China's small farms are fading. The world may benefit.

Zheng Nanda worked the fields that surround this village in the northern province of Shanxi for more than four decades, often behind a plow pulled by cows. He is now in his early 70s and too old for such arduous labor. His children long ago left for jobs in the city and have no interest in farming. So Mr. Zheng became an unlikely agent of change. He has rented almost all of his small plot to other farmers, who work it using modern equipment. The $500 a year he earns in rental income helps keep him comfortable in his neatly manicured courtyard home.“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. As these small farmers bow out, Zheng Chenggong, 27, is taking their place. (As in many rural villages in China, residents of Shanhui share a handful of surnames.) Twenty years ago, his father tilled a small plot of about two acres. Since then, Mr. Zheng and his parents have amassed more than 160 acres by renting plots from the local government and other villagers who have given up.The result is a thriving business cultivating corn and carrots. Mr. Zheng invested in planters, pesticide sprayers and other equipment, including a new, shiny red harvester, parked in a lot behind his modest home. Piles of corn are stored in a warehouse next door. During autumn, he employs over 100 people from about 10 villages to harvest his carrots.