What can I use to fertilize naturally?

What specifically do you want to fertilize? Lawn, plants, veggies, houseplants? Different plants have different needs.

Manure. You can buy it at a garden center.

Assuming that you mean fertilizing garden plants. Compost. Funny, but the answer to so many of life's questions is "compost". If you mean "fertilize" in the context of making children, well, that's a different answer.

depends on what you need to fertilizer.lawn,plants,flowers?

I catch fish and bury them about 4 feet down. It works wonders! !

It depends on what plants/flowers you want to fertilize. Indoor plants? Outdoor plants? Are they acid loving plants?

Check your local zoo to see if they sell disinfected animal dung. They do at my Zoo (called Zoopoopiedo) and it is very popular as fertilizer in my community.

Egg shells, coffee grounds and epson salt.

Chicken droppings, Horse manure, cow manure. But be careful it can burn, it needs to sit a few months before using.