Dissolve yeast in warm water. Stir in whole wheat flour, starter, molasses, butter, and salt. Combine 1 cup flour with baking soda and stir into yeast mixture. Stir in remaining flour a little at a time until a stiff dough forms. On a floured surface, knead dough 5 to 7 minutes, until smooth. Shape into a ball and grease the top. Let rise in warm place until doubled. Punch down and make 2 loaves; put into greased bread pans to rise 1 hour. Bake at 375 F for 35 minutes.