I've wondered this for years, and don't have a good answer. My dad was an agricultural engineer, and he couldn't give me a good answer, nor could my friends wife, who is a soil conservationist. I also asked this of a friend who is a grad student studying something or other about how to feed more people around the world. Nothing doing.
In small tropical islands, sea foam gets everywhere, and makes everything wet. The sea is salty, and the sun evaporates the water, leaving an increasingly toxic level of salts that would be in too high of a concentration for anything to grow. Yet, things do grow on those islands. How?