“Since salt is also a necessary part of the daily diet, and salt was always used in sacrifices to the Lord (Lev 2:13), it was not long before people saw a connection between salt and covenant making.  To “eat salt with” a person meant to share his or her hospitality.  When covenants were made, they were usually confirmed with sacrificial means, at which salt was always present.”

Source: Note on Numbers 18:19. p. 221. Archaeological Study Bible. 2005. The Zondervan Corporation. Grand Rapids, MI.