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With remnants of once-legal lead paint, leaded gasoline and other pollutants from the nation’s industrial past tainting land in U.S. cities, soil researchers warn that the growing number of urban farmers and community gardeners need to test their dirt and take steps to make sure it’s safe.

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.