1. Research Centre Hanninghof, Yara International ASA, Hanninghof 35, D‐48249 Duelmen, Germany
2. Institute of Agricultural Chemistry, Georg‐August University of Goettingen, Carl‐Sprengel‐Weg 1, D‐37075 Goettingen, Germany
3. University of Applied Sciences, Faculty of Agriculture Sciences and Landscape Architecture, Am Kruempel 31, D‐49090 Osnabrueck, Germany