JoAnne Skelly column: Is corrugated metal safe for vegetable gardens?
Submitted by Jeff Munson on Sun, 04/26/2020 - 9:27am
My friend Roni is redoing her vegetable garden and asked me if corrugated metal materials were safe for building raised beds for vegetables. Since I had no idea, I started doing some research.
Corrugated metal, usually made of steel or aluminum, is shaped with alternate ridges and grooves and then galvanized with a zinc coating to make it weather and corrosion resistant. This means that corrugated metal will resist rusting, which is good in a garden application where it comes into daily contact with water. However, I had some concerns.
Does the metal or coating release any compound into the soil that can be absorbed by vegetables and ingested when someone eats the vegetables? Can any toxic compound bioaccumulate in the soil and veggies and create an eventual health hazard? Roni had a concern too: would the metal heat up and harm plants?
Zinc is fairly stable unless it comes in contact with an acid or is heated to 390 degrees F. Under optimal garden soil pH of 6.5 to 7.5, any zinc that might enter the soil as the galvanized metal breaks down over many years would be in very small quantities and might actually be beneficial for plants as a micronutrient (Krans, R., Michigan State University Extension Educator).
A soil high in organic matter can reduce the solubility of zinc, although high levels of salts may make it more soluble. It would be better to use manure-free (less salt) compost or humus than straight manure (high amounts of salt) as organic matter to avoid too much zinc being released into a soil. While ingesting large amounts of zinc is not good for us, small amounts of zinc can actually boost our immune system and metabolic function (Mayo Clinic).
Cadmium can be a contaminant in things manufactured with zinc and is a potential carcinogen. Leafy vegetables like lettuce, cabbage, and spinach, some potatoes, carrots, beets, peas and eggplant can absorb cadmium in roots, leaves, and to some extent in fruits.
But an unusually large amount of produce would need to be eaten over a long period of time for toxicity to occur (Hodel, D.R & A.C. Chang, University of California, Riverside). Cadmium also can release into the soil if the soil is acidic. Since our soils are rarely acidic, neither zinc nor cadmium toxicity in plants or humans should be a problem.
Galvanized containers actually reflect heat so overheating roots also should not be a problem. All in all, it sounds as if corrugated metal may work well.
— JoAnne Skelly is an Associate Professor & Extension Educator Emerita at the University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.
