Put the water, starter, 3 cups flour, rice, oil and sugar in a large bowl. Mix well.
Cover with plastic wrap and leave to ferment in a cool place overnight.
The next morning, deflate the sponge and to it add 2 cups flour, the salt and the baking soda. If the dough seems too loose to handle, add the last 1/2 cup of flour, cautiously. Don’t give in to the temptation to keep adding flour to aid your kneading; it will be very heavy. Oil your hands instead. Let the dough remain a little sticky.
Knead 10 minutes or until you’re sure that everything is well incorporated. Cover the dough again and leave it in a warm place to rise, 2-3 hours.
Deflate the dough and shape your loaf. Cover the loaf and let it rise somewhere warm until it’s light, 1-3 hours.
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, gently remove the loaf from its pan and turn it upside down to finish baking – another 15 minutes. It’s always best to test the loaf with a toothpick before assuming it's done baking. If it seems underdone, give it another 5 minutes, or turn the oven off and come back in 15 minutes.