Since its in the thread title and referenced a few times in the posts, I'll tell a true tale of irony about the salt (we need salt and iron to live)...
At the end of the third Punic War (264-146 BC), the Roman army and Roman fleet sieged Carthage. The Carthaginians resisted desperately, dragging the siege out for three year. Carthage was finally starved into submission in 146 BC, but at great cost to the Romans... who were naturally not happy about that. Of the original 250,000 Carthaginians, only 20% (about 50,000) survived. Gee, sounds kinda like the "exterminate" option in RTW... except in this case, Rome enslaved all the survivors. Then the Romans burned the city for 17 days! After Carthage was destroyed, the Romans cleared the charred ground, and plowed it. Plowing leaves small rows (like mini-ditches), and into this freshly plowed and very fertile soil, the Romans sowed salt. Of course, salt is not a crop... in fact, salted soil will prevent basic crops (like the various wheats which were essential to ancient civilizations) from growing. Then the Romans pronounced a curse on the land. They felt that Salt + Curse would ensure that their bitter enemy and anyone else would never again build a civilization on the site of Carthage again.
The irony is this: Within 25 years, in 122 BC, the Romans themselves wanted to found a new city on the dead site of Carthage. This was primarily because of its strategic position in the Mediterranean. BUT... the curse was actually a psychological barrier for potential settlers, since they really believed in that kind of stuff! They told outlandish tales of things like supernatural wolves attacking even the city's boundary markers... and guess what... within 30 years, the city had failed and was abandoned (but not because of the salting).
It was Julius Caesar that proposed to found a new colony at Carthage, but it was not until after his untimely death that Colonia Julia Carthago succeeded and became a thriving Roman city. In fact, the ultimate irony was that by the mid-first century AD western Roman empire, it was the 2nd largest city... only behind Rome itself!!
So to keep this on topic, it would be historically accurate to exterminate the city again and again, .