Groups

The attributes that were considered to result from the genesis of humus forms were selected by Green et al. (1993) for the characterization of humus orders and groups. The orders are subdivided into groups based on

  • horizon combinations and by the kind of F horizon in the humus form profile inferred to be indicative of essential differences in the nature of humus formation,
  • humification (a synthesis of humus materials), and
  • the kind and degree of incorporation of humus material within mineral soil.

Well-Drained Sites

Hemimor: Description and Conditions for Development

Characteristic Horizon(s): Fm

Description

Fm horizons are prominent in Hemimors, and are thicker than H horizons. Hemimors occur commonly in moisture-deficient boreal or coniferous forests, but are found in various ecosystem types Fungal hyphae occur in noncompact or compact fabric, while faunal droppings are not common. Hemimors are usually between 5 and 10 cm in thickness.

Summary

  • Fm at least 50% of combined thickness of F and H
  • H thin or absent
  • Common on moisture-deficient sites
  • 5-10cm thick
  • Faunal droppings uncommon

Humimor

Characteristic Horizon(s): Fm, Hh

Description

In Humimors, H horizons are more prominent, being thicker than the Fm horizons. Humimors are common in humid or perhumid, temperate, boreal and subalpine climates, under climax coniferous forest. They require environments where fires are infrequent. The Hh horizon usually has massive structure and greasy character when wet, altering to blocky/granular structure when moist. Faunal droppings are found in small amounts in Fm, but may be common in Hh. Humimors range between 5 and 20 cm in thickness.

Summary

  • Hh horizons more than 50% of combined thickness of F and H
  • Humid sites under climax coniferous forest
  • 5-20cm thick (may be over 40cm)
  • Small amounts of faunal droppings in Fm, more common in Hh

Resimor

Characteristic Horizon(s): Fm, Hr

Description
The prominent Hr horizons are thicker than Fm horizons in Resimors. Resimors often occur under climax coniferous forests in humid, mesothermal and maritime subalpine climates. An important feature of Resimors is that they contain intact, skeletal plant structures, which break down with applied pressure. The Hr horizons have blocky structure, are slightly greasy when wet and are often reddish-brown in colour. Often, mineral horizons are enriched in organic matter. Resimors range between 15-25 cm in thickness.

Summary

  • Hr horizons are more than 50% of combined thickness of F and H
  • May have thin Hh and/or Ah, and organic enrichment of organic horizons
  • Present on humid sites under climax coniferous forest
  • 15-25cm thick (may be over 40cm)
  • Reddish-brown colour and some skeletal plant structures

Lignomor

Characteristic Horizons(s): Fmw or Fw, Hhw or Hw

Description

Decaying coarse woody debris accounts for more than 35% of the volume of solids in Lignomor humus forms. Lignomors occur in locations where there is coarse woody debris, often in old-growth coniferous forests.Fw horizons commonly have blocky structure, and Hw horizons are usually massive. Rooting can be common or abundant within decaying wood. Lignomors are often thick (10 to over 40cm).

Summary

  • More than 35% decaying wood by volume of solids
  • Present on sites where coarse woody debris occurs, often old growth coniferous forest
  • Often 10-40+ cm in thickness

Mormoder

Characteristic Horizon(s): Fa

Description

The Fa horizon in Mormoders has a noncompact matted structure, composed of decomposed foliage interwoven with clumps of fungal mycelia. They occur in early or mid-seral coniferous forests most frequently. Ecosystems supporting Mormoders are not favorable to decomposition compared with Leptomoders, but are more active biologically than are ecosystems supporting Mors. Mormoders are usually less than 10cm thick.

Summary

  • Fa horizons, composed of faunal droppings and fungi are diagnostic
  • Present on early or mid-seral coniferous forests
  • Less than 10cm thick
  • More active than Mors, but less than Leptomoders

Leptomoder

Characteristic Horizon(s): Fz

Description

Fz and H horizons combined are thicker than Ah horizons in Leptomoders. They develop under deciduous overstory or understory, where moisture, temperature and aeration are favourable. The Fz horizons of Leptomoders are composed partially of faunal droppings, while fungi are rare. Their thickness is usually between 5 and 15 cm.

Summary

  • Combined Fz and H horizons greater in thickness than Ah
  • • Present on sites with favorable nutrient availability, aeration, moisture and temperature
  • 5 to 15cm thick
  • Faunal droppings common, while fungi are rare

Mullmoder

Characteristic Horizon(s): Fz, Ah

Description
In Mullmoders, F and H horizons combined are thicker than 2cm, while Ah horizons are more than 2cm thick and greater than the combined thickness of F and H. They occur in temperate climates with advantageous moisture and nutrients available for easy and rapid decomposition. Macro faunal incorporation of organic matter is not as extensive as in Mulls.

Summary

  • Ah horizons greater than 2cm, and thicker than combined F and H
  • Present in temperate climates
  • Nutrient availability and aeration favorable to rapid decomposition

Lignomoder

Characteristic Horizon(s): Faw, Fzw or Fw, Hhw or Hw

Description

Decaying coarse woody debris accounts for more than 35% of the volume of Lignomoder solids. They develop in sites where there is coarse woody debris, under conditions favorable to soil fauna activity. Lignomoders are associated with Moder or Mull dominated sites under hardwood, or hardwood-softwood ecosystems. They range in thickness from 10 to over 30cm.

