What we can tell you is there is simply an association of salt with spirit energy, and salt is falsely assumed to confer protective properties, and can form a protective barrier around a location to keep a person safe from spirit attack, when this is not the case. It is strictly superstition, but again is reflecting the reality that spirits are a true hazard and menace of significant power. The widespread practice of putting salt at locations, at the borders around a location, as in the four corners of a room of a home to create a kind of sanctuary, might be self-reassuring as a ritual but that is only a self-soothing gesture and not a significant deterrent in any way to spirits and what they can do. They are unaffected by anything in the physical, being in a somewhat higher energetic vibration to begin with, so what is needed is a better tool, in fact divine intervention, to deal with dark spirits and the risks they represent to the living, and not a superstitious notion that once again is based on an intuitive awareness there is some truth behind the ritual, and that is why it is adopted so widely and has stood the test of time down through the ages. Unfortunately, the level of ignorance has prevented advancing beyond a kind of primitive acceptance of folklore, as being all one needs to live their life with safety and security, when it is a hollow promise and a misdirection in the bargain. The whole notion got started as a lie to disempower humans and keep them vulnerable to spirit manipulation.