Though I'm not a historical expert, salting the Earth does not necessarily mean literally, though can. The mineral salt (sodium chloride) was always prized and I suspect expensive. Not all salts are the same, potassium chloride promotes growth. In the few texts I've read, they often say, 'Plow and salt.' If you turn ground enough, it kills off the underlying biomass and results in sterile ground (e.g., the US Dustbowl), yet that much plowing takes a LOT of work. Herbicides could also be used--any mineral, compound, or disease/fungus which prevents or hinders growth of specific local crops. E.g., potato famine/blight if used as a weapon. There is also a note about sowing weeds which would out-compete with certain crops.
That said, in some cases it might also be symbolic. Salt was always considered pure/purifying. So if say you raze a city and culture, you might then cast salt over the area to ritually purify it of the last remnants of a people. IOW, raze a city, kill the people, plow it under, and salt the Earth.
As to an exact amount and how, plowing would help it reach the underlying biological system, but I suspect, just dumping it on top and allowing the rain to wash it down. I'm not sure if there are texts where they document amounts used and intent.
K2