From the archives of USDA http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=http%3A%2F%2Fwww.nrcs.usda.gov%2FInternet%2FFSE_DOCUMENTS%2Fnrcs142p2_052289.ppt&ei=FafzVK3wK-OwsASt4ILYAw&usg=AFQjCNFvW0WdqJceGqxOQg7wJT1nqmrSxQ&sig2=LjGUMZ8vyjQRP2gZjIwGSg