Gypsum: Should calcium sulfate be applied or not

By Tom Dominguez, Quay County Extension

Homeowners are typically flooded with several sources of information on how to improve soils in the landscape. The claims of some products are mostly miraculous in their ability to improve the quality and workability of soils. Other products stay firmly entrenched in the minds of gardeners and landscape professionals alike for their supposed ability to improve.

Gypsum is one of these products. It is generally thought that gypsum will improve the structure of soils. While this claim is not necessarily false in all cases, in New Mexico many homeowners may not see any positive soil improvements from an application of gypsum.

To determine if an application of gypsum will help your soil, you first need to know a little about gypsum and how it reacts in the soil.
Gypsum is the common name for calcium sulfate, a very water-soluble form of calcium. This makes it a good source of plant-available calcium and sulfur. In most soils, calcium is primarily responsible for helping to hold clay particles together into clumps, clods, or peds, thus ultimately improving soil structure.
In most New Mexico soils, the concentration of calcium in the soil is already high, so an application of gypsum will not significantly improve soil structure. The addition of sulfate sulfur – a good source of plant-available sulfur – will not significantly reduce the pH of the soil.

So, when would an application of gypsum be worthwhile? The most practical use in New Mexico soils would be to encourage the displacement of sodium through the soil.
Many municipal and private water sources in New Mexico contain naturally high levels of sodium.

Irrigation with waters high in sodium in the landscape often leads to the buildup of sodium in the soil. When sodium concentrations in the soil get too high, clay in the soil may become dispersed. When clays disperse, the individual clay particles are no longer held together, thus releasing the clay particles to move through the soil and concentrate in a single dense layer.

Frequently, this layer of dispersed clay is so dense that the movement of water oxygen is severely limited, and roots find it difficult to penetrate the layer. In situations such as this, applications of gypsum can provide a dramatic improvement in returning the soil to its original condition.
The next issue would be to determine how much gypsum you should apply to your soil.

The only effective method is to have your soil tested to determine the concentration of sodium in the soil. The rate of sodium buildup in the soil will be affected by soil type, species of plants grown, irrigation frequency, and the level of sodium in the irrigation water.

New Mexico State University Soil, Water, and Forage Testing Laboratory offers a variety of soil testing services to the public at a nominal charge.