Dissolve soda in buttermilk; stir well. Set aside.
Combine flour, baking powder, and salt in a large mixing bowl; cut in shortening with a pastry blender until mixture resembles coarse meal. Sprinkle buttermilk mixture evenly over flour mixture, stirring until dry ingredients are moistened.
Turn dough out onto a lightly floured surface, and knead 4 to 5 times.
Roll dough to 1/2-inch thickness; cut with a 2 1/4-inch biscuit cutter. Place biscuits on a greased baking sheet. Bake at 450° for 12 minutes or until lightly browned.