A Concept for a Consolidated Humus Form Description—An Updated Version of German Humus Form Systematics

In Germany, the systematics of humus forms has been developed, which is mainly based on morphological characteristics and has been proven via detailed long-term observation. The humus form systematics presented here is an update based on a new approach, clarifying the hierarchical structure into divisions, classes, types, and subtypes. New diagnostic horizons and transition horizons are introduced, uniquely characterising types and subtypes. This paper holds that the humus form is not only a product of decomposition, humification, and bioturbation but also serves as habitat for soil organisms. The processes and the habitat are shaped by soil-forming factors with the main factor being soil water conditions. Thus, on the first level of systematics, aeromorphic and aero-hydromorphic as well as hydromorphic humus forms are differentiated. Many different features of the organic layers and the mineral topsoil can be observed in forests, open grasslands, the mountain zone above the tree line, and natural fens and bogs, as well as degraded peatlands. Features shaping the humus form, such as the proportion of organic fine material and packing of the organic matter as well as the structure of the mineral soil, have now been unambiguously described. However, site-specific soil-forming factors result in typical organic matter characteristics of individual horizons and typical combinations of different horizons. This relationship is illustrated using descriptions of distinct humus forms.