Salt Losing its Savour
“If salt has lost its savour, wherewith shall it be salted?” If men fall from grace, how shall they be restored? The reference is to rock-salt, which loses its saltness if exposed to the hot sun.
“Along one side of the Valley of Salt (that towards Gibul) there is a small precipice about two men's lengths, occasioned by taking away of the salt. I broke a piece off that was exposed to the sun, rain, and air; though it had the sparks and particles of salt, yet it had perfectly lost its savour. The inner part, however, retained its saltness.” —Maundrel, quoted by Dr. Adam Clarke.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894