1. Soil Science Department, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK
2. ADAS Bridgets, Martyr Worthy, Winchester, Hants SO21 1AP, UK
3. Department of Agriculture, University of Newcastle-upon-Type, NE1 7RU, UK
4. Campden & Chorleywood Food Research Association, Chipping Campden, Gloucestershire GL55 6LD, UK
5. IACR-Long Ashton, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS18 9AF, UK