1. Univ. of São Paulo, Luiz de Queiroz College of Agriculture; Av. Pádua Dias, 11, Piracicaba São Paulo Brazil 13418-900
2. Univ. of São Paulo; Center of Nuclear Energy in Agriculture; Av Centenário, 303, Piracicaba São Paulo Brazil 13416-000
3. North Carolina State Univ.; Dep. of Crop and Soil Science; 101 Derieux Place Raleigh NC 27695-7620