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Those of us who celebrate human technological ingenuity, as I do, must admit that human inventions can have dramatic and unpredictable impacts on the biosphere. While our burning of fossil fuels has increased the atmosphere’s concentration of carbon dioxide by 30 percent since preindustrial times, the volume of the terrestrial nitrogen deposited by humans has more than doubled. This has had enormous impacts on land, oceans, and rivers, enough for limitations on nitrogen use to be one of the planetary boundaries recommended by the expert scientific team as necessary to protect the whole Earth system. But the lesson for environmentalists is equally profound, for the story of nitrogen shows that their favored prescription for agriculture—a worldwide switch to organic farming—cannot possibly feed the world. We are left with a quandary: Humanity is already using too much nitrogen, yet we need even more to feed a growing population. Luckily there are ways through the dilemma, but they are ones that many readers may find unpalatable.

First, a quick chemistry lesson. Like carbon, nitrogen is all around us: 78 percent of the air we breathe is composed of N2, an odorless, colorless, and entirely uninteresting gas. In this form it does absolutely nothing that is even vaguely worth writing—or campaigning—about. Even so, nitrogen in a different form is essential to life. Every living cell needs nitrogen: It makes leaves green, constitutes an essential part of all proteins, forms enzymes, and helps encode genetic information in DNA and RNA. Without nitrogen, our crops would die in the fields and our children would develop the awful starved potbellies of African refugee camps.

One of the few ways reactive nitrogen appears naturally is via the instantaneous high temperatures and pressures created by electrical discharges in thunderstorms. Believe it or not, lightning was our first source of fertilizer, depositing small quantities of nitrates in the soil with rain. The second was less conspicuous: It takes place quietly under the soil, in the nodules that can be found attached to the roots of leguminous plants like clover and beans. And that is about it. Apart from thunder and the actions of a few very specialized microbes, there is no other way to get nitrogen into the active biosphere. No wonder nitrogen shortage is the greatest limiting factor on natural biomass productivity both in land and at sea.

That too is why the story of human agriculture is one of repeated—even structural—famine. Throughout history famine has been a constant presence: In Europe “great famines” saw millions starve to death in conditions of appalling suffering between 1315 and 1317 and in the 1590s. In 1600 half a million Russians died of famine-related starvation.1 It was known that manure was a fertilizer, and in more recent times that intercropping with legumes could increase productivity: But this uses more land, for farmland sown with clover could not also yield wheat for much needed bread. Even so, the amount of nitrogen fixed by cultivated legumes doubled from 1860 to 2000 (from 15 to 30 million tonnes) as farmers struggled to raise yields.2

The constant specter of famine was not unique to Europe; it was historically present also in China and other agricultural societies the world over. Stuck with organic farming, preindustrial humans were battling not just nature, but chemistry itself. In the millennium up to 1800, human GDP had hardly changed at all, as a result of chronically low agricultural productivity. Expressed in 1990 dollar terms, per capita GDP in western Europe was $400 in A.D. 1000, $676 in 1400, and $771 in 1500. By 1820, however, after a thousand years of minimal growth, per capita GDP had doubled, to a still paltry $1,204. For comparison, consider that by 2001 the GDP of 1820 had increased sixteenfold, to $19,256.3 Something had happened—something that freed humanity from the iron grip of what had until then been one of the strictest laws of nature.

THE SPECTER OF FAMINE

By the end of the nineteenth century, scientists were