Let’s take a second to review the two major forms of gypsum in the marketplace today in lawn care. The first form is what I mentioned above, calcium sulfate di-hydrate (CaSO4*2H20) and the second form is simply calcium sulfate (anhydrite) (CaSO4). You’re probably thinking, ‘what’s the big deal, gypsum is gypsum and after all it’s only a commodity.’ But there actually is a huge difference. One is very fast acting and extremely water-soluble; the other is not. The di-hydrate form is what you need to look for. This type of gypsum is already infused with two extra molecules of water making it easier to break down. This makes the nutrients available to the plant as soon as it dissolves into solution; with some brands that’s only a matter of minutes. The anhydrite form does not have any water associated with it and therefore is very hard to break down into solution, sometimes taking years to begin working.
Now that we know the two major forms of gypsum on the market today; now you should know how to figure out which type is which. The info can be found on the back of most bags. The following is what you want to see:
Calcium sulfate di-hydrate (CaSO4*2H20)……92%
Derived from naturally mined calcium sulfate di-hydrate.
Don’t let the levels of calcium on labels fool you. The anhydrite forms will contain upwards of 30% calcium, but they are not soluble and do not break down fast enough to be effective. If the calcium is not available to the plant, it doesn’t matter how much calcium or sulfur is in the gypsum.