echinosum wrote:
[...]

I think we need to understand why you think you need to add topsoil to
your vegetable plot, what's there already?

Agreed. People seem to get misled into buying topsoil (or, I'm afraid,
"topsoil") far too often these days: they rarely need it. "Turf.
Rotovators. Landscape fabric. Decking. Blue fences...Mutter, mutter...
telly makeovers...mutter, mutter...ripoff...mutter..." /Old fart mode

Mushroom compost is a soil conditioner, like peat, improves the
texture and moisture holding ability. It actually has few nutrienst
because those were exhausted by the mushrooms. Ideally it would be
mixed in with the soil, though I have heard if you use it as a deep
mulch the worms will do that for you over time. If you are lazy you
can buy conditioned topsoil from some suppliers, ie, premixed.

I'd usually vote for mushroom compost alone, and let the worms do the
work. Probably loosen the top layer of the existing soil to show
willing. Then add nutrients at the appropriate time.

--
Mike.