From geological to soil parent material maps - A random forest-supported analysis of geological map units and topography to support soil survey in South Tyrol

Abstract

Parent material is an important factor of soil formation and consequently plays a dominant role in both traditional field soil survey and digital soil mapping. The emergence of a new generation of detailed geological maps at high spatial resolution in South Tyrol raises the question of how to effectively incorporate these into soil mapping efforts. By comparing the units of these geological maps with the parent material description of soil pits, we evaluate to what extent these can be used as soil parent material maps. Random forest classification and feature selection are applied to highlight those terrain parameters that i) best distinguish between the different surficial geology units, ii) separate soil profile sites with different soil parent material, and iii) can be used together with the geologic map to train a classifier to model the distribution of soil parent material in the study area. The main issue detected by analyzing the differences between the geologic map units and the soil parent material information is the dominant role of till, which acts as soil parent material for a large number of soils located on different geological map units. While slope debris is another class on which geological map and soil pit descriptions often disagree, the issues concerning its misclassification are connected more to categorical transitions between soil parent material classes. Terrain parameters characterizing surface roughness, specifically a combination of vector ruggedness measure (VRM) and topographic roughness index (TRI), were identified as being best suited to join the geological map units in modeling soil parent material and indicate areas where till as soil parent material should be expected. By evaluating these results together with the distribution of soil types, a geologic-topographic characterization is performed for each geological map unit, with the aim of highlighting specific combinations of geological units and topographical situations which should be in the center of future detailed soil surveys, consequently facilitating the soil mapping procedure.