I know the soil should be at least 6-8 inches deep. I did a type of raised bed myself for a vegetable garden. I started by adding a few bags of top soil, about an inch worth (the top soil was a little cheaper than the manure and the compost) I then spread manure on top of the top soil and mixed in a few bags of compost with the manure. So I have mostly manure with a little compost mixed in. It seems to be working well so far. My only problem is keeping the insects away from my plants!