I recently built a large compost bin out of old galvanized metal roofing. I'm concerned that the composting process may leach zinc or other harmful material out of the metal. Is the roofing safe to use?
When it comes to toxins and heavy metals, I always believe it's better to be safe than contaminated. So I called the compost experts at Woods End Research Laboratory. Senior researcher Eric Evans explained that a hot compost pile produces condensation in a closed space. This condensation may contain ammonia or acids, which could contribute to the corrosion of the roofing material. Since your roofing material is galvanized, it could possibly leach zinc, cadmium, and chromium into your compost if the galvanized coating corroded.
While Evans did not state that this material will definitely leach into your compost, I recommend you remake your bin with materials that you can be sure are safe.
The aerobic organisms in compost need oxygen to decompose organic matter. So bins that readily allow air into the pile are best, Evans says. Consider using recycled wood pallets or metal fencing to enclose your pile. There are also manufactured bins made of plastic designed to allow easy aeration. Compost piles don't need to be enclosed—so it's okay if you want to stop using a bin altogether. Whatever you choose to do, I applaud you for being a responsible gardener and questioning the safety of materials used in and around your garden.
New for your e-reader or tablet: Compostology 1-2-3.