Gypsum Facts and Fiction

David M. Kopec, Extension Turfgrass Specialist

Many of us hear the phrases such as add a little gypsum to sweeten your soil, or add a little gypsum to get rid of salts, or add some gypsum to lower the pH. Actually none of these are true!

The real reason why Gypsum is a valuable soil amendment in southwestern soils is that it is used successfully to reclaim a high sodium (sodic) soil condition.

Many of our soils have high amounts of sodium in them, and much of our irrigation water can be high in sodium. Both of these conditions often lead to a sodic soil condition.

A sodic soil is high in sodium. On a soils test, the value for exchangeable sodium percentage (ESP) represents the percent of the soils overall nutrient holding capacity, which is filled with sodium. If the value for ESP is 15% or greater, then the soil may be sodic.

When this happens, the soil tends to loose its ability to form aggregates with other soil particles. The soil is said to be dispersed.

The result is a soil with no tilth at the surface, which appears powdery, has poor water and air penatration, often puddles for days, and supports little or very poor turf growth.

Applications of gypsum (calcium sulphate) act chemically to remove the sodium from the soil particles. This is a two step process. The first step takes about six months. The gypsum is only relatively soluble and is a slow release compound. It takes months of regular irrigation and rainfall for the gypsum to break down into separate pieces of calcium and sulphate alone.

After this happens, the calcium (supplied by the gypsum) trades places with the sodium that is attached to the soil particle.

The sodium then becomes attached to the sulphate (also supplied by the gypsum) and becomes sodium sulphate). This easily washes away below the roots with adequate water (leaching).

The following table shows the amount of gypsum required to remove sodium from high sodium soil .

Amounts are tons of gypsum/acre.

Remember, gyspum is slow to become soluble and takes 4-6 months to be chemically active. It is best to apply gypsum twice a year. Once in the late fall, before the winter rains arrive, and secondly in the early summer, right before the onset of the monsoon. Simply split the yearly requirement of gypsum in half, and apply this amount in each season.

When larger amounts of gypsum are required (heavy textured soils as opposed to sandy) you can aerify the turf first. Then apply gypsum, and water heavily.