   Gypsum: Myth vs. Reality - May 19, 2004 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County 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 gypsum users receive little, if any, positive impacts by using it on their soil. Let’s learn more about gypsum so we can use it correctly and understand its true effect on Arizona soils. 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 Arizona soils, the concentration of calcium in the soil is already high, so an application of gypsum will not significantly improve soil structure. In Arizona, gypsum will only benefit soils when there is excess sodium present. Some water sources have naturally high levels of sodium and, when used for irrigation, sodium can 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, but a white does not necessarily mean that sodium is present. Over time, excess sodium causes clay in the soil to become dispersed. When clay disperses, the individual clay particles are no longer held together, thus releasing them 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 and oxygen is severely limited. Here roots find it difficult to penetrate the layer and water will not percolate into the soil. In situations such as this, applications of gypsum can provide a dramatic improvement in returning the soil to its original condition. The calcium present in gypsum actually displaces the sodium and allows it to be leached deeper into the soil when accompanied by deep irrigation. Soil tests by a reputable lab can tell you whether sodium is a problem in your particular soil. If your soil drains well, then you probably do not have a sodium problem. Conversely, if you think that you have a sodium problem, the soil test will confirm this and gypsum is the best way to correct the problem. The soil testing lab should also provide a recommendation on the correct amount of gypsum to apply. Another myth is that gypsum will help reduce soil alkalinity. Sulfur in the form of sulfate 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. I know this was a technical topic and may not be useful to everyone. However, some nurseries and garden centers recommend gypsum without understanding the science behind it. If we know the real story, then we can direct our gardening time and resources toward more productive ends. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |