What instructions did you rely on for your bin design? Dirt isn't necessary in a worm bin and you certainly don't need to buy any kind of potting mix product. The advantage of shredded paper etc. as bedding is that it is free to obtain and will continue to allow air to permeate throughout the material.

I think you need a procedure in place for harvesting vermicompost and separating it from the worms. It appears from what you said that you relied on some instructions for creating this worm bin but did not follow an instruction that said to put landscape fabric over the gravel layer. I believe the intention of the landscape fabric would be to prevent the castings and vermicompost from filtering down among the gravel which is supposed to serve as a drainage layer. The gravel will serve no purpose at all if nothing is maintaining the air spaces around it - but I doubt, in any case, that landscape fabric would keep the worms from going down among the gravel and filling all the spaces in it with castings. The end result will be a muddy, dense mixture of vermicompost, castings, worms, and gravel on the bottom of the container, which you will need to devise some way of separating out.

jldodson, I think parsley is correct in saying that there is no need for the earth or the rocks at the bottom. Without landscape fabric, or something similar, I think the worms and their castings will fill in the voids and make harvesting very difficult. The good air circulation you are probably trying to achieve (with the gravel) would probably be limited.

For what it's worth to ensure good aeration, this is what I did:

1. built a box approx. 22"x32" using 1/2" plywood

2. using a hole-saw I drilled two holes in each end, just above the plywood base

3. installed two lengths of black ABS pipe (1.5" diam.) end-to-end through the holes and long enough to extend about 1/2" past the ends of the bin.( ABS pipes had numerous 1/4" holes drilled through the sides along the entire length)

4. Then I put aspen chips in the bottom - just enough to cover the pipes and covered the entire bed of aspen with landscape fabric.

5. Finally I added lots of damp shredded newspaper, the worms and some food scraps.

I have had this set-up for a couple of years now and have harvested castings from it 4 to 5 times. It's worked very well so far. Scraping the castings from the top of the landscape fabric has been surprisingly easy and I haven't ever had problems with excess moisture.

Just thought I would share what has worked so well for me. Happy worming.

Harry