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Are store-bought soils safe for growing vegetables?

We put them to the lab test to get the real dirt on dirt.

November 07, 2009|SUSAN CARPENTER

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."


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.


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."

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