1. Dep. of Soil ScienceFederal Univ. of Santa Maria97105‐900Santa MariaRio Grande do SulBrazil
2. Agriculture and Agri‐Food CanadaQuébec Research and Development Centre2560 Hochelaga Blvd.QuébecQCCanadaG1V 2J3
3. Institute of Biological SciencesFederal Univ. of Rio Grande96170‐000 São Lourenço do SulRio Grande do SulBrazil
4. Federal Institute of EducationScience and Technology of Rio Grande do Sul99170‐000 SertãoRio Grande do SulBrazil