1. Institute of Plant Nutrition, University of Bonn, Karlrobert Kreiten Str. 13, D‐53115 Bonn, Germany
2. Natural Resources Institute, University of Greenwich, Kent, ME4 4TB, UK
3. International Center for Soil Fertility and Agricultural Development (IFDC) Africa Division, BP 4483, Lomé, Togo
4. West Africa Rice Development Association, 01 BP 2551, Bouaké, Côte d'Ivoire