For simplicity reasons let's say antiquity ended around 500AD.

It seems the amount of salt needed to make an entire cities worth of land infertile would be ridiculous, especially in a time when salt was so valuable. How many instances were there of large amounts of territory being "salted"? What stopped people from simply setting up farms a mile down the road from where the salt was laid? Is salting a historical practice or is it some sort of allegory? How effective was it if it actually was done ?