Compost piles are a thing, idk how efficient they are vs manure. One of the reasons farmers use manure is they have so much of it anyways and it’s got to go somewhere. There’s also various plants that do similar work when planted in the soil, something about nitrogen fixing. Whether or not something is natural or not gets hard to define but there are ways of getting plants nutrients that are mostly not animal. I say mostly because it also depends on how you’re defining that and how you store and handle things one what ends up inside it but I imagine a meticulous person could manage it.