Can anyone tell me how important charcoal is for a terrarium? Also, how thick a layer should I put in the bottom of a 1 gal terrarium?
I don't think it's important at all. It's really only to absorb odours (the water in the bottom can get pretty smelly), but once it's absorbed all it can, it's useless, and can actually begin releasing things back into the water/air. Besides, you only ever smell the old water at the bottom if you move the soil aside to expose the gravel (which is what i'm assuming you're using for drainage). Since that doesn't happen often at all, what's the point? It's just an extra expense.
I agree that charcoal is not a necessity. However, I will disagree that it is useless. I use horticultural charcoal in all my soil mixes, as I have personally witnessed plant roots selectively attaching to the chunks of activated charcoal in my mix. Activated charcoal is a good medium for ion transfer and may have some beneficial relationship with the selective uptake/metabolism of plant roots (this is all pure speculation based upon my experiences and talking with others--I have no citations to back this up).
As for it releasing toxins back into the environment . . . that is unlikely, as that typically happens only at extremes of pH and/or temps. I use it and will swear by it, but it's mostly a preference thing. At about $3 per gallon, it's not that big of an expense either way.
Actually I didnÂt put any gravel in the bottom of the small terrarium. I used African violet wicking soil. It is very porous. Since I knew nothing about terrariums, I though it would work OK. So far so good after a year. I have been told, though, that I should probably refresh the soil and have a layer of something on the bottom for extra water, thus the question of charcoal.
Thank you both for your input.
Yeah, you really don't need gravel, either. The plant roots grow down into it anyways, so it really doesn't improve drainage.