1. Soils Group, School of Agriculture, Food and Wine and Waite Research Institute, The University of Adelaide, Urrbrae 5064 SA, Australia
2. School of Environmental and Rural Science, University of New England, Armidale 2350 NSW, Australia
3. Present address: Group of Plant Nutrition, Institute of Agricultural Sciences, Swiss Federal Institute of Technology (ETH) Zurich, Eschikon 33, CH‐8315 Lindau, Switzerland
4. CSIRO Agriculture, GPO Box 1600, Canberra 2601 ACT, Australia
5. CSIRO Land and Water, Glen Osmond 5064 SA, Australia
6. CSIRO Agriculture, Glen Osmond 5064 SA, Australia