I have no answer concerning the pressure treated posts.

However, if you can get black locust posts, they need no treatment and will last for a very long time. If they are not in stock at your lumber yard, it is likely that they can special order them. If black locust grows in your area, a local sawmill may even be able to make a small run of them for you. Fence posts have been one of the main uses for black locust because of its great decay resistance.

Jim

Does it have to be wood? Metal pipe is cheaper down here than any wood bigger than a 2x4 so we use it. Not the thinwall stuff like they use in chain-link fences but the thick walled drill pipe. Lasts longer too.

If you have to go with CCA I'd allow for a good size perimeter. Found this at Ecology Center

Leaching of arsenic from PT CCA wood in raised garden beds has been found to be limited to about 6", but that 6" is permanently contaminated. It means that you should not grow vegetables in that strip nor can you turn that soil into the rest of your garden.

You'd want a clear work/walk path between the fence and the outside of the garden anyway, right (wheelbarrow, tiller, hoe handle length, etc.) so if the above is valid then 5 feet or so should be enough.

Dave