There is a growing market for organic foods that are supposed to be free of pesticides, hormones and synthetic chemicals before they can be labeled as such. Consumers, eager for chemical-free products, plunk down close to $14 billion annually for organic fare, according to the Organic Trade Association, a North American organization dedicated to promoting organic farming. But how do they know that the food they're getting—and paying a premium for—is really organic? British scientists have come up with a new test that determines the organic pedigree of products on store shelves by measuring the amount of nitrogen they hold.

To be considered organic, crops must be grown without the use of synthetic fertilizers and pesticides on a farm that has passed a rigorous certification process. The Food Standards Agency in the U.K., worried that nonorganic food might be slipping through inspections, funded a study to come up with a test to confirm the authenticity of finished organic foods, which generally cost more than other products. Scientists from the University of East Anglia in Norwich, England, report in the Journal of Agricultural and Food Chemistry that they have come up with a way to verify purity using nitrogen content as a measure.

Study author Simon Kelly, an environmental scientist, estimates that the test in its current form can reliably characterize tomatoes about 60 percent of the time and sort lettuce and carrots correctly 30 and 10 percent of time, respectively.

"At the moment, the test is not unequivocal," he admits. "What you can say clearly is that the average values for organic tomatoes and organic lettuce and organic carrots are statistically, significantly different than the conventional ones."

Nitrogen makes up over 75 percent of the atmosphere, though the isotope nitrogen 15 (a stable, but heavier form of the element with an extra neutron in its nucleus) accounts for less than 0.5 percent of that amount. The heavy nitrogen, however, is enriched in crops grown with natural fertilizer, like manure. Ammonia in the manure is a source of nitrogen—both the lighter nitrogen 14 and the isotope. Ammonia that is composed of molecular nitrogen 14 is more volatile and typically escapes the manure over time, leaving easily detectable amounts of nitrogen 15 present in the fertilizer as well as in the crops grown in it. Conventional crops are typically grown with synthetic fertilizers, which do not lead to this isotopic enrichment.

The research team, led by Kelly, used a standard technique, employed since the 1970s, involving gas chromatography and mass spectrometry to determine nitrogen 15 levels in different samples of organic and conventionally grown tomatoes, lettuce and carrots. The results show a fairly clear difference in nitrogen isotope values between organic and nonorganic tomatoes—the average value was 8.2 percent higher for the organic version. The other two vegetables showed less distinct differences, with organic lettuce being only a few percentage points higher than regular lettuce and the two varieties of carrots being largely indistinguishable.

Kelly believes a number of factors may be to blame for the discrepancies, such as variations in soil types, weather conditions and varying agricultural practices. For now, he says, the test can serve as a tool to "support or corroborate the suspicion of trading standards commissions" toward one or more manufacturers. Essentially, it is a way to ensure the inspections and accreditation process are working.

In an attempt to boost the test's effectiveness, Kelly says his group will continue to search for signatures from other isotopes, such as oxygen 18, and trace elements of substances like copper, which can creep into organic crops from traditional treatments used to ward off fungal growth in plants.

"What we're doing is just looking at one tiny aspect of organic production," he says, adding that in the U.K., at least, organic farming also involves crop rotation, maintaining hedgerows and encouraging wildlife to come back onto the farm. But, he laments, "you can't develop a test to see if farmer has been looking after the birds in the hedgerows in his farm."