Murray Davidson, of Waverley, asks :-

I have a group of 6 tyre stacks in which I grow potatoes, vegetables and strawberries. I have recently been made aware of some concerns about materials leaching out of the tyres into the soil and the subsequent contamination being absorbed by the plants. I have no wish to be responsible for poisoning my family! Can you advise me about this problem.

Brent Clothier, a production scientist at Plant and Food Research, responded.

I wouldn't recommend growing food in unlined car tyres. They are typically made of (relative weights) natural rubber (100); zinc oxide (3) and stearic acid (5) as activators; carbon black (50) as reinforcer; process oil (5) as self-adhesion; antioxidant (1); accelerator (0.5); and sulphur (2.5) as vulcanising agent.

Also during their original use they pick up, on the outside, hydrocarbons from the road surface and emissions from cars. Lining would be prudent, if not essential.

The U.S. Environmental Protection Agency has no worrys about tyres being used for containing vegetable gardens, but tyre mulch or ash is a definite no-no as the zinc is more readily leached out and can make the soil toxic.

One study of the soil under a 10-year-old tyre dump in the UK showed elevated soil cadmium, lead and zinc concentrations, and these decrease exponentially with distance away from the pile. Where large numbers of tyres have been burnt, nothing grows.

We have raised gardens at home made out of treated timber, and I made plastic linings to separate the wood from the soil inside. In most treated timber it's the CCA (copper, chromium & arsenic) that leaches out. Arsenic is the most mobile!

A 2011 study (Environ Health Perspective, 2011 Jul 1; 119(7): 989–996) showed that of standard new plastic items a large percentage leached traces of chemicals having detectable estrogen activity. A few parts per billion may be detrimental to young mammals.