CHENG TIANQING forages among the stems of a plant and with a loud rustle and a fling of soil pulls out something he says will dispel the hunger of millions of Chinese."We are seeing a revolution in nutrition," says Dr. Cheng, cradling the bit of biota in his palm and stroking away the dirt from its smooth brown skin. People have eaten potatoes for 6,000 years, but never have the one out of five humans who are Chinese dug up so many spuds. For many Chinese farmers and agronomists like Cheng, the potato has become the lodestone of foods. Despite a longstanding popular bias against the potato, millions of Chinese farmers in the past several years have made the tuber their vital source of nutrition. China has quadrupled its potato crop in the past three decades and now grows one out of every two potatoes in the developing world. The potato is especially promising and popular for millions of China's hungriest people: those living in remote mountains on scant arable land, scientists say. If Chinese scientists have their way, the spud in coming years will budge aside staple grains and claim a bigger place in the national larder. China is one of many developing countries that are substituting the potato for rice, wheat, and other grains. As the rise in the yields of staple grains levels off, third-world countries will become increasingly dependent on the tuber to sustain their rapidly growing populations, say Chinese and foreign scientists. "There is no crop that can provide a higher yield or better nutrition on limited land than the potato," says Peter Vander Zaag, director for the International Potato Center (CIP) in Southeast Asia from 1982 until 1990. Developing nations should model their potato promotion after China's systematic program to produce and distribute hybrid potato seeds, say the scientists. For 30 years potato breeders in China have progressively identified hybrids that are more tasty, nutritious, higher-yielding, and resistant to virus and fungus. Then they have relied on China's highly centralized agricultural system to get the hybrid seeds into the ground nationwide. Beijing launched its "potato push" in the early 1950s out of fear of imminent invasion and famine caused by an international trade embargo. Its hybrid seed program anticipated that of CIP - the leading booster of the potato in the developing world - by nearly 20 years, says Elmer Ewing, chairman of the department of fruit and vegetable science at Cornell University. China could hardly have picked a more nutritious crop. The potato yields more food per acre than grains do and is extraordinarily nutritious, brimming with vitamins B and C, complex carbohydrates, niacin, magnesium, potassium, and iron. Less than 1 percent of the food is fat. Moreover, an acre of potatoes produces twice as much protein as an acre of wheat. And it packs a better quality protein than even the soybean. Although millions of Chinese now regard the potato as a gem of nutrition, the tuber still has detractors in China. Compared with staple grains, the potato spoils quickly and is hard to haul and store, critics say. City dwellers and well-off farmers tend to snub the potato as a food for China's minorities and poor. "In the south, people eat rice; in the north, people eat wheat - these habits can't be easily changed," Cheng says. "Chinese people are silly. They think the potato is only a poor man's food," says Kang Yulin, a potato breeder at the Institute of Vegetables and Flowers in Beijing. While urging Chinese to embrace the potato as a staple food, the institute primarily ministers to believers in the tuber's benefits. Under a $377,000 project ending in 1996, the institute aims to raise the area sown with its hybrid seeds from 10 percent to 50 percent of the 7.4 million acres in China devoted to the potato. Meanwhile, back in the institute's greenhouses, potato breeders will continue their quest for the perfect spud. "In the past our varieties achieved very high yields but their shape was very ugly: misshapen with deep-set eyes and bumpy skin," says Cheng, a director of potato breeding at the institute. "We want a full, smooth, oblong potato with shallow eyes and a yellow skin and flesh that makes people hungry."