Calcium sulfate has many benefits as a soil nutrient

This winter's record rainy season caused delay of many field tasks, and growers are still playing catch-up now. One of those tasks is applying calcium sulfate, which is both a plant nutrient and an important soil treatment. However, many growers only know about calcium sulfate as an amendment to correct excessive salts in the soil and are not aware of its other benefits.

California growers use calcium sulfate in two forms: anhydrite, which is pure calcium sulfate; and gypsum, also known as dihydrate because it contains water. There is confusion about the difference between anhydrite and dihydrate. American farmers have been fertilizing with calcium sulfate for over 200 years. Gypsum is widely available across the U.S.; anhydrite, is largely only available in the West.

"Calcium is essential, just as sulfur is essential," said Garn Wallace, owner of Wallace Labs in Southern California.

"If you're being fed zero sulfur in your water supply, you have to introduce sulfur for the crop. It could be 50-60 pounds per acre per cropping cycle," he said.

Calcium is a nutrient that moves slowly through plants, so it must constantly be available to the roots in order to reach the plant tips.

"Calcium is used in a fairly large amount by crops," said Brent Rouppet, head of Sacramento-based Fertile Soil Solutions. "Some crops will remove a couple of hundred pounds per acre annually. When it was virgin soil, it probably had several thousand pounds or more per acre, but if you keep farming it off year after year, you get into a deficit situation."

Beyond fertilizing, calcium sulfate also improves soil structure and chemistry.

"There are two principal bad guys for farming," Rouppet said. "No. 1 is sodium. Not only is it toxic to plants in most cases, it's also terribly destructive for soil structure. If there's a lot of sodium, and not too much calcium, the structure can get very massive. Roots don't want to grow into that massive soil, water certainly doesn't want to penetrate, and oxygen can't get into that soil. Your No. 1 problem in California is sodium, especially in the Southern San Joaquin Valley."

Calcium sulfate breaks sodium loose from clay particles by replacing it with calcium. Then, sodium can leach out of the soil.

"We can drive the sodium down through the soil profile and below the root zone," Rouppet said. "The calcium helps make soil structure much better, where roots can grow and water can move down."

Given that calcium sulfate is so broadly useful, it's natural to ask which form is better, anhydrite (CaSO4) or dihydrate/gypsum (CaSO4.2H2O)?

"The only difference," Rouppet said, "is that gypsum is calcium sulfate with two water molecules attached; anhydrite is just plain CaSO4 with no water attached. Gypsum is 21% water. So, anhydrite has more active ingredient per ton of material. If I was a farmer, without question I'd buy anhydrite over gypsum, because I'd get more calcium sulfate for my money, and I'm not buying water."

"Anhydrite is a concentrated form of gypsum," Wallace said. "Dihydrate dissolves faster because it's already hydrated, versus anhydrite, which dissolves more slowly."

This has led to the false belief that anhydrite is "less soluble," he said. Actually, irrigation water will hold the same amount of calcium sulfate from either source.

Rouppet, who is associated with a Utah gypsum manufacturer Eco-Gem, confirms this so-called difference in solubility "is …not even worth arguing about. I worked with solution grade anhydrite, and when it came out of the sprinklers or the orifices of the drip lines, it was dissolved, and that's the only thing that matters."

Soil applications for calcium and sulfur plant nutrition are made at lower rates (tens to hundreds of pounds per acre) than for correcting chemical imbalances (sodium and magnesium) and correcting soil structure issues, where thousands of pounds per acre are needed.

Both Rouppet and Wallace stress that application rates should be based on soil testing. Wallace said most farmers test only for fertility, but should test for soil structure issues, as well.

The cost of applying calcium sulfate is a combination of material price, shipping cost and spreading costs.