1. Dep. of Crop and Soil Sci. Ithaca NY 14853
2. Current address: Dep. of Soils Federal Univ. of Rio Grande do Sul Caixa Postal 7a76, CEP 91501‐970, Porto Alegre, of Rio Grande do Sul Brazil
3. USDA‐ARS Pasture Syst. and Watershed Manage. Res. Unit Bldg. 3702, Curtin Rd. University Park PA 16802‐3702
4. Dep. of Agric., Resour. and Managerial Econ. Cornell Univ. Ithaca NY 14853