1. Institute of Plant Sciences; Group of Plant Nutrition; ETH Zurich Research Station Eschikon; Lindau Switzerland
2. Acroscope Reckenholz-Tänikon (ART); Reckenholz Zurich Switzerland
3. Research Institute of Organic Farming (FiBL); Frick Switzerland
4. Agricultural Univ.; Dep. of Agrochemistry and Soil Science; Plovdiv Bulgaria