Urban agriculture is booming, but there's often a hidden danger lurking in city soils: lead. A recent University of Illinois study showed universally elevated lead levels in soils across Chicago, an urban ag hotspot.
Scientists don't know much about how vegetables and other crops take up and accumulate lead in real-world settings, but new U of I research in Chicago backyard gardens shows tomatoes are likely safe to eat, even when grown in highly lead-contaminated soils.
"There was so little lead accumulation in the fruits, we estimate the average adult male would have to eat almost 400 pounds of tomatoes per week to reach toxic levels," says Andrew Margenot, assistant professor in the Department of Crop Sciences at U of I and co-author on the new study. "However, a lower body-weight child of about 60 pounds would need to eat 'only' 80 pounds of tomatoes per week—still quite a bit, but a lower threshold of consumption."
It's good news for urban agriculture, but there's a potential caveat for home gardeners and other urban agriculturalists.
"It's not the fruits I worry about, it's the practices of tillage and planting. That's where you get exposed," Margenot says. "If you magically have no exposure to contaminated soils to get to the fruit stage, or if you mulch the heck out of the soil and wear a suit and respirator, you're golden. But, of course, we all know it doesn't happen that way."
That's because when we work contaminated soil, plant into it, or track it into our homes, we end up inhaling it. Lead can also end up in the fine dust on the skin of tomatoes, leafy greens, and especially root vegetables. With improper washing, we eat it right up. And, depending on the soil lead concentration, a little can have big health impacts.
In their study, Margenot and co-author George Watson planted Roma tomatoes in Chicago backyards with soil lead levels between 77 and 1206 parts per million (ppm), exceeding the natural background lead level of 21 ppm and generally surpassing the Illinois EPA threshold of 400 ppm for inhalation risk. The researchers wanted to see how much lead ended up in the fruit with no soil treatment and when the soil was amended with various phosphorus-based treatments shown to reduce lead uptake by humans via the dust inhalation or particle ingestion routes.
The EPA recommends phosphate fertilizers such as triple super phosphate (TSP) at high application rates to mitigate soil lead for human ingestion, but stakeholders told Watson and Margenot they wanted an organic matter amendment, as well.