How to prevent the crockpot burning my soups
A crockpot's keep warm setting is not designed to keep food safe and fresh for days, it's really only good for an hour or two at most. What's happening in your case is that the water is evaporating from your food and then drying out inside the pot. Adding water periodically is not an answer as the temperature of the food is not high enough to prevent foodborne illnesses, you don't want to keep it on warm for long periods of time even if you can keep it from burning. 

If you want to save your soup for later put it in containers and store it in the refrigerator or freezer, re-heating when you want to eat it.