Luvisols (or Alfisols in USDA Soil Taxonomy) are characterized a textural contrast between the A and the B horizon – the Ae horizon has less clay than the Bt horizon. This difference in clay content between the two horizons is presumed to arise from the physical transfer of clay (termed lessivage) from the Ae (eluvial) to the Bt (illuvial) horizon by vertically draining soil water.
The two great groups of the Luvisolic order are distinguished based on the nature of the A horizon.
Gray Luvisol Great Group
Soils of this great group typically have the LFH directly overlying the Bt horizon. The mean annual soil temperature is typically < 8°C.
Gray Brown Luvisols Great Group
These soils have a forest mull Ah horizon and occur in more temperate portions of Canada (that is, where mean annual soil temperature is ≥ 8°C).