Definitely soil. Those tigers really like to dig.
Try to find a potting soil that does not have perlite, vermiculite, or fertilizers added to it.
I use this brand in my vivariums, while it has perlite in it, it is actually soil and not composted peat and pine bark (except in the state of Georgia, USA) I float out most of the larger perlite with water and then let the stuff air dry a bit back into soil.
http://www.scotts.com/smg/catalog/p...proId=prod210004&itemId=cat60016&id=cat140002