1. a Division of Soils, Davies Laboratory , CSIRO , Townsville , Qld. , 4814 , Australia
2. b Division of Mathematics and Statistics, Davies Laboratory , CSIRO , Townsville , Qld. , 4814 , Australia
3. c Division of Soils , CSIRO , Private Bag No. 2, Glen Osmond , S.A. , 5064 , Australia