1. Sustainable Agricultural Systems Laboratory, USDA–ARS, Beltsville, MD
2. Soil Plant Nutrient Research Unit, USDA–ARS, Fort Collins, CO
3. Northern Great Plains Research Laboratory, USDA–ARS, Mandan, ND
4. International Plant Nutrition Institute, Conway, AR and Brookings, SD
5. Soil and Water Management Research Unit, USDA–ARS, St Paul, MN
6. Northwest Irrigation and Soils Research Laboratory, Kimberly, ID
7. University of Arizona, Tucson, AZ
8. National Soil Dynamics Laboratory, USDA–ARS, Auburn, AL