Preheat the oven to 450 degrees. Grease a sheet pan. Combine the flour, baking soda, salt and sugar together. Add the butter and work in the flour mixture until it looks like little pebbles....easier to do in the food processor. Stir in sweet potatoes and pecans. Add half and half a bit at a time until you have a soft dough. Place dough on a lightly foured surface and brush lightly with flour. Press out to 1/2 inch thickness and cut 2 inch biscuits. Place on greased cookie sheet and bake for about 15 minutes or until golden brown.