1. a Plant Production Systems, Wageningen University , P.O. Box 430, 6700AK, Wageningen, The Netherlands
2. d Department of Soil Science and Agricultural Engineering , University of Zimbabwe , MP 167, Harare, Zimbabwe
3. b Soil Productivity Research Laboratory , Post Bag 3757, Marondera, Zimbabwe
4. c Chemistry and Soil Research Institute , P.O. Box CY 550, Causeway, Harare, Zimbabwe