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N. Nitrogen. Atomic number seven. Unnoticed, untasted, it nevertheless fills our stomachs. It is the engine of agriculture, the key to plenty in our crowded, hungry world.

Without this independent-minded element, disinclined to associate with other gases, the machinery of photosynthesis cannot function-no protein can form, and no plant can grow. Corn, wheat, and rice, the fast-growing crops on which humanity depends for survival, are among the most nitrogen hungry of all plants. They demand more, in fact, than nature alone can provide.

Enter modern chemistry. Giant factories capture inert nitrogen gas from the vast stores in our atmosphere and force it into a chemical union with the hydrogen in natural gas, creating the reactive compounds that plants crave. That nitrogen fertilizer-more than a hundred million tons applied worldwide every year-fuels bountiful harvests. Without it, human civilization in its current form could not exist. Our planet’s soil simply could not grow enough food to provide all seven billion of us our accustomed diet. In fact, almost half of the nitrogen found in our bodies’ muscle and organ tissue started out in a fertilizer factory.

Yet this modern miracle exacts a price. Runaway nitrogen is suffocating wildlife in lakes and estuaries, contaminating groundwater, and even warming the globe’s climate. As a hungry world looks ahead to billions more mouths needing nitrogen-rich protein, how much clean water and air will survive our demand for fertile fields?

The nitrogen dilemma is most starkly visible in China, a country that loves its food and worries that supplies might run out. To the casual visitor, that anxiety seems misplaced. There’s a feast, it seems, on every street. In a restaurant called San Geng Bi Feng Gang, on the outskirts of Nanjing, I watch with wonder as dishes parade by: steamed fish, fried mutton chops, chrysanthemum-leaf-and-egg soup, a noodle dish made from sweet potatoes, fried broccoli, Chinese yams, steaming bowls of rice.

“Did you always eat this well?” I ask Liu Tianlong, an agricultural scientist who’s introducing me to farmers nearby.

His boyish smile fades, and for a second he looks grim. “No,” he says. “When I was young, you were lucky to get three bowls of rice.”

Liu grew up in the aftermath of China’s great famine, which lasted from 1959 to 1961 and killed an estimated 30 million people. Drought played a part, but the catastrophe was inflicted mainly by the whims of Chairman Mao. The Chinese leader’s Great Leap Forward collectivized farming and forced peasants to turn their harvests over to a centralized bureaucracy.

The famine passed, but scarcity continued until the late 1970s, when farmers regained control of their own harvests. “Within two years, almost overnight, food was in surplus,” recalls Deli Chen, who witnessed those reforms as a boy in a small rice-growing village in Jiangsu Province. Chen is now a soil scientist at the University of Melbourne in Australia.