 Large areas of Luvisolic soils occur in the central to northern Interior Plains; smaller areas in all regions south of the permafrost zone.The 2 great groups of Luvisolic soils are distinguished mainly on the basis of soil temperature.Gray Brown Luvisols have a dark Ah horizon in which organic matter has been mixed with the mineral material (commonly by earthworm activity), a brown, often platy eluvial horizon (Ae) and an illuvial horizon (Bt) in which blocky structure is common.