Combine potatoes, cornmeal, 1 teaspoon salt, and sugar in a large bowl. Pour boiling water on top. Cover with a dish towel; let sit for 24 hours in a warm place.

Remove potatoes from the yeast water, and add milk, 1 cup water, butter, 1/8 teaspoon salt, and baking soda to the bowl. Add 1 cup flour at a time, stirring constantly, until dough begins to come together and pull away from the sides of the bowl.

Turn dough out onto a floured surface; cut into 3 equal pieces. Knead each piece until soft and elastic. Place dough in 3 well-buttered bread pans and turn to coat. Cover loosely with greased plastic wrap and let rise in a warm place until doubled in size, about 24 hours.

Preheat an oven to 400 degrees F (200 degrees C).

Bake in the preheated oven until tops are golden brown, about 45 minutes.