Well, you are making a container with a solid bottom and therefore will need to think container gardening, not raised bed gardening. Doesn't matter how big the bed is it will still just be a container. So you might want to rethink using soil and sand and whatnot.

Lots of discussions here about pressure treated wood. I have no problem using it. Some do. It's a matter of personal preference.

Rodney

I don't think filling a large bed with peat mix is really an option, that would cost a fortune. If that's what I would have to do I would need to rethink this plan.

Why the solid bottom? We just put hardware cloth under ours which allows some roots to penetrate, but the tunneling pests can't get through it, yes, they can nip at any roots they can get, but all that does is root prune, and it encourages the plant to put out branching roots above the pruning point.

Peter, look on Craigslist for topsoil. I bet there is someplace close to you trying to get rid of some. Just a thought.

Honestly i don't think it is possible to isolate a raised bed from soil borne diseases and have it work well. As you know from all the past discussions here about building raised beds, putting some sort of bottom, regardless of the material used, in them is never recommended as it only creates drainage issues and water ponding and all the problems that leads to.

The only exception maybe the hardware cloth for gophers. But hardware cloth won't effectively isolate unless you can elevate the whole bed well above the ground on legs. EX: the Veggie Trugs and similar elevated beds

Then there are all the fill problems and costs often discussed here. Good raised bed fill can be very costly since it needs much more than your average top soil..

So using containers for the garlic and eggplant is the more obvious (and less expensive) solution. Even then the containers can't sit in direct contact with the ground. Have you considered Earthboxes or the DIY Earthtainers?

Dave