Front-yard garden: Should lead test cause worries when the beds are raised?


Are the author’s raised beds high enough to keep roots from reaching lead-tainted soil in his yard? Depends on which expert you ask. ( Joe Yonan/The Washington Post)

I opened the e-mail in April with trepidation. Attached were the soil-test results I had sent away for weeks earlier, before I had planted peas and lettuce and kale and more as part of my 150-square-foot experiment in front-yard gardening.

The news from the University of Delaware wasn’t good. “This site shows significant contamination with lead,” the report said — 472 parts per million, to be exact. That led to the underlined instruction: “This soil should not be used for vegetable gardening.”

As anyone who has bought or sold a home in the past few decades knows, lead contamination is possible in any home built before 1978. Urban gardens have another source of possible lead contamination if they’re near roadways: fumes from gasoline before we switched to unleaded. Most of the concerns around lead involve children playing directly in the dirt, where they might ingest not only contaminated soil but even flakes of lead paint. Problematic soil can get stirred up by gardeners tilling, tracked into the house on shoes and, apparently, ingested in the form of produce grown in it.

That’s why experts caution gardeners to get their soil tested, and to take precautions if it comes up higher than 400 ppm. Among the suggestions: Raise the acidity of the soil to help bind the lead and make it less available; avoid leafy greens and root crops and focus instead on fruiting vegetables (tomatoes, eggplants, squash and the like), which apparently are less likely to pass along lead in their edible parts; and/or grow in containers or raised beds in new soil.

So why didn’t my heart sink when I read that soil report? Because I had already chosen Door No. 3, the raised-bed option, so I wasn’t really gardening in that underlying soil. I had decided early on that I’d avoid any issues with the ground in my Kingman Park yard by bringing in bag after bag of topsoil, compost and mulch and creating a new ecosystem inside cedar boxes.

But one little thing nagged at me, and that was my decision to not follow the directive of some experts — such as square-foot-gardening guru Mel Bartholomew — who suggested a fabric barrier under those beds. I had started by pulling up sod and replacing it with a “lasagna” system, layering newspaper, a topsoil/compost mix and mulch while I got ready for planting season. But was that enough? Would my plants’ roots eventually reach down through the six inches of raised bed, through the three inches of “lasagna” and into the contaminated soil underneath?

Would I have to pull out my kale, carrots, beets, just as they were starting to get established after a cold start to the spring?

I enlisted the advice of some local authorities. Sandy Farber Bandier, extension agent and master gardener coordinator at the University of the District of Columbia, met me one April morning and immediately expressed concern about the quality of the topsoil mix I had put in the beds; it was full of small rocks and twigs, and she wondered whether it was high-quality enough to support good plant health. She worried that I had planted tomato and pepper seedlings before it was really warm enough — “April 15 might be our average last frost date, but this hasn’t been an average winter” — and asked about how much shade I was getting from a tall tree nearby that was just leafing out. (The answer: several hours a day, which made it good for greens but possibly iffy for tomatoes.)

And then I showed her the soil test. Between the suspect topsoil and the lead numbers, she said, “I think you’ve got two choices: Go into containers, or raise the beds higher and mix in new soil” on top of landscape fabric, and maybe even another layer of bark.

She recommended I talk to her counterpart at the University of Maryland, Jon Traunfeld, who was less concerned. He had just seen eminent USDA research agronomist Rufus Chaney speak, and he didn’t think landscape fabric was worth the trouble. “What you’ve done is the right thing,” Traunfeld said. “You just don’t want to till that underlying soil or mix it.” As he and other experts said, the biggest danger with lead-contaminated soil is when children consume it directly, and mine was safely trapped underneath that “lasagna” layer.

When I spoke to Chaney, he agreed, saying raised beds are sufficient for the few crops that accumulate much soil lead. While his research shows that some vegetables (such as carrots and beets) do pull up lead through their roots, research has also shown that humans absorb significantly less lead from eating produce than agencies such as the EPA assumed when they set the limits based on lead in water. Some of the lead presence on low-growing leafy greens and herbs is traced to contaminated soil splashing on them, which wouldn’t happen in a setup like mine. Plus, the phosphates in the compost I added to the beds would act as a barrier to any lead absorption, he said.

Joe Ludes of the Neighborhood Farm Initiative echoed Chaney and Traunfeld’s lack of worry about my garden’s lead levels. The mulch I put down above the “lasagna” was a better choice than fabric because it would facilitate microbial growth that would make the lead less available, according to Ludes. As for the vegetables he saw growing in my little plot, “Right now there’s no way those roots are touching that subsoil,” he said, “and quite honestly I don’t think they ever would. They’ll grow six inches tops. Once they flower they might go deeper. But even carrots, they’ll shoot through that topsoil, and as soon as they get to the subsoil they’re going to stop.”

That’s because the topsoil — theoretically, anyway — is more fertile, so the roots will stay where they’re getting their nutrition. In fact, Ludes said, if I wanted to make sure the roots didn’t go too deep, I could buck conventional wisdom and give my plants shallow, frequent waterings to keep the roots from searching out deeper moisture. (The usual advice is to water them longer but less often, encouraging them to deepen and become more drought-tolerant.) He also thought I should consider testing the subsoil again — this time to see whether the lead was higher closer to the house.

I took it all in and made a decision. I’d add another three-inch tier to my raised beds, not because I was so worried about that underlying lead-contaminated soil, but because I wanted room to improve the quality of the topsoil (and add more). I’d do it in stages, focusing first on the beds where I had planted herbs, kale, tomatoes and peppers, all of which I figured would handle the temporary disruption as I uprooted them, and waiting on some of the other beds until I had harvested their contents.

One bed at a time, the plot would thicken.

Joe Yonan is the Food and Travel editor of The Washington Post. He got the cooking bug from his Indiana-born mother who let him use her stand mixer when he was 8 years old because it was cool.
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