The project's old pickup needs a push to get it going, but soon Boyd Zimba, the project's assistant coordinator, and Zacharia Nkhonya, its food-security supervisor, are rattling down the road, talking about what they see as the downside of the Malawi Miracle. "First, the fertilizer subsidy cannot last long," says Nkhonya, a compact man with a quick smile. "Second, it doesn't go to everyone. And third, it only comes once a year, while legumes are long-term—soils get improved every year, unlike with fertilizers."

At the small village of Encongolweni, a group of two dozen SFHC farmers greet us with a song about the dishes they make from soybeans and pigeon peas. We sit in their meetinghouse as if at an old-time tent revival, as they testify about how planting legumes has changed their lives. Ackim Mhone's story is typical. By incorporating legumes into his rotation, he's doubled his corn yield on his small plot of land while cutting his fertilizer use in half. "That was enough to change the life of my family," Mhone says, and to enable him to improve his house and buy livestock. Later, Alice Sumphi, a 67-year-old farmer with a mischievous smile, dances in her plot of young knee-high tomatoes, proudly pointing out that they bested those of the younger men. Canadian researchers found that after eight years, the children of more than 7,000 families involved in the project showed significant weight increases, making a pretty good case that soil health and community health are connected in Malawi.

Which is why the project's research coordinator, Rachel Bezner Kerr, is alarmed that big-money foundations are pushing for a new green revolution in Africa. "I find it deeply disturbing," she says. "It's getting farmers to rely on expensive inputs produced from afar that are making money for big companies rather than on agroecological methods for using local resources and skills. I don't think that's the solution."

Regardless of which model prevails—agriculture as a diverse ecological art, as a high-tech industry, or some combination of the two—the challenge of putting enough food in nine billion mouths by 2050 is daunting. Two billion people already live in the driest parts of the globe, and climate change is projected to slash yields in these regions even further. No matter how great their yield potential, plants still need water to grow. And in the not too distant future, every year could be a drought year for much of the globe.

New climate studies show that extreme heat waves, such as the one that withered crops and killed thousands in western Europe in 2003, are very likely to become common in the tropics and subtropics by century's end. Himalayan glaciers that now provide water for hundreds of millions of people, livestock, and farmland in China and India are melting faster and could vanish completely by 2035. In the worst-case scenario, yields for some grains could decline by 10 to 15 percent in South Asia by 2030. Projections for southern Africa are even more dire. In a region already racked by water scarcity and food insecurity, the all-important corn harvest could drop by 30 percent—47 percent in the worst-case scenario. All the while the population clock keeps ticking, with a net of 2.5 more mouths to feed born every second. That amounts to 4,500 more mouths in the time it takes you to read this article.

Which leads us, inevitably, back to Malthus.