Grease a 9x5 inch loaf pan.
In a large mixing bowl, combine the yeast, ½ cup whole wheat flour (70 grams) and ½ cup of the bread flour (63 grams).
Combine milk and water to make 1 cup of liquid, and heat to 125 degrees F. Pour the liquid mixture into the flour mixture and stir well, then add the bubbly sourdough and beat until well blended. Cover the bowl with plastic wrap and set in a warm place for 1 ½ to 2 hours.
Stir in the honey, salt, butter and baking soda, then add the 1 cup (140 grams) of whole wheat flour. Add remaining 1 ½ cups of bread flour half cup at a time, stirring until the dough feels soft but not too sticky.
Using the dough hook, knead the dough until smooth and elastic. You can also do this by hand if you don’t have a dough hook. Cover the dough and let rise until doubled in bulk (about 1 hour). Transfer to the greased 9x5 inch loaf pan and let dough rise again until doubled - -probably another hour.
When ready to bake, preheat oven to 375 degrees and bake for 35 minutes or until loaf sounds hollow.