Assorted rottable garbage also improves soils, so if you dig it up to add gypsum, you could add rubbish under the surface too. Unused plaster will bind the soil, used lumps wont.
I don't really know the answer to your question, but I would warn you not to use gypsum if you want lime-hating plants such as heathers, rhododendrons etc
Run it through a cement mixer and a couple of rocks as a ball mill. So long as you crunch it down enough to go through a sieve, it's fine. Even just soaking it in an excess of water would be enough (this is plaster, not cement).
I wouldn't add "rottable garbage" to soil either, it's much better to rot this down separately. Otherwise you'll be short of nitrogen and lawns in particular would suffer badly.
Our clay overlies enough chalk, and though it helps, it does make the soil excessively alkaline.
We have found copius additions of orgamic material - peats, topsoils and manures - and sand, to be a better bet.
Its back breaking work as well. Hire a rotovator or small digger to break it up and mix it in.
As previous poster says, none really, except it sets hard and can make a mess of drainage in a particular area. The same is probably true of pure gypsum.
I've got areas of lawn that I simply left cement and plaster tailings under and heaps of builders sand, They don't fare well.
The easy way of of making flower gardens in clay, is mulch. That will decompose into decent topsoil, and you can pull annual weeds out of it. Wood or coca shell. Or peaty compost.
Or if its for vegetables, cheat: We made raised beds and filled them with gravel sand and topsoil. After breaking up the underlying clay pan just a little.
I've got one of the gypsum mines and processing facilities round the corner from me too. Ironically the dump bang over the road is unable to accept plasterboard for recycling(!)
I must admit - I don't know if British Gypsum do recycling at the Mountfield/Robertsbridge plant. Though if they can take crap out the ground and turn it into plaster (they do all that here) I would have thought they would have added a preprocessing feed for doing whatever you need to old PB and sticking the result of it in with the raw materials from the mine.
This is a TBE and a FQM (Totally Baffling Effect and Frequently Quoted Misunderstanding). You are chemically correct, but biochemically incorrect.
It isn't helped by the fact that the terms "acid soil" and "alkaline soil" are so misleading - which is the TBE. The reasons that those plants dislike lime has nothing to do with the acidity, but the fact that they have difficulty absorbing iron, and calcium interferes with one common form of plant's lime absorption. Sorry, I don't know the details. Magnesium doesn't have the same effect, for arcane chemical reasons, that I don't understand, either.
Chris, if you can find a Farmers Shop (not a Farm Shop) they may well sell Agricultural Gypsum in 25kg bags, which is what I bought. Mine was old stock, a bit lumpy, and I paid £5. each.
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