Adam Clarke on Numbers 18:19

It is a covenant of salt - That is, an incorruptible, everlasting covenant. As salt was added to different kinds of viands, not only to give them a relish, but to preserve them from putrefaction and decay, it became the emblem of incorruptibility and permanence. Hence, a covenant of salt signified an everlasting covenant. We have already seen that, among the Asiatics, eating together was deemed a bond of perpetual friendship; and as salt was a common article in all their repasts, it may be in reference to this circumstance that a perpetual covenant is termed a covenant of salt; because the parties ate together of the sacrifice offered on the occasion, and the whole transaction was considered as a league of endless friendship. See the note on Lev 2:13.