Urban farmers who have their soil tested for heavy metals and other contaminants can get a nasty shock when they realize what would be coursing through the food they grow on their land. Establish an innocent little vegetable patch and you’ll be serving your family a salad full of fresh lead.
Happily, contaminated soil doesn’t mean farming is out of the question. A relatively small investment in compost and new topsoil can mean a relatively large drop in contaminants. Some urban farmers put in raised beds that keep the plants they intend to eat out of contact with the soil. And then there’s another solution: phytoremediation.
Phytoremediation means using plants to remove contaminants from the soil. The growing plants take up contaminants in the soil and produce and scatter new seeds, at which point the grown plants are harvested and disposed of, leaving the soil that much cleaner. Plus, just because plants take up materials such as lead, doesn’t mean that the lead ends up spread out equally in every part of the plant. As one report states, “Fruits can generally be grown in soil with high lead levels without accumulating dangerous amounts of lead in edible tissues.” The lead bypasses the fruit and accumulates in the leaves, which drop off every year. Small community gardens, or household farms, get the contaminated leaves shipped out and keep the fruit to eat.
If you get phytoremediation right, you can farm contaminated land. To remove contaminants, though, you first have to pick the right plants. If your garden is saturated with arsenic, there are famous hyperaccumulator ferns which evolved in arsenic-rich soils and which take up a thousand times the arsenic of other plants. St. John’s Wort accumulates cadmium and other heavy metals (something to watch out for if you’re taking supplements). Hydrangeas take up aluminum. Water hyacinths take up mercury.
But the work-horses of phytoremediation are Indian mustard and Chinese cabbage. Both in the Brassica genus, they are related to, look like, and are often served like bok choy. You’ve almost certainly eaten them at some point. You may have been eating more than you bargained for, which isn’t necessarily a bad thing. They tolerate, and thus accumulate, nearly anything, including nutritional metals like zinc, so they’re sometimes used as natural supplements.
They also hyperaccumulate cadmium, chromium, copper, selenium, manganese, and lead. And with just a little juice, they’ll start accumulating uranium. Scientists found that, although neither the mustard nor the cabbage were spectacular at accumulating uranium under normal circumstances, adding citric acid to topsoil impressively increased their uptake, and speculated that this might be true for all plants. So dousing your uranium-contaminated farm in acid might actually improve things . . . eventually.
The “eventually” is what is holding phytoremediation back, as a widespread commercial technique. It does work, provided that farmers and gardeners take contaminated plants away to be properly stored every year. It doesn’t work fast enough to be a good financial option for small commercial farms. Farmers tend to have to turn to chemical solutions, or abandon the idea of farming. Urban gardeners, however, can do a lot of good, over the course of their lives, steadily decontaminating the soil underneath them.