I've been watching a lot of Supernatural, and I've been wondering. What is it with salt? They keep bringing up that one needs to salt and burn the bones. Also, salt is used to repel ghosts. This isn't the first time I've heard mentionings about salt being useful, but I'm not sure why.
Salt has had a strong significance in a lot of different religions for millennia.
I believe that salt used for the explicit purpose of repelling demons originates from Buddhism or Shintoism, where salt is used to purify/sanctify a places and ward off evil spirits.
I think Supernatural draws from the Wiccan culture for its usage of salt though, as Wiccan use it almost exactly the way it's used in the show.
Salt is a natural cleanser, and can be used to scour out pots, as toothpaste, to remove rust, etc. (See this page for more.) So it's not a big leap from there to supernatural cleansing.
As for salting and burning the bones, a parallel custom was burning a conquered city and salting the ground, so nothing would grow there. The Romans supposedly did this with Carthage when they conquered it.
Why is salt associated with repelling of demons? This seems like an almost impossible question to answer. The origins are very obscure. Firstly, what does salt symbolize?
Judeo-Christian traditions considered salt a purifier, and the symbol of the eternal nature of God’s covenant with Israel. There are more than 30 references to salt in the Old and New Testaments of the Bible. - Salt in the Bible and in folklore
Salt was added as a seasoning to the composition of Holy Incense of Exodus 30: 35.
35 And thou shalt make of it incense, a perfume after the art of the perfumer, seasoned with salt, pure and holy.
Salt is still used in food preservation and in pharmacology.
It is an essential element in the diet of not only humans but of animals, and even of many plants. It is one of the most effective and most widely used of all food preservatives. Its industrial and other uses are almost without number.
Salt was in general use long before the beginning of recorded history, and dating back to around 2700 B.C. the earliest known treatise on pharmacology was published in China. - History Of Salt
How salt became associated with expelling demons is an open question, but these three elements at least give us a glimpse into a possible answer. Salt purifies, heals and preserves.
History of Salt in Religion
Salt has long held an important place in religion and culture. Greek worshippers consecrated salt in their rituals. Jewish Temple offerings included salt; on the Sabbath, people of the Jewish faith still dip their bread in salt as a remembrance of those sacrifices. In the Old Testament, Lot’s wife was turned into a pillar of salt. Author Sallie Tisdale notes that salt is as free as the water suspending it when it’s dissolved, and as immutable as stone when it’s dry.
Covenants in both the Old and New Testaments were often sealed with salt: the origin of the word “salvation.” In the Catholic Church, salt is or has been used in a variety of purification rituals. In fact, until Vatican II, a small taste of salt was placed on a baby’s lip at his or her baptism. Jesus called his disciples “the Salt of the Earth.” In Leonardo DaVinci’s famous painting, “The Last Supper,” Judas has just spilled a bowl of salt, which is known as a portent of evil and bad luck. To this day, the tradition endures that when people spill salt, they should throw a pinch over their left shoulders to ward off any devils that may be lurking behind.
In Buddhist tradition, salt repels evil spirits, which is why it is customary to throw salt over your shoulder before entering your house after a funeral: it scares off any evil spirits that may be clinging to your back.
Shinto religion also uses salt to purify an area. Before sumo wrestlers enter the ring for a match—which is actually an elaborate Shinto rite—a handful of salt is thrown into the center to drive off malevolent spirits. - History Of Salt
Salt is still used in exorcisms in various Christian Churches:
Holy water has been employed in the religious ceremonies of many peoples as a means of purifying both persons and things, and also to keep away demons. Sprinkling and washing with it were important features of the Greek ritual.
The holy water of the Roman Catholic Church is prepared by exorcising and blessing salt and water separately, after which the salt is dissolved in the water and a benediction pronounced upon the mixture. In the Hawaiian ritual, sea-water was sometimes preferred. - SALT AS A MAGICAL SUBSTANCE
Salt is a preservative. The Tarim Mummies were preserved in salt for 4000 years. If you pack meat in salt it will not rot. (Because it sucks all the water out and kills any bacteria. Water is necessary for life.) Since people could not see microscopic organisms before the invention of microscopes rot, decay, and putrefaction were associated with evil spirits. Since salt prevents rot it must logically drive away evil spirits.
Smoke can also act as a preservative, and has some of the same associations.