Add the salt to the water before you add the potatoes.

If you are boiling potatoes with the intention of mashing them, don't add salt at all.

Old potatoes can be rehydrated by mainataining in water 24 hours. Salt water increase the content of sodium in potatoes but it is not important.

The potatoes become flacid because the salt water drins out the moisture from the cells of the potatoes. This process is called exo-osmosis . The cells lose their turgidity and hence the potatoes become flacid :)

If you water your potatoes with salt water they will die. Don't water anything with salt water. Plants require fresh water.

Old potatoes can be rehydrated by mainataining in water 24 hours. Salt water increase the content of sodium in potatoes but it is not important.