The Sages forbade pickling cucumbers (or olives and the like) in brine or vinegar, because pickling is comparable to cooking. Similarly, the Sages forbade salting foods if doing so fundamentally changes them, as is the case with radishes, onions, garlic, turnips, and cucumbers. Salt helps sweat these vegetables, extracting their bitterness, changing their textures, and improving their taste. Thus, salting is comparable to the melakha of Me’abed (below 18:6), and its impact on foods is similar to that of cooking (SA 321:2-6; MB ad loc. 15).
One may salt an individual piece of a vegetable and then eat it, because this does not resemble pickling. Many also permit salting multiple pieces at once, as long as the pieces will be eaten soon (SA 321:4). However, the custom is to be stringent and eat each piece immediately after salting it. This is because if one waits to eat a salted piece until after additional pieces have been salted, it may look like he is pickling food (MB 321:20; Kaf Ha-ĥayim ad loc. 26).
Salting multiple pieces together is permissible if one adds oil as well. This is because the oil reduces the saltiness, making it clear that one is not pickling, but rather flavoring his food (MB 321:14). Similarly, one may add salt to a cucumber and radish salad being prepared for the upcoming meal. This is because preparing the salad also involves adding oil and spices that weaken the salt, making it clear that one is not pickling (Taz 321:1; MB ad loc. 14). One may also add as much salt as one wishes to vegetables that are not normally pickled, such as tomatoes (SSK 11:2).
The Sages also prohibited things that resemble pickling. Therefore, one may not prepare large quantities of salt water or other pickling agents on Shabbat. However, one may prepare a limited quantity of such mixtures – enough for dipping foods during the meal. Concentrated salt water (i.e., a mixture that is at least two-thirds salt) may not be prepared even in small quantities, since it resembles preparing brine to pickle fish (SA 321:2).