Dissolve yeast in a large bowl with the warm water. Add starter, salt, sugar, and 2 1/2 cups flour. Mix on medium speed 5 minutes. Cover and let rise in a warm place 1 1/2 hours. Mix 2 1/2 cups flour with the baking soda and stir into mixture. Work rest of the flour in with your hands to make a stiff dough. Divide in half and let rise 10 minutes. Shape into 2 round loaves and let rise 1 hour. Bake at 400 F for 35 mintues.