1. North Central Soil Conservation Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Morris, Minnesota 56267; Agricultural Research Service, U.S. Department of Agriculture, 1991 Upper Buford Circle, St. Paul, Minnesota 55108; CSIC-IRNAS, Sevilla, Spain; Department of Plant Sciences, South Dakota State University, Brookings, South Dakota 57007; and University of Minnesota, St. Paul, Minnesota 55108