Mor humus has three distinct organic horizons: an
OL horizon (
litter horizon), atop an
OF horizon (
fermentation horizon), followed by a more or less thick (sometimes absent)
OH horizon (
humus horizon), before a sharp transition to the mineral soil (
E horizon). The OL horizon is very thick and is made up of undecomposed
organic material. In
coniferous forests the litter is composed primarily of
evergreen needles,
conifer cones, and woody debris. In
ericaceous heaths the litter is composed of ericaceous leaves and twigs, included in a dense network of subterranean rhizomes growing more or less horizontally through the
forest floor. In the OL horizon of mor, living (green) and dead (brown) parts of
mosses are often associated with coniferous needles, which do not shade out mosses, contrary to deciduous leaves. The OF horizon below consists of plant remains still in varying degrees of decomposition. A layered and compact-matted structure caused by the interweaving of plant roots and
fungal hyphae make up the fermentation layer, with a very weak contribution of animal
faeces, contrary to the OF horizon of moder. Through their distinct yellow and white coloration, the fungal
hyphae in this horizon are identifiable as
cellulose-decomposing fungi, which are the primary decomposers of organic matter in mor humus, and
ectomycorrhizal fungi (with
conifers) and
ericoid mycorrhizal fungi (with
Ericaceae) which contribute to nitrogen uptake and transfer to their host plant, and act as a sink for carbon. The plant roots present further contribute more organic residues through
exudation. The OH horizon is composed of
humified plant material, resulting from the slow and incomplete process of fungal
decomposition. There is very little blending of material between the humus layer and the mineral soil below resulting in the poor content in organic matter in the top mineral soil horizon. Rather than being intimately mixed (clay, silt) or juxtaposed (sand) with mineral particles, like in the A horizon of
mull or
moder, organic matter, either in
colloidal or
dissolved form (
DOC), is leached (
eluviated) through the
E horizon until deposited (
illuviated) below in the
B horizon, a process typical of
podzolization. Despite the clear distinction between Mor and all other humus forms stemming from the absence of an A horizon and the sharp transition from the forest floor to the mineral soil, mor and moder (in particular
Dysmoder) were often confused under the designation of
raw humus, embracing a wide variety of variants, from poorly to highly biologically active. == Formation ==