Are store-bought soils safe for growing vegetables?
In September I wrote about an unsettling incident in which I’d found high levels of lead in the chard I’d grown in a backyard planter box filled with store-bought soil. According to the head of the lab that did the testing, I shouldn’t have eaten more than one-quarter pound of the leaves a day or I’d risk lead poisoning.
The results were enough to make me rip out all the leafy greens I’d been growing in my custom-built planter and throw them into the black trash bin, not even the green waste bin or my compost pile, because I didn’t want this stuff recycled. The experience also got me thinking: What, exactly, is in all these bagged soils?
I wasn’t alone in my alarm. I received dozens of letters from readers who wanted to know which product was the culprit. The short answer: I didn’t know. I had blended about eight products made by different manufacturers to create a soil mix that would result in luscious leafy greens, and it did. I just couldn’t remember which ones I’d put in that planter box, and which ones I’d used elsewhere.
So I decided to do some testing. I went to three stores and bought six popular brands of bagged soil labeled for fruits and vegetables -- Miracle-Gro, SuperSoil, EarthGro, Kellogg, Sun Land and E.B. Stone -- and sent samples of each to two labs to corroborate the results.
The findings: None of the soils contained toxic levels of lead, zinc or arsenic. The bad news: All contained at least some contaminants, an outcome that, depending on whom you talk to, is not at all problematic or moderately troubling.
I recited the test results to Rufus Chaney, senior research agronomist for the U.S. Department of Agriculture, which sets the country’s policies on farming, agriculture and food. He said the metal levels in the soils were all at “background levels” that weren’t high enough to “affect gardening choices.” In other words, I could grow root vegetables, lettuce, tomatoes and more and feel confident that the harvests could be eaten without adverse health effects over a lifetime.
“Every soil in the world has heavy metals,” said Chaney, “so don’t be automatically afraid.”
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What are heavy metals, and why should we care about them? Although some heavy metals such as iron, copper and zinc are essential to human life in small quantities, others, such as cadmium, arsenic and lead, are usually a result of pollution and are toxic at certain concentrations.
Heavy metals were present in every bagged soil I had tested but only at very low levels. Had they been at significantly higher concentrations, zinc and copper would be more likely to injure or kill a plant before it could bear edibles, according to Chaney. He said cadmium uptake in plants is inhibited by zinc, so it can’t be transferred to edible crop tissues in dangerous amounts when zinc levels are high. Likewise, chromium is not a risk in food because plants can’t draw it out of the soil. Arsenic can be absorbed by plants to some extent and can be toxic at high levels, but again, none of the bagged soils had much.
That brings us to big, bad lead. According to the U.S. Environmental Protection Agency and the Department of Housing and Urban Development, a lead level of 400 parts per million is acceptable in residential dirt -- dirt that might be tracked into a house and accidentally ingested by children. That same level -- 400 ppm -- also is the point at which the USDA says leafy and root vegetables should not be grown. The highest lead level in the bagged soils I tested was 7.85 ppm. The lowest was less than 1 ppm.
Phew.
But the test results didn’t explain the high lead level in my chard. One hypothesis: The lead wasn’t in my chard but on it. According to Chaney, the threat of lead isn’t absorption of the contaminant in fruits or vegetables but rather “soil splash,” water hitting dirt and splashing lead onto food. (I didn’t wash my chard leaf before sending it to the lab this summer.) Chaney also said that lead content in plants can be tempered with phosphate fertilizer, and that lead in food isn’t as terrifying as it sounds because it’s bound to food particles that make it less likely to be absorbed by the human body.
People should be more aware of the lead that comes “from carrying soil into the house on shoes, clothes and tools where it becomes part of house dust and where young children can be exposed through hand-to-mouth play,” he said.
