For years, gardeners and farmers have relied on one of the world's most abundant resources to make fertilizer: animal poop. Animal manure is generally free and easy to obtain, either from your own animals or someone else’s.
Harvesting animal waste for fertilizer has generally been considered a safe practice—most manure is composted before being applied to the ground, which means it has gone through a period of heat treatment that reduces the risk of contamination.
However, a number of current health considerations and warnings are worth considering when using manure at home, including two deaths attributed to the home harvesting of bat guano. Coupled with ongoing outbreaks of H5N1 avian flu, It's definitely a time to proceed with caution when feeding your plants.
While the idea of harvesting poop may sound weird to non-gardeners, manure is an almost perfect fertilizer, provided it has been treated correctly. It is full of nutrients consumed by the animals that produced it, and if they’re your animals, you'll know what that diet included. Manure is also organic as heck, and in most cases can be obtained for free.
Some manure is considered “cold,” meaning that it can be used immediately without harm to your plants (this includes poop from rabbits, ducks, goats). Some manure, including that from chickens, cows, and horses, can burn your plants if not treated correctly, so it is considered “hot”. Simply piling the manure up, ensuring there is a good ratio of carbon to nitrogen, and allowing the sun (and the composting process) to heat the pile will result in processed manure compost, perfect to use as fertilizer. Many types of commercially-produced fertilizers will make note of animal manure they contains.
The problem is that manure can also carry disease, fungus, and other pathogens. Some of these cells can be destroyed by the composting process, but not all of them. While the risk from these diseases transferring to your fruits and vegetables is low, the risk is to the person handling the manure is real. If you’re treating your plants, fruits or vegetables topically with manure and don’t wash it off well, it can remain on the surface.
Recently, two men died in unrelated incidents after inhaling spores from bat guano, which is thought to be particularly beneficial in the production of cannabis. In one case, the infected person hand harvested guano from wild bats, while in the second, the patient applied purchased commercial guano to cannabis he grew and smoked.
In most cases, fertilizer that is sold commercially is regulated and should be considered safe. However, in the U.S., fertilizers are regulated at a state level, which means enforcement and regulations may vary depending on where the fertilizer was produced or sold. In the study of these deaths linked above, two of the scientists, Dr. Sudnik and Dr. Ted Louie, note that commercially sold bat guano fertilizer products do not mention the health risk of histoplasmosis, which both men died of, and that great care should be taken when using them.
The use of masks or respirators can generally protect gardeners from exposing themselves when using animal waste products.
Far more ubiquitous than bat manure, manure sourced from the waste of chickens and other backyard fowl is an incredibly common fertilizer, but this can be a concern at a time when much of the world is facing a highly pathogenic form of avian flu (H5N1). The virus has now been found in cows, pigs, and domestic and large cats, and has infected people as well. When H5N1 is detected in birds, cows, or pigs on farms, those animals must be destroyed to help halt the spread.
The main form of transmission for avian flu is bird manure. Infected birds drop manure around other animals, which eat it and become infected, and thereby infect other animals nearby. While large commercial farms are the greatest concern, backyard flocks are susceptible too. Handling bird manure can thus be dangerous for humans—and because this avian flu is highly pathogenic, it can remain in manure even after composting if it does not reach sufficient temperature for long enough.
“I wouldn’t be handling animal manure unless I was wearing appropriate personal protective equipment, given the current situation,” said Dan Teich, DVM, a vet and owner of District Veterinary in Washington, D.C. “That’s an N95 mask or respirator [and] goggles and gloves, and I’d be very careful washing up afterwards.”
Or, go cold turkey (no pun intended). It's a good idea to consider not repurposing backyard or farm manure in your garden while these threats are ongoing. While commercial fertilizers should mostly be safe, it is worthwhile to check labels for instructions for safe use, and consider only using fertilizers that have been through a treatment process and are labeled as such. Consult with your local garden center to find fertilizers that are appropriate for your use and have been vetted as safe.
