Are you looking at a natural formicarium, foraging area, or both?
I have found that foraging areas with live plants are very hard to clean and maintain with larger colonies. Although I have seen several people use them. I mostly make mine with sand coated grout, plastic plants, rocks, and twigs.
I have used all sorts of substances for formicariums, but prefer the ones with pre-carved tunnels as it makes observation much easier.
I have used sand and let the ants build their own nests - http://www.formicult...d-formicarium/ But I found my ability to observe them was limited and never repeated the experiment.
If you are after a box of dirt that they can form tunnels freely in, I suggest looking up some of drew's threads (dspdrew).