You may not salt certain foods, whether cooked or raw, on Shabbat if the:

• Salt will materially (not just due to the flavor of the salt) change the flavor of the food, as in salting cut or chopped onions or salting tomatoes.
  
  Note You may dip the tomato or other food into salt using your hand as you are eating it.
• Foods have a shell; e.g., corn kernels (on or off of the cob), beans, peas.
• Salt has not been heated previously (e.g., during the processing of the salt) and the food you are salting is hot (over 120° F, or 49° C).

Leniency  If the food has oil in it, you may add salt even if the food contains onions or has a shell. Even a thin layer of oil will exempt the salt.