But these same soils present a sustainability concern on a different front. After millions of years of heavy rainfall, Mato Grosso’s soils have lost nearly all their phosphorus. The soils efficiently remove phosphorus before it reaches the streams but also bind phosphorus added as fertilizer, leaving less for the crops. As a result, farms here require twice as much phosphorus fertilizer as their counterparts in more temperate regions, where the soils are younger and more fertile. And phosphate ore, the source of phosphorus fertilizer, is a finite resource.

The world’s phosphorus reserves are held by a handful of countries — the exact distribution is disputed, but possibly half of it is in Morocco alone. Much of Morocco’s phosphate ore is carried 61 miles across the Western Sahara to the Atlantic, on the world’s longest conveyor belt. A farm might not be sustainable if it depended on political stability in faraway countries. And how should we weigh the negative of phosphorus depletion against the positive of reduced water pollution? We don’t yet know the answers.

The fertilizer story makes it complicated to figure out the sustainability of Mato Grosso’s farms, which send their soy to China, as animal feed, and to Europe. Even more complicated is the question of how these farms will affect the global climate. The link, while not obvious, is important. Global warming results largely from burning fossil fuels. But another important contributor — about 15 percent of our carbon-dioxide emissions — comes from changing land use, primarily the burning of tropical rain forests to make room for food or biofuel crops. If we really want to know the environmental impact of these soy farms, we need to understand their effect on carbon-dioxide emissions.

Natural science alone cannot answer this question, so I increasingly find myself talking to economists and sociologists. Because at the end of the day, the carbon footprint of these farms will depend on whether their economic success leads to more deforestation. If so, their carbon footprint will be huge. This would be a deathblow to their sustainability.

On the other hand, these soy fields produce more calories than the low-productivity pastures they replaced. Remember options one and two: To feed a growing population we will need to either farm more land, or to get more food out of the land we farm. Either option will need to be coupled with efforts to reduce food waste and to feed fewer crops to animals.

For now, deforestation rates in Mato Grosso have slowed, as pastures have been converted to soy. It may be that megafarms, unlike the small-scale landholders they replace, are more reticent to take on the sanctions that come with illegal deforestation, though there are certainly other potential explanations. We don’t know if the slowdown in deforestation will continue. If it does, and if big soy is a reason, one could argue that these fields are a boon to the global environment far greater than your local organic farm.

One thing is clear: In the coming decades we will need to produce a lot more food. I’m not suggesting Mato Grosso’s farms are the answer, far from it. But it’s time to move beyond the oversimplification that large-scale agriculture is incompatible with environmental goals. There are vast areas of the tropics with similar soils. They are likely to be the megafarms of the future.

We need to admit that food production is going to be the dominant use of land in the 21st century, and to decide whether we are going to farm more land or farm more intensively. Then we can move on to the grand challenge of making our farms sustainable.