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…

12 thoughts on “Gypsum Facts”

  1. Hi. Thanks for the great information on gypsum. There is so much nonsense out there, but your information is absolutely right on! I wish everyone out there would read this. Thanks, again.

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