I used to work on a mushroom farm and as far as I know mushroom compost
is sterelised horse shit and straw. When we got it delevered it came in
700 plastic sacks (bin bag size) per shed to which we added 2-3 cm of a
peat/gravel mix on the top of each sack.

That's what I thought ... well-rotted, horse manure + a bit of this and
that.

- h