A tomato from organic/local CSA, Arganica Farm Club, in DC. Organic food is no better than regular food, a study finds. What about the environmental impact of each? (Wendy Galietta/WASHINGTON POST)

For most people willing to pay a premium for organic food, the reason is some nebulous combination of health, environment and flavor. If pressed, however, few could point to evidence that organic food is better than conventionally grown products on any of those counts.

This month, a new piece of evidence emerged on this issue, and the news was bad for organic agriculture. A report out of Stanford University found no consistent differences in nutritional content between organic and conventionally grown crops. So how about the other reasons for buying organic? Let’s start by looking at the environmental differences between conventional and organic farming.

Comparing the environmental impacts of two products involves many considerations: chemical runoff into our water supply, soil health and greenhouse gas emissions, to name just a few. Since there isn’t enough space to consider all of those in a single article, we’ll take one environmental consideration at a time in an occasional series.

Land use is a reasonable place to begin. Part of a product’s environmental impact is the amount of space required to produce it. After all, if a plot of land weren’t required to produce corn or avocados or pigs, it could be used for environmentally helpful purposes such as habitats for wild animals or carbon-eating plants or land for solar panels and wind farms.

Food production requires an incredible amount of land. By some estimates, croplands and pastures now occupy 40 percent of Earth’s land surface. Producing more food with less land could be a major win for the environment.

In May, researchers at Canada’s McGill University and the University of Minnesota published an article in the journal Nature comparing the productivity of organic and conventional farms.

This particular study is known as a meta-analysis. In a meta-analysis, a researcher compiles all of the studies on a particular issue, usually discarding those that are methodologically unsound, then finds a statistical method with which to combine them. Ultimately, a meta-analysis turns a series of smaller studies into a large study, which, if done right, carries more persuasive heft and can bring real clarity to disputed scientific issues.

In their meta-analysis, McGill’s Verena Seufert and her colleagues examined 66 previous studies. Their results are bound to disappoint organic advocates. Overall, they found that organic methods produce 25 percent less food than conventional farming on the same land area.

That headline number, however, tells only part of the story, and the details can help guide your decisions as a consumer.

For some crops, organic methods are nearly as productive as conventional farming. Organic fruit farms, for example, finished in a statistical dead heat with conventional acreage. The yield of organically grown tomatoes (considered separately from other fruits) was statistically indistinguishable from conventional tomatoes as well, as were organic oilseeds such as sunflower and canola. Organic legumes, such as peas and beans, also performed well.

Conventional methods soundly outpaced organic among grains and vegetables. Among wheat, corn and other cereals, organic farms were 26 percent less productive, while organic vegetable growers turned out 33 percent less food per acre than those who used pesticides, herbicides and synthetic fertilizer.

If you’re wondering why organic productivity might be as good as non-organic for some crops but not for others, it has a lot to do with nitrogen. Organic farmers can’t load up their fields with synthetic fertilizer, so crops that use nitrogen more efficiently, such as legumes, perform better in an organic system than those that rely on heavy infusions of nitrogen.

Relative productivity of organic and conventional fields can also depend on more obscure factors, such as the pH of soil. Organic crops perform well when the soil’s pH stays inside a certain range, but they do poorly in high-alkaline or high-acidity environments. Unfortunately, consumers standing in front of a bell pepper display would have a hard time figuring out the acidity of the soil that produced them, so you might as well forget about those details.

Meta-analyses are not perfect, and organic yields were found to be better in a study that predated Seufert et al. Those results, however, came in for serious criticism, and the more recent meta-analysis seems to have solved some of the methodological problems.

None of this is to say you should stop buying organic corn, wheat or vegetables. Overall organic yields in Seufert’s meta-analysis were depressed somewhat by farmers who weren’t applying the right nutrients in the right quantities at the right time. Farmers who manage their crops more expertly come quite close to matching conventional yields.

Over time, it’s entirely plausible that increased demand for organic food will improve techniques further, with investment funding research on best practices and improving farmer know-how. And, as noted above, land use is just one of many considerations when comparing the environmental impacts of two products. Stay tuned to this space for further looks at the environmental impacts of organic vs. conventional.