You know about the natural world and the Cycle of Arda. The topics you may know about include plants, animals, vermin, elementals, weather, seasons and cycles, birth and death, and druids.
Check: In addition to the standard use of Knowledge skills (as per the Knowledge skill description in the 3.5 PHB), you can get along in the wild, protect yourself from severe weather, and predict the weather (as per the Survival skill description in the 3.5 PHB).
Synergy: 5 or more ranks in Knowledge (Nature) gives you a +2 bonus to Search checks involving tracks and natural environments. Also, 5 or more ranks in a Knowledge skill related to the topic being investigated gives you a +2 bonus to Research checks. These bonuses increase by +1 for every additional 10 ranks (that is, +3 at 15 ranks, +4 at 25 ranks, and so on).