building piles on the surface rather than in a trench

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.

Without knowing where you are or the climate you are in. . .

I would go look around abandoned parking lots or other paved over places. The weeds growing there don't need much soil usually. But they might not like the loose structure of leaf piles over wood and sticks. If they don't take, maybe make compacted seed balls or if the lot is really abandoned, crack off some concrete with the plants roots inside.

If there is a good sized forest near you, try looking for weed seeds out of the deepest piles of old leaves, or on the edge of a tree fall. But I have to say please be respectful of number of plants you need to disturb to get the seeds, it is not the same as taking plants/seeds from an abandoned building lot, IMHO.

Could you plant sprawling plants next to the pile and train them over it? Sorta like layering a plant for propagation, but layering them in place.

Does moss grow in your area? While it wont build soil fast it does give weeds a place to take root on bare rocks., you could put down chunks and plant some seeds into the moss.

What about putting (peat pot) potted plants into the bed and letting them move out from their starting soil.

If you just left it for a year, I bet there would be something growing over it by the end.

I ended up digging a shallow trench for the branches, covering them with rotted leaves and grass clippings and piling the dirt back on top. I planted onion sets in the raised bed.

Shannon, I have ten acres in a forest of aspens. There are tons of my trees that have died over the last hundred or so years and are being composted by mother nature. The ones that are on the ground begin to soak up water and are inhabited by all sorts of crawly insects and such. They slowly break down , very slowly. So i would think that your hugels would slowly break down too but without being covered ,it may take a while. Adding grass and leaves on top would start a sort of compost pile which would serve the function of soil, I would think. Your idea of putting buckets of dirt here and there could be a good one. Please try that and let us all learn from your experiment!