Salt residues on the bottom of dish washer

Have you tested the water to see how hard it is? If not, I might start there to confirm this is indeed the issue you are facing.

Although it might not solve your problem, try running cycle or two without dishes but with a a couple of cups of vinegar added. Don't just pour it into the bottom, as it might get purged from the dishwasher's sump when the cycle starts. Instead, put the vinegar in some sort of cup / dish so that it can "slosh out" as the cycle runs.

If this doesn't seem to make a difference, try using a different detergent (there are some, at least in the US, that claim to help with hard water). Else, you might need a water softener.