Preheat oven to 425°F. Spray heavy baking sheet with nonstick spray.
Whisk both flours, sugar, and baking soda in medium bowl to blend.
Add margarine and cut in until margarine is reduced to pea-size pieces.
Add buttermilk; stir until shaggy dough forms. Turn dough out onto lightly floured work surface. Knead until doughcomes together, about 10 turns. Shape dough into 7-inch round.
Place dough on prepared baking sheet.
Cut large X, 1/2 inch deep, in top of dough.
Bake bread until deep brown and bottom sounds hollow when firmly tapped, about 40 minutes.