hugelkultur beds in poorly drained soil

I have two 4' x 25' trenches, about 4 feet apart, that will be slowly filled with freshly cut sweetgums and a huge willow oak that fell down a few years ago. To me, an amature, the soil in this particular spot seems to be of very poor quality with almost no topsoil. To quote from a soil report used for septic system leach field evaluation, "Soils on the property were derived from a mix of intermediate rock parent materials (gabbro) and Carolina slate rock (silty soils)...soils contain expansive clay mineral subsoil textures." So, I don't think that water that seeps into my hugel beds from above will filter on through, is this OK? These beds are at the top of a slope, so a drain could be devised, but is it really necessary? Also, I don't think the dirt that came out of the trenches should be used to cover the wood, it looks like something that was slate rock millions of years ago that has now degraded to slate that I can crush easily with my hand, maybe even work into a clay if I added water and squished it enough. Should I bring in dirt, and if so, what kind? I could put the hugel beds in an area with better soil if need be, but I wanted to take advantage of the hugel technique and make an area that is otherwise not useful, useful. Also, trees seem to thrive in this same soil, with some huge shagbark hickories, many cedars, oaks, sweetgum, pecan, dogwood, sycamore, persimmon, maple, etc. I want to grow a large variety of vegetables in these beds.

I used rather poor subsoil to put on top of my buried wood, but fortified the top layer with old sheep and chicken manure and bedding, with a final layer of sifted topsoil. Not sure if you want to go to that much trouble, but personally I would hesitate bringing in soil, because you just don't know what might be in it in the way of contaminants and weed seeds.

I've found that vertically buried stumps help with drainage at least the first year.
Sounds like you need some organic amendments also.
I dug in lots of wood chips into my clay soil with great results.
You might keep your top 8" to 1' of your bed as just soil, i.e. not wood, branches, or stumps.
Then you can dig in amendments as needed. You could experiment, add wood chips to one part,
other stuff to other parts. Agree with Tyler about not bringing in soil.
I personally don't like to import anything except wood into my garden.


http://lowcostvegetablegarden.blogspot.com/2012/07/wood-chip-bed.html

So, Tyler and Eric - you both mentioned fortifying the soil that will be in the most direct contact with the vegetable roots, with manure or wood chips. Will the addition of these amendments transform my bad soil into something new, or will I now have chunks of good mixed in with chunks of bad? Also, just to be clear, you think I'll be ok just putting that bad soil directly on the lower levels of the hugel bed? What impact would a saturated hugel bed have on my vegetables? Thanks.

incredible blog, Eric. I sent a link to my wife and she is super happy with your step by step explanations of your experiments. thats going to help us a bunch.