Many people use epsom salt dissolved in warm water for soaking various body parts. Epsom salt is chemically known as magnesium sulfate. Years ago it was taken by mouth as a laxative in small quantities. There appears to be no scientific reason why epsom salts would be any better than table salt for common soaking. Soaking in plain warm water can be very soothing, simply due to the warmth of the water.
Did you know that soaking in epsom salt can actually make your foot problem worse?
Soaking your foot in epsom salt (or any other salt--even the Pacific Ocean) does one thing very well: it dries the skin on your feet by drawing out the moisture. So if you already have dry skin, your skin will become even dryer after soaking them in epsom salt. Soaking your feet in Betadine is also very drying, and should be avoided if you have dry skin. The cracks in your heel callouses will deepen and your skin will itch more.
If there is a draining ulcer or infection in your foot, epsom salt and warm water (not hot) soaks can be helpful. But other treatments are also good, if not better.
If your skin is dry, a good soaking solution is warm water and a small amount of liquid dishwashing solution. Dishwashing liquids are antibacterial and contain skin softeners. Think Madge the Manicurist who soaked her client's hands in TV commercials. She was right!
For most situations, forget epsom salt. Leave the box on the shelf in the pharmacy. Better to spend that money on a frozen yogurt and go for a walk on the beach. The exercise will likely make you feel better than soaking in epsom salt.