I have watched Supernatural through season seven and always thought the salt thing was particular to the world of "Supernatural." I have not watched or read a lot of "ghost stories."
While watching an episode of Merlin with my daughter today, I saw that they referenced the same idea. In episode 49, A Herald of the New Age, Merlin is tossed a bag of salt with the edict that a ring of it will protect him from evil spirits. The comment is made in jest and Gaius later says the practice is rubbish, but the superstition is referenced.
Upon researching the subject (briefly,) I discovered that some people belive the superstation to be true because salt is supposedly a pure substance, while others believe only blessed salt would work, and others only that salt directly from the sea will work. I did not, However, find anything that suggested an origin for the superstition.
So, what is/are the origins for beliefs that salt will protect against evil spirits? If it is widespread because of various origins, or difficult to know because of its ancient existence, I'd still love to know the countries/regions where the belief seems to have started. Please offer references wherever possible.