Salting the earth, how much salt does one actually need?

Give the California fires and the planes scooping sea water, there have been concerns about salting the ground. How much salt is needed to make the soil barren? And how did they do it, if they even did it, in ancient times when salt was a form of currency, and thus expensive?

I have a coastal house that has had sea water go across the lawn several times and it looks fine after 2 weeks even without rain or irrigation.