1. USDA-ARS; National Soil Tilth Laboratory; 2150 Pammel Dr. Ames IA 50011-4420
2. USDA-ARS; Soil and Water Management Research Unit; St. Paul MN 55108
3. Dep. of Physics and Astronomy; Drake University; Des Moines IA 50311
4. Dep. of Agronomy; Iowa State University; Ames IA 50011