Toxicity of Gypsum: Some other folks are still teaching about “gypsum toxicity.” Facts:
1. There has to be approximately 20,000 ppm Ca2+ in the soil for calcium toxicities to exist…
2. There has to be approximately 800 ppm SO42- in the soil for sulfur toxicities to exist.
Both are extremely rare in productive, agricultural soils, even soils where gypsum has been applied in extreme excess.
Solubility of Limestone vs. Gypsum:
The other gypsum myth I have heard is that as soil pH conditions reach values near 7.0, agricultural limestone becomes is a better choice for adding calcium to the soil rather that gypsum or anhydrite. First of all, agricultural limestone is always 150 times less soluble than gypsum or anhydrite under any conditions. Gypsum/anhydrite is never in any soil reaction (pH) or oxidation/reduction condition any less soluble than this.
However, note that all soluble soil calcium becomes insoluble at pH values near 8.0 So at these pH values, calcium is basically nonexistent in the soil for promoting good soil structure, and as an essential plant nutrient. This is why we need to keep the soil pH values for most soils in California and the west at pH values about equal to 6.4.
Much more on this subject later….
And please write me with your gypsum/anhydrite/limestone questions…