Air prune bed filler is (roughly):

-bottom wire
-~1/2" of wheat straw
- couple inches of aged chicken bedding
-inch or two cow manure mixed with dirt
-6" of potting soil
-2 layers of walnuts
-~2" of potting soil
-layer of straw. 

Thought process behind that layout was that I want to reuse this bed for a long time with minimal work. The manure will break down and feed the seedlings over time, insulation will help keep everything cool in our brutal summers and keep soil life alive. 

As I pull and sell seedlings, planting new ones is as easy as raking the bed level with my hands, adding new nuts, cover with soil, cover with straw. By the time I retire it, if ever, I'll have enough time and money (Lord willing and the aquifer don't dry) to have built 10x the production space.