Jamieson, Fausset and Brown on Numbers 18:19

19. it is a covenant of salt--that is, a perpetual ordinance. This figurative form of expression was evidently founded on the conservative property of salt, which keeps meat from corruption; and hence it became an emblem of inviolability and permanence. It is a common phrase among Oriental people, who consider the eating of salt a pledge of fidelity, binding them in a covenant of friendship. Hence the partaking of the altar meats, which were appropriated to the priests on condition of their services and of which salt formed a necessary accompaniment, was naturally called "a covenant of salt" (Le 2:13).