John Wesley on Leviticus 2:13

Salt - To signify that incorruption of mind, and sincerity of grace, which in scripture is signified by salt, Mark 9:49, Col 4:6, and which is necessary in all them that would offer an acceptable offering to God. Or in testimony of that communion which they had with God in these exercises of worship; salt being the great symbol of friendship in all nations is called, either, because it represented the perpetuity of God's covenant with them, which is designed by salt, Num 18:19, 2Chr 13:5. Or, because it was so particularly required as a condition of their covenant with God; this being made absolutely necessary in all their offerings; and as the neglect of sacrifices was a breach of covenant on their part, so also was the neglect of salt in their sacrifices.