Time for nonilluvial Bt horizons?

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<sup>o</sup>N and 55<sup>o</sup>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 non-illuvial coarse-over-fine texture-contrast horizons and that the criteria for the Luvisolic order also be broadened to include these non-illuvial Bt horizons.

Advances in soil survey and classification in Canada

“…These proposals, albeit generally straightforward, have a cascading effect on the keys to soil classification since any new taxon will need to be integrated into the existing taxonomic system. Two manuscripts target the Luvisolic Order (Dyck et al 2023;Pennock and Fisher 2022). Pennock and Fisher (2022) challenge the concept that the diagnostic horizon of a Luvisolic soil, the Bt horizon, is purely the result of lessivage, and examine development of a Bt horizon because of a lithological discontinuity.…”

“…Two manuscripts target the Luvisolic Order (Dyck et al 2023;Pennock and Fisher 2022). Pennock and Fisher (2022) challenge the concept that the diagnostic horizon of a Luvisolic soil, the Bt horizon, is purely the result of lessivage, and examine development of a Bt horizon because of a lithological discontinuity. Dyck et al (2023) propose a polygenetic pathway for development of Gray Luvisols, with emphasis on the intergrade soils between the Luvisolic and Chernozemic that are so common in the Aspen Parkland, the nexus between the Great Plains and the Boreal.…”