ELEMENTAL NEED

Phosphorus helped save our way of life—and now threatens to end it.

- ELIZABETH KOLBERT

In the fall of 1802, the German naturalist Alexander von Humboldt arrived in Callao, Peru’s major port, just west of Lima. Humboldt had timed his visit to coincide with a transit of Mercury, which he planned to observe through a three-foot telescope, in order to determine Lima’s longitude. He set up his instruments atop a fort on the waterfront, and then, with a few days to kill before the event, wandered the docks. A powerful stench emanating from boats loaded with what looked like yellowish clay piqued his curiosity. From the locals, Humboldt learned that the material was bird shit from the nearby Chincha Islands, and that it was highly prized by farmers in the area. He decided to take some home with him.

When human beings invented agriculture, some ten thousand years ago, they were, almost immediately, confronted with a conundrum. Crops need nutrients to grow, but harvesting them removes the nutrients, leaving the soil unfit for future harvests. Early farmers got around this bind by letting some fields lie fallow; spreading animal waste, including their own, on the land; and planting legumes, which possess restorative properties. But they had no clear idea why these practices worked. By Humboldt’s day, savants in Paris and London were starting to figure out what it was, exactly, that crops required. A Prussian chemist analyzed some of the clay Humboldt had brought home and found that it contained high concentrations of two essential nutrients: nitrogen and phosphorus. Guano offered an answer to the age-old problem of soil exhaustion; as Gregory Cushman, a historian at the University of Kansas, has observed, it “was the Miracle-Gro” of its moment.