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Gypsum for calcareous soil
Do the fizz test to determine whether you have calcareous soil.
August 26, 2015
5:33 pm
Near Meadview, between Lake Mead and Grand Canyon West (Skywalk)
Forum Posts: 177
Member Since:
May 6, 2015
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The fizz test: 

Squirt some vinegar on a small sample of your native dirt.  If you hear it sizzling and it bubbles, you have calcareous soil.

One of our local gardeners had poor results until he amended with gypsum after he got a soil test since we have so much excess calcium in the soil.

Our native dirt is extremely compacted (we now need to use a PICK to loosen up dirt piles that were excavated in spring before we can shovel it).

A comprehensive publication about gypsum by the University of Arizona Extension:

Using Gypsum and Other Calcium Amendments in Southwestern Soils, Dr. James Walworth, Revised 08/12

A few excerpts:

...Gypsum is a good choice for Ca addition because it is inexpensive, non-toxic, and safe to handle, and it is relatively
soluble. We are interested both in solubility (how much of the salt will dissolve in the soil water) and the rate of dissolution
(how fast the salt dissolves in water). Mined gypsum is well crystallized, having formed over millions of years. Waste
gypsum, on the other hand, is formed rapidly during industrial processes, and is less crystallized. Although they have the same
chemical formula, the waste gypsum materials dissolve more rapidly than mined gypsum. Sometimes powdered gypsum
is prilled in order to reduce dust and to improve handling properties, and this slows its rate of dissolution.

A study that compared dissolution rates of gypsum sources found that flue gas gypsum dissolved 3.6 times faster than mined gypsum, whereas phosphogypsum dissolved 2.2 times faster than mined gypsum. The rate of dissolution is particularly important for treatment of soil crusting, which is caused by dispersion of clay particles at the soil surface. In this situation, rapid dissolution is critical to maintain a high level of dissolved Ca2+ in the surface soil as raindrops or irrigation water leach cations from the uppermost layer of soil. However, for general treatment of soil structure, the rate of dissolution is less important than the overall solubility. ...

I've been buying my gypsum at Star Nursery, the only place in Kingman where I found OMRI listed mined gypsum.  It's a nice slightly pinkish very fine powder and costs about $6.50 for I think 40lbs.  

Star has another brand that costs over $30 because it is fast acting.  I checked the label and it had identical ingredients.  I asked what the difference is, but they had no clue.  A couple weeks later I got more gypsum and inquired again with the same guy and he still had no clue.  And that's just how it is these days -- cheap help couldn't care less about what they sell.  Very disappointing for a "local" Nevada nursery.  You might as well be at Walmart or at the Home Depot.

I've read about people putting drywall scraps in their gardens, but that's not for me.  See about toxic drywall.  Most of the drywall in our house came from Lowe's in 2006.  We've been using adobe since 2008, a much better choice for many reasons.

So, I'm certainly not paying 5 times more for what might be flu gas gypsum or whatever industrial waste!

As an organic grower, I wouldn't use it if it was free.

Currently there is the cheaper "Arizona's Best" gypsum at the Home Depot, but it's not OMRI listed and I've heard reports of people finding pieces of drywall in it.   No thanks!

The university publication contains a nice chart that tells you how much gypsum to apply per acre and per 1000 sqft.

I'm going to contact Dr. Walworth to see if he has recommendations for a soil testing lab specializing in our calcareous soil (our pH is 8.2 to 8.7).

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