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Guano is the name given to the collected droppings of seabirds and bats. It can be used as an effective fertilizer due to its high levels of nitrogen. Soil that is deficient in organic matter can be made much 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 is harvested on various islands in the Pacific (for example Nauru) and in other oceans (for example Juan de Nova Island). These islands have been home to mass seabird colonies for many centuries, and the guano has collected to a depth of many metres.