1. University of Bonn, Agroecology and Organic Farming Group, Auf dem Hügel 6, 53121Bonn, Germany, at present: University of Kassel, Faculty of organic agricultural sciences, Nordbahnhofstraße 1a, 37213Witzenhausen, Germany,
2. University of Bonn, Agroecology and Organic Farming Group, Siegaue 16, 53773Hennef, Germany