"Is organic agriculture polluting our food with heavy metals," asks James E. McWilliams in Slate. I suppose anything is possible, but McWilliams’ article doesn’t give us much reason to worry.

McWilliams worries that dirt used to grow organic foods contains some of the same toxins as the soil used to grow non-organic crops:

Whereas conventional agriculture follows the law of supply and demand, organic agriculture follows what its founder, Sir Albert Howard, called "the law of return." Potential waste, according to this dictum, ends up enriching the soil.

The law of return, however, has a loophole. One issue frequently overlooked in the rush to embrace organic agriculture is the prevalence of excess arsenic, lead, cadmium, nickel, mercury, copper, and zinc in organic soil. Soil ecologists and environmentalists—and, to some extent, the concerned public—have known for more than a century that the synthetic pesticides of conventional farming leave heavy metals in the ground. But the fact that you’ll find the same toxins in organic soil has been something of a dirty little secret.

This passage seems to imply that composting last year’s crop waste back into the soil concentrates these toxins with each successive iteration. Later in the essay, McWilliams points out that organic fertilizers like manure can also concentrate heavy metals in topsoil. He admits that scientists don’t know whether organic soils are dirtier, on average.

Even if it’s true that organic soils contain more heavy metals, it doesn’t follow that organic food is a greater health hazard. McWilliams doesn’t show that increasing soil metal translates into increased metals in the food.

McWilliams notes that produce is a leading source of lead exposure for the average person. However, he doesn’t say how close the average person is to lead toxicity. Even if produce contributes more heavy metals than any other source, it’s
probably still the case that we don’t get that much of it. Nobody gets lead poisoning from fruits and vegetables now. For all we know, we could double these trace quantities and still be in the clear.

As always, the dose makes the poison. McWilliams rattles off the symptoms of acute lead poisoning (anemia, brain damage) as if they might be threats from organic food. He doesn’t give any evidence that lead exposure from produce could be anything close to the levels that cause these acute symptoms (e.g., in children who eat a lot of lead paint chips).

Whether trace metal accumulation is a problem for organic agriculture is a legitimate scientific question to be explored by nutritionists and agronomists. McWilliams concerns seem entirely premature and overblown, given the speculative nature of his concerns.