Most gardeners understand that the soil in big cities is often contaminated with lead — and know to get their soil tested. But the researchers' interviews with 70 urban gardeners in Baltimore revealed that most are pretty clueless about how to avoid other types of contaminants — like heavy metals and asbestos — from getting into their vegetables.

Part of the problem is that "there might be contaminants that [gardeners] can't test for," says Brent Kim, a program officer at the Center for a Livable Future. Most soil tests look for lead, cadmium and arsenic, he tells The Salt. But they don't test things petrochemicals left behind by cars, or cleaning solvents, which might have seeped into the soil from an old laundromat.

Many of these chemicals — including the cleaning solvents lautromats used back in the day, and chemicals found in the exhaust of cars — are carcinogenic, and they're dangerous to ingest or even breath in. Asbestos left over from a building demolished years ago cause lung problems, as well. And children are especially vulnerable to all of these substances, Kim says.

So if you're thinking of starting an urban garden, Kim says, once you've have found a plot of land, you should learn its history. What's now an empty plot or a backyard might once have been a parking lot, a gas station or the site of a chemical spill, he says. "Knowing the site history will give you some clues about what might be in that soil," Kim says.

And testing the soil and investigating the land's history are crucial whether or not you're planning on using raised beds. "People tend to think raised beds are going to solve their contamination problem," Kim says. But contaminated soil could easily get kicked onto your plants, especially if the beds are low to the ground.

"Another consideration is you have to be careful about the materials that you're using to build a raised bed," Kim says. Recycling wood from an old construction site might seem like a good, eco-friendly idea. But that wood could be treated with chemicals you don't want touching your fruits and veggies, Kim says.

Even after taking all those precautions, gardeners can never be 100 percent sure that they're in the clear, Kim says. So it's always a good idea to use gloves while gardening, and wash all your produce thoroughly. And if your young kids like to help out in the garden, you might want to clean them up as well.

All of this advice applies to all urban gardeners, Kim says, whether you're in charge of a small backyard plot, or a giant operation, like 140-acre Hantz Farms in Detroit.

According to the report, in Baltimore, over a quarter of the gardeners were relative newbies, with less than five years of experience.

Kim says there's no quick way to bridge the knowledge gap. But providing gardeners with the right resources is a first step. He and his colleagues have created an urban gardening guide. Baltimore residents can also check out an interactive map of the city, which highlights sites where the risk of contamination is high.

Other great resources include the U.S. Department of Agriculture's online guide.

"I see these urban growing spaces as these oases in the middle of these urban environments," Kim says. They bring communities together, and they help people save money on fresh produce.

"Urban growing spaces are amazing," Kim says. "Let's keeping doing this, but let's do it safely."

Copyright 2014 NPR.

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Newbie Urban Gardeners May Not Be Aware Of Soil's Dirty Legacy