Place sourdough starter in a crock or glass bowl, along with the water, sugar, salt, and the first portion of flour. Mix well, cover lightly with a towel or cheesecloth folded into several thicknesses, and let stand at room temperature for approximately 18 to 24 hours or until the dough has doubled in size.|
Mix half of the remaining flour with the baking soda and stir this into the risen dough. Turn the dough out onto a floured board and knead enough remaining flour into it to make a very stiff dough. Knead it for 5 to 10 minutes or until the dough is smooth. Shape into long or round loaves, place on a lightly greased baking sheet, cover, and let stand in a warm place for 3 to 4 hours or until almost doubled in size.
Preheat oven to 400°F. Brush a little water on top of the loaves and make a few diagonal slits across the top with a sharp knife. For a crustier bread, place a shallow pan of water in the bottom of the oven. Bake for approximately 45 to 50 minutes or until the crust is a medium dark brown.