Gypsum for Agricultural Use in Ohio—Sources and Quality of Available Products

Gypsum is hydrated calcium sulfate (CaSO 4 • 2H 2 O), and is often marketed as a soil " conditioner " for improving soil " tilth. " Compared to most other calcium-rich soil amendments, such as limestone, gypsum is relatively soluble in water, dissolving up to 2 g per liter. The solu-bility of gypsum, when either incorporated or surface applied, permits a quick release of calcium (Ca 2+) and sulfate (SO 4 2-) ions into the soil solution. The supply of dissolved salt and Ca 2+ ions, in particular, may reduce soil crusting (Figure 1) and otherwise benefi t soil structure. The aggregation of clay particles that help to form and stabilize soil structure is clearly enhanced by the presence of calcium on clay exchange sites. It is important to note that pure gypsum is not a liming agent, and it cannot be used to raise soil pH. However, gypsum has the potential to relieve aluminum (Al 3+) toxicity in acid soils and to supply calcium and sulfur (S) for plant nutrition. Some natural and synthetic sources of gypsum also contain other chemical compounds, such as calcium carbonate (agricultural lime), calcium oxide (burned lime), or calcium hydroxide (hydrated lime). These materials do have a liming effect when applied to soil, but are not discussed further in this publication. The objectives of this fact sheet are to review possible sources of gypsum for agricultural use in Ohio, and to report results from chemical and mineral analyses of representative samples. There are several possible sources of gypsum currently available for agricultural use in Ohio. These include: • Natural gypsum mined from geologic deposits • Synthetic gypsum produced as a by-product of electricity generation • Recycled casting gypsum from various manufacturing processes • Recycled drywall gypsum Figure 1. Inhibition of soybean seedling emergence by severe surface crusting.