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.

thanks Slimy_Okra,

If i understood well, iôll have gypsum if i:

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

or

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?

thanks

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.

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.

I thank you all for all your inputs, but the problem really is to know how people call gypsum here. I canôt 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 donôt know if it is ready for agricultural needs.

we call "gesso" for two things cardboard- "gesso", most famous brand is Pladur (i think itôs like drywall) and gesso when you broke an arm or leg. I donôt 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 , donôt 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 itôs confusing me is that in brazil, like i said before, the call it agricultural plaster, i wonder what they call it here... any thoughts??

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.

"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.

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.

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