Does Your Soil Need Gypsum?

Gypsum is a popular soil amendment in Arizona. However, there is a lot of misinformation out there about its use and effect on soils. The most commonly, I hear people say “gypsum improves soil structure”. This is true, but only under specific soil conditions. I would contend that many northern Arizona gardeners receive little, if any, benefit by incorporating gypsum to their soil. Let’s learn more about gypsum so we can use it correctly and understand its effect on 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 in clumps, clods, or “peds” (the term soil scientists use), thus ultimately creating good soil structure. In most Arizona soils, the concentration of calcium in the soil is already high, so an application of gypsum has little or no effect.

Gypsum is most beneficial to soils where excess sodium is present. Some water sources have naturally high levels of sodium and, when used for irrigation, will cause sodium to accumulate. This is especially true when sodium containing irrigation water is not applied deeply and losses to evaporation are high. The first signs of this may be a white crust on the soil surface when it is dry.

Over time, excess sodium causes clay in the soil to become dispersed. When clay disperses, the individual clay particles are no longer held together in aggregates, thus allowing them to fill in spaces preventing water infiltration and percolation. Frequently, this layer of dispersed clay is so dense that the movement of water and oxygen is severely limited. In these situations, applications of gypsum can provide a dramatic improvement in returning the soil to a more permeable condition. The calcium present in gypsum actually displaces the sodium and allows sodium to be leached deeper into the soil when accompanied by deep irrigation.

In general, most soils in central AZ will not benefit from gypsum additions. If your soil drains well, then you probably do not have a sodium problem. Conversely, if you observe very poor soil drainage that is not caused by compaction or an impenetrable soil layer (caliche), it is best to confirm the need for gypsum additions through a soil test. The soil test parameters needed are sodium adsorption ratio (SAR) and salinity (electrical conductivity or EC). The University of Arizona Cooperative Extension publication titled Using Gypsum and other Calcium Soil Amendments in Southwestern Soils provides recommendations of amounts of gypsum to add based on SAR and EC.

Gypsum applications have no effect on soil alkalinity because the sulfur is in the “sulfate” and will not significantly reduce the pH of the soil. Only elemental sulfur (soil sulfur) or some other acidifying agent will reduce soil alkalinity. Acidification occurs when elemental sulfur and water chemically react to form sulfuric acid. The resulting effect is a slight acidification of the soil near the individual sulfur particles. This is a temporary effect so sulfur can be incorporated on a yearly basis in garden soils, flower beds, and other areas where alkaline soils may impact plant performance.

Agricultural lime is another calcium-containing soil amendment. The primary use of lime in gardening and agriculture is to raise soil pH (to reduce acidity). Unlike gypsum, lime solubility is dependent on soil pH. Its solubility increases in acid soils and decreases as soil pH increases. When soil pH is above approximately 8.2, lime becomes very insoluble. Arizona soils rarely, if ever, benefit from lime applications.

Sometimes gardeners add gypsum to treat blossom end rot: a disease of tomatoes and peppers caused by calcium deficiency. This is usually ineffective as calcium is rarely deficient in Arizona soils. To minimize blossom end rot, avoid water stress by using infrequent, deep irrigation to keep the soil moist and avoid water stress at night. Use mulch to prevent wide fluctuations in soil moisture. Do not over-fertilize plants as higher nitrogen availability can decrease uptake of calcium. Avoid injuring roots during cultivation near plants and do not overwater, especially in heavy clay soils. Blossom end rot is usually an early season problem and becomes less prevalent as the growing season progresses.