The word “guano” originates from the Andean indigenous language Quechua, which refers to any form of dung used as an agricultural fertilizer. Archaeological evidence suggests that Andean people collected guano from small islands and points off the desert coast of Peru for use as a soil amendment for well over 1,500 years and perhaps as long as 5,000 years.
Spanish colonial documents suggest that the rulers of the Inca Empire greatly valued guano, restricted access to it, and punished any disturbance of the birds with death. The Guanay cormorant is historically the most abundant and important producer of guano. Of course, if it wasn’t for fish, the cormorant wouldn’t have any food to eat.
With its high levels of nitrogen, guano from bats and birds has been harvested through the ages as a natural fertilizer. Wars have even been fought over the stuff: In 1864 a naval conflict broke out between Spain and Peru over the Chincha Islands, covered in guano deposits said to be over 100 feet tall. Guano played its part in wars, too. When dried, it contains the necessary ingredients for saltpeter, a key ingredient of gunpowder. During the American Civil War, the Confederate Army mined guano from caves to bolster their supplies.
Guano is an ideal fertilizer as it’s made entirely of nitrogen, phosphate and potassium. It contains the chemical elements that improve growth and productiveness of plants. Bat and seal guano are lower in fertilizer value than bird guano, which contains about 11-16 percent nitrogen, 8-12 percent phosphoric acid, and 2-3 percent potash.
Before Europeans “discovered” guano, the Inca took advantage of these properties for hundreds of years. The seacoast Peruvians used no other manure but the dung of sea birds. Thanks to the social habits of the birds that produce it, guano tends to be available in huge, concentrated, perpetually regenerating heaps.
The guano mining process resulted in ecological degradation through the loss of millions of seabirds. Unsustainable guano mining in caves alters cave shape, causing bats to abandon the roost. Guano mining also involved the poor treatment and enslavement of workers such as Chinese immigrants, Native Hawaiians, and Africans. The demand for guano spurred the human colonization of remote bird islands in many parts of the world, resulting in some of the first examples of U.S. colonialism and the expansion of the British Empire.
The arrival of commercial whaling on the Pacific coast of South America contributed to scaling of its guano industry. Whaling vessels carried consumer goods to Peru such as textiles, flour, and lard; unequal trade meant that ships returning north were often half empty, leaving entrepreneurs in search of profitable goods that could be exported. In 1840, Peruvian politician and entrepreneur Francisco Quirós y Ampudia negotiated a deal to commercialize guano export among a merchant house in Liverpool, a group of French businessmen, and the Peruvian government. The biggest markets for guano from 1840–1879 were in Great Britain, Germany, and the United States.
By nationalizing its guano resources, the Peruvian government was able to collect royalties on its sale, becoming the country’s largest source of revenue. Some of this income was used by the state to free its more than 25,000 enslaved Black people. Peru also used guano revenue to abolish the head tax on its indigenous citizens. This export of guano from Peru to Europe has been suggested as the vehicle that brought a virulent strain of potato blight from the Andean highlands that began the Great Famine of Ireland.
The demand for guano led the United States to pass the Guano Islands Act in 1856, which gave U.S. citizens discovering a source of guano on an unclaimed island exclusive rights to the deposits. In 1857, the U.S. began annexing uninhabited islands in the Pacific and Caribbean, totaling nearly 100. Several of these islands are still officially U.S. territories. Conditions on annexed guano islands were poor for workers, resulting in a rebellion on Navassa Island in 1889 where Black workers killed their white overseers. In defending the workers, lawyer Everett J. Waring argued that the men could not be tried by U.S. law because the guano islands were not legally part of the country. The case went to the Supreme Court of the United States where it was decided in Jones v. United States (1890). The court decided that Navassa Island and other guano islands were legally part of the U.S. American historian Daniel Immerwahr claimed that by establishing these land claims as constitutional, the court laid the “basis for the legal foundation for the U.S. empire.” The Guano Islands Act is now considered “America’s first imperialist experiment.”
The 19th-century guano trade played a pivotal role in the development of modern input-intensive farming, but its demand began to decline after the discovery of the Haber–Bosch process of nitrogen fixing led to the production of synthetic fertilizers. In 1913, a factory in Germany began the first large-scale synthesis of ammonia using German chemist Fritz Haber’s catalytic process. The scaling of this energy-intensive process meant that farmers could cease practices such as crop rotation with nitrogen-fixing legumes or the application of naturally derived fertilizers such as guano. The international trade of guano and nitrates such as Chile saltpeter declined as artificially synthesized fertilizers became more widely used. With the rising popularity of organic food in the 21st century, the demand for guano has started to rise again.
Enough history. In my next article, I’ll write about animal manure and compost — on the list of organic fertilizers.
Ron Krupp is the author of The Woodchuck’s Guide to Gardening and The Woodchuck Returns to Gardening. He is working on his third Vermont garden book, The Woodchuck’s Guide to Landscape Plants and Ornamentals.