1. USDA-ARS, Pasture Systems and Watershed Management Research Unit; Building 3702, Curtin Rd. University Park PA 16802
2. Soil Science Centre Alterra; Wageningen Univ. and Research Centre (WUR); P.O. Box 47 6700 AA Wageningen the Netherlands
3. Dep. of Soil Quality; Wageningen Univ.; WUR, P.O. Box 47 6700 AA Wageningen The Netherlands