Humus Forms

Humus forms are classified on different levels in accordance with distinct criteria, such as soil hydrologic balance and oxygen availability, the occurrence of diagnostic horizons, and distinct sequences of horizons. Criteria are designated by specific morphological features and the occurrence of specific vegetation. Changes to site conditions may result in an imbalance of the dynamics of organic matter, which demonstrates the importance of defining transition humus forms. Depending on the scale and purpose, differentiation between humus forms can be carried out on different levels: Division, class, type, subtype an variety . On the first level, the hydrologic balance of the soil is considered. Under well-aerated conditions, aeromorphic Mull humus forms such as Typical L Mull and Typical F Mull and humus forms with an Oh horizon such as Typical Moder and Typical Mor develop. In Figures 1 and 2, selected aeromorphic humus forms and typical sequences of horizons are listed. Moderately aerated and poorly drained conditions will lead to the aero-hydromorphic humus forms Moist Mull, Moist Moder, and Moist Mor. Anaerobic conditions resulting from prolonged water saturation will lead to hydromorphic humus forms: Anmoor and Moor with different humification levels of the peat. Aeromorphic and aero-hydromorphic humus forms dominate on mineral soils but may also occur on primeval peat-forming organic soils if the conditions for peat accumulation are no longer fulfilled.

Structure of German Humus Form Systematics

Mull humus forms

Humus forms with an Oh horizon

Initial humus forms

Naturally and Anthropogenically deteriorated humus forms

Hydromorphic humus forms