I culture springtails on wet coconut husk (the big chunks). For grow out containers for little dart frogs, I've taken to using just that and then a layer of leaves from "Live Oak" (a species of tree). It works superbly, particularly if I add a few white worms from an old grow out container.

For long term terrariums for frogs that aren't at risk of ingesting chunks of substrate, I use a thin layer of coconut husk above the drainage layer. This is covered in a layer of ABG soil mix (search the forum for Atlanta Botanical Gardens). Not all of the exact components are natural to the Appalachians, but they have very close counterparts there.