Is it possible to create your own Enriched Terrain (also known as Fertile Terrain), or are you stuck with the ones provided? And related, where do I get more Compost?
You can enrich existing terrain by throwing Compost on it (note that this will not work for Acidic or Barren terrain). Terrain can be enriched even if there is already a plant growing there.
Compost is producded when a Phyta is eaten by a Larian; the Larian then spits out the corpse of the Phyta which you cannot pick up. You need to wait about 10 seconds before the corpse breaks down, and leaves behind Compost which you are able to pick up.
The best level to produce compost on is Larian Gauntlet, where there are plenty of places to corral the Phyta into the Larian tentacles. It's a dangerous level, so make sure to keep the Phyta on that level well fed so they aren't all killed off.