They say Gypsum will help that but you need a lot of it. We see it often when there are extended periods of drought where there is irrigation. We have to have a good rain to wash those salts away. I think they are just evaporative calcareous salts that build up in Alkaline soils. We have relatively hard water, even the surface water and the high pH of the soil just combine and form the that crust. A good heavy low pH rain will fix it.