Both solar salt and pellets are available for your softening needs.
Pellets are a mined salt that is washed and compressed into pellets thus making them a purer, cleaner product.
They leave little to no residue in your day tank. Solar salt is as effective at regenerating a water softener as pellets and is a lower price alternative. It is not as clean as pellets and does require more frequent cleanings of your day brine tank due to the sediment left behind.