The ground of Carthage clearly was not salted, as Carthage was able to reestablish itself as a major city.
However, a number of historians note that "salting" in this context is ritual, a small plot of land in a defeated city was ritually salted but the agricultural lands were not since the Romans expected to one day occupy those lands for themselves. Indeed, there is no historical evidence that any culture actually salted the lands of conquered foes, both because it would have been an incredibly stupid thing to do when wars were fought over the right to control agricultural lands, but also because salt was extremely expensive and difficult to acquire in ancient times. ("Salary" is derived from salt, which was so valuable in Roman times that it was used as a measure of value.) The amount of salt that would have been required to salt the lands of Carthage would have equaled the salaries of the entire Roman army. "Salting the earth" as a meme didn't actually arise until the 17th or 18th century, when industrial salt production finally made salt cheap and plentiful.