Outside/Inbox: I found coal in my garden. Are my vegetables safe to eat?
Every other week on NHPR's Morning Edition, the Outside/In team answers a listener question about the natural world.
This week’s question comes from Maureen McMurray, in Concord, New Hampshire. Full disclosure: Maureen used to work on Outside/In, but these days she's just a listener.
For the past few years, Maureen’s been growing vegetables in a backyard garden. Sometimes, she finds chunks of coal in the soil when she’s digging.
“I’m growing stuff that’s in the same soil as all of this coal… am I poisoning myself and my family?”
Maureen’s house was built in 1880, so these coal chunks could be leftover from the days when most of the houses in the city were heated with coal.
“My first reaction is that I think it's probably a fairly common issue,” said Nate Bernitz, public engagement program manager at the University of New Hampshire (UNH) Cooperative Extension.
Mysterious coal chunks notwithstanding, Nate said that getting your home garden soil tested is always a great idea.
“There's kind of a thought that food is safer when you grow it yourself, which may or may not be true,” said Nate.
Nate recommended testing the soil for heavy metals, which can be associated with coal ash.
The term "heavy metal" is a bit of a loose one, but generally, heavy metals are a class of elements which are toxic even in tiny amounts. Exposure can cause cancer, organ failure, or impact brain development.
Since this is an old house in an old city, one of the biggest to watch out for is lead. Lead is the most common urban soil contaminant because of its historical use in lead paint and leaded gasoline. The latter wasn’t fully banned for passenger cars until 1996.
The method to sample your soil is pretty simple: dig up a few samples from six inches deep from around the garden, mix and air-dry the soil, pour it into a labeled, plastic sandwich bag, and mail everything off to the lab.
The standard home and garden test costs $20, but we added a couple extra tests, including heavy metals, so our lab fee totaled $152. When the lab emailed the results a few weeks later, the report showed that heavy metals were present.
Interpreting the results
If ingested, heavy metals like lead, arsenic, and mercury can be toxic even at low doses. But just because they’re in your soil doesn’t mean they’ll wind up in your salad – and interpreting soil results can be a little confusing.
Take lead, for example.
This test found that Maureen’s soil contained lead at a concentration of 453 parts per million (ppm), which the lab described as a “medium” level. That’s higher than the background level in New Hampshire soils, and above the recommended limit for bare dirt in children’s play areas.
Sounds scary, right? But if you try 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 to see numbers coming 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 come up with unrealistically low [levels]... keeping gardeners away from gardening.”
While there is some guidance available, like this table from the Cornell Small Farms program, Ganga explained that it’s complicated to set straightforward safety limits for soil contaminants because “parts per million” are only part of the story. A lot depends on soil chemistry.
“When it comes to any of these soil contaminants, what is important is their bioavailability, not their total concentration in soil,” said Ganga.
Plants have the ability to absorb, or uptake, these elements, and that ability changes depending on the specifics of the soil.
- Soil pH, the measure of soil acidity and alkalinity. Ideally, the pH should be near neutral, around 6.5 or above.
- Adequate soil organic matter, which is essentially built by adding compost.
If those two conditions are met, heavy metals bind to the soil better... which is a good thing! Contaminants that are bound to the soil are less likely to wind up in your veggies.
“Most of the time, the lead uptake by plants is going to be pretty insignificant, in most soils,” said Ganga. “Except root crops… like carrots, radishes, turnips.”
The biggest risk for lead poisoning comes from direct ingestion: mostly, kids eating dirt, or surface contamination on inadequately washed produce.
So … is this garden safe?
Based on these results, it’s okay for Maureen to keep gardening in this soil. Her lead isn't low as it could be, but her pH and levels of organic matter are helping to keep things in check.
“The levels are not going to be high enough to cause any type of health issue… based on the science we know at the moment,” said Ganga.
But home gardeners like Maureen can take steps to manage their risk.
Mulch. Again, the biggest risk of exposure comes from
direct ingestion, especially when it comes to kids.
"When it comes to children, since they are more vulnerable, we have to minimize every possible pathway, so that we should consider any pathway, like even touching," said Ganga.
- Install raised beds. “Twelve inches deep would be really comfortable,” said Nate Bernitz at UNH Extension.
- Select crops with care. If contaminants are a concern, avoid root vegetables and opt for fruiting plants like tomatoes.
- Wash produce thoroughly, especially low-lying crops like lettuce.
- Get your soil tested, and if necessary, create a soil amendment plan to manage factors like soil organic matter, pH, and texture.
Remember: you do not need to figure this out alone. If you choose to test your soil with the UNH Cooperative Extension, the program offers consultations to help gardeners understand their lab results and make a plan for their garden.
Getting your soil tested helps you understand more about the ground beneath your feet. And even if you do find something that sounds scary – there are ways to manage it. You don’t necessarily have to stop gardening.