The use of manure and composts as fertilizers to replace the nutrients that crops remove from the soil is probably almost as old as agriculture.
One historically important manure is guano. It is exceptionally high in nitrogen, phosphate and potassium, which are all key nutrients essential for plant growth.
Guano is the accumulated excrement of sea birds and bats in places where they tend to reside and nest. Andean people collected guano from small islands and points off the desert coast of Peru for use as a soil enrichment for well over 1,500 years and perhaps as long as 5,000 years.
In the past, this guano was important to the prosperity of the Inca Empire by increasing agricultural productivity. The Inca implemented regulations to safeguard the breeding grounds of the sea birds, and violations were punishable by death.
Today, the Peruvian government has very strict rules governing the mining and distribution of this guano source. The area has a rainless-hot-dry climate that prevents the leaching of key ingredients from the accumulating guano. The birds that live in this area feast on the numerous oily anchovies from the nearby waters.
Harvesting the guano is grueling work that hasn't changed in hundreds of years because of the rugged and dung-slick terrain and to protect the birds. Teams use picks, shovels, and brooms to loosen the guano which can be several yards thick in some spots. Workers have to cover their faces against swirling dust and the stench of the droppings.
Guano can be found on islands around the world, but unlike Peruvian guano, its potency is often of poor quality due to being leached by high levels of rainfall and humidity.
During the 19th century, before the advent of industrially manufactured fertilizers, a huge guano trade, involving both the U.S. and Europe, played a pivotal role in the development of modern input-intensive farming. It also led to the ruthless exploitation of thousands of people from all parts of the world who were used and often enslaved to mine this valuable export product.