Dan Boone wrote:It's not so much what's in them as what manages the journey out of them and into the edible portions of your plants.
The relative durability of tires in the environment is a clue there; they do not break down rapidly or easily, but horde their constituents tightly within themselves.
Virtually all of the research I've been able to find about the safety of recycled tire material has focused on machines that chop the tires into small pellets (for use as mulch or an ingredient in concrete or pavement). Chopping up the tires exposes many fresh surfaces to air and water, and has been found to liberate undesirable stuff (mostly metals) in undesirable quantities.
Whole tires? There's no research. So we don't know. Paul Wheaton, in the definitive thread about this here on Permies.com, says the question triggers the inner conflict in him between his organic tendencies and his recycler tendencies but in the end he rejects the tires.
For me, in my environment, planters made from triple-stacked tires offer the virtues of tall raised beds (protection from small nibblers, easier access to the gardener's attention without undue stooping, depth of soil) at essentially zero cost. And when I parse the risks, I find them small enough. But I honor and respect a different conclusion from a person who puts different weights on the irreducible uncertainties.
Casie Becker wrote:Actually, does anyone here know how much research was done before they started the labor intensive process of recovering the tires they had tried to use to establish new coral reefs?