Is organic agriculture polluting our food with heavy metals?

News and commentary about environmental issues.
Sept. 8 2008 12:02 PM

Rusted Roots

Is organic agriculture polluting our food with heavy metals?

If I'm reading the banana peels correctly, the prospects for organic agriculture have never been better. Indeed, the most recent batch of bananas I bought came adorned with a "certified organic" sticker that confirmed the virtue of my purchase. It explained, "When you purchase organic produce you are taking part in the HEALING of our land."

This makes intuitive sense. Conventional agribusiness, after all, is a chemically dependent, resource-intensive venture that contributes to global warming, aquatic "dead zones," and massive land degradation. Organic systems, by contrast, restore soil health, foster biodiversity, and recycle organic matter rather than lading the land with synthetic chemicals. 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.

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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.

The implications of this fact cannot be overlooked. The human body naturally contains trace amounts of heavy metals, but when they accumulate faster than the body can excrete them, several serious health problems can follow, including cardiovascular and neurological disorders as well as kidney and liver damage. The Agency for Toxic Substances and Disease Registry, a division of the U.S. Public Health Service, cites fruit and grain consumption as the leading cause of lead exposure in the general population. Lead exposure has been proven to cause severe anemia and permanent brain damage. It's not yet clear whether organic (or conventional) soil contains enough of these metals to pose a genuine risk to human health. But continuing research on this topic weighs heavily on the future of what we've come to assume is a sustainable alternative to conventional agriculture.

Scientists have known since the 1920s that organic fertilizers used by farmers to supplement conventional systems—composted animal manure, rock phosphates, fish emulsions, guano, wood ashes, etc.—further contaminate topsoil with varying concentrations of heavy metals. Organic advocates, who rely exclusively on these fertilizers, remain well aware of the problem today, although they rarely publicize the point.

No one is saying that organic soil has higher heavy-metal counts than conventional soil as a rule—scientists have not conducted enough research to make such a determination. Still, some evidence indicates that organic soil can, in some cases, be more contaminated. George Kuepper, an agriculture specialist with the National Center for Appropriate Technology, observed in a 2003 report that composting manure actually concentrates the fertilizer's metal content, which could lead to greater levels of the contaminants in organic soil.

Recent studies have lent Kuepper's concern tentative support. For example, in 2007, researchers conducted an analysis of wheat grown on various farms in Belgium; based on the results, they estimate that consumers of organically grown wheat take in more than twice as much lead, slightly more cadmium, and nearly equivalent levels of mercury as consumers of wheat grown on conventional farms.

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