If considering a purchase of calcium sulfate dihydrate (gypsum), it is important to understand the many varied benefits and when they take effect in agriculture. As both a soil amendment and a crop nutrient source, gypsum will remediate sodic soils, help manage saline irrigation waters, provide calcium and sulfur as crop nutrients, and improve soil structure. All of these benefits occur at different rates.
I invited a friend to a gypsum symposium in Kansas this summer because I knew he had tried it, and he has an open mind when it comes to trying new ideas. However he declined and reminded me, “I haven’t had the best results from using gypsum in the past.” I recall a conversation we had about gypsum, and his expectation was that a gypsum application should create a response just like applications of nitrogen, phosphorus, or potassium. His was an unrealistic expectation, and my telling him so didn’t undo his perception.
Gypsum works to improve drainage in soils dominated with dispersive clays in four ways. One is immediate, one nearly immediate, the third occurs with a flush of root growth and soil biological activity, and the last is a change in structure.
First – When gypsum dissolves into soil water, it helps water move more freely. That’s an immediate effect, and it can be substantial as it improves drainage. Since gypsum doesn’t dissolve immediately, the effect is persistent. If surface ponding is an issue, gypsum should be concentrated within the first few inches of soil.
Second – Calcium displaces sodium on the clay’s exchange sites and reverses the clay’s tendency to disperse. As flocculation and structure improve, drainage improves. This can’t happen until the gypsum has gone into solution and ion exchange occurs. The effect isn’t immediate but will happen during the season it applied. This benefit is even more persistent than the salt effect above.
Third – Once drainage improves, root growth renews, and the biology of the soil kicks in. The root flush occurs before the top growth flush can occur. This will occur in-season.
Fourth – Lastly, the process by which gypsum amends the soil is somewhat slow. It can take anywhere between 3 months and 3 years depending on the clay and organic content and the initial starting point of the soil. You might have to amend the soil for a few years to get the desired results.
If you apply gypsum to improve soil structure, set the right expectation. By 3 to 5 years after a large application of 1 to 2 tons or multiple applications of 1,000 to 2,000 pounds you should begin to see improvements in tilth.
Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at email@example.com or call 402-649-5919.