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Judging by the number of telephone calls in recent weeks, use of gypsum for crop production has been a hot topic of conversation. Because of the variety of questions, it's appropriate to take time to look at the value of this material for crop producers.

The Basics

Gypsum is calcium sulfate. Historical accounts suggest that it was applied to land in colonial times. When applied to soil, it dissolves somewhat slowly and separates into the Ca⁺⁺ and SO₄^-- ions. Using chemistry terminology, this separation is called dissociation.
The chemistry equation is written as: CaSO₄ <==> Ca⁺⁺ + SO₄^--.
The calcium (Ca), being positively charges, becomes attracted to clay particles and is not leached from soils. The sulfate (SO₄) is not attracted to clay particles and remains in the soil water films. It can leach through soils just like NO₃ although not as rapidly.

A Source of Calcium for Plant Growth?

Yes, the calcium from gypsum can be taken up (absorbed) by plants. Amount of calcium needed for crops, however, is relatively small. Minnesota soils have high levels of calcium. Therefore, calcium from an outside source will not be needed to achieve optimum crop production in Minnesota. Thus, we don't need to think of gypsum as a fertilizer source of calcium.

A Liming Material?

There has been some suggestion that the application of gypsum will raise the pH of acid soils. Although gypsum, like lime, contains calcium, the calcium is not responsible for an increase in pH where soils are limed. It is true that the use of gypsum increases pH when applied to acid soils in the southwestern United States. That increase is due to complex reactions with soluble aluminum. Soluble aluminum is certainly not a concern in Minnesota soils. Therefore, we can't justify the application of gypsum as a liming material.

Will Gypsum Use Reduce Soil pH?

Soybean growers who have problems with iron deficiency chlorosis in western Minnesota would like to discover a soil amendment that would lower pH. Some have suggested that the application of gypsum will do this. Gypsum, however, is a neutral salt. When it dissolves (dissociates) in soil, there is no change in soil pH.

Gypsum is used to reclaim saline or saline/alkali soils in the western United States. With these soils, there is an excessive amount of sodium (Na) that must be removed. This reclamation requires:

1. Application of gypsum at very high rates (hundreds of pounds per acre)
2. Large amounts of irrigation water
3. Drainage to remove this water from the soil system.

The reclamation process can be described as follows:

In this process, the calcium from gypsum replaces the sodium associated with the soil clay particles. To be successful, the sodium must be removed from the soil system. This is accomplished by flushing with high rates (in excess of 12 inches) of irrigation water and drainage.

In Minnesota, a small number of acres are classified as saline or saline/alkali. In addition, large amounts of irrigation water and good drainage are not found where these soils are present. So, use of gypsum on these soils would have no benefit in Minnesota.

The pH of calcareous soils can be reduced temporarily by the application of high rates of elemental sulfur. When elemental sulfur is added to soils, it is converted (oxidized) to sulfate sulfur. This is a biological reaction that produces hydrogen (H⁺).

Elemental Sulfur --->--->sulfate sulfur + H⁺

To achieve any reduction in pH, high rates of elemental sulfur are needed and the pH reduction is temporary. Results of research in Chippewa County show this (see following table).

Results of this trial show that there is no economical way to reduce soil pH for any length of time.

Gypsum does contain sulfur (approximately 17.5%). But the sulfur is in the sulfate form and there is no reduction in soil pH when this material is added to calcareous soils. So, we don't need gypsum to reduce soil pH.

A Sulfur Fertilizer?

Gypsum has been recognized as a source of sulfur for several years. Use of gypsum will have a positive effect on crop yield where sulfur is deficient in soils. In Minnesota, sulfur fertilizers are needed where crops are grown on soils with a sandy texture (loams, sandy loams, loamy sands). Use of gypsum and other sulfur fertilizers will not have a positive effect on yield when crops are grown on soils that are not sandy.