1. Dep. of Soil Science; Federal Univ. of Santa Maria; 97105-900 Santa Maria RS Brazil
2. Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre; 2560 Hochelaga Blvd Quebec QC Canada G1V 2J3
3. Institut de recherche et de développement en agroenvironnement; 2700 EInstein St. Quebec QC Canada G1P 3W8