Boil 3 or 4 potatoes until they disintegrate and cool them to room temperature without draining. Stir vigorously to make a gruel of the softened potatoes, adding water if needed to make about 2 cups of liquid. Combine potato gruel with 2 – 2 1/2 cups flour and 1-2 TBSP sugar in the sourdough pot and mix with a non-metallic spoon. The starter should be thick and creamy, not stiff. Thin with water or thicken with flour if needed. Cover pot loosely and set in a warm place. When mixture begins to bubble after a day or two, nourish the starter by adding and stirring in 1/2 cup each of flour and water, and 1 tsp. sugar. Starter is ready to use when it is full of bubbles and gives off a good, clean sour aroma, usually 3-5 days.
Leave at least 1/2 c. starter in pot when using in a recipe. Rebuild by adding equal amounts of flour and water to the pot, stirring in 1/2 c. of each for each cup of starter used. Store the replenished starter in the refrigerator, covered but not sealed shut and use it at least once every 2 weeks. Cold will make the starter sluggish, revive it the night before by adding a little flour and warm water and letting it stand at room temperature overnight. Add an occasional spoonful of sugar if starter needs reviving.
A clear liquid forming on top of the starter is natural, just stir it in. If mold appears or if it turns color, toss out and start again. Occasionally transfer starter to another container and scrub out the pot. Avoid using metal bowls and utensils, metal baking pans are ok.