In ancient times salt was valued very highly. It was used for many varied reasons including pottery-making, mummification and food preservation. It was historically used as a method of trade and currency, including the purchase of slaves. The phrase “not worth his salt” comes from that fact.
It originates from the Latin ‘salarium argentum’, the quota of salt given to Roman soldiers. This was their salt ration, and not their actual pay, but the value of the salt was understood.
According to Pliny the Elder, it actually was their pay, but perhaps he was referring to the fact that it was a portion of it. According to Wikipedia, in his “Natural History”, he wrote that “[I]n Rome. . .the soldier’s pay was originally salt and the word salary derives from it…”.