Combine all ingredients except salt in an 8-quart stockpot. Bring to a boil; reduce heat, and simmer, uncovered, for 3 hours. Strain mixture through a cheesecloth-lined colander into a large bowl, pressing vegetables with the back of a spoon to remove as much liquid as possible; discard solids. Stir in salt.
Note: Store broth in refrigerator in an airtight container for up to 1 week. Pour 2 cups broth into freezer-safe containers; freeze for up to 3 months.