1. Centre for Ecology & Hydrology; Wallingford OX10 8BB UK
2. National Center for Water Quality Research; Heidelberg Univ.; Tiffin OH 44883
3. Crop, Soil and Environmental Sciences; Univ. of Arkansas; Arkansas AR 72701
4. Grassland, Soil and Water Research Laboratory, USDA-ARS; Temple TX 76502
5. International Plant Nutrition Institute; Guelph ON NG1 1L8 Canada