Gypsum as a soil additive: use it or lose it?

Gypsum, or calcium sulfate, has become a hot topic in the fertilizer industry and while this soil additive is widely advertised, it is not well understood. The following is a quick reference list about gypsum.

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Gypsum is NOT lime.

• In order to adjust soil pH, liming products must contain carbonate (CO3-) which reacts with hydrogen ions to neutralize soil acidity.
• Gypsum is calcium sulfate (CaSO4). While the calcium will displace hydrogen ions, these ions will remain in solution and will not adjust soil pH.

Gypsum can be used as a source of calcium and sulfur, however, remember:

• Gypsum is more soluble than lime and can add calcium more rapidly to the soil. This may result in decreasing potassium or magnesium levels in the soil. Monitor this by soil testing.
• Michigan soils generally are calcareous-based soils meaning they are derived from materials high in calcium resulting in soils naturally higher in calcium.
• Gypsum can be used as a sulfur source, however, it tends to be less soluble than other sources such as ammonium sulfate.

Gypsum can improve water and root infiltration in sodic soils.

• Sodic soils are very uncommon in Michigan.
• Sodic soils are high in sodium, low in calcium and have problems with water and root penetration due to the effects sodium has on structure.
• The calcium in gypsum, along with drainage and tillage, has been shown to reduce sodium levels in sodic soils.

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• Reduced sodium levels will improve soil structure resulting in greater water and root penetration in sodic soils.

Gypsum has been shown to be effective at treating aluminum toxicity on soils with a pH lower than 4.5.

• At pH lower than 4.5, aluminum in soil overpowers the ability of hydrogen ions to increase pH.

Sulfate may act as a counter ion on soil particles increasing aluminum absorption from the soil solution. Aluminum sulfate is less toxic to plants than the aluminum ion individually.