Gypsum is composed of calcium, sulfate, and water, has a chemical symbol of CA SO4 2H2O. It is an abundant natural mineral, which originated from the drying out of ancient seas and is quarried or mined in many parts of the world. Plants use almost as much calcium as phosphorus.
Until fairly recently, gypsum’s value as a fertilizer was not widely known or appreciated. Thirty years ago, when Dr. Jacob Mittleider taught people to use it as a Pre-Plant fertilizer in high pH or alkaline soils, he was even sometimes ridiculed by fertilizer sellers, who asked if he had “gypped some folks today.”
Gypsum now enjoys an excellent reputation as a fertilizer. It is even used as a ‘clean green’ or organic soil conditioner and fertilizer.
While other calcium sources raise soil pH, gypsum has a substantial advantage for use in high pH or alkaline soils, because of being pH neutral. This is because the sulfur in the compound lowers soil pH.
Gypsum is particularly useful in treating heavy or clay soils, where it is used to improve the soil’s texture, drainage and aeration. It is also used to improve the structure of soil that has been compacted by the traffic of heavy animals or machinery, in the improvement of sub-soils exposed by the removal of topsoil, and in reducing soil salinity caused by dairy waste matter or alkali conditions.
In large-scale farming, gypsum is sometimes applied initially at the rate of 2 tons per acre (depending on soil type), and 1 ton per acre is common as an annual maintenance application.
For the home gardener, gypsum should be used by those with less than 20″ of annual rainfall at the rate of 2 pounds in each 18″ X 30′ soil-bed or Grow-Box – per crop – incorporated into the soil. For shorter beds use 1 ounce per running foot. Those with more than 20″ of annual rainfall should use agricultural or dolomite lime in the same amounts.