Whisk together all ingredients in a large bowl. Cover tightly with plastic wrap. Store at room temperature. You may use after 1 day at room temperature. Add 1 cup warm water, 1 cup bread flour, and 1/2 cup rye flour every other day. Discard remaining Sour Starter if you don't intend to use it after a few days.
Note: Sourdough starters may be stored at room temperature indefinitely, as long as you "feed" the starter with water and flour. In fact, some sourdough starters are more than 20 years old.