Luvisolic soils are one of the three main orders for forested soils in Canada. (Orthic Gray Luvisol) Luvisolic soils are dominant in forested landscapes underlain by loamy tills derived from underlying sedimentary rocks or on clayey lacustrine deposits (the latter primarily in the Boreal Shield Ecoregion). The other two orders, Podzolic and Brunisolic, are primarily found on sand-dominated parent materials through the Boreal forest region.
The parent materials of the Luvisolic soils are typically well supplied with base cations such as calcium and magnesium, and have loamy or clay dominated soil textures. Both properties result from the pulverizing of the underlying sedimentary rocks by the glaciers and then the deposition of the pulverized sediments either directly by the glacier as loamy till or in glacial lakes, where sorting of the sediments into silty and clay lacustrine sediments occurs. Because of the high base cation content these soils typically have neutral or alkaline pH values although some acidic Luvisols are found, especially in eastern Canada.
The diagnostic feature of Luvisolic soils is a textural contrast between the A and the B horizon the Ae horizon has less clay than the Bt horizon. The specific rules about how much of a textural contrast is required are defined in the rather complex criteria for the Bt horizon.
This difference in clay content between the two horizons is presumed to arise from the physical transfer of clay from the Ae to the Bt horizon (termed lessivage) by vertically draining soil water the Ae is an eluvial horizon and the Bt is an illuvial horizon. Other evidence of clay illuviation is deposits of clay along the walls of pores that are visible either to the eye (called clay skins) or in soil thin sections examined under the microscope. (Lessviage) Often the Ae horizon has platy structure associated with it.
While calcium carbonate is present the fine clay particles remain in larger sized aggregates and cannot be readily transferred by the soil water. The transfer of clay from the A to the B begins after any calcium carbonate present in the A horizon is lost from the horizon by decalcification. If the parent materials are very high in calcium carbonate its presence can retard formation of the texture contrast horizons.
In some cases the B horizon may be overlain by a different parent material for example, where a silty or sandy eolian cap overlies a loamy till-derived B horizon. (Brunisolic Gray Luvisol) This contact between the two parent materials is called a lithological discontinuity. In cultivated landscapes the B horizon may be directly overlain by the plow layer (Ap). In either of these two cases the B horizon is designated as a Bt horizon if evidence of clay skins is present.
If the B horizon does not meet the specific criterion for the increase of clay relative to the Ae horizon, or if clay skins were not present under a lithologic discontinuity, the B horizon should be designated as a Btj or a Bm horizon and the soil classified into the Brunisolic order. These distinctions are difficult to determine in the field, and often a tentative classification into the Luvisolic order would be made until a subsequent laboratory analysis is completed.
Luvisolic Great Groups
The nature of the A horizon is the distinguishing criteria between the two great groups of the Luvisols. In more temperate forests such as in southern Ontario or parts of the interior of B.C., sufficient earthworm and burrowing animal activity occurs to mix part of the organic leaf litter (LFH) into the upper part of the A horizon, creating a mull Ah horizon. Elsewhere in Canada the less temperate conditions limit the mixing that occurs, and the forest litter layer (LFH) has a sharp contact with the underlying Ae horizon. Both great groups have an eluvial A (Ae, Ahe) and Bt present.
Soils of this great group typically have the LFH directly overlying the Bt horizon. The mean annual soil temperature is typically below 8°C, which means soils of this great group are found throughout the Prairie provinces but also in parts of eastern Canada. (Orthic Gray Luvisol)
Gray Brown Luvisols
These soils have a forest mull Ah horizon and occur in more temperate portions of Canada (that is, where mean annual soil temperature is greater than or equal to 8°C). (Orthic Gray Brown Luvisol)
The subgroups of the Luvisols mainly recognize transitional soils that have characteristics of the Luvisols and of the other main soils found in the two main regions where they occur. Both great groups have transitional subgroups to the Brunisols, Podzols, Gleysols, and Vertisols; the Gray Luvisols also have transitional subgroups to the Chernozems and Solonetzic orders as well as a special subgroup for Luvisols with a fragipan.
Brunisolic (Gray and Gray Brown)
These soils have a Bm or thin (< 10 cm) Bf horizon overlying the diagnostic Bt horizon. The Bm or thin Bf is often formed in a sandy mantle overlying a loamy or clay dominated till. (Brunisolic Gray Luvisol)
Podzolic (Gray and Gray Brown)
These soils have a Bf horizon at least 10 cm thick overlying the diagnostic Bt. If the Bt is found at a depth greater than 50 cm the soil is classified as a Podzolic soil. (Podzolic Gray Luvisol)
Vertic (Gray and Gray Brown)
These soils would typically be found on high clay lacustrine deposits and the Bt must be underlain by a horizon with slickensides (Bss, Ckss) within 1 m of the mineral surface.
Gleyed (Gray and Gray Brown)
Soils of this subgroup are associated with non-permanent water saturation in the profile, leading to faint to distinct mottles within 50 cm of the surface or prominent mottles between 50 and 100 cm. Any of the other subgroups (except the Orthic) may also have a Gleyed prefix added to it if they meet this criterion plus the criteria for the other subgroup (for example, Gleyed Podzolic Gray Luvisol).
Dark Gray (Gray great group only)
These soils are transitional to the Chernozemic order. They have an Ah or Ahe that does not meet the criteria for a Chernozemic A horizon.
Solonetzic (Gray great group only)
Soils of this subgroup are transitional to the Solonetzic order. The B horizon meets the criteria for a Bt horizon but does not have a fully developed Solonetzic B horizon. Typically the B horizon would be classified as a Bntj. The subgroup is associated with saline parent materials or groundwater salinization.
Fragic (Gray great group only)
These soils have a fragipan within the B (Btx) or in the underlying transitional horizon (BCx).
Soils that have an Ae and Bt and lack any of the features outlined above are classified into the Orthic subgroup. Often if the Luvisol is cultivated, the horizons overlying the Bt (Bm, Bf etc.) are mixed together, and the resulting soil is also classified into the Orthic subgroup.