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Gypsum 101

Posted by succulent_succotash 10 (My Page) on
Tue, Jul 9, 13 at 20:34

I am a newbie gardener and need some help understanding gypsum in Al's Gritty Mix. My intention is to use the complete gritty mix in small pots for succulents. But, I'm sure this applies for all container gardening.

I bought all the ingredients for the gritty mix with the exception of gypsum.

Questions about Gypsum:
1. Is it absolutely necessary to add gypsum to the gritty mix?
2. Can you mix gypsum into the entire batch and let it sit? Or should I add on an need-more-soil basis? (I have a non-sifted 50lb bag of Turface, 75lb granite, 1.5cut ft bag of fir bark)
3. How often do you need to add gypsum to the gritty mix?
4. When "refreshing" soil, do you simply add to existing mix, or do you have to dump out old soil and replace with new mix with gypsum?
5. Do you have to use epsom salt in gritty mix, too?
6. What are some common brands of gypsum? Based on other threads: CalMag, B-1, Foliage Pro...

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Here is the information I gathered from other threads to consolidate:

-1 tablespoon of gypsum per gallon of gritty mix
-Gypsum is Calcium Sulfate (CaSO4)
-It is not the same as Epsom salt (magnesium)
-It serves as a fertilizer in the gritty mix
-You can buy at select nurseries, big box stores, and Amazon.com, hydroponics stores
-Some may use Plaster of Paris as a substitute, but opinions are split on this

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Definition: Gypsum is a mineral consisting of hydrous calcium sulfate that is used as a soil amendment to add sulfur and calcium and counteract salt damage. It is added to cement to regulate setting.

Source: http://glossary.gardenweb.com/glossary/gypsum.html

It is also used to loosen up stubborn, compacted or clay soils. Gypsum works by pulling together clay particles in the soil to make bigger particles, creating porous spaces for air, water and plant roots. For saline-infused soil, gypsum removes sodium and replaces it with calcium. For all soil types, gypsum adds calcium and sulphur, which are necessary elements for plant growth. Gypsum also helps soil retain water and helps decrease soil erosion.

Source: http://www.homedepot.com/webapp/catalog/servlet/ContentView?pn=Lime_Gypsum&storeId=10051&langId=-1&catalogId=10053

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Thank you :)


Follow-Up Postings:

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RE: Gypsum 101

Here are answers to a several of your questions...

1. No. Gypsum is only necessary if your fertilizer doesn't supply calcium.
2. You can go ahead and mix the full batch and let it sit.
5. No. Like with gypsum, epsom salt is only necessary if your fertilizer doesn't supply magnesium.
6. Brands will probably vary regionally. Try the lawn fertilizer section at Lowes -- the bags will be big, but they're very affordable. Foliage-Pro isn't gypsum. It's a soluble fertilizer that provides all the macro and micronutrients (including calcium and magnesium).


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RE: Gypsum 101

S...you are on the right track. We are all doing science experiments...but often times these issues are not clearly defined scientific formulas. If you know what I mean....Certain potting ingredients (like bark and peat) make the mix acidic. Calcium neutralizes the acidity for plants not wanting an acidic medium. Hope this helps.


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RE: Gypsum 101

While what fireduck says is true with regard to adding lime to the 5-1-1 (bark/peat/perlite) mix, it doesn't apply to the gritty mix (turface/granite/bark). Al's gritty mix has a pH that's suitable for most plants, which is why he uses gypsum (a calcium source that, in this context, doesn't affect pH) rather than lime (a calcium source that raises pH).


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