Q. I keep seeing bags of gypsum in the stores, and hear all these wonderful claims on what it will do to my clay soil. It sounds almost too good to be true. Is it?

A. Years ago, I remember my soils instructor pounding his fist on the lectern and harping on us to remember that gypsum won't work on Iowa soils. Since I was probably half asleep and the pounding roused me from a junior-year morning slumber, his words still resonate somewhat in my ears and bounce around in my mind, but with less than absolute assurance.

Yet, Iowa is not Illinois and not Missouri. But for all these years, I've turned my nose up on the issue of gypsum in soil, pooh-poohing those who sold the notion of instant high-quality soil. But, being mindful of the need to have education behind me, and not half-awake memories, research is needed.

Several websites tout the wonderful benefits of gypsum as a soil conditioner and a source of calcium for the soil. Yet, scrolling through all the websites, one thing comes across time after time: all the websites claiming gypsum benefits are located in the southern part of the United States. A key phrase that pops up is, "If you have an acid clay subsoil, gypsum may have an effect on the roots of plants within three years."

That is one clue why gypsum doesn't work well on Midwest soils - our parent material or subsoil is different. In the South, and even west of the Rockies, soils are more sodium or iron-based. On sodium and iron-based soil, the calcium will replace those elements, which can drive soil particles apart. Thus, you get some loosening action of the soil.

Here, our subsoil is limestone-based, or calcium-based. It's not acidic, but just the opposite. Gypsum is calcium-based; you can't replace calcium with calcium and expect any results.

Gypsum is sometimes used on agronomic soils where heavy crop productions can cause problems, particularly with clay soil. But urban soils are much more complex, being combinations of subsoil, organic matter, native and added topsoil and other additives. They are also heavily layered, and gypsum doesn't work well on layered soil.

Research also shows that gypsum can cause aluminum to leach faster, though if you're trying to grow the blue hydrangeas, that's not what you want. It also can cause leaching of iron and manganese, two other elements we really would rather stick around.

On sandy soil, gypsum cuts down on the movement of phosphorus, copper and zinc. It's been advertised to improve water holding capacity, but research doesn't show that to be true. There are reports that gypsum can raise the soil pH. There are similar reports that it lowers the soil pH. Both probably are based on the calcium and sulfate that make up gypsum, and you end up with free-floating calcium and sulfate ions, but the latter attracting hydrogen, creating hydrogen-sulfate, or sulfuric acid.

If you need calcium to raise the pH, which is seldom needed in most Missouri and southern Illinois soils, limestone is much better and longer lasting. But first, always get a soil test. Sadly, there is no magic pill, or chemical, that will improve poor structure, other than the addition of organic material year after year.