1. Department of Soil Science and Plant Nutrition and Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 10, 6703 HB Wageningen, The Netherlands, British Geological Survey, Wallingford, Oxon, OX10 8BB, U.K., Laboratoire de Géochimie et Métallogénie, Université P&M Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Univer Algarve Unidade de Ciencias Exactas e Humana, Campus de Gambelas, 8000 Faro, Portugal