Guano is, in a nutshell, animal poop—but with one important qualifier. It is useful animal poop. It is so rich in nutrients that it can be used as fertilizer.
The term was first used in Peru, where people had to make a clear distinction between the ordinary excrement from the “beneficial” ones collected from pelicans and other sea birds. Today, guano is also used to refer to the droppings of seals and bats.
South American farmers were one of the first to harvest animal droppings for crop fertilizer; the practice went on for several centuries before the “industry” was formalized and actually made into a leading source of economic revenue. In fact, Cuba’s agricultural and economic history is strongly linked to guano, and bat guano from Asia, United States and South America is still ranked as the best and safest fertilizer in the world (“safe” because it is organic).
Guano is rich in nitrogen and phosphorous, which can help plants thrive. It also contains fungi and bacteria that, ironically, protect the plant because it kills off more predatory microorganisms. Bat guano is considered the best of all guano because it is found in dark, deep caves, shielded from sunlight and less “fragile” and prone to breaking down.
Interestingly, guano varies from animal to animal and region to region—apparently the diet and environment do affect chemical composition. Farmers can even ask for a particular kind of bat guano, based on the species and area from which it was harvested.