1. Dep. of Agricultural Resource Management Kenyatta Univ. P.O. Box 43844‐00100 Nairobi Kenya
2. Dep. of Crops, Horticulture and Soils Egerton Univ. P.O. Box 536‐20115 Njoro Kenya
3. Dep. of Agronomy Univ. of Fort Hare Private Bag X1314, King William's Town Road Alice 5700 South Africa
4. Dep. of Soil Chemistry and Plant Nutrition, Institute of Soil Water and Environmental Sciences, ARO, The Volcani Centre P.O. Box 6 Bet‐Dagan 50250 Israel