Is there a reason to not add salt when making a soup?
Because as you are cooking your soup, water in your soup is evaporating away as steam. You might salt a soup perfectly halfway through, but after evaporation, your now thicker soup is too salty.


  When adding salt, wait until the end of the cooking process, as soups will reduce and concentrate the flavors as the liquid evaporates.


[ Source: http://allrecipes.com/howto/cooking-questions-soup/ ]