1. a Department of Soil Science, Faculty of Agriculture , Ahmadu Bello University , PMB 1044 , Zaria , Nigeria
2. b Plant Production Systems Group , Wageningen University , PO Box 430, Wageningen , 6700 , AK , The Netherlands
3. c Animal Husbandry in the Tropics and Subtropics , University of Kassel and Georg-August Universitat , Göttingen, Steinstrasse 19, 37213 , Witzenhausen , Germany
4. d Organic Plant Production and Agroecosystems Research in the Tropics and Subtropics , University of Kassel , Steinstrasse 19, 37213 , Witzenhausen , Germany
5. e Plant Research International , Wageningen University and Research Centre , PO Box 616, Wageningen , 6700 , AP , The Netherlands