EcoGEM. Correcting poor soil structural conditions with an application of calcium sulfate (gypsum/anhydrite) 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 calcium sulfate 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 more surface area, 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 area 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 calcium sulfate application on a routine soil test because they weren’t designed to make these recommendation. For lime, 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 calcium sulfate rate. A heavier soil with a higher CEC would require more calcium sulfate 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.
Calcium sulfate 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 calcium sulfate 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 calcium sulfate is applied at 200 to 400 lbs. per acre every 1 to 2 years.