This article is taken from our Sefer
Salting salad on Shabbos:
All foods which salt helps to change its natural state, [such as] to soften it, or harden it, or remove its bitterness, and other [changes] of the like, which includes all foods which are commonly pickled, may only be salted one piece at a time, and must be eaten immediately after the salting. It is thus forbidden to salt another piece prior to eating the first piece, as doing so delays the eating of the first piece. Amongst foods included in the above category are: radish, onions, garlic, beans and lentils that were cooked in their peels, cucumbers, and lettuce. Accordingly, it should be forbidden to salt a salad on Shabbos being that doing so transgresses both of the conditions, as it involves salting more than one piece of food at a time and one does not eat the salted food immediately. There is however one method available in which it is permitted to salt a salad on Shabbos, and that is through adding oil or vinegar to the food either before, or immediately after, the salting. If one adds oil or vinegar to the food immediately after salting, then it is permitted to salt even many pieces at the same time and they may even be eaten later on, on Shabbos, although one must do so in close proximity to the meal. Practically, although one may place the salt first and then immediately add the oil, nonetheless, it is proper to first place the oil and only then place the salt.
 Admur 321:4; Michaber 321:3-6; Shabbos 108b
 Admur ibid Taz ibid
 Admur ibid; Taz 321:6
 Admur ibid; Rashi ibid
 Stam opinion in Admur; Michaber 321:3; Rambam
The reason: The reason for this is because when one salts two pieces together, and certainly [when he salts] more [than two pieces], it appears like one is pickling pickled foods [which itself is forbidden due to cooking, or do due it being similar to tanning].
 2nd opinion in Admur ibid; M”A 321:6; Raavan 352; Elya Raba 321:6; Bach; M”B 321:14; based on reason of Rashi behind the prohibition
The reason: According to the 2nd opinion, the reason [the Sages] prohibited to salt a few pieces at the same time is not because it appears like [one is] pickling pickled [foods], but rather is because that until one [finishes] eating the first [piece] the second piece remained in the salt and since the salt benefits it, this is similar to tanning.
 Admur ibid; Michaber 321:3; Shabbos ibid; See Shabbos Kehalacha 20 footnote 69 regarding radishes of today which are not very sharp, although he nevertheless concludes to be stringent.
 Admur ibid; Taz 321:2; M”B 321:13
 Admur ibid; Michaber ibid; Tur in name of Rabbeinu Peretz
 Admur ibid; Taz 321:1; M”A 321:6; M”B 321:14; Shiltei Giborim; Rashi ibid; Regarding why pouring vinegar is not considered like pickling-see Shabbos Kehalacha 20 Biurim 11
The reason: Even though salting many pieces together is forbidden [to be done] even to eat right away, nevertheless since [the pieces] do not remain at all alone with the salt, as one immediately pours on it vinegar and other species, it is [therefore] not similar to tanning. [ibid]
 So is wording in Shiltei Giborim; M”A 321:6; Admur ibid; M”B ibid; However see Shaareiy Teshuvah ibid in name of Shvus Yaakov which says to first place in the oil, although in truth in the Shvus Yaakov the word “first” is omitted.
 Ketzos Hashulchan 128 footnote 3 and 5; Menorah Hatehorah 321:6; Piskeiy Teshuvos 321:4; See Shaareiy Teshuvah 321:1 in name of Shvus Yaakov that “if one first places the oil, it is permitted”; See however Shabbos Kehalacha 20:31 and Biurim 12 and Tosefes Biur that there is no need to first place the oil, as is the simple implication of Admur ibid
The reason: As perhaps if one were to place the salt first he may come to forget to place in the oil immediately after. [Ketzos Hashulchan ibid 5] As well, this should be done being that according all opinions when the oil is placed first there is no question of a prohibition, and it is better to do an act that will go in accordance to all. [Ketzos Hashulchan ibid 3] As well, some Poskim explicitly write to place the oil first. [Shaareiy Teshuvah 321:1]