While the data are incomplete, most studies suggest that organic farming uses less energy than conventional. (William Wan/WASHINGTON POST)

A few weeks ago, after a major study showed that organically grown food offers little or no nutritional benefit over the cheaper, conventionally grown equivalent, I began investigating the other major reason people buy organic: saving the environment.

The environmental impact of a product is too complex to cover comprehensively in a few hundred words, so I began with one aspect of it, land use, and looked at how recently released data shows that conventional farming produces more food on less land than organic farming.

Several organically inclined readers of this column were disappointed with that finding, and a few dismissed it as insignificant in the larger picture of environmental impact. Before moving to other aspects of the ecological analysis, I want to briefly discuss the dangers of that viewpoint.

The things we produce are neither wholly good nor wholly bad for the environment. Most of the choices we make involve balancing different kinds of environmental harms. Consider the quandary of paper bags vs. plastic bags. The former require large amounts of water and tree farms on land that could be put to other uses, while the latter involve the extraction of petroleum and take ages to biodegrade. Choosing between the two can seem like a kind of environmental Sophie’s choice.

The same goes for food. As world population grows and the need for calories grows along with it, the environmental benefits of organic farming won’t matter if we have to sacrifice precious acres of biodiversity hot zones and old-growth forest to organic farms.

Let’s move on to energy use. Although the data are incomplete, most studies suggest that organic farming uses significantly less energy than conventional.

The Rodale Institute, which promotes organic farming, has been investigating this question for more than 30 years. It grows organic and conventional corn, wheat and soy side by side on test plots and measures the energy inputs for each. According to the nonprofit organization’s numbers, farming one hectare (about 2.5 acres) of organic corn requires 10,150 megajoules of energy. (That’s the approximate amount of energy in 78 gallons of gasoline.) By contrast, one hectare of conventionally grown corn requires 17,372 megajoules, 71 percent more than the organic crop.

What accounts for this enormous difference? It’s not the pesticides and herbicides that some consumers are most concerned about. Rather, it’s nitrogen-based fertilizer, which represents 41 percent of the energy used in the conventional technique.

“Corn is a heavy feeder, and conventional farmers have to pound their plots with nitrogen,” says Mark Smallwood, executive director of the Rodale Institute. “The manufacture and transport of synthetic nitrogen requires a tremendous amount of oil.”

Organic corn requires nitrogen as well, but it comes from less energy-intensive sources. Some nitrogen comes from the composted manure of dairy and beef cattle. The only environmental impact of the manure is the diesel required to bring it to the farm, because environmental analysts allocate the other energy inputs required to make manure — including feed for the livestock — to the cows’ intended products, such as milk and beef. Organic farmers also use nitrogen-fixing cover crops such as legumes, which not only add nitrogen to the soil but also provide legumes to our grocery stores.

So what about those pesticides and herbicides? From an energy standpoint, they’re not as significant as the fertilizer. Synthetic herbicides account for only 10 percent of the energy consumption of conventional corn production in the Rodale tests, and the pesticides even less than that. (Such chemicals are off-limits to organic farmers.) However, herbicide use may be on the rise on conventional farms, which would increase energy costs.

“The conventional farmers at Rodale are beginning to see super-weeds that are resistant to Roundup,” says Smallwood, referring to a popular weedkiller. “They now have to go back and apply a secondary herbicide.”

A handful of larger meta-analyses, which combine the results of many different studies, have also found organic farms to be better energy misers than their conventional counterparts, and they put the organic advantage at closer to 20 percent per hectare. (In a couple of sectors, including poultry, conventional farming used slightly less energy than organic farming. Organically raised birds don’t grow as quickly.)

It should be noted that farming accounts for only 35 percent of the energy embedded in your food, according to some estimates. The majority of the energy goes to transportation, cooking and the disposal of waste. Going organic can cut your energy consumption and greenhouse gas emissions, but other changes, such as more energy-efficient cooking methods, may have a greater impact.

The same caveat that applied to land use analysis also applies here. Environmental impact cannot be reduced to a single number such as productivity or energy consumption per acre. There are other factors to be considered, and I’ll cover those in a future column.