1. USDA‐ARSNational Agric. Water Quality Lab.P.O. Box 1430DurantOK74702‐1430
2. CADSWES, Univ. of ColoradoBoulderCO80309‐0421
3. Wisconsin Dep. of Nat. ResourcesMadisonWI53707
4. Dep. of Plant ScienceUniv. of DelawareNewarkDE19717‐1303
5. Dep. of AgronomyUniv. of ArkansasFayettevilleAR72701
6. Soil Science Dep.Univ. of FloridaGainesevilleFL32611