In high mountain zones, highly variable conditions such as bedrock, climate, and the type of vegetation affecting litter decomposition lead to great differences in the morphology and the thickness of organic surface layers. Two different zonal subtypes of Moder are so far known.
The subtype Tangel is typically characterised by an Ovh horizon occurring in the contact zone with the bedrock. The Ovh horizon is finely friable and has a loose structure. Compared to the overlying O horizons, the Ovh usually has a higher pH and a lower CN ratio. Often, the Ovh horizon also shows a higher Ca content and contains chippings of limestone. The mineral content in the Ovh may also be increased due to deposition of aeolian dust or mixing of mineral soil via tree throw. The Obh horizon overlaying the Ovh is usually >10 cm thick and has a much lower pH, mostly <4 (pH CaCl2).

The thickness of the organic layer may vary from >15 cm up to >100 cm. The occurrence of thick organic layers is described predominantly on limestone and dolomite, rarely on silicate rocks. Tangel occurs particularly in montane to subalpine zones with high precipitation. The typical Tangel frequently occurs where transitions to moist or hydromorphic humus forms are observed. So far, Tangel humus forms are insufficiently studied, and soil data are rarely presented. Therefore, no further subdivision is provided, and in the first instance, only Tangel on limestone with an Ovh horizon is defined. It can be expected that transitional types to Mor or other Moder subtypes will occur, as well as Tangel humus form on unconsolidated rock. Distinct features are:

  • Ol horizon is present all year round;
  • Of horizon is present all year round and thin (<5 cm);
  • Obh horizon is present, with diffuse boundary with underlying Ovh; and
  • Ovh horizon is present and directly overlies the C horizon.

Organic surface layer of a Tangel under mountain pine (Pinus mugo) on limestone
(Photo: Gerhard Milbert).