salt: the everyday mineral used to season and preserve food, and a byword for both lasting loyalty and utter ruin
Salt was one of the most ordinary and most valued substances in the ancient world. It flavored food, kept meat from spoiling, and was required with the grain offerings brought to God. Because it was cheap, plentiful around the Dead Sea, and part of daily life, it easily became an image for other things too.
Its two symbolic uses pull in opposite directions. Salt does not rot, so a bond sealed with salt was a bond meant to last: a covenant of salt is a promise that will not be broken. But salt in the soil kills crops, so scattering it over a conquered city or finding it crusted across a wasteland pictured barrenness that would never recover. The same substance could say either this will endure forever or nothing will ever grow here again.
Salt also names places. The Dead Sea, whose water is far too salty for life, is simply called the Salt Sea, and a valley near it is the Valley of Salt. In those names the word keeps its plain meaning: it is the salt itself that gives each place its name.