little benefit," even though it will decrease sodium levels in soil. In other words, southwest desert climates can benefit from some gypsum but, for the most part, adding gypsum to the soil underneath your lawn does not yield beneficial results.
Negative Effects
The list of negative effects gypsum has on lawn soil or any other type of soil to which it's applied is a lengthy one. Chalker-Scott concludes, citing extensive research, that aluminum leaching can contaminate watersheds, can lead to deficiencies in plants of iron, manganese, magnesium and other nutrients and will not improve the fertility of acidic or sandy soils. Even the positive effects gypsum does have on soil are short-lived, rarely lingering longer than a few months.
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