Since calcium sulfate is so broadly useful, it’s natural to ask which form is better for agricultural purposes, anhydrite gypsum or dihydrate gypsum. The answer depends on how you define “better.”

In terms of what they contribute to plants and to the soil, available data suggests that they provide the same calcium sulfate benefits, and provide them equally well. They are both calcium sulfate. The water in dihydrate gypsum does not alter that, although it does dilute the potency of dihydrate gypsum by more than 20%.

One difference between them is that, when immersed in water, the soft dihydrate gypsum dissolves faster than harder anhydrite gypsum. Biochemist Garn Wallace explains it this way: “Anhydrite gypsum is a concentrated form of dihydrate gypsum, because it’s not diluted by the water of hydration. Dihydrate dissolves faster because it’s already hydrated, vs. the anhydrite gypsum, which dissolves more slowly.”

This has led people to believe, incorrectly, that anhydrite gypsum is “less soluble,” which is not true. Anhydrite gypsum and dihydrate gypsum have the same solubility in water at normal temperatures—2.531 gr/L at 20°C— meaning that water can hold the same amount of calcium sulfate in solution, no matter which source it comes from.13

The difference in dissolution speed has little effect if calcium sulfate is being applied through the irrigation system. According to Brent Rouppet, who has worked with irrigation systems all over the world, it is a non-issue. He states emphatically that in practical terms, the faster or slower dissolution didn’t matter.

“It’s not even worth arguing about. I worked with anhydrite gypsum. I worked with a solution grade product. They delivered it to the silos. They injected it into the machines that injected it into the water, and when it came out the other end at the sprinklers or the orifices of the drip lines, it was dissolved, and that’s the only thing that matters.”

Because finer particles dissolve more quickly, particle size can be used to control the speed of dissolution. The speed difference between conventionally ground dihydrate gypsum and a finely-ground anhydrite gypsum product can little if any difference at all.

When anhydrite gypsum or dihydrate gypsum are spread in dry granular form, the different dissolving rates can actually be an advantage, if they are used intelligently. Dry ag fines get dissolved (and thus enter the soil) by repeated applications of water from rain or irrigation. The number of cycles necessary to fully dissolve the particles depends on their size and their chemistry.

Faster-dissolving dihydrate gypsum releases quickly. It can release a fast burst of calcium and sulfur into the soil to restore balance and bring up the base levels of nutrients in the soil. However , plants use calcium over time, and if the calcium and sulfur get flushed out of the soil more quickly by subsequent rain or irrigation. The result is that the calcium and sulfur may not be available at a time with the plants need it.

Anhydrite gypsum, dissolving more slowly, feeds the soil more slowly, and for a longer period. Its slower dissolution gives it a kind of “timed release” effect. It can keep feeding the crops, replenishing the soil, and restoring structure and chemical balance over the long term. This also means that an anhydrite gypsum application can last longer, reducing how often it needs to be spread.