Gypsum is calcium sulfate, and lime is calcium carbonate. Both are soil amendments, and both provide calcium. Only gypsum provides a source of sulfur.
Gypsum is actively marketed as a soil amendment. Gypsum can be mined. Mined gypsum is a relatively pure and clean product depending on how many other minerals are present. It can range from 65 to 98% purity.
Gypsum is also an industrial byproduct from coal-fired power plants, citric and lactic acid plants and water treatment plants. In coal-fired power plants, lime removes sulfur oxides from flue gas, forming calcium sulfate, or gypsum. And the higher the sulfur content of the coal, the greater the amount of synthetic gypsum produced. This form of gypsum creates a disposal issue for power plant operators, and finding uses for it has been an ongoing process. This synthetic gypsum, while relatively pure, could carry some heavy metals, so be sure of the quality of the material that is available for land application. Be sure to do your homework and ask.
Gypsum is a soil amendment, and it doesn’t impact soil pH. Lime corrects low pH in soil, but it has low solubility. Fall is the best time to apply lime so that it has time to react in the soil. Lime is sold based on its grade, calcium carbonate equivalent and grind size. The smaller the grind size, the more reactive it is. The major selling point for lime is to raise the pH of an acid soil.
The major selling points for gypsum are that it can serve as a source of calcium, and it can flocculate (bind) soil particles together in order to improve soil structure. It is also a source of soluble calcium that is immediately available to roots. It is also a source of sulfur, and with crops pulling more nutrients out of the soil and less atmospheric deposition, sulfur deficiencies are looming. Two hundred pounds of gypsum per acre every two years will provide enough sulfur for a corn soybean rotation, for example.
Gypsum and lime both contain calcium, and both are soil amendments. Unfortunately, some folks still think that gypsum, like lime, can change pH when applied to soil. But gypsum can’t raise or lower pH. It is the carbonate in calcium carbonate that raises pH and provides the neutralizing effect. Calcium sulfate can’t raise or lower pH. While elemental sulfur can lower soil pH somewhat, sulfate in gypsum will not.
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 firstname.lastname@example.org or call 402-649-5919.