1. Department of Environmental Systems Science, Group of Sustainable Agroecosystems, Swiss Federal Institute of Technology; ETH Zürich, Tannenstrasse 1; CH-8092 Zürich Switzerland
2. Department of Environmental Systems Science, Group of Plant Nutrition, Swiss Federal Institute of Technology; ETH Zürich; Eschikon 33, CH-8315 Lindau Switzerland
3. Research Institute of Organic Agriculture (FiBL); Department of Soil Sciences, Ackerstrasse CH-5070 Frick Switzerland