1. School of Physical and Chemical SciencesUniv. of Canterbury20 Kirkwood Ave., IlamChristchurch 8041New Zealand
2. AgroscopeReckenholzstrasse 191CH‐8046 ZürichSwitzerland
3. Dep. of Soil and Physical SciencesLincoln Univ.LincolnNew Zealand
4. Manaaki Whenua–Landcare Research54 Gerald StLincolnCanterburyNew Zealand