Grease and flour a 13x9-inch pan. Sift together first three ingredients; set aside. Cream butter and sugar, then mix in buttermilk and sour cream. Add the baking soda to the wet batter. Slowly mix dry ingredients into the wet. Pour batter into the pan and bake at 350 degrees F for 30 minutes. Remove from oven and pierce cake liberally with a toothpick. Mix orange juice and powdered sugar and pour over the warm cake. Serve as is, or topped with orange slices.