Should you use bat guano? Is it a good fertilizer? Does harvesting it from caves harm bats? How does it compare to seabird guano? Learn more about this valuable resource and understand organic products better.
What is Bat Guano?
Bats live in caves and spend a good part of every day there. They hang from the ceiling and their feces drop to the cave floor where it accumulates forming bat guano. The cave protects the guano from rain and so the material builds up over time forming deep layers. Commercial bat guanos have an NPK of about 7-3-1 but the value varies depending on where it is collected, the age of the guano, the species of bat and what the bats were eating.
The word “guano” means fertilizer in Quechua, a language of the indigenous people of Peru.
What is Seabird Guano?
Seabirds also poop and if they live in a restricted area like a small island, the feces accumulate forming large mounds of guano, as much as 200 feet deep. The food source for birds is different and the guano is exposed to the elements, so it has a different NPK. Fresher material, also called white guano, has an NPK of about 10-10-2. Older material, known as red guano, has been weather by sea and rain which reduces the nitrogen level to an NPK of 0-10-0.
Since most garden soil has lots of phosphate, red guano should not be used unless you know you have a phosphate deficiency.
Some of the best high-nitrogen seabird guano comes from the Chincha Islands off the coast of Peru. This is a very dry region which preserves the nitrogen.
A guano mine on Peru’s Chincha Islands, circa 1860. Arrows point to the workers. source: public domain
Is Guano Organic?
The term organic has several meanings which I have discussed in What Is Organic Fertilizer? A lot of Guano fertilizer is certified organic. Does it contain organic matter? It does have fairly high levels of organic matter but I found very little information that described it. Since some of these deposits are very old one has to wonder what form it takes and whether it is of value to the garden? None of the products I reviewed gave any information about the organic matter in their products.
Most organic fertilizers have low nutrient levels, in the order of 1-1-1, while guano is quite high. Guano is also one of the most expensive organic fertilizers. A 25 lb bag of bat guano (7-3-1) is $124 US on Ebay, or $71 for one pound of nitrogen. A 20 lb bag of urea (45-0-0) is $56 or $6 per pound of nitrogen.
Concerns Over Variability
Guano changes a lot as it ages and since piles of it can be very deep, there is a lot of variability. It also reacts with the surrounding rock and is affected by the presence or lack of water. Nutrients levels vary, pH varies, the actual chemical form of the nutrients varies and the amount and type of organic matter changes. Fresh guano has a pH of 8.5 – 9.0 and rapidly becomes acidic (5.5 – 4.0) with age and depth, some being reported as having a pH of 3.
Commercial products don’t do a good job of providing detailed analysis of their product; none of the ones I looked at provided the pH, so it is hard for the gardener to know what they are buying.
Does Guano Work?
It is a fertilizer. It does contain nutrients. So it does work for adding nutrients to soil, which will grow plants. So what? Any fertilizer will do that. Does guano work any better than other fertilizers?
I found one study that compared bat guano to synthetic fertilizer on five economically important species and found no difference. Another study looking at hybrid sunflowers also found bat guano worked as well as, but not better than synthetic fertilizer. Bat guano has slightly higher macro and micro nutrient levels than vermicompost. Commercial products don’t provide any evidence that guano is any better than other fertilizer.
Testing of bat guano showed that phosphorus is released fairly slowly with a peak at about 3 months after application. But this can vary depending on source.
Bat guano is a good source of nitrogen and phosphate but is no better than synthetic fertilizer. I found no evidence that the organic component of guano has any special benefits for plant growth.
Does Guano Spread Diseases Like Rabies?
Bats can spread rabies, but only if they bite you. You can’t get rabies by simply touching a bat, nor can you get rabies from their feces.
Histoplasmosis and Cryptococcosisis are fungal diseases contracted through airborne spores in bat and bird droppings, as well as normal garden soil. Exposure to relatively fresh material in your attic or in caves is a concern, as is mining the material from caves. It is not clear if this is an issue with commercial products.
