1. International Institute of Tropical Agriculture, Humid Forest Ecoregional Centre, BP 2008 (Messa), Yaoundé, Cameroon
2. International Institute of Tropical Agriculture (IITA), Oyo Road, PMB 5320, Ibadan, Nigeria
3. Institute of Plant Nutrition, University of Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany