eli5 why does manure make good fertiliser if excrement is meant to be the bad parts and chemicals that the body cant use
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Faeces are leftovers your body doesn’t use, mixed with bile and dead bacteria and blood cells. While your body has no need for these, it just so happens that they are like vitamins for plants. Plants feed animals, animals feed plants.
Faeces: it’s what plants crave.
To put it very simply: animals and plants have different dietary needs. What’s waste for the one is nutritient for the other. You can also think about how we breath oxygen and exhale co2, while plants do the exact opposite.
Manure contains phosphorus and nitrogen, along with other things, that plants need to thrive really well.
In addition to the answers given, poop is a lot more than just waste. A lot of helpful stuff ends up in it, which is why fecal transplants are a thing.
Manure isn’t just anybody’s poo. It’s cow poo. Cows are ruminants – they have multiple stomachs for digesting plant matter in like grass, which is not very nutritious to us single-stomach havers. What’s so special about these stomachs? They have bacteria, like that gut microbiome you’ve been hearing about lately. Cows have four bioreactors full of bacteria capable of breaking down and extracting energy from that tough stuff. Mix that digested crap and the bacteria ecosystem that comes with it with some dirt, and you have more than just dirt, you have something living and life-sustaining: soil.
Although the answers here are good, I feel compelled to point out that excrement isn’t *meant* to be anything.
Feces mostly contain leftovers. Not bad stuff, just stuff that didn’t get used. (Although possibly bad stuff as well, since the animal can’t use it.) Before the concept was sanitized, what we now call trickle down economics was called horse-and-sparrow economics. As in, if you feed the horse enough oats, there will be some for the sparrows to eat.
But animals are really not designed. They just happen.
Manure does two things, improve structure of the soil and adds nutrients.
Manure is mostly herbivore shit, and herbivores generally have a lot of fibers and carbon still in their poop and the poop is often blended with straw used as bedding for the animals. This makes the soil more of a sponge as both fiber and decaying straw keep a nice amount of water but also let acces water flow out. It makes clay less dense and makes sand less dry.
As for the nutrients, plants mainly need water and carbondioxide to grow, but also quite a bit of nitrogen. Animals dump out a lot of nitrogen in their pee. Plants also need phosporus, sulfur, potassium and a bunch of metals in small amounts. Animals eat plants and in their diets these elements are generally present to some degree, enough for new plants to grow and often more then the native soils. This is because of supplementairy feed, salt licks and accumulation of elements in animals.
Fresh manure does *not* make good fertilizer. It has to be composted to make the nitrogen available to plants, and that means soil bacteria, and maybe earthworms, need to work on it for awhile. Like any other organic material, it’s not fertilizer until it has gone through that process.
There’s a few good reasons for this, but the main one is that plants produce their own energy from sunlight, and can use that energy to build all of the complex molecules they need essentially from scratch.
Manure is very useful to plants because it already contains little fragments of things that they need (mostly Nitrogen compounds) to rebuild larger molecules that are essential for like. Sure they’re smashed to bits by the time they’re in poop, but it’s still much better than breaking N2’s extremely powerful triple bonds that hold nitrogen gas (air) together.
Animals, on the other hand, do not produce their own energy and instead need to eat things they can rip apart and burn (oxidize) for fuel. What we poop out contains whatever is left over after this process that we didn’t need, plus a bunch of complex nitrogen compounds that are broken and no longer useful to us.
Such is the cycle of life
Because evolution doesn’t optimize efficiency universally, just locally.
If there’s plenty of nitrogen, for example, in the environment, in the diet, the body doesn’t have to be punctilious about using every last molecule of it. Piss it out. Some plant will pick it up and feed it back to us.
If there are plenty of citrus fruits available, the body may well stop producing its own Vitamin C.
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