Outside/Inbox: I found charcoal in my garden. Am I safe to eat my vegetables?

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Biweekly in the Morning Edition of NHPR, the outside in Team answers a listener question about nature.

This week’s question comes from Maureen McMurray of Concord, New Hampshire. Full Disclosure: Maureen used to work on Outside/In, but these days she’s just a listener.

For the past several years, Maureen has been growing vegetables in a backyard garden. Sometimes she finds lumps of coal in the ground while digging.

“I’m growing stuff that’s in the same soil as all this coal… am I poisoning myself and my family?”

This story is adapted from Yardwork, a summer yard and garden series by Outside/In. To listen to a longer version of this story, click here.

Maureen’s house was built in 1880, so these chunks of coal may be left over from when most houses in town were heated with coal.

“My first reaction is that I think it’s probably a pretty common problem,” said Nate Bernitz, public engagement program manager at the University of New Hampshire (UNH) Cooperative Extension.

Mystery chunks of coal notwithstanding, Nate said it’s always a good idea to have your backyard soil tested.

“There’s a notion that food is safer if you grow it yourself, which may or may not be true,” Nate said.

Nate recommended testing the soil for heavy metals that can be associated with coal ash.

The term “heavy metal” is a bit vague, but in general, heavy metals are a class of elements that are toxic even in minute amounts. Exposure can cause cancer, organ failure, or impair brain development.

Being an old house in an old town, lead is one of the biggest things to look out for. lead is the most common urban soil contamination because of its historical use in lead paint and leaded gasoline. the latter was not banned entirely for cars up to 1996.

The method for testing your soil is fairly simple: dig up some samples from a depth of six inches from the garden, mix and air dry the soil, pour into a labeled plastic sandwich bag and send everything to the lab.

The standard home and garden test is $20, but we added a few extra tests, including heavy metals, for a total of $152 in our lab fee. When the lab emailed the results a few weeks later, the report showed heavy metals were present.

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A ziplock bag containing the soil sample.

interpreting the results

Ingested heavy metals such as lead, arsenic and mercury can be toxic even in small doses. But just because they’re in your soil doesn’t mean they end up in your salad—and interpreting soil results can be a little confusing.

For example, take the lead.

This test found that Maureen’s soil contained lead at a concentration of 453 parts per million (ppm), which the lab described as an “intermediate” concentration. This is higher than background levels in New Hampshire soils and above the recommended limit for bare dirt in children’s play areas.

Sounds scary right? But if you’re trying to compare that number to the EPA safety threshold for soil contamination, well… you can’t.

“It doesn’t exist,” said Ganga Hettiarachchi, a professor of soil and environmental chemistry at Kansas State University.

“People really want numbers to come from somewhere like the EPA…but at the same time, I have some concerns about that.” The reason I’m worried [is if] we would arrive at unrealistically low values [levels]…keep gardeners away from gardening.”

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Maureen’s laboratory data from the UNH Cooperative Extension soil test. Note: Refer to the total lead result using the EPA method, not the Mehlich 3 test result. When comparing these heavy metal concentrations to those from the Cornell Small Farms Program and the NC State Extension, the concentrations of most other heavy metals are relatively low. The other elevated result in their soil is arsenic.

There are some guides such as this table from the Cornell Small Farms programGanga explained that it is difficult to set simple safety limits for soil contamination because they are only “parts per million”. Part from the story. Much depends on soil chemistry.

“When it comes to these soil contaminants, what matters is their bioavailability, not their total concentration in the soil,” Ganga said.

Plants have the ability to absorb or assimilate these elements, and this ability changes depending on the specifics of the soil.

  1. Soil pH, a measure of the acidity and alkalinity of the soil. Ideally, the pH should be close to neutral, around 6.5 or above.
  2. Sufficient organic matter in the soil, which is essentially built up by adding compost.

When these two conditions are met, heavy metals bind better to the soil… and that’s a good thing! Pollutants tied to the soil are less likely to end up in your vegetables.
“Most of the time, lead uptake by plants will be fairly insignificant in most soils,” Ganga said. “Except for root crops…like carrots, radishes, beets.”

The greatest risk of lead poisoning comes from direct ingestion: mostly children eating dirt, or surface contamination on inadequately washed products.

So… is this garden safe?

Based on these results, Maureen is fine to continue gardening in this soil. Their lead isn’t as low as it could be, but their pH and organics content help keep things in check.

“Levels will not be high enough to cause any kind of health problem… based on the scientific evidence we currently know,” Ganga said.

But home gardeners like Maureen can take steps to manage their risk.

  1. mulch. Again, this is where the greatest risk of exposure comes from
    direct ingestion, especially when it comes to children.

    “When it comes to children, since they are more vulnerable, we need to minimize every possible way, so we should consider every way, like even touching,” Ganga said.

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    Maureen is standing in front of her garden in overalls and all the trimmings.
  2. Install raised beds. “Twelve inches deep would be really comfortable,” said Nate Bernitz of UNH Extension.
  3. Choose the plants carefully. If contamination is a problem, avoid root vegetables and opt for fruiting crops like tomatoes.
  4. Wash produce thoroughly, especially low-lying crops such as lettuce.
  5. Have your soil tested and, if necessary, create a soil improvement plan to manage factors such as soil organic matter, pH and texture.

Remember: you don’t have to figure this out on your own. If you would like to test your soil with the UNH Cooperative Extension, the program offers consultations to help gardeners understand their lab results and create a plan for their garden.
Having your soil tested will help you better understand the ground beneath your feet. And even if you find something that sounds scary, there are ways to deal with it. You don’t necessarily have to stop gardening.

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