A few dashes of soy sauce can be used in place of salt in a soup recipe. Soy sauce has a salty flavor and can add the same taste as a few dashes of salt.
Bouillon cubes can also be used to replace salt in soups. Bouillon cubes and granules are high in sodium and can add a good flavor to many types of soups. Bouillon cubes are made in a variety of flavors, including chicken, pork, beef and vegetable.
For those watching their sodium intake fresh herbs can replace salt in a soup. Good substitutes include basil, thyme, rosemary, oregano and crushed red pepper.