In a bid to bring clarity to what has too often been an emotional debate, environmental scientists at McGill University in Montreal and the University of Minnesota performed an analysis of 66 studies comparing conventional and organic methods across 34 different crop species. “We found that, overall, organic yields are considerably lower than conventional yields,” explains McGill’s Verena Seufert, lead author of the study to be published in Nature on April 26. (Scientific American is part of Nature Publishing Group.) “But, this yield difference varies across different conditions. When farmers apply best management practices, organic systems, for example, perform relatively better.”
In particular, organic agriculture delivers just 5 percent less yield in rain-watered legume crops, such as alfalfa or beans, and in perennial crops, such as fruit trees. But when it comes to major cereal crops, such as corn or wheat, and vegetables, such as broccoli, conventional methods delivered more than 25 percent more yield.
The key limit to further yield increases via organic methods appears to be nitrogen—large doses of synthetic fertilizer can keep up with high demand from crops during the growing season better than the slow release from compost, manure or nitrogen-fixing cover crops. Of course, the cost of using 171 million metric tons of synthetic nitrogen fertilizer is paid in dead zones at the mouths of many of the world’s rivers. These anoxic zones result from nitrogen-rich runoff promoting algal blooms that then die and, in decomposing, suck all the oxygen out of surrounding waters. “To address the problem of [nitrogen] limitation and to produce high yields, organic farmers should use best management practices, supply more organic fertilizers or grow legumes or perennial crops,” Seufert says.