I was watching a homesteading show where they were designing a small structure to capture waste from their goats to use it as fertilizer and it got me thinking about what makes some poop safe to grow food and others not so much.
In: Biology
First of all, human waste **isn’t** harmful to use in the same way, and has traditionally been used as fertilizer in many places in history (look up the term “night soil” to learn more).
That said, there are two main reasons why human poop is worse than other poop for fertilizer:
The first is that the poop of animals like goats, cows, and horses is better fertilizer than that of humans because of differences in our diet and digestive system.
Second and probably more importantly, the poop of other animals carries the diseases of other animals – most of which don’t call disease in humans. Human poop on the other hand is chock-full of human disease. So close contact with human poop is a great way to get sick, while the risk is much lower with non-human poop.
Human feces contain germs that grow well inside human bodies. This is why germs coming from poop to your mouth (called fecal-oral transmission) is such a common way to get sick. Putting those pathogens into the soil your food is growing in means they will go in your mouth when you eat the food. This will make you sick very easily.
Human waste can and is used for fertilizer. Diet and health matters, and humans can have diseases other humans can get, so handling it might require more care. Bird shit is used and makes you sick. Also pharmaceuticals can be in an issue in human waste.
Human waste can be and is often used as fertilizer. However, there is risk. To be clear, there is risk associated with *any* animal waste. The waste may have pathogens: viruses, bacteria, and especially parasites which can harm humans. Pathogens tend to be pretty specific to their host, though. Our immune systems have been evolving for hundreds of millions of years. Pathogens have co-evolved with us to escape our immune systems. Pathogens that have adapted to infect other species rarely have the adaptations needed to infect humans. In fact, the adaptations needed to infect other species often make them much easier for our immune systems to find them and kill them.
As such, the waste from animals, while somewhat risky, is not as dangerous because most of the pathogens in that waste will not be able to infect humans. Hopefully, you can see where this is heading: human waste has human pathogens in it. That makes it far more dangerous. Those pathogens already know how to infect us.
Our waste is how we get rid of pathogens that have infected us. Cells gobble them up, rip them apart, and send them out with our poop. That doesn’t mean all of the pathogens are all the way dead, though. Plus, our guts are full of bacteria, most of which is helpful *as long as it’s in our guts*. The bacteria doesn’t know or care where it is. As long as it’s in an environment where it can thrive, it will. Our intestines have a lot of adaptations to allow the bacteria to thrive without letting it infect the rest of our bodies. If those bacteria species are allowed to get into other parts, though, it can cause serious infections. Those bacteria are alive and well, and a lot just happen to get caught up in waste as it’s passing through and passing out. Again, many of these species are found in the guts of animals, but like other, more infectious pathogens, the bacteria in human waste is already adapted to living in a human body, even if it’s usually confined to our intestines.
Handling human waste gives all of those pathogens plenty of opportunities to come into contact with other parts of our body and infect us.
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If you compost the waste properly, it is generally harmless, and there are many composting toilet designs available. Human composted waste has been used in many areas of the world.
Poop from herbivores breaks down much quicker than poop from carnivores. Carnivore poop has lots of fats and oils in it that creates little sealed microbiomes that allow very pathogenic bacteria survive for much longer.
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The story of Milorganite from Milwaukee Wisconsin is appropriate here and interesting. They’ve been taking municipal sewage and making it a very good fertilizer sold throughout the United States.
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