I have been asked on several occasions if there is a concern about heavy metals leaching out of tires and affecting the vegetables. In short, the risk appears to be minimal.
Here is a somewhat longer answer. Your comments and especially any information you might have would be greatly appreciated.
the question was about heavy metals leaching, I worked at finding
information on that. The studies that have been done have been on
shredded tires used as a base for play areas for children. I asked in
ECHO, which has been promoting the use of tires
and other basins for many years now. The consensus seemed to be that
there have been no studies done on using tires for vegetable production,
but that the risk is minimal (versus the advantages outweighing the
negatives) because the tires are used whole, not shredded. That limits
the surface area that is exposed to water/humic acids and also changes
the physics of how water and humic acids interact with the surface area it does
encounter. Also, at least part of any metals that might leach out would
wash out with the daily watering, affecting the soil under the tires,
but not the vegetables in the tires. Finally, we are worried
specifically about heavy metals and heavy metals are not absorbed by
most plants at all easily. The concern would be greater for vegetables that can (beets strike me as one that can absorb some heavy metals?) and particularly if those vegetables are grown consistently in the same tires. The odds would then increase that at least some of those crops are are contaminated.
Again, anyone reading this blog, please let us know if you have any information about studies that have been done. Martin Price at ECHO noted that one of the problems is that chemical studies like this are expensive and the people who use tires to grow vegetables are often poor, which makes it hard to find the funds to do the expensive studies.