1. a Escola Superior de Agricultura “Luiz de Queiroz”, Soil Science Department , Sao Paulo University , C.P. 9, 13.418–900, Piracicaba , SP , Brazil E-mail:
2. b Escola Superior de Agricultura “Luiz de Queiroz”, Soil Science Department , Sao Paulo University , C.P. 9, 13.418–900, Piracicaba , SP , Brazil
3. c Instituto Agronomico de Campinas , C.P. 28, 13.001–970, Campinas , SP , Brazil