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Orders: Luvisolic

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) (LUVISOLIC CANANDA MAP). 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.

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