A natural mineral, gypsum is a safe and inexpensive soil additive used to break up compacted earth. Used in agriculture, lawn and garden care, gypsum is effective but also the subject of controversy in agricultural circles.
A byproduct of sulfide oxidation, gypsum is known by several names including hydrous calcium sulfate, satin spar and selenite. Mined worldwide, gypsum is used in the manufacture of chalk, plaster of Paris, Portland cement and in sheetrock. For agriculture, the mineral is ground into small flakes, pellets or powders and worked into the soil as an agent to loosen soil.
Gardens with clay or compacted soil see the greatest benefit from gypsum because the mineral penetrates the clay particles, causing them to clump together and allow air and moisture slots to develop. Agriculture consultant and gypsum proponent Brent Rouppet says the use of gypsum improves compacted soils by attracting sand, clay, silt and other particles together. In addition the mineral helps replace detrimental salts and can reduce water runoff.
Root growth can be limited because of the hardness of the soil. Gardeners around new homes find the soil highly compacted from construction vehicles. In addition, arid areas and coastal gardens will often find soil too firm for proper root growth. Malcolm Sumner of the University of Georgia says that gypsum applied to the surface or worked into the soil will ameliorate soil and allow for roots to proliferate where water was previously beyond their reach.
Gypsum is not a fertilizer, although it does contain both sulfur and high amounts of calcium, elements critical to plant growth. There is no danger of over-fertilization with gypsum and it can be applied on the surface or worked into the soil. Application can be achieve with a drop or broadcast spreader on large gardens or just tossed by hand in small areas.
The recommended application rate is 40 pounds per thousand feet. A granular form of gypsum is best for gardens. Application is only needed once a year and the gypsum can be applied to the surface of the soil. Water immediately and allow the dissolved gypsum to leach into the soil.
While nontoxic and safe to handle, gypsum remains highly controversial. The Agriculture Center at Louisiana State University considers gypsum unnecessary. The center contends that most soils in the United States are a common mixture of non-native and native subsoils. These "layered" soils are already loose and show little effect from an application of gypsum. The LSU Agriculture Center recommends core aerifying in spring and summer to reduce compaction instead of applying gypsum.