Other fungal and viral diseases can also be spread by guano, but most are of minor concern. Claims that bats spread Ebola seem to be incorrect, “after spending millions of dollars disproportionately testing large numbers of bats, there is still no scientifically credible evidence linking bats to Ebola”
Microbes in Guano
Guano creates its own ecosystem of organisms (viruses, bacteria, algae, fungi, and protists) that like to hang out in high nutrient conditions. Manufacturers of guano fertilizer claim benefits from these microbes but provide no evidence that they are still alive in the commercial product. If the microbes are still alive – would the pathogens mentioned above not also be alive?
The number and type of organisms in any product will depend on manufacturing and storage processes – no one seems to have any data on this, so we should not assume any benefits. Besides, adding extra microbes to soil does not increase the number in soil.
Guano Rules the World
It’s 1820 and developing countries are growing fast with more and more people moving to cities. There is a growing need for more food but the limited amount of manure is just not enough to give farmers high yields. More fertilizer is needed and it was found on the islands off the coast of Peru which are covered with seabird guano – a rich source of needed nitrogen.
This started the “Golden Age of Guano” which took place from 1840 to 1880. Huge amounts of guano where shipped from the west coast of Peru to Europe. In the 40 year period, Peru exported 20 million tons of guano, earning them around $2 billion in profit. The trip from Peru to Britain, a distance of 10,000 nautical miles that would take 40-50 days. No wonder guano was called “white gold”.
The price of guano skyrocketed and in the late 1800’s other forms of commercial fertilizer started being developed. In 1909, a German chemist, Fritz Haber, successfully fixed atmospheric nitrogen in a laboratory and by 1913 Carl Bosch had developed the first industrial-scale application of the Haber–Bosch process. This process is still used today to make synthetic nitrogen fertilizer. With an inexpensive and quick way to make fertilizer anywhere in the world, the guano trade almost disappeared.
Proponents of organic methods criticize synthetic fertilizer, but it is important to understand that the development of the Haber–Bosch process prevented the destruction of the seabird’s habitat. As it was, several of the islands were so heavily mine for guano, the the birds disappeared. An account of one island had 450 ships remove so much guano in one year that by the following year the island was 25 feet shorter and completely deserted. Synthetic fertilizer saved the seabirds.
The Peru government did finally step in to control guano exports in an effort to prevent an environmental disaster.
However, the story may repeat itself. The world is now running out of soil-based phosphate deposits and some are eying guano as a source, which would again threaten bats, seabirds and their habitat.
Are Bats Harmed by Collecting Guano?
Does the collection of guano from bat caves harm the bats? I have been hearing that this is a big environmental problem, but the facts are not that clear.
A report that looked for examples of damaged bat caves concluded that “Despite its widespread occurrence, accounts of the impact of guano harvesting upon cave-dwelling bats appear to remain largely anecdotal”. There is no doubt that removing too much guano from caves harms bats. For example, Bristol Cave in Jamaica was once a colony of tens of thousands of bats with a great variety of troglobytic invertebrate species. Now there is nothing, thanks to guano mining. Without concrete data, it is hard to know how big or significant this problem is.
Standards are now in place for harvesting guano from bat caves in a sustainable way. They limit the amount that can be removed and it requires that removal be done when the bats are not there – either during the night, or when they migrate and leave the cave empty. Details can be found in “Guidelines for Minimizing the Negative Impact to Bats and Other Cave Organisms from Guano Harvesting” developed by International Union for Conservation of Nature (IUCN). A study based on the IUCN guidelines found that if the harvest rate does not exceed the deposit rate of guano, the population of bats is unaffected.
Unfortunately there is no certification that manufacturers can use to let the buyer know that their guano has been harvested sustainably. That means that all products are suspect.
Should You Use Guano?
Guano can be harvested sustainably, but at the moment you don’t know how the product you are buying is harvested. For all you know, that bag of guano is killing bats or seabirds and other organisms in their environment.
There seems to be no evidence that guano is any better than other forms of organic fertilizer or even synthetic fertilizer. Just because the product is stamped “certified organic” does NOT make it a good option.
From a North American or European point of view, guano has to be shipped a long way to get to you (none of the products I found are mined in the US). That is never good for the environment. Always, use local sources of organic matter first.
Given the high cost of the product, the possibility of harming the environment and the lack of any real benefit over other options, I personally feel that it is a very poor choice. Use some local cow shit instead!