Guano: the fertilizer of yesteryear

I hadn’t really thought much about fertilisers before – I merely assumed they were either cow manure, human excrements, or if a farmer could afford it, synthetic.


That all changed when I was editing some photos of the Japanese Cormorant (Phalacrocorax capillatus) viewing cliff. What looked like a standard brown cliff with slathers of calcium carbonate turned out to be bird droppings. My imagination stretched a little further and created a unique species of bat as I stared at an inverted version of a close-up shot: Japanese Cormorant Bat (Chiroptera phalacrocorax fusafeces), a bat that lives in external caves.

The latrine of my web-winged mammal raised a question though – can bat poo really defy gravity?


Cormorant face-off (from left to right): Japanese Cormorants (Phalacrocorax capillatus) and the Common Gull (Larus canus) on a cliff in Jogashima; Japanese Cormorant Bats (Chiroptera phalacrocorax fusafeces) and the Common Gull Bat (Chiroptera larus fusafeces) on a cliff in Jogashima

I later learned that, no, bat poo can’t defy gravity and guano is the proper term for bat and seabird droppings.

Guano became the world’s most valuable fertiliser after Alexander von Humboldt preached its agricultural merits during a trip to Peru in 1802. Even countries that used sea-originated fertilisers couldn’t resist guano; Japan, which traditionally applied Pacific Herring (Clupea pallasii) on their fields, targeted Christmas Islands during WWII for its vast guano deposits, and Ireland replaced Bladderwrack (Fucus vesiculosus) with Peruvian bat guano on their tuber plots (bat guano has been linked to the blight that caused the Irish potato famines in the 19th century!). The concentration of nitrogen and phosphate in guano is much higher than other fertilisers because of the fish and insect-heavy diets of seabirds and cave-dwelling bats.

Diet isn’t the only factor that influences guano quality: biology, habitat and geography play a part as well. The nitrogen content in seabird guano is high relative to other mammals because it excretes nitrogenous waste in the form of semi-solid uric acid, which is much more concentrated than urea. Although bats excrete urea, they’ve adapted ways to excrete hypertonic urine to prevent heavy loss of water. Both seabirds and insectivorous cave-dwelling bats reside in colonies that seldom relocate en masse. As such, excrements accumulate over time and solidify to form guano. Seabird colonies in dry regions produce guano with particularly high concentrations of nitrogen, as lack of precipitation minimises nitrogen leaching.

The yield gains that resulted from application of guano further fuelled the Western world’s insatiable demand for the fertiliser. Explorers searched far and wide for ever-more amounts of this precious excrement. Guano expeditions soon expanded from Peru to the Caribbean, Central Pacific, and the islands off the coast of Namibia, Oman, Patagonia and Baja California.

Despite significantly increasing food supply and indirectly eradicating starvation for millions, the global guano trade caused countless controversies. Conservationists established programs to protect seabirds and their habitat – the Guanay Cormorant (Phalacrocorax bougainvillii), Peruvian Pelican (Pelecanus thagus) and the Peruvian Booby (Sula variegata) were among the most vulnerable species. The use of slaves and convicts with minimal safety precautions on the Chincha Islands raised human rights issues. Indigenous communities who inhabited the guano-rich islands didn’t go unscathed either – they experienced ‘grave cultural and social dislocation‘, the effects of which remain to this day.

Fortunately, the demand for guano has since dwindled. The commercialisation of the Haber-Bosch process in 1913 allowed large quantities of ammonia to be produced from atmospheric nitrogen. Driven by population growth and ever-increasing demand for food, fertiliser consumption increased from 104kg per hectare of arable land in 2002 to 138kg in 2014 – we’ll have to be careful though, synthetic fertilisers are a mixed blessing.


Silly Japanese Cormorant (Phalacrocorax capillatus) staring into a guano covered cliff in Jogashima



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