ANSWER: The word 'salt' occurs 31 times in the Old Testament and only 10 times in the New. Only three of its many references in the Bible are in relation to a covenant. The first reference is in the book of Leviticus.
And you shall not allow the salt of the covenant of your God to be lacking from your grain offering (Leviticus 2:13, HBFV throughout)
The reference in Leviticus is in regards to the mineral being an essential ingredient in offerings made to God. The mineral's ability not only to ward off decay but also to preserve made it an excellent symbol to represent the perpetual agreement between God and his people.
The location of the second reference in Scripture is in Numbers 18, where it is made in relation to God promise to provide for the needs of the priests who serve him.
19. All the heave offerings of the holy things, which the children of Israel offer to the LORD, I have given you and your sons and your daughters with you by a statute forever. It is a covenant of salt forever before the LORD to you and to your seed with you (Numbers 18:19)
The last mention of the phrase in question is in 2Chronicles 13. King Abijah of Judah used the term to refer to the royal Davidic dynasty's right to rule over Israel in place of the (rebel) King Jeroboam of Israel.
5. Should you not know that the LORD God of Israel gave the kingdom over Israel to David forever, to him and to his sons by a covenant of salt? (2Chronicles 13:5)
There is a definite Biblical linkage between this commonly found mineral and the making of agreements or contracts. Partaking of it in a group seated around a table was an ancient symbol of unbreakable friendships and enduring alliances.
Refrigeration as a means of preserving large quantities of food did not begin to grow until the latter part of the 19th century. One of the most common ways of preserving food before this time (including the period of the Old Testament) was to use salt. This property of physical preservation (cf. Matthew 5:13) led to this mineral being used in terms to symbolically represent preservation in general.
Taken together, a 'covenant of salt' is an agreement or contract between parties that endures regardless of the circumstances. Such agreements are solid, unbreakable and everlasting.