Use a Phiomia to make mass amounts of medium poop by feed it large amounts of Stimberries.

3 poop and 50 thatch makes one bag of fertilizer after 50 real life minutes on base rates.

Some better alternatives:

Dung beetles convert feces to fertilizer in 15 minutes.

If you make a man-made toilet poop in it and flush, it will automatically give you a bag of fertilizer.