Salt

Rock Salt/Rock Salt Gun Non-lethal weapon and ammunition often used by police to control crowds or riots. Spirit hunters also use them as a defense against ghosts and other supernatural spirits. The fired salt alone will not destroy the spirit, but it does act as a deterrent, forcing the spirit to dissipate briefly. – Pad of Definitions, (1.07 Hook Man), Official Website

Ring of Salt A positive symbol of protection used against demons and evils protecting or trapping anything within it. – Pad of Definitions, (1.20 Dead Man's Blood), Official Website

Salt in "Supernatural"

In Supernatural, salt is used:

• In shotgun cartridges, to disperse ghosts.
• As protection of space - laid in an enclosing ring or around the boundaries of a room to prevent evil from entering.
• To purify a corpse before burning its bones.

In The Supernatural Book of Monsters, Spirits, Demons and Ghouls Sam and Dean mention that they know hunters who salt their beer. This practice stems from an old Scottish tradition in which salt was added to a batch of mash to keep witches away.^BoM

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Salt as a barrier

The practice of putting down an enclosing ring of salt (or salting the doors and windows of a room) for protection lies in the lore that a vampire or demon cannot cross the line without counting each grain. The impossibility of this task thus prevents the demon from crossing the threshhold/line, and thus a thicker line is used where the threat is greater.

In vampire lore, it is not required to be salt - it can be any form of grain (rice, millet, etc).

Christian uses of Salt (from Wikipedia)

Although demons, being spiritual beings, do not need food, according to Christian demonology, demons and especially the Devil hate salt, and thus no food is served with salt during the Sabbaths. Bread, if not made with rye, and oil, are prohibited. This is what the Christian church believed.

This can have an explanation in the fact that salt is a natural preservative and antiseptic, which has long been used as a purifying agent in folk magic. In some Christian rituals of baptism, especially in Catholicism, salt is put on the lips of the child during the ceremony of the baptism as a symbol of wisdom. The dislike for bread can be explained because it represents the body of Jesus for Christians and is transubstantiated into his flesh during the Mass.

Secular instances of the use of salt to combat supernatural evils

• Salt used as a talisman against evil (rock salt worn as an amulet)
• Grains of salt in milk to prevent spoiling by witchcraft
• The salting of a new house to prevent witchcraft
• The use of salt to cast "a spell over obnoxious individuals"
• Used in a spell to summon one's love interest
• "The ancient Teutons believed that the swift flight of birds was caused by certain powerful spirits of the air. Now salt is a foe to ghostly might, imparts weight to bodies, and impedes their motion."
• Salt used to impede bird flight
• To prevent imps from entering stables
• "Common salt has long enjoyed a reputation as a means of procuring disenchantment." - in therepeutic magic.
• Carrying salt as protection against spirit enemies of any kind

Source and more detail

The Folk-lore of Common Salt: VIII. Salt As A Magical Substance