1. Agriculture and Environment Div.; IACR-Rothamsted; Harpenden Herts AL5 2JQ UK
2. Dep. of Chemistry, Queen Mary; University of London; London E1 4NS UK
3. AAT Consultants; 15 Clerk Street Edinburgh EH8 9JH UK
4. Dep. of Soil Science and Agricultural Chemistry; Szent István University; 2103 Gödöllő Hungary