1. Dep. of Soil Science, Luiz de Queiroz College of Agric. Univ. of São Paulo Av. Pádua Dias, 11, Caixa Postal 9 CEP 13418‐900 Piracicaba SP Brazil
2. Lab. of Stable Isotopes, Center for Nuclear Energy in Agric. Univ. of São Paulo Av. Centenário, 303, Caixa Postal 96 CEP 13400‐970 Piracicaba SP Brazil
3. Res. Center Hanninghof Yara International ASA Hanninghof 35 D‐48249 Dülmen Germany