Another possible culprit behind the high-lead levels in my chard: fish fertilizer. Every couple of weeks, I applied a liberal dose, thinking it was a healthier alternative than traditional petrochemical fertilizers. Unfortunately, I have no idea what brand of fish fertilizer I bought; I used it all up and recycled the bottle. I went back to Home Depot to look for the product, but it was no longer there. When I asked the sales clerk to identify the brand by looking up the product number on the receipt, the search didn’t yield anything more specific than “fish fert.”
Fertilizers are overseen by the USDA but regulated by individual states’ agriculture departments. In California, the Department of Food and Agriculture oversees fertilizers such as the fish-derived product I had used. The department also regulates bagged soils, but only if they contain fertilizer. When these fertilizer-enhanced bagged soils are tested, the department is merely verifying that the contents listed on the label do what they claim to do. The soils are not tested for potential contaminants.
The California Department of Food and Agriculture does random sampling of the 5,400 products registered with the agency, but only 22% are tested each year. In 2008, 28% of those products failed to live up to claims on the label or were mislabeled, according to Steve Lyle, director of public affairs for the agency. The agency retests violators several times a year until they comply, Lyle added.
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Such random testing brings up an interesting question: Do you trust the government to identify toxic pollutants and to stop violators? My guess is that a lot of the 8.6 million Americans who became food gardeners this year do not. Food safety was cited as a concern by 48% of gardeners polled in a 2009 National Gardening Assn. report on home and community gardens. About half of the people who started food gardening this year were young mothers between 18 and 34, according to Bruce Butterfield, who conducted the research.
Then consider that about 17 million Americans garden only in containers, and what they put in those containers is mostly bagged soil -- a $1.5-billion industry.
So what’s a consumer to do? You can mix your own soil, which is difficult because it takes far more organic material and time than most casual gardeners and urban farmers have. (Plus, if you’re composting, there’s a chance your compost ingredients could be high in toxics.)
If you buy bagged soil, you can read labels or run a lab test. Although reputable manufacturers do extensive testing of their own, consumers who want more guarantees can look for certifications from organizations such as the Organic Materials Review Institute, an Oregon-based nonprofit that conducts checks of products to ensure they comply with the USDA’s National Organic Program. Certification also can come from the Mulch and Soil Council, an industry-backed group that tests its members’ products to ensure they live up to the billing on the bag and are low in heavy metals such as chromated copper arsenic, a pesticide used to treat wood.
Wood byproducts are one of the main ingredients in bagged soils. Much of the wood in bagged soils is leftover bark from paper and lumber mills -- bark that is stripped off before wood is treated with chemicals. Bark holds up plants and improves drainage. What type of bark is used in bagged soils varies by region: Because soil is so heavy to ship, it’s typically made within a few hundred miles of where it’s sold. In California, much of the bark is from fir and redwood.
As an agricultural region, California provides other kinds of organic materials. Scotts, which makes soils under the Miracle-Gro, EarthGro and SuperSoil brands, uses leftover skins and seeds from Napa Valley, rice hulls from Stockton, as well as pecan and walnut shells, among other things. The company, based in Marysville, Ohio, recycles 5 billion pounds of 100 raw materials, mixing them into 1,000 recipes for soils sold around the country.
Kellogg, which is based in Carson and has soil-making facilities in Ontario, uses ingredients such as bat guano from caves in the Southwest and worm castings and chicken manure from local farms.
Other materials commonly found in bagged soils include Canadian peat moss (to suppress disease and provide soil acidity, which helps plants to absorb nutrients), Sri Lankan coconut coir (frequently blended with peat moss to retain moisture), Norwegian kelp meal (contains trace materials) and perlite (a mineral that improves drainage).
It’s safe to say, I’ve got all these things in my planter box, which is now growing herbs that I wash carefully before eating. My son and I eat so few herbs in a single sitting that there isn’t any chance we’ll reach the quarter-pound-per-day limit. As for the bagged soils I had tested: I felt confident enough to use the remainder of all six in my raised beds, the fruits of which I’ll continue testing as my soil evolves.
