On 7 June, 09:31, NT wrote:

Unused plaster will bind the soil, used lumps wont.

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.