The benefits of gypsum

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean check-off.

At the 2022 Ohio No-Till Conference, Dr. Warren Dick, retired soil scientist at The Ohio State University, discussed the benefits of gypsum and how it may play a key role in water quality issues when properly applied to soils. Gypsum can help capture phosphorus and prevent it from leaving the field. Gypsum is a soft mineral composed of calcium sulfate dihydrate. The word gypsum is derived from a Greek word meaning “chalk” or “plaster”. Gypsum is moderately water-soluble. Gypsum can be mined or synthetically sourced.

Several possible sources of gypsum for agricultural use are currently available in the United States. These include mined gypsum from geologic deposits, phosphogypsum from wet-acid production of phosphoric acid from rock phosphate, recycled casting gypsum from various manufacturing processes, recycled wallboard gypsum, and flue gas desulfurization (FGD) gypsum from power plants.

“Gypsum is beneficial because it is a source of calcium (Ca) and sulfur (S) for plants,” said Dick. “The sulfur in gypsum is beneficial for plants. For many years, crops received more than enough sulfur from rainfall, but monitoring of sulfur deposited by rainfall onto soil has revealed significant decreases in sulfur inputs. In 1979 about 31 lbs of sulfur per acre were deposited onto our soil in Ohio, and this decreased to about 16 lbs of sulfur per acre in 2007. This decrease, coupled with other decreases, in S inputs due to the use of highly concentrated fertilizers containing little or no sulfur, intensive cropping systems, and increased crop yields that result in more sulfur removal from the soil every year, is leading to more and more reports of sulfur deficiencies in crops.”

Calcium moves very slowly, from one plant part to another, and fruits at the end of the transport system get too little. Calcium must, therefore, be constantly available to the roots. Additions to soil of a good source of calcium, such as gypsum, can improve the quality of horticultural crops. “It is a source of S and exchangeable Ca to improve subsoil acidity and aluminum toxicity,” said Dick. “Gypsum can cause clay particles to form into small masses to improve soil structure and reclaim sodic and high magnesium soils. It can also improve soil and water quality. Soil dispersion is mainly caused by highly hydrated ions, such as sodium and magnesium, attracted to the surface of clay particles. This is a benefit of the calcium in gypsum.”

“The cationic bridging effect of the calcium ion stabilizes organic matter on clay surfaces. The stability of microaggregates is also enhanced by multivalent cations which act as bridges between organic colloids and clay,” said Dick. Gypsum has been shown to improve surface infiltration rates by inhibiting or delaying surface seal formation. Gypsum application to soil can also reduce soil erosion by forming larger clay aggregates by binding clay particles so that they settle out of surface water and thus are less prone to be moved offsite.