Trapping Phosphate with Gypsum

It’s time to think about applying gypsum in the fall to build soil structure while trapping phosphate on the landscape so it doesn’t end up moving off the field into surface waters.

Gypsum, or calcium sulfate, is best described as a soil ammendment. It contains 20 to 24 percent calcium and 18 to 20 percent sulfate. It has been used in California for decades to remediate sodium and salinity. It is also a source of soluble calcium that can improve soil structure by flocculating soil particles together, helping to form peds and improve soil structure. Lastly, it is a source of calcium and particular sulfur, both important crop nutrients. But today it is a tool to reduce nutrient loss.

Nutrient loss reduction: Today growers are learning how to better manage N and P to keep it in the soil for the crop and out of surface waters. It turns out that gypsum can trap P on the soil and stabilize it over winter so that it does not detach with surface runoff.

Calcium and phosphorus (phosphate) have an affinity for each other. Soluble phosphate is an anion, meaning it has a negative charge. Any free calcium (cation) reacts with phosphate to form tricalcium phosphate (Ca3(PO4)2), an insoluble (or very slowly soluble) mineral that is not readily available to plants. Soils have ample amounts of calcium either in the soil solution or on the exchange complex and they can interact to form calcium carbonate or calcium phosphate minerals. Unfortunately much of the calcium is unavailable to tie up surface applied phosphate.

Gypsum can trap phosphate on the surface because it is soluble—dissolving into its calcium and sulfate salts. The free calcium attaches to surface applied phosphate—forming calcium phosphate. Gypsum’s inherent salts increases soil’s ionic strength, causing better phosphorus retention capacity by soil particle surfaces and lower phosphorus concentration in soil solution, which means less leaching. And these soil particles are part of larger aggregates which are less easily removed by water runoff. Particles stay on fields, and waters in ponds and ditches are much less turbid.

Spreading gypsum is the first abatement practice measure which binds phosphorus by reducing transportation of both particle-bound phosphorus and dissolved phosphorus while maintaining plant availability of phosphorus at the same time.

Sodium remediation: Gypsum’s most common purpose is to remediate sodium in the soil by displacing it with calcium and then letting irrigation water flush it below the root zone. This is a very common practice in California where growers apply mined gypsum directly to their fields or inject solution grade gypsum into irrigation. It is pretty safe to say that without gypsum, California growers who irrigate using pumped aquifer water would not produce many crops.

Soil amendment: Outside of California, gypsum is most commonly used as a soil amendment. This is due to the nature of calcium to help flocculate soil particles. Cations like magnesium and sodium deflocculate soil particles so aggregate structure virtually collapses, sealing off the soil and reducing porosity and permeability. When you apply gypsum and improve soil structure, this starts a domino reaction and many benefits begin to accumulate, such as improvements in soil tilth, health and productivity. But this doesn’t happen overnight and you have to be patient and let the benefits accumulate to improve and stabilize soil structure.

Nutrient source: Gypsum is a source of calcium and sulfur and as a mineral it is very soluble. As we have cleaned up our air there has also been a significant reduction in sulfur emission and deposit on the soil. Sulfur, while used in small amounts compared to N, P and K, is now being applied routinely. Gypsum is a good and inexpensive source of sulfate. Generally no one thinks much about applying calcium. Soil tests reveal huge amounts of exchangeable calcium in the soil, so no thinks that applying it will make a difference. However, while there are large measurable amounts of exchangeable calcium, the actual amount of soluble calcium is low. Applying gypsum actually increases the amount of soluble calcium significantly in the soil solution. How this benefits crop growth and yield is uncertain.

Agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.