The Strait of Hormuz isn’t fertilizer’s first starring role — and 1856 law still defines U.S. overseas territory

The closure of the Strait of Hormuz, through which roughly one-third of the world’s fertilizer normally passes, has made clear that access to nutrients for crops remains as strategically important in the 21st century as access to oil.

As an environmental historian, I study how fertilizer transformed the modern world. That history is about more than scientific innovation or agricultural progress. It also reveals the hidden costs of modern food production, including the brutal treatment of workers and the expansion of imperial power.

Even before the Iran war began, Russia’s invasion of Ukraine had disrupted natural gas markets and the global trade in nitrogen fertilizers; sanctions on Belarus had unsettled global markets for potash, one of agriculture’s principal sources of potassium; and China had restricted phosphate exports in an effort to stabilize domestic prices.

Even further back, long before synthetic fertilizers transformed farming in the 20th century, farmers increasingly relied on a scarce and geopolitically contested resource: seabird guano. Here is how bird droppings reshaped global agriculture and helped build an overseas empire for the United States.

The first Green Revolution

During the 19th and early 20th centuries, fertilizer companies shipped millions of tons of dried bird excrement from islands in the Pacific and Caribbean to farms across Europe and North America. Rich in nitrogen and phosphorus, guano transformed agricultural productivity at a time when expanding populations and industrializing societies demanded more food than traditional farming systems could produce.

Guano became the first fertilizer commodity traded on a truly global scale. For centuries, farmers had sustained soil fertility by recycling locally available nutrients, including animal manure, crop residues and human waste. Those nutrients circulated within largely closed ecological systems linking soils, crops, livestock and people.

As industrialization accelerated during the 19th century, the local nutrient cycles that had sustained farming for centuries began to unravel. Expanding cities drew food from increasingly distant farms but returned few nutrients to the countryside. Instead, material that used to fertilize farms accumulated in urban streets, rivers and sewers, while rural soils grew steadily poorer with each harvest. Farmers searched for new ways to replenish depleted fields as they struggled to feed rapidly growing populations.

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