Gypsum and Clay Soils

Let’s look at the science of using gypsum to improve clay soils rather than the popular garden mythology. This demands a bit of attention but the bottom line is that it might work on your soil and it might not. It depends on your soil.

Note from Doug: This is an older article and it’s on my to-do list to research some of the chemistry of this gypsum/soil interaction on various soil types. But the bottom line still holds true for applications and effectiveness.

What is Gypsum?

Gypsum is Calcium sulfate, a mineral that is used in a wide variety of applications ranging from gypsum or plaster board to medical casts.

Is it Acidic or Basic

It is actually neither – it is pretty much neutral.
You may be confusing it with limestone which is alkaline and raises soil pH (makes soils less acidic)
And to confuse you a bit on this 🙂 There are several kinds of limestone:

  • Dolomitic Limestone that is calcium carbonate and magnesium.
  • Calcined limestone is lime that has been “calcined” or heated in its manufacture.
  • While good old-fashioned limestone neither contains magnesium or is heated

So unless there are impurities in the Calcium sulfate, it should not influence the pH of your soil when added to the soil. (There have been published reports of impurities affecting pH)

How Does it Work?

Smaller soil particles lump together (floculate) to form tiny soil balls. In clay soils, this has not happened to any degree. Gypsum aids the clay particles to lump together (it’s a chemical reaction/bonding kind of thing beyond the scope of this page)
Bottom line – adding gypsum helps the tiny particles to group together to form larger particles and because you have larger particles, you have more air space and water movement. Both of these things are good for the tiny feeder roots.

Does it Work on All Soils?

Here’s where the garden mythology takes over. Proponents say, “add it”, “it works”. But no – it doesn’t work on all soils.
The reality is if your soil is already high in Calcium, then adding more calcium isn’t going to do a darn thing. So a soil such as mine – that’s based on limestone rather than granite has all the calcium it needs and no amount of gypsum is going to improve it.
If your soiil is based on Limestone parent material (like most of the mid-west) then gypsum isn’t going to work for you.
If your soil is a Southern soil – based on different parent material like iron, the calcium can be used to floculate the tiny clay particles into larger ones.
But it’s all about whether your soil can absorb any more calcium.
If is also about whether your soil is sodic. This is a measurement of soil sodium content. Soils that are high in sodium have a different electrical charge to them that prevents the electrical bonding of calcium to soil particles. So a sodic soil will not be helped by gypsum either.
You have to know your soil type and you have to have a soil test to know the soil’s ability to absorb calcium (or not)

But my Neighbor Claims it Works for Her

OK. What I often hear (and read) is that the neighbor digs the garden every spring (aerates it) adds organic matter (aerates and separates the soil particles) and adds some gypsum while digging.
And then gives gypsum the credit.
Heck, digging and adding organic matter are going to do the job here – not the gypsum.

How Do I Add Gypsum

Most agricultural applications simply dust it onto the soil surface and then water it in (or allow rain to drive it into the ground). It is then worked in during regular fall plowing.
So in the garden, the easiest way is to sprinkle it onto the soil in the spring and water it in.

How Much Do I Add?

Now here’s the rub – it depends on your soil. One often repeated amount is 40 pounds per 1000 square feet of garden. But agricultural rates indicate that for crop production, a rate of 45 pounds per acre is sufficient.
The answer is between these two numbers and is determined by a soil test.

How Fast Does It Work?

Again, published reports say it’s going to take at least three years for applications to have an effect.

Do I Have To Repeat It Every Year?

Here’s another myth. Gypsum or Calcium sulfate in powdered form is not particularly water soluble so it’s not moving downward and out of your garden very quickly if at all. This means that once you have the application rate (from a soil test) then it’s almost a one-time event to add it to your working garden soil-depth.
Apply and retest the following spring to determine whether your soil can absorb any further calcium.

Bottom Line

Gypsum will work for some clay soils but it is not the cure-all promoted by some websites. It all depends on your soil.

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