When being salt of the earth

“You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people's feet.” (Matt. 5:13 ESV)

Impurities can cause flavor loss 

“Jesus taught His followers to be careful not to lose the flavor of their Creator. Most of the salt used in Israel came from the Dead Sea, which contained impurities. Impurities can cause flavor loss. Right before this verse, in the conclusion of the Beatitudes, Jesus proclaimed blessings over those who are persecuted, not only for living in sake of righteousness but for living for Him. He described His followers, and believers today, as salt and light, clearly called to influence, improve and make the world a better place.” (The Moody Bible Commentary).

Don’t lose your saltiness 

“Salt is good, but if salt has lost its taste, how shall its saltiness be restored?” (Luke 14:34 ESV)

“For everyone will be salted with fire. Salt is good, but if the salt has lost its saltiness, how will you make it salty again? Have salt in yourselves, and be at peace with one another.” (Mark 9:49-50 ESV)

Therefore…

“Let your conversation be always full of grace, seasoned with salt, so that you may know how to answer everyone.” (Col. 4:6 NIV)

Quotes:

“We should not ask, ‘What is wrong with the world?’ for that diagnosis has already been given. Rather we should ask, “What has happened to salt and light?” – John Stott 

“Salt is a preservative and is tasty. The Christian’s conversation is to be wholesome and helpful.” - NIV Study Bible Notes