When Sheet Metal Meets Soil

Can I use galvanized sheet metal to build raised beds?

November 13, 2014

Q. Is it safe to use galvanized sheet metal to build raised garden beds? —Susan Taylor, Monticello, Utah

A. Over time, compounds used in the galvanizing process will leach from galvanized metal into surrounding soil. Climate and soil conditions such as moisture and salinity affect the rate and the amount of leaching. While the by-products of corrosion are unlikely to occur in amounts that pose any risk to human or plant health, gardeners who are considering growing in galvanized containers or metal-framed beds should be aware of the potential for zinc and other materials to transfer into the soil.

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Zinc, the main ingredient in the galvanizing “bath” used to prolong the life of steel, is an essential micronutrient that occurs naturally in North American soils at an average background level of 0.07 milligrams of zinc per gram of soil. For the sake of comparison, the Daily Value (an approximation of our dietary need) for zinc established by the FDA for adults is 8 to 11 milligrams.

While studies of zinc levels in the soil next to galvanized structures have found increased amounts of the element, those levels often are comparable to background levels and within EPA guidelines, says Dan Barlow, a corrosion engineer with the American Galvanizers Association. Zinc does not migrate readily through soil, so elevated zinc levels tend to be found only in the immediate area of a galvanized container or structure. Soil pH, organic matter content, and other soil characteristics affect zinc’s ability to be taken up by plant roots. As much as 90 percent of zinc in soil may be unavailable for uptake by plants.

Due to zinc’s limited bioavailability in soil, there is little chance of ingesting too much zinc through plants grown in proximity to galvanized metal, says Eric Van Genderen, Ph.D., manager of environment and sustainability for the International Zinc Association. “You will likely never get even your recommended daily allowance from your produce, much less too much,” he says. Because galvanized metal corrodes faster as pH decreases, Van Genderen says it’s probably not the best container material for plants that require acidic conditions.

Other corrosion by-products may show up in the surrounding soil, Van Genderen says. He notes that levels of other metals found in galvanized surfaces, such as nickel and bismuth, typically would be “so low that you’d probably never see a difference in the amount coming from the galvanized metal versus the background levels.”

The health of beneficial soil microorganisms that are exposed to galvanized metal is another consideration. “There is no question zinc can kill some of the soil’s microbes and that others love it,” says Jeff Lowenfels, author of Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web, Revised Edition (Timber Press, 2010), and Teaming with Nutrients: The Organic Gardener’s Guide to Optimizing Plant Nutrition (Timber Press, 2013). “I am willing to let the arbuscular mycorrhizal fungi take up excess zinc, feed the plants what they need, and hold the rest,” Lowenfels says. His research has convinced him that “any damage done to the soil food web [by excess zinc] is quickly corrected by it if the soil food web is a healthy one.”

 

Photography by Lena Gabrilovich
Originally published in Organic Gardening magazine, December 2014/January 2015

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