Watercress and Buttermilk Soup

Ever since settlers in the first Southern colonies churned butter derived from a sour milk called clabber and drank the leftover nutritious liquid, buttermilk has had a special status in the South that few outsiders appreciate. Gone are the days, however, when you can find natural buttermilk like that I drank as a child. Today it is replaced by the very respectable commercial product made from low-fat milk transformed with special bacteria. For centuries, buttermilk has been a vital ingredi