**Acknowledgements**

The work was funded by Regional Universities Forum (RUFORUM), Uganda for Capacity Building in Agriculture under grant RU 2009/GRG 15. The Isotopic study was funded by the International Atomic Energy Agency (IAEA), grant number CRP 15399 for A.O Olaleye. The cooperation of the staff and students of the National University of Lesotho, Roma, Lesotho is also acknowledged.
