A system for classifying subsolum geological substrates as a basis for describing soil formation

Abstract

The research topic ‘parent material for soil formation’ is at the interface between the disciplines of geology and soil science; interdisciplinary research will be of mutual benefit. In order to foster such research and knowledge transfer, this paper presents the development of a novel geochemical-physical classification system for subsolum geological substrates as a basis for both understanding soil formation and estimating and modelling soil properties. It is based on information from classical geological surveys and maps and additional specific field data and laboratory analysis of surficial geology and is applicable to solid bedrock and unconsolidated deposits. In summary, the units of the classification system consist of three evaluation levels encoded in entities describing their lithogenetic, geochemical, and physical characteristics. Beside the geochemical-physical characterisation, the classification approach also considers the layer structure of unconsolidated deposits. The basis for the classification is mineral component groups of the subsolum geological substrates, namely dolomite, calcite, and felsic, mafic, and clay minerals. The categorisation is implemented in three distinct triangular diagrams with a scope of application according to the proportion of carbonate. The practical application includes the processing and classification of geological reference samples and was applied to 321 analysed field samples of unconsolidated deposits. In order to describe and evaluate the influence on soil formation and soil properties, we examined the soil analysis data of 389 and field descriptions of 1664 legacy forest soil profiles in the Tyrolean Alps. With the compiled dataset we were able to cover 50 different classification units representing approximately 92% of the forested area of Tyrol. We were able to show clear differences in the chemical and physical soil properties between units. Beside the mineral components of the parent material, the different lithogenetic entities play an important role in soil formation and soil properties. This is explained by the preliminary fragmentation of rock through different transport processes and distances as advanced soil genesis is facilitated by the presence of finer grain size. Hence, parent material is a key factor in soil formation, determining soils physical and chemical properties. The valorisation of geological information for soil science bridges the gap between the disciplines and contributes added value to other research fields. Due to the openness of the proposed classification system for the further differentiation of lithogenetic, geochemical, and physical entities, it is transferable to the entire Alpine arc and other mountain regions.