Category Archives: Soil/Water Science

More Gypsum Facts

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:

Gypsum--Hydrous Calcium Sulfate

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…

Gypsum Facts

I have written and lectured a lot about the benefits and uses of gypsum products in the past [see the publications tab to your left].  Gypsum [and its more concentrated and prevalent form, anhydrite] remains the “miracle” amendment…it has been documented by many scientists as having over 40 (yes, forty!) beneficial uses in production agriculture.  Now, as time moves on, I’ll talk more about the benefits and uses, and dispel many myths about gypsum usage vs. that of agricultural limestone, etc…. .  For now, here are some quick facts about “gypsum.”

Gypsum Rock

There are two calcium sulfate minerals found on earth: the first is called gypsum.  The chemical formula for gypsum is (CaSO4·2H2O).  The second/much more prevalent calcium sulfate mineral on earth, is anhydrite (CaSO4)…almost identical to gypsum, but sans the two molecules of water.  By nature, gypsum with its associated two water molecules is 21% water, and 79% solids.  Calcium sulfate in the pure anhydrite form (no water) is 29.4 percent calcium (Ca) and 23.5 percent sulfur (S), while pure gypsum with its water associated in the molecular structure (CaSO4·2H2O) is approximately 23.3 percent Ca and 18.5 percent S.  However, agricultural anhydrite and gypsum, as a soil amendment/conditioner/fertilizer usually has other impurities, so grades are approximately 22 percent Ca and 17 percent S for a decent quality product.

There has been some serious confusion concerning the solubility of calcium sulfate products. The fact is: both anhydrite and gypsum are indeed water soluble, and both essentially have the same solubility: 0.205 grams per 100 grams water (2003-2004 CRC Handbook of Chemistry and Physics).  This has also been verified by many field trials by Soil Scientists [including myself], worldwide farmers and others.  The solubility of 0.205 grams per 100 grams water equates to:

0.205 grams/100 grams water         =  0.0171 pounds per gallon

=  5,575 pounds per acre foot water (325,850.58 U.S. gallons)

Therefore: the maximum solubility of anhydrite or gypsum in one acre-foot water 23.8 milliequivalents per liter (or 5,575 pounds per acre foot water [or basically 12″ of rainfall will dissolve approximately 2.8 tons of agricultural gypsum/acre)

Gypsum and anhydrite are the neutral salts of a strong acid and strong base and do not increase or decrease acidity.  Dissolving gypsum and anhydrite in water or soil results in the following reaction: (CaSO4·2H2O)  =   Ca2+ + SO42- + 2H2O.  They add calcium ions (Ca2+) and sulfate ions (SO42-), but do not add or take away hydrogen ions (H+). Therefore, they do not act as a liming or acidifying material.  The Ca2+ ions simply interact with exchange sites in soil and sulfate remains dissolved in soil water.

More on gypsum this coming Monday March 8. Please send me your gypsum, limestone and other questions…