“When people talk about gypsum, they really should be talking about calcium,” says David Holden, president, Holden Research and Consulting of Camarillo, Calif. “For years we've always talked about gypsum as being synonymous with calcium, but people don't realize that the real issue is not gypsum, the issue is soluble calcium.”
As Holden explains it, westerners have a bounty of calcium in their waters. But that calcium is a tease; it's inaccessible, tied up by carbonates and bicarbonates. Once the water dries, it produces limestone in the soil. Limestone itself is insoluble; more calcium, but no more helpful than a doorstop. “So there's still a need for more soluble calcium than what our soils and our water can provide,” Holden says.
“Another thing people are confused about is that gypsum will acidify their soil. It does not. Limestone will bring the pH of an acid soil up; gypsum does not bring the pH of an alkali soil down.”
Holden says he's been preaching to anyone who will listen. “When we say we are adding gypsum, it's not gypsum, it's calcium we're adding to the soil. When we talk about adding limestone, that may be about calcium, but it is also an adjustment for an acid soil (which is not a major issue in California.)”
Holden says if you have soil that is highly alkaline with a lot of sodium and not a lot of water dispersion, those soils may need to be pre-treated with gypsum. “But in permanent crops when you have sodium built up, putting gypsum on top of the soil is not an efficient way to get sodium reduction, because there is very little soluble calcium in it. And that's what displaces sodium.
“If you broadcast gypsum on soils where you have a permanent crop and can't work it into the soil, it's going to be a long time before that gypsum can do a whole lot of work. As a pre-plant application where you can work it into the ground, it's a great solution,” Holden says.
For an immediate source of calcium, Holden recommends using Thiocal calcium thiosulfate. You will see sodium reduction with Thiocal use, Holden says, within two to three months. The economics work out, too. “It doesn't take that much Thiocal to equal a ton of gypsum. I've found that about 30 gallons of Thiocal equals about a ton of gypsum. It pencils out,” he says.