1. Soil, Crop, and Atmospheric Sciences Department, Cornell University, Bradfield Hall, Ithaca, New York 14853, Institut national de 1a recherche scientifique-Institut Armand Frappier, 245 Hymus, Pointe-Claire, H9R 1G6, Quebec, Canada, U.S. Plant, Soil, and Nutrition Laboratory, Tower Road, Ithaca, New York 14853, Department of Natural Resources Sciences, McGill UniversityMacdonald Campus, Ste-Anne-de-Bellevue, H9X 3V9, Quebec, Canada