Humus Forms

Humus in Ecosystems

“Climate, parent rock and vegetation can be considered as distal factors setting the stage for the formation of humus forms, of which plant roots, soil invertebrates and microbes are the agents.” – J.F. Ponge 2013

Humus forms are made up of soil horizons located at or near the surface, which have formed from organic residues (separate from or mixed with mineral particles). Horizons that may comprise a humus form include L, F, H, and Ah, but not B or C.

Humus form classification is based on the sequence, properties, and inferred origin of organic horizons (usually just the H horizon) and the Ah horizon (if one is present). Several systems of humus form classification have been proposed, often causing confusion by adopting and redefining established terminology. Despite the differences, all these classification systems tend to put humus forms into the following three broad categories:

1. mull,
2. moder (or “duff mull” in the U.S.), and
3. mor.

Based on the relationships among morphological, biological and physicochemical features of humus forms, several attempts have been made to classify them on the base of characters noticeable to the naked eye directly on the field, and to derive from them properties at the ecosystem level. Among many classification systems it is worth mentioning those developed by Delecour (1980), Brethes et al. (1995), and Green et al. (1993), the latter being the main source of information on humus form classification presented at this website.

Mull is characterized by an intense mixing of organic matter with mineral soil particles as a result of soil faunal activity. Mulls generally form under deciduous trees with more nutrient-rich litter, where gut-passage through soil animals followed by predominantly bacterial decomposition creates granular structures.

Moder is characterized by a less rapid transformation of litter done by litter-dwelling animals and fungi, resulting in the accumulation of organic residues. Moder humus forms have intermediate properties of the mull and mor humus forms.

Mor is characterized by the slow transformation and accumulation of undecayed plant residues. This humus form is generally present under conifers and is dominated by fungal decomposition leading to a compact-matted structure of fungal hyphae, low pH and an abrupt boundary between organic and mineral horizons.

Soil animals, microorganisms and plants are involved in positive (building forces) and negative (stabilizing forces) feedback relationships most of them taking place in the humus profile (Ponge et al. 2013). Consider the following two examples:

Example 1 – sites located on parent rocks comprised of minerals that weather easily and in mesic climates (not too cold, not too dry), will end up with plants (including trees) that grow rapidly. Those sites will also include plant species that have high demands for nutrients (i.e., flowering plants, with nutrient-rich and lignin-poor foliage). Consequently, these sites end up having nutrient-rich litter that favours microorganisms (bacteria) and animals (earthworms) that contribute to tree growth and diverse vegetation, typical of multi-layered forests. In such sites, mulls are more likely to form.

Example 2 – the same set of causes and consequences as those described in the example 1 explains why mor humus form is present on sites with smaller numbers of microbial, faunal and plant species and is characterized as less productive.