The initial assumption can be taken on salt being a valuable commodity in the ancient world. Indeed Roman soldiers were often paid in salt (this is where we get the word Salary from!). Taking ‘Earth’ as the planet then I can see where people were coming from.

However if we take earth as the soil we get the opposite meaning, for pouring salt onto soil makes it barren and lifeless.

We could perhaps combine these two meanings and come to the conclusion that we are indeed the salt of the planet, but wherever we spread we inevitably spoil the environment as we are doing as a species right now.