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Gypsum vs agricultural gypsum

Posted by nooba none (My Page) on
Fri, Apr 18, 14 at 22:54


I´m trying to get a Calcium source for some soil mixes i´m experimenting right now and the recommend source of calcium, that shouldn´t mess up my ph level, is gypsum.

My problem is that i don�t know what tha is?
I�m in Portugal, and i believe it some sord of plaster but i�m not really sure about that. So i googled it, and i found that it is widely used in Brazil (im searching in portuguese) and the call it agricultural plaster (my translation of "gesso agricola").

So i google that and nothing, went to my local shop of fertilizers, and they never heard about that!

So my question is: Is gypsum that ordinary white powder in ordinary plasterboard? or is it something diferent that can be called "agricultural plaster"


Follow-Up Postings:

RE: Gypsum vs agricultural gypsum

Plaster of paris is also calcium sulfate but a dehydrated form. When mixed with water, it forms gypsum. However, this would have to be milled back into a powder. Therefore, if you applied plaster to moist soil, it would absorb the moisture and form large clumps of gypsum that would take forever to dissolve. So try to find true gypsum, i.e. CaSO4.2H2O, or what you refer to as "agricultural plaster".
If you only need a small amount, you can use blackboard chalk, which is actually gypsum and unrelated to the actual mineral chalk, which is calcium carbonate. To test this, mix some blackboard chalk with vinegar. If it doesn't fizz, it's gypsum.

This post was edited by Slimy_Okra on Fri, Apr 18, 14 at 23:25

RE: Gypsum vs agricultural gypsum

Gypsum is a mineral found in some places. How it is prepared determines its end use with some being processed for use as wallboard, plasterboard, sheet rock, drywall, and some more being processed for use in growing things, agricultural gypsum. Essentially the same stuff except the gypsum used for interior walls has additives that make it not too desirable in soil. Gypsum, CaSO4.2H2O, and lime, or Calcium Carbonate CaCo3, are
two different minerals. Everything I have found about blackboard chalk tells me it is CaCo3, Calcium Carbonate not gypsum.
As near as I can tell Gypsum is readily available in Portugal since it is the base of many murals, gesso, painted in many of the churches and public buildings there.

RE: Gypsum vs agricultural gypsum

thanks Slimy_Okra,

If i understood well, i´ll have gypsum if i:

1. mix plaster of paris with water and then mill it back to powder.


2. make powder from blackboard chalk.

is that it?

One more question, whats there a diference between plaster and plaster of paris, in terms or uses?


RE: Gypsum vs agricultural gypsum

You need to try other places that sell fertilizers. Gypsum is a VERY common source of calcium and sulfur for anyone who grows crops of cabbage, mustard, kale, or other Cruciferous vegetables.

RE: Gypsum vs agricultural gypsum

  • Posted by ericwi Dane County WI (My Page) on
    Sat, Apr 19, 14 at 10:44

Here in the USA, plaster is anything that you smear on your walls, and plastering is the act of applying plaster, to the walls. The plaster can be made from mud, clay, slaked lime, manure, iron oxide(for color), and de-watered, powdered gypsum. Gypsum that has been prepared for plastering is finely ground, it has been heated to drive off the water, and it is white in color. Since it has been processed, it will cost more than gypsum that has been coarsely ground for agricultural application.

RE: Gypsum vs agricultural gypsum

I thank you all for all your inputs, but the problem really is to know how people call gypsum here. I can´t believe that it is not available, like kimmsr said, there are a lot of churches, and "gesso"(i believe this is plaster) is very well know here, but i don´t know if it is ready for agricultural needs.

we call "gesso" for two things cardboard- "gesso", most famous brand is Pladur (i think it´s like drywall) and gesso when you broke an arm or leg. I don´t know if it the same or not, or even if any of those is gypsum?

I do believe too that it is not common to use it in soils , don´t know if it because the ph is two low arround here, as i searched in many garden centers none of them sells it. Everybody recommends me to add lime if i need calcium, or even oyster shells, but that will ruin my ph levels, and i want to keep it low, as it is now.

One thing i found was Calcium superphosphate which is P-18% Ca - 10% and SO3 - 27%, will this be ok if i cant find gypsum? If the awnser is yes, the only problem is that it will mess arround with my ferlilizer for acid loving plants that already has P- 5% and S-25%.

The other thing that it´s confusing me is that in brazil, like i said before, the call it agricultural plaster, i wonder what they call it here... any thoughts??

RE: Gypsum vs agricultural gypsum

Calcium superphosphate is actually a combination of monocalcium phosphate and calcium sulfate (gypsum), so it's the next-best thing you can use if you can't find pure gypsum.

It should not affect the pH much. I also wouldn't worry about the extra phosphorus - most excess P ends up being locked up anyway.

And yes, your other option is to combine plaster with water, let it harden and dry, and then grind it back into powder. I would just use the superphosphate instead of going to all that trouble.

RE: Gypsum vs agricultural gypsum

"Therefore, if you applied plaster to moist soil, it would absorb the moisture and form large clumps of gypsum that would take forever to dissolve. "

In fact I have applied old bags of finish plaster to soil many time and never had that happen. Yes, if you dumped it in a pile, but if the material is still fairly dry then it is quite easy to broadcast. Plastering materials usually are about equal amount calcium sulfate and calcium carbonate. An excellent amendment for acidic soils.

Nooba, do you have a soil test? If Ca levels are adequate, there is little reason to add gypsum. If you are trying to lower Ph compost is probably a better method
unless dealing with a very large area.

RE: Gypsum vs agricultural gypsum

Nooba is making his own potting mix. Thus the need for an added source of calcium. A peat based mix may be fine with regular lime, but if he's trying out something with alternative ingredients, gypsum can provide the required calcium without affecting pH.

RE: Gypsum vs agricultural gypsum

Ah, I guess I should have read the OP better. On such a small scale a bag of plaster will do the trick, then.

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