Correcting poor soil structure conditions with an application of gypsum can have a significant impact on soil tilth and crop yields. However, many producers overlook this practice because they are not aware of the benefits to soil structure as well as a readily available source of calcium and sulfur.
One important aspect of gypsum is that it is soluble and reacts quickly regardless of particle size. Compare that to lime that requires time since it is not very soluble. The general practice for lime is to apply in the fall, lightly incorporate and then during the next 6 to 8 months it will dissolve and react in the soil.
The exact amount of time it takes for lime to react is based on particle size and soil moisture. Smaller particles have a high surface to volume ratio, and react faster in the soil. For powdered ag lime, particles smaller than 60 mesh will dissolve and react within 30 days. As particle size increases, the dissolution rate slows, since the particle’s surface to volume decreases as particle size increases. Particles between 30 and 60 mesh take 1 to 2 years to react, while particles between 8 and 30 mesh take as long as 5 years. Powdered ag gyp (even when pelletized) is often smaller than 100 or 200 mesh, and in addition it is soluble, so gypsum’s solubility is measured in days if moisture is available, not weeks or months as is the case with lime.
You can’t base a gypsum application on a routine soil test because it wasn’t designed to make a gypsum recommendation. A soil test measures water pH and buffer pH, and this information is used to determine a lime recommendation to correct an acid soil pH. A soil test can calculate a soil’s cation exchange capacity, and that information can be used to set a gypsum rate. A heavier soil with a higher CEC would require more gypsum than a lighter (sandier) soil with a low CEC. A routine soil test will also measure calcium and sulfur, but it doesn’t measure soil structure. Soil testing to determining a gypsum application rate is not a well-defined science yet.
Gypsum can come from several sources, including mined and synthetic by-products, and all are substantially equivalent in turns of calcium sulfate but differ in purity. What about the calcium sulfate content? Most ag gyp products contain calcium sulfate in varying amounts from 60% to greater than 95% – this is referred it as purity. Deposits often contain some limestone and other minerals.
Applying gypsum as a source of calcium and sulfur and to improve soil structure is a good practice. Determining the appropriate rate depends on economics and product type. By-products are usually applied at 0.5 to 1 ton per acre every 3 or 4 years while pelletized ag gypsum is applied at 200 to 400 pounds per acre every 1 to 2 years.
Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.