From Scientific American:
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.
More here.