For sponge: dissolve yeast in 1/4 cup lukewarm water in large bowl and let stand 5 minutes to proof. Add starter, dry milk and remaining warm water and blend well. Add wheat flour, all-purpose flour and wheat germ and beat 4 to 5 minutes. Cover with plastic and let stand in warm draft-free area until doubled in volume, about 1 hour.
Add molasses, margarine, salt, baking soda and 1 1/4 cups all-purpose flour and beat until dough is stiff, adding remaining 1/2 cup all-purpose flour as necessary. Turn dough out onto very lightly floured surface and knead 10 to 15 minutes. Transfer to bowl. Cover and let stand in warm draft-free area until doubled, about 1 1/2 hours.
Grease 9x5-inch loaf pan. Punch dough down and shape into loaf. Transfer to pan. Cover and let stand in warm draft-free area about 45 minutes.
Preheat oven to 375 F. Bake until loaf is nicely browned, about 35 minutes.