Guano (via Spanish, ultimately from the Quechua wanu, meaning "dung") is the feces and urine of seabirds, cave-dwelling bats, and seals. Guano manure is a highly effective fertilizer due to its high phosphorus and nitrogen content and its relative lack of odor compared to other forms of organic fertilizer such as horse manure.
Guano consists of ammonium oxalate and urate, phosphates, as well as some earth salts and impurities. Guano also has a high concentration of nitrates. Seabird guano typically contains 8 to 16 percent nitrogen (the majority of which is uric acid), 8 to 12 percent equivalent phosphoric acid, and 2 to 3 percent equivalent potash. Bat and seal guano are lower in fertilizer value than seabird guano because of their lower nitrogen contents.
The word "guano" originates from the Quichua language of the Andes and means "the droppings of sea birds". Andean peoples collected guano from small islands located off the coast of Peru for use as soil enricher. On the basis of archaeological objects recovered from some of the Peruvian guano islands, which display stylistic elements characteristic of the Moche people, Andean people had visited the islands for well over 1,000 years. Spanish colonial documents suggest that the rulers of the Inca Empire assigned great value to guano, restricting access to it and punishing any disturbance of the birds with death.
Guano has been harvested over several centuries along the coast of Peru, where islands and rocky shores have been sheltered from humans and predators. The Guanay Cormorant has historically been the most important producer of guano; its guano is richer in nitrogen than guano from other seabirds. Additionally, the guanay cormorant has historically been much more abundant than the other species of guano birds. Other important guano producing species off the coast of Peru are the Peruvian Pelican and the Peruvian Booby.
During the guano boom of the 19th century, the vast majority of seabird guano was harvested from the Peruvian guano islands. During the height of the guano boom, massive deposits of guano existed on the islands, in some cases more than 50 m deep. Today, the majority of islands contain less than 5 cm of accumulated guano, with the Peruvian government heavily regulating the current harvesting and exportation of the guano.
The high concentration of nitrates also made guano an important strategic commodity. The discovery during the 1840s of the use of guano as a fertilizer and its Chile saltpetre content as a key ingredient in explosives made the area strategically valuable.
In this context the United States passed the Guano Islands Act in 1856, giving citizens discovering a source of guano the right to take possession of unclaimed land and entitlement to exclusive rights to the deposits. The guano, however, could only be removed for the use of citizens of the United States. This enabled U.S. citizens to take possession of unoccupied islands containing guano.
Control over guano played an important role in the Chincha Islands War (1864–1866) between Spain and a Peruvian-Chilean alliance since Spain occupied with its navy the Chincha Islands depriving Peru of lucrative income.
Chinese coolies played an important role in guano harvest. The first group of 79 Chinese coolie arrived in Peru in 1849. Before the coolie trade ended almost a quarter of a century later, over 100,000 of their fellow countrymen had been imported. Almost all the coolies recruited were employed by the recruiting agents ranging from use of opium to deception and violence.
In the second half of the 19th century guano extraction was eclipsed by saltpetre in the form of caliche extraction from the interior of Atacama Desert, not far from the guano areas. After the War of the Pacific (1879–1883) Chile seized much of the guano as well as salpeter-producing area making its national treasury grow by 900% between 1879 and 1902 due to taxes coming from the newly acquired lands.
The importance of guano deposits faded after 1909 when Fritz Haber developed the Haber-Bosch process of industrial nitrogen fixation (nitrogen gas from the air converted into liquid ammonia fertilizer). The Haber process is important today because the fertilizer generated from ammonia is responsible for sustaining one-third of the Earth's population. It is estimated that half of the protein within human beings is made of nitrogen that was originally fixed by this process, the remainder was produced by nitrogen fixing bacteria and archaea.
The ideal type of guano is found in exceptionally dry climates, as rainwater drains the guano of nitrates. Guano is harvested on various islands in the Pacific Ocean (for example, the Chincha Islands) and in other oceans (for example, Juan de Nova Island and Christmas Island). These islands have been home to mass seabird colonies for many centuries, and the guano has collected to a depth of many metres. In the 19th century, Peru was famous for its supply of guano.
Bat guano is usually mined in caves and this mining is associated with a corresponding loss of troglobytic biota and diminishing of biodiversity. Guano deposits support a great variety of cave-adapted invertebrates, that rely on bat feces as their sole source of nutrition. In addition to the biological component, deep guano deposits contain local paleoclimatic records in strata that have built up over thousands of years, which are unrecoverable once disturbed.
The greatest damage caused by mining to caves with extant guano deposits is to the bat colonies themselves. Bats are highly vulnerable to regular disturbance to their roosts. Some species, such as Phyllonycteris aphylla, have low fat reserves, and will starve to death when regularly disturbed and put into a panic state during their resting period. Many species will drop pups when in panic, with subsequent death, leading to a steady reduction in population. Research in Jamaica has shown that mining for bat guano is directly related to the loss of bat species, associated invertebrates and fungi, and is the greatest threat to bat caves on the island.
In agriculture and gardening guano has a number of uses, including as: soil builder, lawn treatments, fungicide (when fed to plants through the leaves), nematicide (decomposing microbes help control nematodes), and as composting activator (nutrients and microbes speed up decomposition).
See also 
- "Seabird Guano and Bat Dung". 2004-11-25. Retrieved 2013-05-11.
- Szpak, Paul; Longstaffe, Fred J.; Millaire, Jean-Francois; White, Christine D. (2012). "Stable Isotope Biogeochemistry of Seabird Guano Fertilization: Results from Growth Chamber Studies with Maize (Zea mays)". PLoS One 7: e33741.
- Szpak, Paul; Millaire, Jean-Francois; White, Christine D.; Longstaffe, Fred J. (2012). "Influence of seabird guano and camelid dung fertilization on the nitrogen isotopic composition of field-grown maize (Zea mays)". Journal of Archaeological Science 39 (12): 3721–3740. doi:10.1016/j.jas.2012.06.035.
- "h2g2 - Seabird Guano and Bat Dung". Bbc.co.uk. Retrieved 2013-01-26.
- Crow, The Epic of Latin America, p. 180
- Wolfe, David W. (2001). Tales from the underground a natural history of subterranean life. Cambridge, Mass: Perseus Pub. ISBN 0-7382-0128-6. OCLC 46984480.
- BBC: Discovery - Can Chemistry Save The World? - 2. Fixing the Nitrogen Fix
- "Jamaican Bat Guano and Cave Preservation", by Jamaican Caves Organisation.
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