1. Contribution from the Institut für Biophysik und Physikalische Grundlagen der Medizin, Uniklinikum Homburg, Universität des Saarlandes, D-66421 Homburg/Saar, Germany, Institute of Agricultural Chemistry, University of Bologna, Viale Berti Pichat 10, I-40127 Bologna, Italy, and Department of Soil Sciences and Plant Nutrition, University of Florence, Piazzale Cascine 28, I-50144 Florence, Italy