Potting soil will have addatives I wouldn't want my ferrets exposed to.  If
you want to make a digging box and don't have access to any suitable dirt
(these days who does?) someone recently posted that Toys R Us sells sand
for childrens sand boxes.  Pet stores sell tiny little gravel in bags for
aquariums, and this is easier to clean up than sand or dirt.
 
Georgia
[Posted in FML issue 2259]