This is a fairly common topic around here. Not to discourage new answers, but you might find some interesting comments in this thread, which itself has a link to the section in our popular questions list to a number of others.

This is almost certainly an invention based on stories about the end of Rome's Third Punic War and the destruction of Carthage popularized in the 19th century. (In reality, the land around Carthage was declared ager publicus and the city itself rebounded).

Something about the badassery involved in doing something like salting the earth clearly resonated with people from then to now, but I'm not aware of any evidence it was ever actually done anywhere by anyone.

If you're really interested, you can take a peek at R.T Ridley's article on the subject in the journal Classical Philology (1986), entitled "To be taken with a pinch of salt: The destruction of Carthage".