1. Dep. of Crop and Soil Sciences North Carolina State Univ. Raleigh NC 27695 USA
2. Dep. of Crop, Soil, and Environmental Sciences Univ. of Arkansas System Division of Agriculture 1371 West Altheimer Drive Fayetteville AR 72704 USA
3. USDA‐ARS 2413 Nashville Rd B5 Bowling Green KY 42101 USA
4. Plant Nutrition Canada Guelph ON Canada
5. Univ. of Kentucky Research and Education Center Princeton KY 42445 USA
6. Dep. of Soil Science Univ. of Wisconsin‐Madison 246 King Hall, 1525 Observatory Drive Madison WI 53706 USA
7. Dep. of Plant and Soil Sciences Univ. of Kentucky N122‐G Ag Science North Lexington KY 40506 USA
8. Dep. of Agronomy Iowa State Univ. Ames IA 50011 USA
9. Dep. of Agronomy Kansas State Univ. 2708 Throckmorton Manhattan KS 66506 USA
10. LSU Agricultural Center, Scott Research & Extension Center 212‐A Macon Ridge Road Winnsboro LA 71295 USA
11. Division of Plant and Soil Sciences West Virginia Univ. Morgantown WV 26526 USA
12. Agricultural Analytical Services Laboratory Penn State Univ. University Park PA 16802 USA