Abstract
The Bt horizon is the diagnostic horizon of the Luvisolic Order in Canada. According to the Canadian System of Soil Classification (CSSC), the Bt must be formed from clay illuviation through the processes of lessivage (i.e., physical transport of clay). In a study of a Luvisol catena in the central Saskatchewan, we demonstrate that Ae/Bm horizons overlying IIBt horizons are formed in a sandy mantle overlying till (i.e., a lithological discontinuity) and that the sandy mantle contributed negligible amounts of illuvial clay despite the presence of clay skins on ped surfaces in the IIBt horizon. We extended the results of this study to the regional scale by examining sand fractions in 63 pedons of Luvisol-dominated soil associations from soil surveys in the Northern Forest Reserves (between latitudes 53°N and 55°N). Of the 63 pedons, 13 had lithological discontinuities identified in their profile description and a further 27 had discontinuities identified through shifts in the sand fractions between horizons. For the profiles with discontinuities, inherited particle size differences are a more likely cause of coarse-over-fine textural contrasts than lessivage. A regional analysis of the distribution of Luvisol-dominated associations showed distinct zonations that account, in part, for the differences in the occurrence of lithological discontinuities. Based on these results, we suggest that the criteria for Bt horizons in the CSSC should be broadened to include nonilluvial coarse-over-fine texture-contrast horizons and that the criteria for the Luvisolic order also be broadened to include these nonilluvial Bt horizons.
Highlights
Soils of the Luvisolic Order are widespread in forested landscapes throughout Canada and normally have developed in glacial parent materials derived from sedimentary rocks
The data used are drawn from nine soil surveys of the Provincial Forest Reserves of Saskatchewan published between 1970 and 1996 (Figure 1) (Anderson and Ellis 1976; Ayres et al 1978; Crosson et al 1970; Head et al 1981; Padbury et al 1978; Rostad and Ellis 1972; Staff, Saskatchewan Centre for Soil Research (SCSR) 1996; Saskatchewan Institute of Pedology 1983; Stonehouse and Ellis 1983)
Not applicable to the great majority of soils discussed in this paper it may be useful to include the buried soil-naming protocol in the revised CSSC. Both the field study of a Luvisol catena and an analysis of 63 modal profiles from soil surveys in Saskatchewan indicate that lithological discontinuities are common in Saskatchewan Luvisols
Summary
Soils of the Luvisolic Order are widespread in forested landscapes throughout Canada and normally have developed in glacial parent materials derived from sedimentary rocks. Research from other regions with Luvisolic-like soils has, increasingly questioned the assumption that lessivage is the dominant process responsible for texture-contrast soils in a high proportion of Luvisols. This counter argument was originally developed in the soil genesis volume by Paton et al (1995), who challenge the primacy of eluviation-illuviation in the formation of these soils and instead develop a bioturbation/soil geomorphic explanation for the soils. In later papers Phillips (2007) and Phillips and Lorz (2008) demonstrate how multiple processes can interact to form texture-contrast horizons in field sites in North Carolina and Arkansas, USA and in Germany. The need to examine multiple pathways is well established in the literature (e.g. Bockheim and Hartemink 2011)
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