Low carbon levels leave the ground nutrient-poor, requiring ever-greater amounts of fertilizer to support crops. They also make for thin soil that is vulnerable to erosion and less able to retain water, so yields suffer quickly in times of drought.

To bring levels back up, a set of techniques known as carbon farming, or regenerative farming, encourage and complement the process by which plants draw carbon dioxide from the atmosphere, break it down and sequester carbon into soil. They include refraining from tilling, or turning, the soil; mixing crops together rather than growing large fields of just one type; planting trees and shrubs near or among crops; and leaving stalks and other cuttings on fields to decay.

Mr. Brown keeps his fields planted for as much of the year as possible to minimize nutrient loss. When he mixes clover and oats in the same field, the clover fixes nitrogen into the soil. After the oats are harvested, livestock graze the clover and leave their manure behind.

Such strategies have allowed him to stop using synthetic fertilizers and pesticides, reducing costs. And the rich soil not only yields higher volumes, but the crops are more nutritionally dense than those grown on depleted land, he says.

“Economically, it’s much, much, much more profitable,” he said.

Mr. Brown’s approach is very different from the techniques of industrial-scale farming that have taken hold in the United States and other wealthy countries, where single crops stretch over many acres, and fertilizers and pesticides are used heavily.

Things are worse in poorer nations, where farmers’ desperation often means they are unable to care for the soil, Mr. Lal said. He recalled seeing a Mexican sharecropper carting corn straw away from the fields to sell: “I said, ‘Why don’t you leave it on the land? The land will be better next year.’ And he said, ‘This land will not be mine next year, and I need money now.”’

There is some momentum behind a shift. The French government, which helped broker last year’s landmark Paris Agreement on climate change, is pushing an effort to increase soil carbon stocks by 0.4 percent annually, which it says would halt the rise in atmospheric carbon dioxide levels.