1. Dep. of Agronomy; Iowa State Univ.; Ames IA 50011
2. USDA-ARS; National Soil Tilth Laboratory; 2150 Pammel Drive Ames IA 50011
3. USDA-NRCS; Soil Survey and Resource Assessment; P.O. Box 2890, 14th Independence Ave. S.W. Washington, DC 20250
4. Dep. of Agricultural & Biosystems Engineering; Iowa State Univ.; Ames IA 50011