Whole regolith pedology classification: examples from Brazil

Authors

J. Juilleret, A. Azevedo, R. A. Santos, and S. Dondeyne

Reference

in 5th International Soil Classification Congress (SSC 2016), 1-7 December 2016, Bloemfontein, South Africa, 2016

Description

A clear interest for classifying regolith material was expressed during the “Whole–Regolith Pedology” symposium organized by the “Soil Science Society of America” in 1992. Buol (1994) subsequently proposed a Saprolite-Regolith Taxonomy (SRT) but this classification system has hardly been adopted by the soil science community. In principal subsolum layers (C and R layers) within 2 m from the soil surface ought to be considered as they are part of the object of classification of both World Reference Base (WRB) and Soil Taxonomy (ST). As these classification systems focus on diagnostic features in the surface horizon (A, H) and/or pedogenic subsurface horizons (E, B), they lack concepts and definitions to include subsolum layers when naming soils. In WRB, the only way to explicitly refer to subsoil material is when shallow continuous rock (WRB) occurs (defining Leptosols and Leptic qualifier) or in ST as a lithic contact or the Lithic subgroup. However, many researches have stressed the importance of considering features of C and R layers in soil classification for their ecological function as roots can extend deep below the solum and for their hydrological function as infiltration of water and contaminants is controlled by geogenic features such as cracks or sedimentary beds. To address these shortcomings, a new classification system has been proposed (Juilleret et al, 2016). Based on the weathering stage and properties of the C and R layers, four Subsolum Reference Group (SRG) are distinguished: Regolite, Saprolite, Saprock and Bedrock. Intergrades of these SRG’s can be categorized with principal qualifiers, while morphologic and lithologic characteristics can be categorized with supplementary qualifiers. To foster the discussion on the need for a classification system which encompasses the whole regolith, we compare the classification of four soil profiles from Brazil according to the SRT and SRG classifications. The cases illustrate that SRT emphasizes geo-mechanic properties and is restricted to the classification of soft materials with a special focus on the mineralogy and lithology. The SRG allows to also cover hard materials and to convey informations on morphological features such as texture, rooting capacity and the nature of rock fragments. We also argue that there is a need for creating a subsolum working group within the soil classification commission, to further test, develop and assess the need for integrating subsolum features into the current soil classification systems.