I bought my Soul 2 Diesel in February. Here in the UK the roads get a healthy dose of rock salt every winter and some areas are using magnesium phosphate.
As already stated above, there is nothing under the car that will take much harm but I can assure you that some of the tin shields and lighter metal components will corrode and at best will get a coating of surface rust.
I like the bottom of my car to look good and be well preserved. I bought a cheap underseal gun off ebay (about 15 USD) and a gallon of Waxoyl. I warmed it up and sprayed it liberally underneath. I covered absolutely everything except the exhaust system. It does no harm to rubber and I sprayed it even onto the back of the hubs (a little gets on the brakes but it soon burns off. I guess it will not last long onder the wheel arches but for the rest it is still nice and waxy. I took the front bumper off, masked the radiators and did all the exposed metal;
To do the rear you have to remove the rear lamps, then the bumper;
Incidentally, those square holes in the rear valance go all the way forward to the suspension outriggers. The spray gun comes with an extension tube so you can get into cavities like that so all the chassis runners have been done in that way. In this close up you can see the consistency of the waxoyl. It dries a bit candle wax consistency and softens in hot weather. It stinks for the first week but it soon subsides;
There is about 2 litres under the whole of the car.
I put a thick coat of meguires gold behind the rear lights when putting it back. I'm away from home at the moment but will take some photos of just how good it is underneath over the weekend.
I know its a bit OCD but I intend keeping this vehicle so I will do my best to preserve it!