Guano (from the Quechua ‘wanu’, via Spanish) is the excrement (feces and urine) of cave dwelling insectivorous bats, seabirds, and seals. Guano manure is an effective fertilizer and gunpowder ingredient due to its high levels of phosphorus and nitrogen and also its lack of odor. Superphosphate made from guano is used for aerial topdressing. Soil that is deficient in organic matter can be made more productive by addition of this manure.
Guano consists of ammonia, along with uric, phosphoric, oxalic, and carbonic acids, as well as some earth salts and impurities. Guano also has a high concentration of nitrates.
Currently vast volumes of phosphorus are needed to produce fertilizer, as it is an essential plant macronutrient. Guano is rich in phosphorus and is an intensely effective phosphorus fertilizer.
The word “guano” originates from the Quichua language of the Inca civilization and means “the droppings of sea birds”. Incas collected guano from the coast of Peru for use as soil enricher. The Incas assigned great value to guano, restricting access to it and punishing any disturbance to 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. Other important guano producing species off the coast of Peru are the Peruvian Pelican and the Peruvian Booby.
In November 1802, Alexander von Humboldt studied guano and its fertilizing properties at Callao in Peru, and his subsequent writings on this topic made the subject known in Europe.
The high concentration of nitrates also made guano an important strategic commodity. The War of the Pacific (1879 to 1883) between the Peru-Bolivia alliance and Chile was primarily based upon Bolivia’s attempt to tax Chilean guano harvesters and over control of a part of the Atacama Desert that lies between the 23rd and 26th parallels on the Pacific coast. 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 US 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. However, the guano could only be removed for the use of citizens of the United States. This enabled US citizens to take possession of unoccupied islands containing guano.
By the end of the 19th century, the importance of guano declined with the rise of artificial fertiliser, although guano is still used by organic gardeners and farmers.
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 invertebrate species, which rely on bat faeces as their sole nutrient input. 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).