The passuk says in Vayikra (2:13)
על כל קרבנך תקריב מלח
You shall offer salt on all your sacrifices.
The Gemara in Menachos (21a) says that this passuk is coming to teach us that:
ואפילו בשבת תקריב
Even on shabbos you must salt the korban before offering it on the altar
Why do we need an extra passuk to allow salting the animal on shabbos? What prohibition or melacha is there in salting the korban?