Put the water, starter, 3 cups flour, oil and sugar in a large bowl. Mix well.
Cover with plastic wrap and leave to ferment in a cool place overnight.
Next morning, deflate the sponge, and to it add 2 cups of flour, the salt and the baking soda. If the dough seems too loose to handle, add the last 1/2 cup of flour, cautiously. For a loaf that’s lighter than the usual sourdough, keep the dough sticky. Oil your hands to knead.
Knead 10 minutes. Cover the dough again and leave it in a warm place to rise. This will take 2-3 hours.
Deflate the dough and shape your loaf. Cover the loaf and let it rise somewhere warm until it’s light. It may not rise to double in size, but you should be able to see gas blisters under the surface skin of the dough. This third rising takes anywhere from 1 hour to 3 hours, depending on how warm the room is.
Slash the top of the loaf to avoid “flying crown.” This is especially important if the loaf is to be free-form, not baked in a pan. Give it about 5 minutes to recover, then bake in a preheated 350° F (180° C) oven for 1/2 hour.
When the top has a firm, golden crust, test the loaf with a toothpick. If it seems underdone, give it another 5 minutes and test again. If you're baking it free-form, it will probably be done after 1/2 hour; it might take another 5 minutes or so if baked in a pan.
Plain white sourdough bread from Unpacked – https://jewishunpacked.com/how-to-make-plain-white-sourdough-bread-2/