"Salting the Earth" was really more a symbolic ritual upon conquest that was reportedly practiced in the ancient Near East. I don't think the sources for the most famous instance of this (the razing of Carthage) are considered factual. The story of the Romans "salting the earth" of Carthaginian lands may be a later invention or interpolation of biblical themes into Roman history.

Even if one did salt the earth, you would need an incredible amount of salt to have any significant impact on crop yields over an entire state's lands, and salt wasn't a cheap commodity for much of the ancient world. In addition, unless the site you salt is a terminal basin of a watershed (i.e. Dead Sea), the next heavy rains will leech much of the salt back out of the soil. Even the most heavily salted soil would likely recover within 5 years.