“I won’t want to join my children in the city,” he said. “There is a Chinese saying that ‘fallen leaves return to the roots.’”

As young people leave for the cities, more small farmers like Mr. Zheng are leasing their land for others to work. That is a monumental shift for a country where small family farms have dominated the rural landscape for centuries.

Other wealthy countries, like the United States, saw farms grow as the rural population shrank. Only relatively recently has that begun to happen in China. In the 1980s, the government broke up the giant communes favored by Mao Zedong and redistributed the rights to farm individual plots to households. Further changes in government policy in the mid-1990s made those land rights secure enough for farmers and others to have the confidence to rent land on a wide scale. China’s agriculture sector is far from being dominated by big commercial farms, as it is in the United States, but the process has begun.

It may sound tragic, as a traditional way of life gives way to modernization, much like the disappearance of the small American family farm. But the transformation is good for China and the entire global economy.