Why, if someone isn’t up to the job, do we say he isn’t “worth his salt”?

Thousands of years ago, before money was introduced, workers and soldiers were often paid with a negotiated quantity of salt. More than as a seasoning, salt’s value was in its use as a preservative or cure for meat, as well as a medicine. The early Romans called this payment a “salarium,” which gave us the word salary. If a man wasn’t worth his salt, he wasn’t worth his salary.