By Kelly Robertson

There doesn’t seem to be a week or even a few days that go by that I don’t get a question on gypsum. Gypsum does have some good points when managed properly and used properly, but it can do more damage quickly when used wrong. I am beginning to see some red flags show up in fields where repeated gypsum applications have been made: fields that are low in pH, moderate in BpH and have a high percentage of Ca and a low percentage of Mg and K. Gypsum itself is not bad when used properly and all other soil test elements are monitored.

Gypsum is calcium sulfate dihydrate (CaSO₄·2H₂O), sourced either from mined deposits or as a byproduct of flue-gas desulfurization at coal plants. Agronomically, it’s valued as a source of both calcium and sulfate-sulfur, and unlike ag lime it’s pH-neutral — it won’t raise soil pH, since it’s a neutral salt rather than a carbonate. But it will raise your calcium levels in the soil, which can cause issues if not monitored. I don’t know how many times I hear that gypsum adjusts pH. It does not do a thing to pH.

In row crops, it’s used for three main things:

1) Correcting sulfur deficiency (increasingly common as atmospheric S deposition has declined)

2) Improving soil physical properties — flocculating clay to reduce crusting and surface sealing and improve water infiltration (Usually black, high magnesium, lakebed or river bottom soils, but not clay knobs or silt loam soils.)

3) At high rates, it’s also used to reclaim sodic soils, since the calcium displaces sodium on exchange sites so the sodium can leach out as sodium sulfate.

Rates vary a lot by goal: a couple hundred pounds per acre for S correction versus a ton or more per acre for soil structure or sodic remediation work. The risk with corn, wheat and soybeans comes mostly from cation competition and leaching, not toxicity. Calcium, magnesium and potassium all compete for the same exchange sites and the same root uptake sites, and calcium generally wins that competition. Over-apply gypsum and you push soil Ca levels up enough that Mg and K uptake gets suppressed even when soil test levels for those nutrients look adequate. This shows up as induced Mg or K deficiency, and corn and soybeans are both sensitive.

Sulfate is also a mobile anion; it pairs readily with Mg²⁺ and K⁺ in the soil solution and can carry them down past the root zone in leaching rain events, which is a real concern on sandy or coarse-textured soils. On phosphorus, the mechanism is different — excess soluble calcium in solution can react with phosphate to precipitate it as insoluble calcium-phosphate compounds, tying up P and reducing availability, an effect that’s more pronounced on high-pH or calcareous soils. High rates or repeated applications of gypsum push soluble calcium concentration in the soil solution up sharply — gypsum dissolves readily, so a heavy application (think tons per acre rather than a few hundred pounds) floods the soil solution with far more Ca²⁺ than plants need or than the CEC can buffer.

That excess Ca²⁺ then reacts with phosphate ions in solution (H₂PO₄⁻ and HPO₄²⁻) to precipitate out as calcium phosphate minerals — starting as more soluble forms like dicalcium phosphate, but able to age over time into less soluble compounds like those found naturally in calcareous soils. This reaction is favored more at higher soil pH, since phosphate shifts toward the HPO₄²⁻ and PO₄³⁻ forms that bond more readily with calcium into insoluble salts. So, the risk compounds on ground that’s already neutral-to-alkaline. The practical effect is that some portion of your soil P gets converted from a plant- available form into a mineral form that roots can’t easily access — it still shows up in a total P test, but it drops out of the labile pool that Bray or Olsen soil tests measure and that corn and soybean roots can actually draw from. It’s not instantaneous; it plays out over the season as Ca²⁺ stays elevated in solution.

On the sulfur side: gypsum delivers sulfate-S directly, so it’s plant-available immediately with no dependence on soil biology, temperature or moisture. Elemental sulfur (S⁰) has to be oxidized by sulfur-oxidizing soil bacteria (Thiobacillus and related genera) through intermediate forms to sulfate before a plant can use it — a process that takes weeks to months depending on soil temperature, moisture, microbial population and how finely ground the sulfur is (finer particles oxidize faster). That oxidation also releases hydrogen ions and acidifies the soil, which is a common reason elemental S is chosen deliberately — to lower soil pH — whereas gypsum has no acidifying effect at all.

We have fields that are beginning to show elevated Ca levels to a point where they could be limiting P, K and Mg uptake. Some fields with repeated high applications could be binding those elements and suppressing them or making them unavailable. When we get to this point, there is no way to fix it other than time – a long time. The soil must repair itself. There are no magic products or treatments to fix it. This is why I am diligent in monitoring and limiting gypsum applications, especially on soils that are on the border line of a train wreck.