Guano
Phosphate- and nitrate-rich accumulated excrement (phosphates, nitrates)
An accumulated deposit of bird or bat droppings rich in nitrogen and phosphate, historically a prized natural fertilizer.
- Mohs hardness
- 1-2
- Color
- white, yellow, brown to black
- Type
- sedimentary
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Overview
Guano is the accumulated excrement of seabirds, bats, or seals, built up over long periods into thick, organic- and mineral-rich deposits. Though it begins as waste, hardened ancient guano is treated as a sedimentary deposit because of the way it accumulates, lithifies, and reacts chemically with underlying rock.
It is extremely rich in nitrogen, phosphate, and potassium, the key nutrients in fertilizer. In dry climates the nitrates are preserved; in caves and wetter settings the deposits become dominated by phosphate minerals.
Guano was so valuable as fertilizer and a saltpeter source that it drove exploration, mining booms, and even wars in the 19th century.
Formation & geology
Guano accumulates where huge colonies of seabirds nest on arid islands or where bats roost in caves over thousands of years, dropping excrement faster than it can decompose. Arid coastal climates (as on islands off Peru) preserve soluble nitrates, producing nitrogen-rich guano.
In humid or cave settings, rainwater and groundwater leach the nitrogen, leaving behind insoluble phosphates. These phosphate-rich solutions react with limestone cave floors and host rock to precipitate a suite of phosphate minerals, sometimes lithifying into hard phosphorite.
Major historic deposits occur on Peru's Chincha Islands, on Pacific islands such as Nauru and Christmas Island, and in countless bat caves worldwide.
How to identify it
Fresh or young guano is a soft, light, crumbly deposit ranging from white and yellow to brown and black, often with a strong ammonia or musty smell and visible layering. Bat-cave guano forms dark, earthy accumulations on cave floors.
Ancient, mineralized guano is harder and may resemble pale phosphorite or earthy rock; it is associated with caves, islands, and limestone. Phosphate-rich material can be confirmed chemically in the lab.
Its setting (bird colonies or bat caves), low hardness, organic odor, and association with phosphate minerals distinguish it from ordinary sedimentary rocks.
Uses & significance
Guano's overwhelming value has been as a high-grade natural fertilizer, rich in nitrogen and phosphate, and historically as a source of nitrates for gunpowder and saltpeter. The 19th-century guano trade was a major global industry.
Today it is still mined and sold as an organic fertilizer and soil amendment prized by gardeners, and processed guano supplies phosphate for agriculture. Phosphate minerals formed from guano are of interest to collectors and to industry.
It has no gem or metaphysical use. Mining must be managed carefully because guano deposits are tied to fragile bird and bat ecosystems.
Frequently asked questions
What is guano made of?
It is accumulated bird or bat excrement, very rich in nitrogen, phosphate, and potassium, sometimes mineralized into phosphate-rich rock over time.
Why was guano so valuable historically?
It was an outstanding natural fertilizer and a source of nitrates for gunpowder, sparking a major 19th-century trade and even territorial conflicts.
Is guano a rock?
It begins as organic waste but accumulates and lithifies like a sedimentary deposit, and ancient guano can harden into phosphate-rich rock such as phosphorite.
What is the difference between bat and seabird guano?
Seabird guano from arid islands is rich in preserved nitrates, while bat-cave guano in wetter settings tends to be leached and dominated by phosphate minerals.
Is guano still used today?
Yes, it is mined and sold as an organic fertilizer and soil amendment, and processed for its phosphate content in agriculture.
Guano guides
In-depth guides for identifying, valuing, and understanding Guano.
Guano identified by the community
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