The Bible refers to “a covenant of salt” and “the salt of the covenant.”
Lev 2:13 (NIV) Season all your grain offerings with salt. Do not leave the salt of the covenant of your God out of your grain offerings; add salt to all your offerings.
According to the New Oxford Annotated Bible, “of salt” most likely means that the covenant is “a perpetual covenant, because of the use of salt as a preservative.”
However the term “covenant of salt” doesn’t just illustrate the perpetual nature of the covenant but also the friendly nature of it.