Can I store a soup in my cast-iron pot for up to 24 hours in the refrigerator?
Perhaps there are some types of soup that would be ok, but many foods can easily pick up a metallic taste from cast iron if left in the pot overnight. Examples are tofu fried in a cast iron frying pan, egg omelet (egg sometimes even becomes gray), and anything tomatoey. I'm sure there are others, especially mildly flavored foods, and acidic ones.
So in general, rather than find out the hard way with your particular type of soup, I recommend changing containers for storage in the refrigerator.