Sprinkle yeast over warm water in a small bowl; let stand until foamy, about 5 minutes.

Place butter or margarine and buttermilk in a small saucepan. Heat slowly until the butter or margarine melts; cool to lukewarm.

Place sugar, salt, baking soda, buttermilk mixture, and yeast in large mixing bowl. Add 3 cups flour one cup at a time, and mix with the dough hook attachment of an electric mixer. Gradually add the remaining flour while continuing to mix. When dough is not sticky, turn out on a lightly floured surface. Knead for several minutes, until the dough is soft and smooth. Place in a greased bowl, and turn once. Allow to rise until doubled in size.

Punch down the dough; divide and shape into 2 loaves. Place loaves into two well-greased 8x4-inch loaf pans. Let rise until dough is 1 inch above the tops of the pans.

Preheat the oven to 375 degrees F (190 degrees C).

Bake in the preheated oven until loaves are golden brown and sound hollow when tapped on the bottom, 30 to 35 minutes.