Having done a lot of work with pest management in museums over the last 15
years, I have had a couple of queries about decontaminating soil.  It can be
baked at 200 - 250 degrees F. for  about an hour to effectively kill any
critters.  However, as an earlier post said, this does smell terrible.  I
would not, however, replace soil with ground coffee -- you can get critters,
but most of all, the scent of coffee might be strange for the exhibit.  I
recommend, creating a form of the landscape of some dark-coloured base
material, coating the base material with an adhesive, and then sprinkling it
with sterilized soil.

Good luck.

Wendy Claire Jessup
President and Conservator
Wendy Jessup and Associates, Inc.
210 Little Falls Street, Suite 203
Falls Church, VA  22046
(703) 532-0788
(703) 532-1661

P.S.  I can't remember what the original posting said about the types of
artifacts/specimens which will be in the exhibit.  Depending upon what will
be exhibited, care will need to be made in the selection of the base material
and the adhesives to reduce the potential risk from gaseous pollutants from
off-gassing of the materials as well as any contact corrosion to objects that
may be sitting directly on the base.  This is another thread that we may wish
to discuss...

w.