There are two sides — active, vocal sides — to just about every food-supply issue on the planet. Are genetically modified organisms, organics, pesticides or conventional livestock good or bad? Depends whom you ask. There is one issue, however, that gets universally bad press. Nobody, but nobody, defends monocrops.

I’m not exactly going to step into the breach — this month, monocrops; next month, Stalin! — but I think any discussion of our food supply has to include a look at just what monocrops are, why farmers sometimes choose them, and the degree to which they’re risky.

A monocrop is exactly what it sounds like. A monogamist has one spouse, a monoglot speaks one language and a monocrop is one plant growing in the same place, year after year.

There are two problems with monocrops. The first is that they are not conducive to good soil health. The second is that, when all your eggs are in one basket, you’re vulnerable to a devastating loss; think Irish potato famine. Half of our 300 million farmed acres are planted with corn and soy, and that’s a very big basket.

Of the two issues, famine sounds scarier, but it’s actually less likely to be a problem. Tim Griffin, director of the Agriculture, Food and Environment program at Tufts University’s Friedman School of Nutrition Science and Policy, says our vulnerability is limited, mostly because we don’t eat those crops directly. An event like the drought of 2012 affects meat and dairy (and ethanol) prices, but humans still have plenty to eat. He also points out that staple crops such as corn, soy, wheat and rice provide most of the world’s calories, so it makes sense that they take up a big slice of our farmland.

Soil health is another matter. Growing only one plant tends to deplete the soil’s nutrients over time, and leaving fields bare for the winter can hasten erosion. Monocrops also provide a friendly home for pests that happen to like that crop, since it shows up reliably, every spring.

“There’s a consensus that monocrops are bad,” says Griffin, but not all monocrops are the same, and a monocrop today isn’t necessarily an arid wasteland tomorrow. Take the mother of all monocrops: the wheat that has been grown continuously in English fields since the 1840s. In some areas, the wheat stalks were left in the fields. In others, they were removed. “They have data on characteristics of the soil that go back to 1840,” Griffin says, “and they show that growing wheat for 175 years is a bad idea, and removing the straw is worse, but it reaches an equilibrium.” That equilibrium isn’t as productive as well-managed cropland, but neither is it a dust bowl.

If monocropping is unequivocally bad for soil health, why would farmers choose to do it? Most of the time, here in the United States, they don’t. Steven Wallander, an economist with the United States Department of Agriculture, tracks which crops are grown on what land, and it turns out that the vast majority of land under cultivation supports a rotation of two or more crops. The most recent data indicate that 16 percent of corn, 14 percent of spring wheat and 6 percent of soybean acreage is continuously planted with one crop over a three-year period.

Much more common is what I’ll call a duocrop. Although precise numbers aren’t available, Wallander says it’s reasonable to estimate that more than half of our corn acres are in a rotation that includes soybeans. I asked Griffin how much better that duocrop is than a monocrop. “It’s a little better,” he said, unenthusiastically. He points out that you still have the problem of crops being planted in the spring and harvested in the fall, with fields bare over the winter. “Ecologically, and in terms of soil management, it’s still a simple system.”

So why do it? Wallander and Griffin have the same answer: economics. Planting only one or two crops can make sense for some farmers in some situations. “There’s an economic advantage to specialization,” says Griffin. “One of the reasons for the duoculture is that the equipment for corn and soy is identical. If you add one more crop, and grow wheat, just that one change requires a specialized planter.” He adds that there are marketing concerns. The farmer who takes corn and soy to a local grain dealer might not have an outlet for potatoes.

It strikes me, though, that if you want to know why farmers do something, it makes sense to ask farmers. Garry Niemeyer grows corn and soy on 2,100 acres in Illinois, and he sometimes plants corn continuously because he can yield 230 bushels an acre, which makes corn more profitable for him than soy. He’s perfectly aware that continuous planting will degrade his soil, and he rotates in other crops before that happens. “Two years of corn and one year of soy works pretty well for us,” he says.

Richard Wilkins also grows corn and soy, as well as a variety of vegetables, on the Delmarva peninsula, and all of his 1,000 acres get rotated. Some of them are in a corn-soy rotation, for the simple reason that those acres aren’t irrigated and can’t support other crops. When irrigation isn’t a problem, says Wilkins, something else is. “There are parts of the country where farmers might grow other crops if the market provided for them to be able to do it,” he says. “And there some regions in the Midwestern states that do have some vegetable production, but there are different types of soil,” some of which don’t lend themselves to growing vegetables.

Soybean plants grow in Illinois. Many farmers rotate soybean and corn crops, planting them in alternate years, because they can be grown using the same equipment. (Seth Perlman/Associated Press)

Maintaining soil health is the central tenet of organic farming, but I’ve never met a farmer, organic or conventional, who wasn’t concerned about it. Crop rotation, even if it’s just the two crops, is one way farmers of commodity crops are balancing the need to keep their farms healthy with the need to grow the plants they can sell. According to the USDA, no-till systems, which help prevent soil erosion and nutrient runoff, are on the rise, and about a quarter of corn acres, and almost half of soy acres, are farmed that way. Although the USDA doesn’t track cover cropping (planting an interim crop like rye grass or clover, specifically to enhance soil health), every source I spoke with says it appears to be on the rise.

Still, a system in which two crops dominate is distinctly sub-optimal, and it’s perfectly reasonable to point to monocropping as a problem. What isn’t reasonable is expecting farmers to lead the charge for change. “Farmers will produce what the market asks them to produce,” says Wilkins, and I think that’s the crux of the issue. A complex series of factors, from government subsidies to consumer preferences, has built a food supply with an almost insatiable appetite for corn and soy.

If farmers can’t change things, who can? I can think of two ways to start tackling our monocrop problem. The first is to re-jigger farm subsidies (and regulations on ethanol; 30 percent of the corn crop goes to fuel), which could change the economic reality for farmers. The second is to cut back on our consumption of the meat and processed foods that most of our corn and soy goes into. Worried about monocrops? Look in your pantry, and see if you can’t help solve the problem.

Haspel, a freelance writer, farms oysters on Cape Cod and writes about food and science. On Twitter: @TamarHaspel. She’ll join today’s Free Range chat at noon: