1. Soil-Plant-Nutrient Research Unit; United States Department of Agriculture-Agricultural Research Service; Suite 100, 2150 Centre Avenue, Building D Fort Collins CO 80526-8119 USA
2. Agroecosystems Management Research Unit; USDA-ARS; University of Nebraska; 118 Keim Hall Lincoln NE 68583-0937 USA
3. Grain, Forage, and Bioenergy Research Unit; USDA-ARS; University of Nebraska; 137 Keim Hall Lincoln NE 68683-0937 USA