We could use a tar/ creosote type product on the base of the post. How much of this would ACTUALLY end up INSIDE a tomato growing ten feet away? As far as I can tell, the creosote should KILL things, so any toxins would not make it back.
We could use asphalt emulsion. Is this stuff toxic, or not?
We could use wood tar, but that is very quite expensive.
We could use linseed oil, but this is expensive. Would any other oils work?
We could soak them in borax. Would this just leach way?