Salt has always been important and a valuable commodity. In some Arab countries today if two men eat salt together they are sworn to protect one another even if before this they have been sworn enemies. In other places people will throw salt over their shoulder when they have made a promise.
Look up 2 Chronicles chapter 13, verse 5. God makes a "salt covenant", that is a binding agreement with David and his sons.
In Numbers chapter 18, verse 19-20, Aaron and his family are to care for the sanctuary in the tent of the covenant, and they will not own land, but God declares that they will receive a "covenant of salt" for ever.