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. Québec G1V 2J3 QC Canada
3. Inst. de Recherche et de Développement en Agroenvironnement; 2700 Einstein St. Québec G1P 3W8 QC Canada