Salt has long been used as a symbol of purity and as a substance to ward away evil spirits. In Christianity, salt is associated with eternity and divine protection. Salt and holy water are used in baptisms and to bless the church sites, as well as to protect unbaptized babies prior to baptism and the dead in their journey from earth to the next world.
Witches and demons were traditionally said to be repelled by the presence of salt. It is thus used as protection against witchcraft. The evil eye, and it has the power to break evil spells. When trying to conjure demons or spirits, it is recommended that one avoids salt as it can interfere with the connection. Salt has been utilized in regard to those accused or suspected of being witches. It was a form of torture to feed the accused heavy doses of salt. Women have been suspected of being witches on the mere fact that they complained of their food being overly salted.
When one spills, borrows, or runs out of salt, it is considered unlucky and is said to make one susceptible to the powers of the Devil. It is possible to negate this situation by pinching salt in the right hand and tossing it over the left shoulder.
Source: Witchcraft today: an encyclopedia of Wiccan and neopagan traditions, by James R. Lewis, http://books.google.ch/