Mind the gap: A classification system for integrating the subsolum into soil surveys

• Classifying material below diagnostic soil horizons for addressing environmental issues
• A field based morphogenetic classification of the subsolum is proposed.
• The classification is compatible with WRB and Soil Taxonomy.
• Complementing soil survey data to 2 m

As soil surveys were traditionally conducted in support of agriculture, soil classification tended to focus on upper soil horizons and their characteristics. However, when dealing with environmental issues – such as vegetation ecology or water quality – an integrated knowledge of the soil, soil-to-substratum, and deeper substratum continuum is required. In both World Reference Base for soil resources (WRB) and Soil Taxonomy (ST), the lower boundary for soil classification is arbitrarily set at 2 m, including weathered and continuous rock. However, as soil classification hinges on diagnostic horizons and characteristics, which often occur within the first 100 cm, collecting data on the subsolum is often neglected. We propose a classification system of the subsolum, the structure of which is inspired by WRB. We define Regolite, Saprolite, Saprock and Bedrock as four subsolum reference groups corresponding to different weathering stages. Intergrades of these reference groups can be qualified with principal qualifiers, while morphologic and lithologic characteristics can be presented with supplementary qualifiers. The proposed subsolum classification system is not intended to substitute geological surveys, but rather to complement existing soil classification systems such that at least the whole 2 m can be categorized. Still, whenever desired the system can also be used for deeper materials.