'Salt' symbolizes the desire of truth and good to be conjoined with each other. So, a salt can conjoin water, which corresponds to truth, with oil, which corresponds to good.

Salt can also have a negative meaning. 'Lot's wife becoming a statue of salt' signifies all the good of truth being vastated, or laid waste, in the church that Lot represented. In that story, Lot's wife represented a form of 'truth' that turned itself away from 'good,' and looked instead to doctrinal matters. Because 'salt' can signify vastation, and 'cities' signify doctrinal aspects of truth, in old times they sowed cities with salt when they were destroyed to prevent them from being rebuilt, as in Judges 9:45

(Reference: Arcana Coelestia 10300)

1023. That 'and I, behold I am establishing My covenant' means the Lord's presence in charity becomes clear from the meaning of 'a covenant', dealt with in 666, where it was shown that 'a covenant' means regeneration, and in particular the conjunction of the Lord with a regenerate person by means of love. It was also shown there that the heavenly marriage is the actual covenant itself, and that consequently the heavenly marriage exists with every regenerate person. The nature of this marriage or covenant has also been shown already.

[2] The heavenly marriage with the member of the Most Ancient Church existed in the area of his own will, but the heavenly marriage with the member of the Ancient Church came into existence in the area of his own understanding. Actually when the will part of man's mind had become utterly corrupted, the Lord in miraculous fashion separated the area of his own understanding from that corrupted will part, and within the area of man's own understanding He formed a new will, which is conscience. Into conscience He infused charity, and into charity innocence, and in this manner joined Himself to man, or what amounts to the same, established a covenant with him.

[3] To the extent that the area of a person's own will can be separated from this understanding part the Lord is able to be present with him, that is, to join Himself or establish a covenant with him. Temptations and similar means of regeneration render inactive the area of a person's own will, so that it is reduced to nothing and so to speak dies. To the extent that this happens the Lord can operate within charity by means of conscience implanted in the area of a person's own understanding. This is what is called 'a covenant' here.