They point to cities like Indianapolis, where nine out of 10 urban gardens tested by one researcher had problems with lead in the soil. Or the Boston area, where a recent study suggests that even clean, trucked-in soil can end up contaminated, perhaps by windblown dust or dirt splatted by rain, in a few short years.
Agriculture and other experts say such problems don't outweigh the benefits of urban gardening, but those growing food should make sure their soil has been tested and take appropriate steps to address pollution so their fruits and vegetables are safe.
"You can control these things once you're cognizant of them," said Nicholas Basta, a soil and environmental chemistry professor at Ohio State University. "But nobody can underestimate the benefits of . . . fresh-grown food."
While lead paint and leaded gasoline were outlawed decades ago, experts say lead remains the biggest problem for urban growers when it comes to soil contamination. While most plants don't draw up lead from the dirt, there's a danger - especially to children - from soil tracked indoors or left on food that isn't washed well.
Other concerns are cancer-causing chemicals such as arsenic, once used to treat lumber and put off by coal-burning plants, and polyaromatic hydrocarbons, a byproduct of burning materials like oil, coal, wood and garbage.
Tim Beckman, 44, had been gardening on the east side of Indianapolis for more than 15 years before he saw researcher Gabriel Filippelli on public access TV and asked him to test his dirt. The results were somewhat of a relief: Low lead levels where he gardens. But other parts of Beckman's yard had extremely high levels, and he's since reconsidered where he lets his chickens roam.
Beckman said the test results weren't a surprise. His neighborhood is mostly made up of homes built in the 1940s, when lead paint was in wide use.
"I probably should have been more aware of it at the time, but it (the TV show) was one of those 'ah-ha moments,'" Beckman said.
Filippelli, an earth sciences professor at Indiana University-Purdue University Indianapolis, said Beckman's test results were typical of what he sees around the city: Lead levels were higher in soil near the street, where cars burning leaded gasoline once drove, and near the area where water runs off the house, known as the drip line. Based on tests at about 60 gardens around the city, Filippelli said about 90 percent need some kind of work to make gardening safe.
Beckman said the tests made him think about steps, such as putting down mulch near the house, to keep dust from lead-tainted areas from blowing into his garden beds. With the planting season approaching, other alternatives for gardeners include trucking in clean soil that can be placed on top of potentially contaminated land to create raised beds and moving their plants away from contaminated areas.
While no one knows exactly how many urban residents are growing food, the U.S. Department of Agriculture estimates there are thousands of community gardens nationwide. The American Community Gardening Association said it has more than 2,600 active listings in its online database and has seen a steady increase in people inquiring about community gardening in recent years.
In the Boston suburbs of Roxbury and Dorchester, where four out of five backyard gardens tested had high lead levels, new research has suggested that a one-time fix isn't enough to keep soil safe. The nonprofit Food Project installed raised beds filled with freshly composted soil, but tests showed the lead content in some tripled in just four years. Researchers say that while more study is needed, the early results suggest growers need to change the way they think about city soil and test not only when they first plant but as years go by.
"It's not a static situation," said one of the researchers, Daniel Brabander, an associate professor at Wellesley College. "It's very prudent to characterize it at the start, but depending on neighborhood where you're doing this, it is evolving."
The Food Project has recommended growers also take simple yet potentially effective steps to reduce exposure to contaminated soil by washing their hands after gardening, washing vegetables thoroughly and trying not to track soil indoors.
Murray McBride, director of Cornell Waste Management Institute, said its analysis of garden beds in New York City generally has been encouraging, with one pilot study of 44 gardens finding less than 10 percent had high lead levels in the soil. He said efforts there to bring in clean soil and compost for raised beds may be why lead was less of a problem.
A lack of standard practices as urban agriculture expands has made the problem difficult to assess. Dave Weatherspoon, an associate professor at Michigan State University who studies food issues in Detroit, where urban farming is taking off, said more research is needed to provide a better understanding of what soil contamination could mean for crops and what should be done about it.
"We don't want people to feel that their food isn't safe," Weatherspoon said. "That is the worst thing that can happen to the U.S. food system."