we did a mix of topsoil, vermiculite, mushroom compost, and peat moss. the square foot gardening sites all recommend the 3 things that aren't topsoil, but the guy at home depot convinced dhtoy that he HAD to have topsoil. so we added that too.
For our soil-less salad tables we used vermiculite, perlite, and peat moss. I never imagined it would work, but they're doing wonderful.