The assumption is simple: If you have vacant land in the city, plant vegetables. There's a lot to be said for local produce and healthy eating.
The reality is more complex: Most urban soils are contaminated with lead, a neurotoxin of special concern for children, whose brains are still developing.
There's baseline lead from the region's rocks, and lead on some former industrial sites, and lead was emitted through vehicle tailpipes during the era of leaded gasoline. Leaded gas was phased out and ultimately banned after 1995, but its legacy remains in urban dirt.
But fortunately, even if you suspect - or know - that your urban garden soil has lead in it, there are steps you can take to limit exposure.
Nevertheless, the concern is not only that the lead might get into plants - that depends on the plant itself, and the alkalinity of the soil (the higher the alkalinity, the more lead binds to the soil, and the plants can't take it up) - but also that gardeners might breathe dust with lead in it, or inadvertently eat lead-contaminated dirt.
Some of the latest data comes from geologist Stephen Peterson of Aston, who analyzed lead concentrations in city soil for a master's degree project at Temple University.
He had intended to sample and map the entire city - as scientists did in London in 2011. But that seemed too daunting. So he settled on Fairmount Park, the nation's largest inner-city park system.
Peterson took hundreds of soil samples from all over - the Northeast near the Tacony Creek to North Philadelphia to the Wissahickon Creek to West Philadelphia near Grays Ferry.
He knew that the region's soils have higher levels than the national average because of its lead-bearing rocks. It's 33 parts per million here, vs. 19 ppm in the U.S. overall.
But the Fairmount Park soil levels were even higher than the region's norm. This held true everywhere he went, even in the park's deep-woods locations, largely undisturbed for more than a century.
The ubiquity of the lead told him the biggest culprit was, indeed, leaded gasoline.
"You can't really get too far from the road in Philadelphia," he said. "Even in Fairmount Park."
The exception was when soil levels spiked near a building, but dropped dramatically a step or two away. In those cases, he blamed lead paint.
The good news is that when he tested the urban gardens in the park, all were well below the limit of 400 ppm recommended by the Environmental Protection Agency for soil that children play in.
"Most of them didn't even register lead, he said. "That was a really good sign." He credited the fact that many urban gardeners make raised beds with clean soil.
Peterson would love to see more testing, but for now, he said, "making people aware of the legacy of this pollution that's happened for decades, that's the first step."
Urban gardening advocates have paid attention.
Sarah Wu, a policy manager in the Mayor's Office of Sustainability, also cochairs a city food policy advisory council. Members have been focused on vacant land. One of their big questions is soil safety, and a goal is to devise recommendations for soil-testing and mitigation.
"For the vast majority of land in the city, growing food in it and eating it is good," Wu said. Then again, "everyone knows we live in an urban area . . . if we're trying to promote urban agriculture, we want to promote it in the safest way possible."
Meanwhile, here's a nugget to ponder as you head into the garden: A few years ago, the Pennsylvania Horticultural Society decided to test collard greens grown in the city, in Fairmount Park compost.
PHS also had grocery store collards tested as a control.
The lab found that the city-grown collards had no lead. The store-bought ones did.
GreenSpace: If You Think, or Know, Your Soil Has Lead
Here are some steps you can take to lessen exposure:
-- Cover the existing soil with landscape cloth, build a raised bed, and fill it with new soil. Or garden in containers.
-- Wear gloves when you garden. Wash your hands.
-- If your clothes get excessively dirty, wash them separately from other laundry.
-- Garden on days when the soil is moist so you are less likely to breathe in dust.
-- Thoroughly wash all vegetables.
Soil test kits are available from Penn State extension offices, including Penn State Center-Philadelphia, 675 Sansom Street. The basic kit, with directions, costs $10. Lead test is $27 extra. Information: 215-471-2200.
The regional EPA office also occasionally has soil-testing events, called "soil kitchens." Community center hosts usually publicize them.
Note: Lead in soil is also a concern if young children play in the yard. In that case, mulch the yard or plant a grass cover.
Source: U.S. Environmental Protection Agency; Pennsylvania Horticultural Society.
"GreenSpace," about the environment and health, appears every other week, alternating with Art Carey's "Well Being" column.