Well, humanity has been using salt in their various religious tendencies forever. It's a natural part of the earth. Generally used to mix in with things for cleansing and the sort. Negative entities and energies can't pass a salt line.

[ So much for the we don't know why it works. ]

The wards I put on the doorways haven't kept the buggers out, so, at least they can't come over and blow out the fire.