Rusted Roots

Is organic agriculture polluting our food with heavy metals?

By James E. McWilliams
Posted Monday, Sept. 8, 2008, at 12:02 PM ET

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.

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.

Beyond the comparative impact of organic vs. conventional systems on the soil's heavy-metal concentration, there's the question of how easily these trace elements enter crops. Although the research here is also relatively thin, what has been done suggests that the problem of plant uptake is equally serious in both organic and conventional systems. For example, 14 percent to 28 percent of New Zealand's cattle (destined to be organic beef) were found to have kidney cadmium levels exceeding limits set by the New Zealand Department of Health because of a diet of plants grown in contaminated soil. Similarly, a 2007 study of Greek produce found that organic agriculture does not necessarily reduce the cadmium and lead levels in crops. As it turned out, "certified" organic cereals, leafy greens, pulses, and alcoholic beverages had slightly less heavy-metal contamination than conventional products, but "uncertified" organic products had "far larger concentrations" than conventional ones.

These findings might be preliminary and inconsistent, but pressure is mounting on the organic community to take action. Under rules set by the USDA's National Organic Program, responsibility has been left to the individual farmer to manage plants and animals in a way that does not contaminate crops with heavy metals. The question of how to monitor that responsibility, however, is complicated by the fact that there are as yet no federal limits on heavy-metal concentrations applicable to all fertilizers.

Organic farmers thus work with broad suggestions rather than concrete federal regulations. They're routinely forewarned by organic watchdog groups such as the Organic Trade Association about dangerous levels of copper and arsenic in poultry manure. They're reminded of proper "nutrient management planning" and encouraged to experiment with the relationship between soil pH levels and rates of heavy-metal contamination. They're advised to test soil regularly for heavy metals and to adjust fertilizer combinations and relative nitrogen, phosphorous, and potassium levels in the soil when metal concentrations rise. A handful of states—California, Oregon, Washington, and Texas among them—have established loose legal guidelines. But the fact remains: The decentralized sprawl of information about fertilizers and heavy metals fosters a far-flung approach to the problem.

The Organic Materials Review Institute, a nonprofit organization that provides certifiers with an independent review of products intended for organic use, is working to change this situation by framing a standard of fertilizer use. OMRI officials have evaluated models from Canada, Washington state, and the Association of American Plant Food Control Officials. Initially, the organization established regulations that, according to Patty Martin, director of Safe Food and Fertilizer, would have been like "driving a hazardous waste truck into organic agriculture." After her 2006 testimony, however, OMRI revised its recommendations to include a somewhat more stringent set of limits on heavy metals in fertilizers.

No matter how they end up, OMRI's guidelines may ultimately come to naught. Scientists are currently documenting another cause of heavy-metal pollution in global agriculture. "Atmospheric deposition"—the transfer of pollutants from the air to the earth—has nothing to do with organic practices per se but is, rather, the result of industrial processes beyond the farmer's control. Farmers already know that fertilizers are not the sole source of heavy-metal contamination. They've inherited a landscape once pummeled with arsenic and lead insecticides. Heavy metals can leach from wooden fences treated with copper chromium arsenate and from lead paint on houses. But the idea that these contaminants can come directly from the air takes all this to a more bewildering level.

Mercury from tooth fillings incinerated with humans corpses and magnesium dust blown east from the Gobi Desert can now join heavy-metal residues wafting from smelting plants and arsenic from coal mines to contaminate soil. Needless to say, airborne contaminants don't land on conventional farms alone. As atmospheric deposition grows more widespread, the difference between organic and nonorganic farming will become ever smaller.

Fortunately, complete despair may still be avoided. Consider this: Plant biologists are working to genetically modify a fern plant that, when ashed and dusted on soil, is capable of sucking up zinc. The dust can then be gathered so the recovered zinc may be recycled and put to better use. Of course, in order for bioremedial technologies such as this to move forward, the dichotomy between organic and conventional agriculture will have to be collapsed, a sober view of organic agriculture will have to be adopted, and we'll have to read the banana leaves with greater skepticism.

James E. McWilliams is the author of American Pests: Our Losing War on Insects From Colonial Times to DDT and an associate professor of history at Texas State University.

Article URL: http://www.slate.com/id/2198756/