Why is someone who is not doing their job said to be "not worth
his salt?" 

At one time soldiers in ancient times were paid, in part, with a
ration of salt called a salarium, from the Latin word "sal"
meaning salt. If a soldier's performance was not up to standard,
the soldier was said to be "not worth his salt." Later, when
salt was replaced with money to purchase the salt, this money was
called a salarium. Later, salarium came to mean the wages
themselves which led to what we now call one's pay a "salary."