Urban agriculture is booming. For many of us who live in urban areas, a backyard garden can provide a mini oasis from city life. However, the hidden danger of lead can be found in city soils.
In a recent study from the University of Illinois, researchers have detected elevated lead levels in soils across Chicago, an urban agriculture hotspot.
It is not well understood how vegetables and crops take up lead in real-world settings, but the new research is shedding some light on the topic. The experts found that tomatoes, even when grown in lead-contaminated soil, are safe to eat.
“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,” said study co-author Andrew Margenot.
While this is good news for urban agriculture, there are potential caveats for home gardeners. Specifically – exposure during tillage and planting.
“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,” said Margenot.
When we work in 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 that we ingest. Depending on the soil concentration, a little bit of lead exposure can have big health impacts.
The researchers planted Roma tomatoes in Chicago backyards with soil lead levels between 77 and 1,206 parts per million (ppm), exceeding the natural background lead level of 21 ppm and surpassing the Illinois EPA threshold of 400 ppm for inhalation risk.
The researchers measured lead concentration in fruit with no soil treatment. They compared this level of contamination to fruit grown in soil that was treated with phosphorus-based treatments, which are developed to reduce lead uptake by humans.
The EPA recommends phosphate fertilizers such as triple super phosphate (TSP) to mitigate soil lead for human ingestion, but some wanted an organic matter amendment, as well.
“We chose to test TSP as well as composted and air-dried biosolids, which are human feces processed by Chicago wastewater treatment plants. They’re Class A biosolids, which means they’re tested for pathogens and heavy metals,” explained Margenot. “I know there’s an ick factor, but they’re likely safer than steer manure you can buy at a hardware store.”
The results showed that none of the amendments lowered lead in the tomatoes. Lead uptake by tomato plants was already so low, even in highly contaminated soil, that the amendments had no detectable effect.
Across the two years of the study, lead levels remained very low. However, variation between study years was of interest to the research team.
“In the second year, we saw an order of magnitude increase in lead in the fruit at two of the three sites. It was totally unexpected, and we couldn’t explain it. But the soil lead levels didn’t change across years and the fruit lead levels were still extremely low,” said Margenot.
“So to me, it’s two things. First, there’s still so much basic research to be done on plant uptake of lead – we didn’t even know to expect a seasonality effect. Second, and importantly, there’s a very poor correlation between total soil lead and lead uptake.”
So – what can you do as a backyard tomato grower? Don’t panic!
“If you minimize dust with a heavy mulch, you can safely grow tomatoes, so not all hope is lost. In Illinois, the EPA sets the inhalation risk at 400 ppm, but we found you can be up to three times above that in the soil and safely grow tomatoes,” said Margenot.
Overall, it is advised to till the soil with care, cover the surface, and wash the fruit to minimize lead exposure.
Despite the minimal change in applying phosphorus treatments, the researchers see opportunity in minimizing lead uptake.
“If we want to sustainably reduce lead ingestion and inhalation risk across the city, we should be looking at phosphorus and at local sources such as biosolids more closely,” said Margenot. “Biosolids are locally produced in Chicago and there are programs to get them into the hands of users. So if we’re talking about low-cost ways to deal with lead, this would be one good resource in the city.”
The researchers also recommend agricultural practices that require minimal soil disturbance, such as agroforestry and perennial fruit production.
The study is published in the journal Science of The Total Environment.