I started a hugelkultur bed today, by piling up branches and wood logs in a pile about five feet wide and ten feet long. I have a truckload of leaves and grass clippings to pile over that; then I have to figure out what to put on top. I didn't excavate the spot before building the pile, so I don't have any soil. I am thinking of getting a load of leaves from the city leaf dump to put on top of the leaves and grass clippings. Those leaves have a bit of soil mixed in with them, because of the front end loaders scraping up soil as they push the piles around at the dump.
My question is: how thick does the layer of soil on top have to be? I can get some soil from elsewhere on the farm, but as I don't have any machinery, I will have to do it by hand, in buckets, which would be very slow. What if I just left the leaves and grass clippings to rot down on top of the wood? Would that work? Or do I really, really need some soil?
Here's another idea: I could have pockets of soil in the rotting leaves and I could plant stuff in those pockets of soil. That way, I would not have to cover the entire surface of the hugel with dirt. I could just put it here and there. I know from experience that eventually the leaves will rot down to humus (in about a year) and things will start growing in that humus spontaneously.