Is internal dirt counted among radiation shielding?

Most studies I've seen always put a great deal of emphasis on an external (usually non-rotating) radiation shield. Yet inside the rotating habitat, we have landscaping, which implies at least a meter or two of dirt. Elsewhere, I've seen that a meter or two of dirt provides robust radiation shielding.

So, it would seem to me that you could forego the external radiation shield, at least for the areas of the habitat which are "underfoot." A person might have to take precautions (e.g. wear a radiation monitor) when digging holes, but at most times the dirt should provide sufficient shielding by itself. If an extra safety factor is wanted, then add more dirt -- this only enables the planting of bigger or more varied trees, and deeper foundations for tall structures.

What am I missing here?

Thanks,
- Joe

P.S. Is there a Wiki anywhere dedicated to space habitat topics? It seems like that would be an ideal place to collect information like this, along with primary references. Wikipedia is a bit too general for the level of detail I'd like to see.

Joe Strout -- joe@...
http://www.verex. com/