Summary

  • 35% coarse woody debris by volume of solids
  • Prsent in hardwood, or hardwood-softwood ecosystems
  • From 10 to more than 30 cm in thickness
  • Associated with sites containing Moders or Mulls

Vermimull

Characteristic Horizon(s): Ah

Description

In Vermimulls, thin L and F horizons overlay well formed, Ah horizons, at least 2cm thick. Vermimulls develop in highly productive forests, from easily decomposable litter. They have granular structure, earthworm casts are common, and faunal activity is high.

Summary

  • Well developed Ah over 2cm thick
  • Thin L and F, thin H if present
  • Characterized by a rapid decomposition
  • Present in productive forests
  • Granular structure

Rhizomull

Characteristic Horizon(s): Ah

Description

In Rhizomulls, Ah horizons are formed from the decomposed roots of herbaceous plants. Fz horizons are thin when they occur. Rhizomulls develop where there are perennial grasses, in subhumid to semiarid climates. They are dark in colour, with dense systems of roots in the Ah horizon. Earthworms tend to be absent. Rhizomulls can be as thick as 25cm.

Summary

  • Ah developed from decomposition of dense root network
  • Grassland ecosystems
  • Dark colour
  • Up to 25cm thick

Poorly-Drained Sites

Hydromor

Characteristic Horizon(s): Fm

Description
In Hydromors, O horizons may occur due to poor drainage and excessive moisture, though Fm and H horizons account for more than half of the thickness of the organic horizons. Hydromors occur in areas of prolonged saturation of soils, such as boreal, cool mesothermal or temperate climates. They are associated with Gleysols or wetland organic soils. Fungal and faunal activity occurs in upper layers, above excessive moisture and occasionally anaerobic conditions. Hydromors are less than 40cm thick.

Summary

  • Fm and H horizons greater than 50% of thickness
  • O horizons may be present
  • On sites with excessive moisture, occasional saturated, anaerobic conditions
  • Less than 40cm thick

Fibrimor

Characteristic Horizon(s): Of

Description

The Of horizons of Fibrimors encompass more than 50% of the total humus profile thickness. Fibrimors occur in arctic, subarctic, subalpine, boreal and temperate climates with a high permanent water table. Usually, a layer of living mosses occur mixed with litter, creating an S horizon. Due to low temperatures, low nutrient content, excessive water and poor aeration, Fibrimors are the least biologically active humus form. They are often more than 40cm thick.

Summary

  • Of horizon greater than 50% of thickness
  • S or L, F and H may be present
  • Present in cold climates and saturated soils
  • More than 40cm thick
  • Least biologically active humus form

Mesimor

Characterisitc Horizon(s): Om

Description

Dominant Om horizons make up more than 50% of the thickness of the organic horizons in Mesimors. They are found in arctic, to temperate climates under prolonged saturation. Mesimors frequently have S horizons comprised of living mosses interwoven with litter. Decomposition is advanced due to high nutrient contents of water, but is inhibited by cold temperatures and anaerobic conditions. Mesimors are commonly over 40cm thick.

Summary

  • Om horizon greater than 50% of thickness
  • S or L, F and H may be present
  • Present in cold climates, saturated soils, but high nutrient content in water
  • More than 40cm thick
  • Advanced decomposition of organic matter

Hydromoder

Characteristic Horizon(s): Fz or Fa

Description

O horizons may develop due to excessive moisture, but F and H horizons make up more than 50% of the thickness of Hydromoder organic horizons. They occur in various climates, where soil saturation occurs regularly. Fauna exist in the unsaturated upper horizons, as do small amounts of fungal mycelia. They have higher pH, lower C:N ratios and more available nutrients than Hydromors. Usually, Hydromores are between 10 and 20cm thick.

Summary

  • F and H horizons greater than 50% of thickness, O horizons may be present
  • Occur where there is prolonged but not permanent saturation
  • 10 to 20cm thick
  • Higher pH, lower C:N ratios and greater nutrient availability than Hydromors

Saprimoder

Characteristic Horizon(s): Oh

Description
In Saprimoders, Oh horizons make up more than half of the thickness of the humus profile. Saprimoders exist where there is permanent saturation, in fens or swamps with scarce forest cover of deciduous, cedar, spruce or larch. Low temperatures and poor aeration hinder decomposition, but relatively high pH, Ca availability, and anaerobic biota contribute to more rapid humification.

Summary

  • Oh horizons greater than 50% of thickness
  • L, F and H may be present
  • Occur in fens or swamps with sparse forest cover
  • Low temperature and poor aeration
  • High pH and Ca, relatively rapid humification

S (Sapric) – A distinct living layer of bryophytes that can be intermixed with litter suspended in the moss. It is part of Saprimoder, which is distinguished by the presence of a substantial thickness of Oh horizon.

Hydromull

Characteristic Horizon(s): Ah

Description

Hydromulls have Ah horizons that are usually very dark, from incorporation of well-humified organic matter. Usually, L, Fz or H horizons are present but thing. Hydromulls form where soil saturation occurs for prolonged periods, in temperate climates. They are often associated with Humic Gleysols. Hydromulls have high base saturation and near neutral pH. Decomposition is rapid in Hydromulls, due to the action of anaerobic and aerobic organisms.

Summary

  • Ah developed from well-humified, dark organic matter
  • Form in areas of prolonged, but discontinuous saturation
  • Associated with Humic Gleysols
  • Rapid decomposition, high base-saturation, near neutral pH