I try to fill the bottom with something besides black dirt. I like to use junk wood if I can find it like cottonwood, boxelder, willow, etc. Anything but pine. If I can't get that, I'll use whatever decomposable material I can find such as square bales, brush from tree trimming and the like. If I were in town I would go look at the recycle place for branches and other woody residue. I don't like leaves and lawn clippings as they break down to fast and there are chemicals in the grass clippings I don't want my veggies in. For me out here in the sticks, after the wood, I like to fill the bottom with composted manure from the neighbors cattle. The price is right and I can get as much as I want. I leave the bottom of the bed in direct contact with the ground. That way worms and other necessary critters can find there way into the bed to do their thing. Good Luck!!