1. a Department of Soil Science , University of Nairobi , Box 30197, Nairobi , Kenya
2. b Agriculture and Agri‐Food Canada , Pacific Agri‐Food Research Centre , P.O. Box 1000, Agassiz , British Columbia , VOM 1AO , Canada E-mail:
3. c Department of Soil Science , University of British Columbia , Vancouver , British Columbia , V6T 1Z4 , Canada