With potting soil, you would have to check the bag, to make sure no water
holding additives or pH enhancers were added.  Also, potting soil is not
actually a soil, but a planting medium.  Most manufactures do use additives
in their potting soil.  The additives are usually necessary for plants but
not safe for ferrets I suggest using a mix of equal parts of peat,
vermiculite, and perlite.  Moisten just enough so it lightly stick together.
They love this.  And all you have to do is to keep it mixed and moistened.
Don't close it off.
 
I am a horticulture major.  This is what we use for planting media in our
greenhouse and there are no additives to it.
 
Margie
[Posted in FML issue 2259]