Sift together the flour, sugar, salt, and soda.
Place in large bowl and cut in the butter and shortening until mixture resembles coarse meal.
Dissolve the yeast in the water.
Add yeast mixture and buttermilk. Mix.
Turn out onto a floured board.
Knead for 3-4 minutes, kneading in more flour if necessary to make a smooth dough.
Roll out to 3/4-inch thickness and cut with biscuit cutter.
Place on an ungreased baking sheet. Allow to sit at room temperature for 30 minutes.
Bake in a preheated 450°F. oven for 12-15 minutes.
Makes about 32 medium biscuits.
Note: These biscuits may be made in the food processor using the steel blade.