Actually, based on my observation last year with some hugel pots, they don't need a lot of soil and the seem to do better with dry, not rotted wood.
The logs wick water down to roots right above the bottom 3-4" soil layer. So the bottom roots get a slow nutrient drip in a wet aerated soil layer. The roots do great in this layer.
I actually think that after the wood rots a lot, the pots will perform less well, but we'll see in a few years.
http://lowcostvegetablegarden.blogspot.com/2012/09/eggplant-stump-branch-pot-comparison.html