Just south of Downtown on Meridian Street, steam billows from two heaping piles of tree mulch on the 3-acre site of South Circle Farm, Indianapolis' latest urban agriculture enterprise. Though April showers slowed their work, volunteers and farmhands managed to level the mulch piles into beds, about a foot thick with a 24-inch-high berm.

Now, they're topping the beds with 12 inches of topsoil, mined and trucked in from another farm site beyond city limits.

Pointing west of where we're standing, Amy Matthews, an Indy native and urban farm entrepreneur, says, "That's where the berries are going."

Behind me, rows of robust greens and herbs stand tall, seemingly emboldened by the season's rains. Some are still covered with a lightweight Agribon cover to provide protection from late-season frosts.

The precautions do more than provide shelter from the conditions — they protect both plants and people from hazards in the soil. South Circle Farm is surrounded by crumbling industrial infrastructure and scrap yards; contamination of the soil is a significant threat to the work done onsite. Luckily, farmhands have a veteran of the agricultural trade leading their efforts. Matthews developed the methods she uses today over the course of 12 years' experience in Illinois, Ohio and, most recently, Alaska.

Her work caught the eye of Steve Rock, an environmental engineer with the U.S. Environmental Protection Agency's (EPA) Office of Research and Development.

"I have not seen an urban site that would not benefit by what [Amy's] doing," said Rock as he toured the farm with a group of city officials, urban gardeners, and landscape architects. They had gathered for the informal Phyto-Forum Indy 2011, a daylong conference to discuss phytoremediation, in which producers use plants to mitigate soil contamination on brownfield sites.

According to EPA urban gardening sources, phytoremediation doesn't remove lead from soil — plant roots can't absorb it. So, the lead lingers. To prevent direct exposure to the toxic element, Dr. Gabriel Filippelli, a professor of earth sciences and director of IUPUI's Center for Urban Health, recommended capping South Circle Farm's growing site with six inches of topsoil.

Filippelli explained that the lead threat doesn't come from the plants themselves, but rather exposure from dry soil dust or contamination of crops by soil residues. This typically affects people through either ingestion or inhalation.

Chris Harrell, Indianapolis' brownfield redevelopment coordinator, agreed. "Research shows that heavy metals are not readily absorbed into plants," he said. "Researchers have spent years of trying to get trees to suck things out of the ground... It's hard. Growing fruit trees [on lead-contaminated sites] is not a problem."

In other words, don't eat the dirt.

Unsettling patterns

Urban agriculture has historically waxed and waned in cities as conditions shift. For now, Indianapolis finds itself in a period of urban agricultural expansion, but this has ramped up soil contamination concerns. Lead tops the list of those concerns, in part because it's the most studied contaminant, according to Harrell.

Dr. Filippelli identified three primary sources of lead contamination: leaded paint, used to paint homes; leaded gasoline (which was finally banned in 1996 by the Clean Air Act); and lead emissions from smelters.

"Cities have a long legacy of lead deposition," said Filippelli. His research in urban soils has shown contamination levels 10 to 100 times what they should be. "Natural levels are 20 parts per million," he noted.

Drawing from 1,000 yard sites and 60 urban gardens, Filippelli found that the pattern of lead contamination radiates from the city center like a bull's-eye. The highest concentrations are near the center, expanding to the 2000 and 3000 blocks in all directions. The next radius extends to I-465.

Near his Broad Ripple home, levels are 10 times higher than normal, though "not a level most would consider unsafe," he said. Still, he's constructed 8-inch-deep raised beds in his yard.

Just as important as knowing the content of soil is knowing a property's past use. For example, if your home is located on or near a former dry cleaner or gas station, this information can be helpful in predicting what kind of contaminants might be found around your home or urban garden. To that end, the city is creating an inventory website with online tools to determine past land use.

"The closer you are to the urban center, the more historic records are available," according to Chris Harrell. The website is currently being tested and will be available later this summer. Additionally, Dr. Filippelli said a guide for gardening in urban settings will be available in a couple of months.

Precautionary measures

Filippelli, Harrell and their fellow advocates worry that fears of soil contamination will drive Hoosier urbanites to abandon backyard garden plots. In this case, knowledge is power.

For residents who want to grow food on property within city limits, Filippelli recommended testing soil for nutrient content, organic matter and lead contamination. As part of his research, he's offered free lead testing to the public since early 2010.

"I will ask you to take your own samples and send them to me," Filippelli explained. "I will then send your results back via email with some suggested guidelines for planting. You will need only things you already have in your house for the sampling." He added that since lead is common in the top five inches of soil, it's nearly impossible to submit a bad sample.

These tests provide feedback on what may be helping or hindering plant growth such as organic matter and the pH balance. According to the EPA's Steve Rock, knowing your soil's pH is important.

"Keeping soil pH in the neutral range means heavy metals are bound up in the soil so that they are not available to plants to take up through the roots," he said. So whatever you do to mitigate lead in soil will address other potential heavy metal contaminants. Indiana soil tends to be alkaline, or have a high pH, often above 7.8. A neutral range would be around 6.5 to 7.

Ultimately, knowing your soil conditions, taking appropriate action, and understanding what conditions your plants prefer will ensure not only safer soils but also healthier plants and more produce for city-dwelling families.

These measures can seem like a lot to expect of the average Indianapolis resident, and the city's urban growers are sympathetic to that.

But they also say the work is worth it. Back at South Circle Farm, Amy Matthews has done what was necessary to reduce the lead exposure so that she could feel confident about her growing conditions.

"The way lead was spread across the site, we had several areas of high concentrations scattered across two acres. Doing it this way, we've increased our barrier without increasing the cost of our growing. All of the mulch comes for free," Matthews said. "It makes building the site initially a little more work, but what you come out with is so much better."