Covenant of Salt

In Number 18:18-19, God allots a portion of offerings to Aaron and his family. God says, "It is an everlasting covenant of salt before the LORD for both you and your offspring." References to salt in the New Testament ("you are the salt of the earth" - Matthew 5:13) usually point to its being a preservative. That doesn't quite seem to fit here. Actually, there doesn't seem to be a strong answer for what this means. The best I found came from the Concordia Self-Study Bible, which pointed out that salt was often a part of offerings/sacrifices, so it might be talking about God's covenant with Aaron's family to carry out the priesthood.