If the soup can stand it, add some lemon juice. Salt and lemon juice tend to argue for supremacy with each other. If the soup is not too salty, a dose of lemon juice that is not so large it causes other problems for the soup may be the best solution.

If lemon juice is not an acceptable addition to this particular soup, will it benefit from some cream, milk, water, chicken stock, beef stock, tomato juice, or other liquid that will water it down and disperse the salt within a greater quantity of soup? If you choose cream or milk, don't let the soup return to a boil or the milk may curdle. If you use stock, make sure it is a low-salt version, or better yet, your own homemade, unsalted stock. If you use a bouillon cube to make stock, you'll only be compounding the problem, since they are full of salt.