The manure comes ultimately from hay. So you are taking the nutrients from another man’s land and putting it on your field. Eventually someone somewhere has to fertilize. Plowing clippings, etc… back in is good, but it doesn’t get you to 100%. If you remove potassium (which you eat in your vegetables) it is gone. It has to be added back. Same with meat protein, or even vegetable protein. It comes from nitrogen.

As I wrote, you can plant clover, then plow it in. This will add nitrogen, but you lose that growing season. Much better to spread urea (which is rotted piss, though in modern cases it is man-made in a chemical plant. Same stuff).