For "natural" in the sense of "occurring in nature":
* Bio-: Meaning "life" or "living," often used for things related to living organisms. Example: Biodegradable.
* Geo-: Meaning "earth" or "ground." Example: Geochemistry.
* Eco-: Meaning "environment" or "habitat." Example: Ecosystem.
* Phyto-: Meaning "plant." Example: Phytochemistry.
* Zoo-: Meaning "animal." Example: Zoology.
For "natural" in the sense of "not artificial" or "authentic":
* Un-: Meaning "not." Example: Unprocessed.
* Non-: Meaning "not." Example: Non-synthetic.
* True-: Meaning "genuine" or "real." Example: True color.
* Organic-: Meaning "derived from living organisms." Example: Organic farming.
For "natural" in the sense of "characteristic of nature":
* Natur-: Meaning "nature." Example: Naturalism.
* Natu-: Meaning "nature." Example: Naturopathy.
Ultimately, the best prefix to use depends on the specific meaning you want to convey and the context of your word.