system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can some explain how poop is so effective as fertilizer? I don't understand how the waste can still be rich in nutrients, the digestion system had ONE job which is to absorb the nutrients. Are our bodies that bad at it?
gpt
The body excretes a lot of stuff that it either can't use or that it already absorbed enough of. 

You only need so much nitrogen, phosphorous, and potassium (main fertilizer components), so your body excretes the rest. Some of the stuff your body excretes would be toxic if it didn't get pushed out.

As far as efficiency goes, your digestion and excretory systems are incredibly efficient. How many other machines go 70, 80, 90 years with minimal maintenance?

Aside from that, excrement is not just food, but dead cells. And the nutrients you ate helped build new cells as the old ones are replaced. And then of course, feces is largely dead bacteria that fed on the food in your gut. And as they fed, they broke down that food, created compounds your body could absorb, and some it could not.