What is Bat Guano?

Bat guano is the carbon and nitrogen-rich feces of bats, guano may also come from birds.

Along with carbon and nitrogen fertilization, it contains micronutrient minerals and microbes that are easily taken up by plants. Because of this guano is used in organic farming to enrich soil fertility and improve soil texture. Like worm castings, bat guano has the added advantage that it will not cause nutrient burn to plants.

One study also found that when guano is combined with worm castings, the mixture significantly increased plant growth and development. There is some evidence that it may act as a booster for other manure fertilizers, and provide the soil with beneficial microbes that increase plant growth.

Uses of Bat Guano Overview

• Plant Fertilizer
• Soil Builder
• Compost Activator
• Beneficial Microbe Amendment
• Possible Soil Cleanser
• Possible Antifungal and Anti-nematode Activity
• Best for high-value crops
• Safety and health risks to be aware of

While some health and safety risks of bat guano will be discussed, it is relatively safe when properly used. Since guano is considered raw manure, the 90-120-Day Rule set by the USDA applies to its use.

Benefits of Bat Guano Use

• Plants fertilized with bat guano tend to have reduced-sized root systems due to the high availability of nutrients, especially phosphorus, at the top of the soil.
• Better growth, more biomass production, and higher nitrogen levels have been reported in guano fertilized plants. Some plants like corn have higher overall yields with guano fertilization.
• Phosphorus rich bat guano has been shown to increase shoot length in some plants.
• There is some evidence of guano having microorganisms beneficial to plant growth.
• Bat guano can be used in smaller concentrations than other manures, such as cow manure, and provides the same amounts of nitrogen.

Nutritional Value of Bat Guano

The nutritional value and effect of bat guano on soils vary with the source location of the bat guano, the bat species, and its diet of insects or fruit. Guano age will also create variation in quality. Guano may be fresh, semi-fossilized, or fossilized (highly mineralized). Nutrients become more available to plants when the guano is more mineralized with age.

Bat guano requires a very small amount to be applied to increase plant growth. 0.5 to 1 part guano to 20 parts soil is usually sufficient. Using 1 or more parts bat guano to soil won't harm plants, but it may lose its beneficial effect when applied in excess (nutrient lock).

There are two main categories of bat guano. The first is high phosphorus guano from blood or fruit-eating bats, and the second is high nitrogen guano from insect-eating bats. Bat guano may be slightly alkaline or slightly acidic, such as in insect feeding bats depending on bat diet.

Reported Retail Bat Guano Nutrient Profiles

Bioremediation and Pathogen Suppression

Some claim bat guano contains beneficial bacteria and can mediate soil toxins and control fungi and nematodes in the soil, but little published research is available to support claims or characterize bacterial communities in guano. If bat guano truly does have the microbes present to mediate soil toxins it makes for an ideal fertilizer in farms transitioning from chemical fertilizer and pesticide use to organic conventions.

Like nutrient composition, the microbial composition of bat guano will vary based on the source. One study that did look at the microbial communities in bat guano noted that there were significant levels of microorganisms present and the species were much different than those found in worm castings. The bat guano analyzed was generally much lower in bacteria.

Human Health and Safety Risks

Due to the small body of academic and published research studying bat guano, beneficial claims can be difficult to support. Overall, guano has not gained much popularity among conventional farmers but has recently gained popularity as an organic amendment for crops. The use of bat guano is most justified in cultivating high-value crops such as those because it is not the cheapest.

With the COVID-19 pandemic of 2020, this natural fertilizer may fall even further out of favor. The coronavirus variants (some harmless, others very dangerous) are very common to bats.

There is a concern with pathogen exposure to workers harvesting guano. Bat guano is a vector for fatal viruses like Nipah Virus (NiV), Coronavirus (CoV), and Severe Acute Respiratory Syndrome (SARS). At a concentration of 300g/kg-1, one study also found bat guano to significantly increase fungi that were pathogenic to plants. Bat guano may also contain the fungi responsible for the serious human illness histoplasmosis, which presents flu-like symptoms in mild cases, or pneumonia in severe cases.

Bat guano should not be handled by people with compromised immune systems. Likewise, the fungi can be found in chicken manure, which care should also be taken with.

When using bat guano, it is always safest to wet it down when handling to reduce dust and avoid inhalation. 

Growers do report very positive results from fertilization with bat guano, but users should be careful to take proper safety precautions and make sure they acquire guano that has been properly aged and collected from a reputable source.