They will stay in the area they were composted in. If you want to in ground compost like that, you have to dig some holes in and around the garden and bury stuff in between the plants, or every few feet. I have a friend who composts this way.

We compost that way. We dig holes and put in our vegtable scraps and coffee grounds. We've even done it with shredded paper, but that is more trouble. Egg shells take a while, but everything else disappears in a few weeks.

I think it helps the gardening areas by feeding micro-organisms and worms, but I don't think it spreads too far.