Grass and weeds have a way of really rusting out a car. As far as dirt/soil it all depends on the moisture content of the soil. Which usually depends on the elavation as compared to the ground water. You could park a car on a sand hill and be fine. Park it on a sandy beach and have the car rust. It's even like that with parking on cement. A low lying cement slab will rust a car. A 'high and dry' cement slab would be much better. You could lay down a sheet of heavy plastic sheeting to make a vapor barrier under your car.
