Obviously gypsum works in the Eastern Midwest soils. There has been a lot of work that document it.

In the big picture Ca⁺⁺ and Mg⁺⁺ have the same ionic charge so in this are roughly equally attracted to clay & organic matter.

The Mg atom is smaller than the calcium atom so is held tighter to the clay than the calcium.

The kicker is the soil will be in equilibrium and the Mg atoms can not hog all the sites. To kick off the Mg atoms a large surplus of Ca must flood the system.

A also run effect is K⁺ can and will be kicked off the clay particles, and will be kicked off before the smaller Mg atoms.


Obviously gypsum works as advertised in Ohio and Indiana. Does it apply in Arkansas? I do not know. I do know the gypsum addition to my calcareous soil with it's excessively high Ca levels and moderate Mg levels is considered to be counter productive. (Ca/Mg <10/1) ( a conservative <5,000 ppm Ca and roughly 300 ppm Mg)

I suspect, and this is wild supposition, where gypsum works is on land that requires lime to maintain pH. Where elemental sulfur will adversely effect the pH, while gypsum has a neutral effect on pH. The idea is to form sodium sulfate to on sodic soils and magnesium sulfate for high Mg soil. Most crops remove far more calcium than they do Magnesium, so after generations of applying lime that also contains magnesium, the Mg levels become excessive.

Remember anhydrous NH₄⁺ can also displace Mg, as well as Ca, Na, and K, if you flood the system.

A number of years back I had a Professor  in Indiana insist I need gypsum to flush out excessive Mg. Problem was and is I have an excess of calcium, and a modest amount of Mg. My tissue test and hay testing tell me our hay has close to double the usual level of Ca and is reasonably close to the desired Mg level.

Hopefully Ontario Keith will chime in. He has a very good understanding of this whole subject. Martin as well as fertilizerguy also have a better understanding of the subject than I.