The Covenant of Salt as based on the significance and symbolism of salt in primitive thought
http://www.archive.org/stream/thecovenantofsal00trumuoft#page/n1/mode/1upThis book goes into to showing and explaining how salt in ancient times was believed to be in comparison to that of life and equal if not more valued and powerful to that of blood. It has been used to preserve life and flesh, to cleanse and cure, believed to restore life and vigor and to denote wisdom and wit. Also it has been compared to that of 'light' in its attributes.
Excerpt:
Pg. 17
While the word covenant appears more than two hundred and fifty times in the OT, it is a remarkable fact that the term "covenant of salt" occurs in only three instances and then in such obviously exceptional connections. The Lord's covenant with Aaron and his seed in the priesthood and with David and his seed in the kingship, is as a covenant of salt, perpetual and unalterable. And God's people in all their holy offerings are to bear in mind that salt is a vital element and factor, if they would come within the terms of the perpetual and unalterable covenant.
Pg. 75
As the sun shines upon the water drawn from the sea or lake, the water is evaporated and the salt remains. This is the ordinary process of salt-making with all its benefits in various countries to the present day. What thought is more natural, in view of this recognized fact, than that the sun is the generator, or the begetter of salt which is life? If the sun is supposed to bring life, in what way does it more directly accomplish this than by this salt creation. This would seem to give added significances and force to the words of Jesus as to salt and light. If in the days of Jesus it was held as Pliny says, that there was nothing that could help the life of humanity like salt and sun, life and light, the disciples of Jesus must have recognized a peculiar meaning in the teachings of the Great Physician as he sent them out into the world to heal the sick, and raise the dead and cleanse the lepers, and cast out demons, when he suggested that it was what they were, rather than what they did, that was to be the help of humanity. In the same teaching he said, "Ye are the salt of the earth, Ye are the light of the world"