Humus Form Orders
The purpose of humus form classification is to organize knowledge about humus forms and to enhance our understanding about relationships between humus forms and factors involved in their formation. Information presented at this website follows the system developed by Green et al. (1993) focused on the humus forms known to occur in British Columbia and other regions under the influence of comparable climate.
As in soil classification, forest floor types are divided into humus form orders. The main orders are mor, moder and mull. They are subdivided further into humus form groups, while groups are subdivided into subgroups.
- Part 1
- Part 2
- Part 3
Humus Form Orders – Part 1
Humus Form Orders – Part 2
Humus Form Orders – Part 3
Humus Form Orders – Part 3
Mor: Description and Conditions for Development
In a mor, plant litter is decomposed more slowly and accumulates on top of mineral horizons with a sharp transition between organic and mineral horizons. Fauna are uncommon and undecomposed plant material accumulates over time to form a compact matted Fm horizon containing scarce or no faunal droppings. Without soil fauna, decomposition is often incomplete and accumulation of organic material occurs. A mor forms due to incomplete fungal decomposition, resulting in some immobilization of nutrients in the forest floor – removing them from cycling and accessibility to plants. Mors tend to occur under less optimal climatic conditions over nutrient poor parent material. In boreal and some coastal forests of British Columbia, humus accumulations can immobilize a large portion of the nutrient capital of a site.
Characteristic Horizon(s): Fm
Mors exhibit a layered, compact-matted structure caused by interweaving of fungal hyphae throughout the Fm horizon. The cellulose-decomposing fungi responsible for this fabric structure are the primary decomposers of organic matter in the mor order. Often, roots are prevalent within the humus layer, contributing to organic residues within the Fm horizon. Bacterial and faunal decomposition is limited due to acid conditions and high C:N ratios so that nutrients become available to plants at a slow rate, often resulting in relatively thick Fm horizons and rarely developing Ah horizons. Mors occur under conditions unfavorable to development of moders or mulls that are more biologically active.
- Layered, compact-matted structure
- Chemical properties: very acid to acid, high C:N ratios leading to a storage of nutrients which are slowly released and made available to plants
- Present under unfavorable climatic or other conditions
- Fungal dominated decomposition, limited bacterial and faunal activity
- Roots prevalent in the plant residues of the Fm horizon
- Thick Fm horizons and usually lacking Ah horizons
Moder: Description and Conditions for Development
Moders tend to have upper organic horizons with fungal mycelia and faunal castes, overlying Ah horizons of mixed organic faunal castes and mineral soil.
Characteristic Horizon(s): Fz or Fa
Friable Fz horizons in moders are made up of plant residues fragmented by soil fauna into non-compact, loose arrangements, largely composed of faunal droppings. Like mors, the moder order tends to accumulate organic matter above the mineral horizons. However, moders have more faunal activity, similar to the mull order. These characteristics place the moder order as intermediary on a continuum between mor and mull, but also make them distinct from the other orders. Fungal decomposition is important in moders, but bacteria, actinomycetes, protozoa and fauna play a more important role than in mors. Compared with mors, moders have higher pH, lower C:N ratios and greater nutrient availability. Usually, they occur below deciduous forest stands.
- Loose, non-compact structure comprised largely of faunal droppings
- Chemical properties: Higher pH than mor, lower C:N ratios, higher mineralizable N and base saturation than mors, indicating better nutrient availability
- Fungal decomposition important, but decomposition by bacteria, actinomycetes, and fauna is even more important
- Limited mixing of organic matter into mineral soil (Ah)
Mull: Description and Conditions for Development
The mixing of organic material with mineral soil is carried out by soil fauna followed by rapid bacterial decomposition, results in the formation of a mull. The mull is characterized with more complete decomposition and higher nutrient availability than the other two orders. Generally, mulls are associated with more fertile soil and greater species richness. Macro-fauna create casts and burrows, distributing and providing access to food for smaller organisms. Mulls have Ah horizons where soil organic matter is intermixed with mineral soil in crumbly organo-mineral aggregates resulting from the activities of roots, fauna and microbes. These organo-mineral aggregates are generally associated with earthworm activity but can also result from the actions of other agents such as other animals, roots, white-rot fungi, termites, ants or mechanical disturbances. Soil fauna can increase decomposition by fragmenting or comminuting litter and altering the microbial community.
Characteristic Horizon(s): Ah
Incorporation of organic matter into the mineral soil leads to the formation of well-developed Ah horizons, which are diagnostic of the mull order. L and sometimes Fz and/or Hz horizons with combined thickness of 2 cm or less overlay the Ah. Mulls develop in grassland ecosystems, or on nutrient rich soil in forests with easily decomposable litter. The activity of soil fauna and plentiful bacterial colonies rapidly decompose and mineralize organic matter that is then mixed into the mineral soil by soil fauna – especially earthworms. Decomposition of roots within mineral soil also contributes to organic matter enrichment. Mulls have the highest pH, lowest C:N ratios and highest nutrient availability of the humus forms. They occur in a wide range of climates where temperature, moisture and nutrient levels are prime for high rates of biological activity. Mulls are generally found on base-rich parent materials.
- Well decomposed organic matter that is incorporated in mineral soil (Ah)
- Chemical properties: Among the three orders, mull has the highest pH, lowest C:N ratios, nutrient availability and biological activity
- Rapid decomposition by bacteria and soil fauna
- Thin L, F and H if present