What's All the Hype About Gypsum?

Do you use it in your garden? For what purpose? It's one of those things like bone meal that has a lot of folklore attached to its use. I have seen directions for using it to (1) prevent dog urine from making brown spots in the lawn, (2) correct the pH of the soil, (3) improve the texture of heavy clay soil and (4) counteract the effects of salt in our soil from winter road maintenance. That's a lot to expect from simple calcium sulfate aka gypsum. Is it true? Will it do all those things? The answer to these questions is yes and no. As with most things, one size does not fit all. Most folk wisdom, while very good for the situation and place it originated, is not universally true. Let's examine these statements and see how they stack up for our northern gardens.

First, the dog urine thing: if it works for you, use it. I have friends who swear that when they sprinkle it on the spot where the dog urinated, it prevents browning. In my case, we had 5 dogs in the family when we left the farm and moved into this urban garden. The children each had responsibility for one and I can guarantee that no one ever put anything on any of the spots. Our less-than-5000-square-foot lawn has never shown the first brown spot without gypsum. We are now down to one dog and we still have no spots in the lawn.

Next, correcting the pH of the soil depends on the pH of your soil before you apply it. Gypsum will lower an alkaline pH (over 7.0) toward neutrality, but not below. If your soil pH is high enough to need lowering, the best alternative will be sulfur, but don't go down that road without having your soil tested so that you know where you are starting and how much it will take to achieve your goal. Many gardeners believe their soil is acid and that they need to add lime because they have spruce or pine in their yard. In Minnesota more soils are neutral to alkaline than the other way round. Test and be sure.

Third, gypsum is widely recommended and used for amending heavy clay soils, but here is one of the cases where the folklore is not always true. Gypsum will help with the soil texture in clay soils that are low in calcium. In that case, the calcium in the gypsum (calcium sulfate) will displace the sodium in the clay forming a new more open bond between the soil particles. If the soil is not low in calcium, this reaction cannot happen and your effort is wasted.

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