1. Department of Soil Science, IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2JQ United Kingdom, Department of Plant and Soil Sciences, Cruickshank Building, University of Aberdeen, Aberdeen, AB24 3UU United Kingdom, and ADAS, Gleadthorpe Research Centre, Meden Vale, Mansfield, Nottinghamshire, NG20 9PF United Kingdom