Guano, most often associated with bats, is nutrient-rich manure from bats, seabirds and even seals. It is popular with organic gardeners because of its high levels of slow- and fast-release nitrogen and phosphorus.

Depending on the source, guano also often contains usable levels of micronutrients due to the wide variety of diets of the birds and other animals that contribute to guano formation. Some types of guano come from carnivores that eat insects and small fish, and some types come from herbivores that eat algae and plankton.

Unlike other natural and synthetic sources of nutrients, guano does not contain high levels of salt, which can potentially burn plants if over-applied.

How is Bat Guano Made?

Guano is most often found in caves on the windward side of bodies of water. It needs to be aged for extended periods of time before it can be used by growers. Caves provide the perfect arid environment for the excrement of bats and seabirds to become guano as there is little to no precipitation, so nutrients are not leached away by rain. Guano can also be harvested from coastal areas that do not receive much rainfall. Once the excrement has been aged and subjected to biological activity from guano beetles and decomposing microbes, the risk of plants contracting a soil-borne disease from the application of bat guano is almost zero.

Horticultural Applications for Bat Guano

Guano’s many applications in the garden make it desirable for organic growers. It can be used on both field- and indoor-grown crops as a soil builder. Guano is an odorless manure that can be diluted with water and used in hydroponic applications. It can be used for lawn treatments and as a fertilizer for landscape plantings.

Guano even has natural pest-control properties, meaning it can be used as a fungicide when fed to plants through a foliar application, and as a nematicide, as its decomposing microbes help control nematodes. This same microbial activity makes guano an excellent composting activator as well.

Different types of guano can be used at different times in the growing season, and on different types of crops. Leafy vegetables or plants in their vegetative stages benefit from a higher-nitrogen guano. Plants that are setting their fruit or seeds are better served with a higher-phosphorus gua

no. As with all application recommendations, refer to the actual mixing rates printed on the packaging. In the absence of that, the following rates can be used as a guide:

  • When used as a soil amendment, powdered guano should be incorporated at a rate of about 5 lbs. per 100 sq. ft. of soil area.
  • When used as a tea, mix 3 tsp. of the powder per 1 gal. of water, or 1-2 tsp. per 6-in. pot.
  • When used as an amendment in compost piles, simply apply a thin layer of powder every time the pile is turned or moved.

What is the Nutritional Content of Guano?

Seabird guano has the highest nutrient analysis of the guanos, with a typical fertilizer analysis of 10-16% nitrogen, 8-12% phosphorus and 2-3% potassium. The nutrient levels of guano from bats and seals are slightly lower, but they are still among the most nutrient-dense, natural fertilizers available.

Peruvian guano is often regarded as the most desirable, as the cold water and warm air on Peru’s coast prevent rainfall and subsequent nutrient-leaching of the guano. This results in higher levels of nitrogen in particular.

Bat guano is often marketed with either high nitrogen or high phosphorus values. It is usually powdered so it can be spread as is, or mixed with water for spray or hydroponic applications. It also comes in pellet form or ready-to-use liquid form.

The main drawback with using guano is not its ease of use, but its cost. Guano can be 10 or more times higher in price per pound than other organic sources of either nitrogen or phosphorus, due mostly to its limited availability and high demand.

High-nitrogen bat guano usually has an N-P-K analysis of about 10-3-1, while high-phosphorus bat guano usually has an N-P-K analysis of about 3-10-1. High-nitrogen seabird guano usually has an N-P-K analysis of about 12-18-1, and high-phosphorus seabird guano usually has an N-P-K analysis of about 1-10-1.

They both tie up the majority of their nutrients as slow-release fertilizers, needing four months or more to release their goods. Guano also stimulates the soil’s microbial activity and aids in the decomposition process. Guano’s interaction with soil can be sped up by introducing an enzyme that increases the biological activity of the breaking-down process.

A Bit of Guano History

Guano, or “wanu” in the original Quechua language of the Andean peoples, was a strategic commodity for the people of Peru and Chile, not only for its agricultural benefits, but also for its uses in warfare. The nitrates in guano (salt peter) were used as a primary ingredient in the manufacturing of gun powder and other explosive ordinance.

Guano was so important to the Incan people that any citizen of the empire caught causing harm to seabirds could receive a punishment as harsh as death. The caves and sources where it was harvested were entrusted to caretakers and were well-guarded. Guano became Peru’s most valuable export in the 19th century and continues to be one of its most valuable exports.

Outside of South America, guano’s popularity, specifically in Europe and the United States, can be traced to an influencer of Charles Darwin, noted bio-geographer Alexander von Humboldt, who published studies on guano and its fertilizing properties in 1802 at Callao, Peru.

Guano became so popular in the 19th century that US President Millard Fillmore made its collection a priority in his 1850 address to the nation, when he stated, “…guano has become so desirable an article to the agricultural interest of the United States that it is the duty of the government to employ all the means properly in its power for the purpose of causing that article to be imported into the country at a reasonable price. Nothing will be omitted on my part toward accomplishing this desirable end.”

These days, various types and qualities of guano can be found and harvested all along the western coasts of Central and South America, and southwestern US.

Sourcing Your Garden Guano: Things to Consider

Guano is not a rapidly renewable resource. It can take hundreds of years for the raw material to develop into a usable form for plants. Guano caves can be thought of like peat bogs. The material was formed naturally, and more will be formed as time goes on, but we are using the material faster than it can be replaced.

There are ecological costs with the harvesting of guano as well, such as the loss of biodiversity at the sites where it is collected. Specialized ecosystems survive and thrive in and around the caves where guano is formed, and are disturbed—sometimes at detrimental levels—during guano harvesting.

Guano deposits support a great variety of cave-adapted invertebrate species. Some of these invertebrates rely on bat feces as their sole nutrient input, but the bats are the ones that experience the most damage from the harvesting of guano. Bats are highly sensitive to regular disturbances in their roosting areas.

Some vulnerable bat species will starve to death, as disturbances to their homes put them in a panicked state and their low fat reserves are unable to sustain them. Other bat species may abandon their young.

In addition to the biological damage that can be caused by guano mining, historical data can be lost forever in the process. Deep guano deposits contain local paleo-climatic records in the strata that have built up over thousands of years.

Studying these strata tell scientists about the historical climate, and give them insight into the history of flora and fauna of the area. This valuable information is irreplaceable once guano has been harvested.

Fortunately, bat guano can be sustainably harvested in ways that do not damage bat colonies and do not destroy valuable climate data. By mining or harvesting guano only from caves or habitats of migrating bats, the stress or panic induced to the bats by the harvesting operation can be avoided.

Prior to the harvest, core samples can be collected from the site to ensure there is a profile on record of the stratigraphy at each site. However, achieving and maintaining these levels of sustainable practices will not come without a price.

Retail prices of responsibly sourced guano will continue to rise, which could also lead to a rise in unethically sourced material that is harvested without consideration for the ecosystem it was taken from. Buyer beware!