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.

Peru’s Indigenous people had been collecting guano from the Chincha Islands for centuries. (The word “guano” comes from the Quechua wanu.) But once Europeans decided to exploit the islands—they were delayed for a few decades by the Napoleonic Wars and the campaigns of Simón Bolívar—the Peruvian government enthusiastically extinguished all Native claims. In 1840, it agreed to a monopoly arrangement with some European merchants, and in the next fifteen years more than a million tons of guano made their way from Peru to the United Kingdom. The miserable work of harvesting the stuff was largely performed by Chinese laborers, under conditions of near-slavery.

By the mid-eighteen-forties, American farmers, too, had become gung ho for guano, and they were furious that the United States had failed to secure a steady supply. In 1850, President Millard Fillmore moved to rectify this situation, declaring that guano had “become so desirable an article” that it was incumbent on Washington to use “all the means properly in its power” to obtain it. In the spring of 1856, William Henry Seward, then a senator from New York, proposed what would become known as the Guano Islands Act; the bill, which became law later that year, deputized U.S. citizens to claim for their country any poop-covered “island, rock, or key not within the lawful jurisdiction of any other government.”

A rush to some of the world’s most remote landmasses ensued. Within three years, the United States had staked claims to nearly fifty islands, including those of Midway Atoll, in the North Pacific. The Baltimore American and Commercial Advertiser described these islands as the equivalent of “a new El Dorado” and proclaimed that although they possessed no actual gold, they would cover this country’s “wasted fields with golden grain.” (Seward would later engineer the purchase of Alaska, which critics dubbed Seward’s Icebox; by analogy, one historian has suggested that the U.S.’s guano islands might be considered Seward’s Outhouse.)

Guano exports from Peru peaked in 1870. Then they dropped dramatically. The shit exported to farms in Europe represented the cumulative output of millions of birds in the course of hundreds of generations. Once it had been shipped off, the birds that remained—many had seen their nesting grounds destroyed—couldn’t poop fast enough to keep up with demand. America lost interest in its de-guanoed islands. Most were eventually ceded to other countries; only a handful, like Midway, remain U.S. possessions.

But the end of the boom proved to be the beginning of something much bigger. Chemists identified other deposits of nitrogen and phosphorus, which replaced guano. When these sources were, in turn, exhausted, others were discovered, or, in the case of nitrogen, invented. Farmers can now purchase fertility as readily as they might buy seeds or plows. The result is a world awash in nutrients. This has created a new conundrum: How do we feed the planet without poisoning it?