Make a batter of the half cup of starter, cup of milk, and 1 cup of the flour. Let sit overnight or 8 to 10 hours. Mix the biscuits sifting together the remaining cup and a half of flour and all other dry ingredients except the baking soda.
Work in the shortening with your fingers or a fork. Add baking soda dissolved in a little warm water to the sponge and then add the flour mixture. Mix into a soft dough. Knead lightly a few times to get in shape.
Roll out to about 1/2-inch thickness or a little thicker,and cut with a biscuit cutter. Place close together in a 9 x 13-inch pan, turning to grease tops. Cover and set in a warm place to rise for about 45 minutes. Bake in a 375F oven for about 30 to 35 minutes.