1. Dep. of Soil Science; “Luiz de Queiroz” College of Agriculture Univ. of São Paulo; 11 Pádua Dias Avenue Piracicaba SP 13418-900 Brazil
2. USDA-ARS; Natl. Lab. for Agric. and the Environment; 2110 University Boulevard Ames IA 50011
3. Dep. of Biology; Colorado State Univ.; 200 West Lake Street Fort Collins CO 80523
4. Dep. of Agronomy; State Univ. of Maringá; 5790 Colombo Avenue Maringá PR 87020-900 Brazil
5. Center for Nuclear Energy in Agriculture; Univ. of São Paulo; 303 Centenário Avenue Piracicaba SP 13400-970 Brazil