1. Dept. of Soil Biology and Plant Nutrition, Faculty of Organic Agricultural Sciences, University of Kassel, Nordbahnhofstr. 1a, D‐37213 Witzenhausen, Germany
2. Present address: Institute of Plant Nutrition, University of Hohenheim (330), D‐70593 Stuttgart, Germany
3. Dept. of Organic Farming and Cropping, Faculty of Organic Agricultural Sciences, University of Kassel, Nordbahnhofstr. 1a, D‐37213 Witzenhausen, Germany