1. Chair of Plant Nutrition, Technical University of Munich, Am Hochanger 2, 85354 Freising‐Weihenstephan, Germany
2. Present address: Überlinger Str. 39, 78628 Rottweil, Germany
3. Bavarian State Research Center for Agriculture, Institute for Agricultural Ecology, Organic Farming and Soil Protection, Vöttinger Str. 38, 85354 Freising‐Weihenstephan, Germany