Buying organic food is an exercise in personal virtue: You pay more to consume food that's healthier for you and less damaging to the environment because it's grown without artificial or toxic chemicals.
This powerful perception, based more on belief than facts, goes a long way toward explaining why demand for organic products has grown so much. Organic sales have more than tripled in the past decade, to more than $30 billion a year, while sales of conventional food products have dawdled along at an annual growth rate of about 2 percent.
There's just one huge problem: Neither of the main assumptions driving the growth of organic farming are grounded in science. In fact, there is evidence that organic farms produce as much, or more, pollution than conventional farms and that organic products might actually contain more toxins than other foods.
Like all farms, those that grow organic products rely on fertilizer. Often, organic farmers use animal manure rather than chemicals derived from petroleum or minerals.
In one study of greenhouses in Israel, the use of manure led to much more nitrogen leaching into groundwater compared with use of conventional fertilization. Nitrogen contamination, the study noted, is one of the main reasons for closing drinking-water wells.
And by the way, nitrogen from all sorts of farming is one of the main pollutants behind algae blooms, fish kills and dead zones in bodies of water from local farm ponds to the northern Gulf of Mexico.
A broader study of 12 different farm products in California found that in seven cases, those using conventional methods had lower greenhouse-gas emissions. A big reason for the difference? Conventional farming tends to be more efficient than organic farming, meaning fewer inputs are needed to generate the same amount of food.
That hits on a critical issue for organic farming, as noted in a 2012 analysis of more than 100 studies of farming methods across Europe: Getting the same unit production from organic farming tended to lead to "higher ammonia emissions, nitrogen leaching and nitrous oxide emissions." And while organic farming tends to use less energy, it also leads to "higher land use, eutrophication potential" — that's the dead zones mentioned above — "and acidification potential per product unit."
The main author of the study, Hanna Tuomisto, a professor at Oxford University, said: Many people think that organic farming has intrinsically lower environmental impacts than conventional farming, but the literature tells us this is not the case. While some organic farming practices do have less environmental impact than conventional ones, the published evidence suggests that others are actually worse for some aspects of the environment. People need to realize that an "organic" label is not a straightforward guarantee of the most environmentally friendly product.
Organic animal production also can cause problems. Unlike conventional farms, organic farms usually let animals wander around. No surprise that animals then do their business wherever nature calls. Rain, in turn, washes waste into local streams and rivers. Think of that next time you see free-range something on the menu.