Chapter 9: Organic Order

A diagrammatic representation of the depth relationships of tiers, and of Typic and Terric subgroups of Organic soils, is shown in Figure 34. Diagrammatic sketches of profiles of some subgroups of the Organic order are shown in Figures 35 and 36. Individual subgroups may include soils that have horizon sequences different from those shown. In the description of each subgroup, presented later in this chapter, a common horizon sequence is given; diagnostic horizons or layers are underlined and some other commonly occurring horizons are listed.

Figure 34 Diagrammatic representation of depth relationships in the control section used to classify Fibrisol, Mesisol and Humisol great groups.

Figure 34 is a diagrammatic representation of depth relationships in the control section used to classify Organic soils. The tier is used in the Canadian System of Soil Classification as a depth stratification in the classification of Organic soils; the surface tier refers to the upper 40 cm of peat; the middle tier, the tier just below the surface tier, is 80 cm thick or extends to a lithic or hydric contact and the great group classification of Organic soils is usually based on this tier; the bottom tier is the tier below the middle tier, it is 40 cm thick or extends to a lithic or hydric contact, and the subgroup classification of Organic soils is based partly on the characteristics of this tier.
The soils of the Fibrisol great group have a dominantly fibric middle tier, or middle and surface tiers if a
terric (unconsolidated mineral substratum underlying organic soil material), lithic (bedrock under the control section of a soil, in Organic soils, bedrock occurring within a depth of between 10 and 160 cm from the surface), hydric (layer of water in the control section of Organic soils, extending from a depth of not less than 40 cm to a depth of more than 160 cm, or cryic (perennially frozen layer) contact occurs in the middle tier. Fibric refers to the relatively undecomposed organic material. The Mesisol great group refers to those soils that are at a stage of decomposition intermediate between Fibrisols and Humisol and have a dominantly mesic middle tier, or middle and surface tiers if a terric, lithic, hydric, or cryic contact occurs in the middle tier. The Humisol great group describes those soils that show the greatest signs of decomposition or humification and have a dominantly humic middle tier, or middle and surface tiers if a terric, lithic, hydric, or cryic contact occurs in the middle tier.
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Figure 35 Diagrammatic horizon pattern of some subgroups of the Fibrisol, Mesisol and Humisol great groups.

Figure 35 is Diagrammatic horizon pattern of some subgroups of the Fibrisol, Mesisol and Humisol great groups.
Typic Fibrisol (TY.F)
Common horizon sequence: Of or Om, Of. Soils of this subgroup have the general properties specified for the Organic order and the Fibrisol great group. They are composed mainly of fibric material that is commonly derived mainly from mosses. 
These soils are identified by the following properties: 
1.	If present, the middle and bottom tiers are dominantly fibric. A lithic contact may occur. 
2.	They have neither subdominant humic layers with a total thickness of greater than 12 cm or subdominant mesic layers with a total thickness greater than 25 cm in the middle and bottom tier, or in the middle and surface tiers if a lithic contact occurs in the middle tier. 
3.	The middle tier lacks terric, hydric, cumulic, and limnic layers. 

Humic Fibrisol (HU.F)
Common horizon sequence: Of, Om or Oh, Of, Oh, Of or Om. Soils of this subgroup have the general properties specified for the Organic order and the Fibrisol great group. They differ from Typic Fibrisols by having a subdominant humic layer thicker than 12 cm in the middle or bottom tier. They may also have a subdominant mesic layer. The control section lacks terric, hydric, cumulic, and limnic layers. 

Hydric Fibrisol (HY.F)
Common horizon sequence: Of or Om, Of, W. Soils of this subgroup have the general properties specified for the Organic order and the Fibrisol great group. They differ from Typic Fibrisols by having a hydric layer (a layer of water that extends from a depth of not less than 40 cm to a depth of more than 1.6 m). They may also have mesic, humic, cumulic, terric, or limnic layers.

Typic Mesisol (TY.M)
Common horizon sequence: Of, Om or Oh, Om. Soils of this subgroup have the general properties specified for the Organic order and the Mesisol great group. They are composed mainly of organic materials at an intermediate stage of decomposition.
They are identified by the following properties:
1.	If present, the middle and bottom tiers are dominantly mesic. A lithic contact may occur.
2.	These soils do not have terric, hydric, cumulic, or limnic layers within the middle tier.
3.	They lack subdominant humic or fibric layers with a total thickness greater than 25 cm in the middle and bottom tiers or in the middle and surface tiers if a lithic contact occurs in the middle tier.

Limnic Mesisol (LM.M)
Common horizon sequence: Of, Om or Oh, Om, Oco, Om. Soils of this subgroup have the general properties specified for the Organic order and the Mesisol great group. They differ from Typic Mesisols by having a limnic layer beneath the surface tier. A limnic layer is a layer or layers at least 5 cm thick of coprogenous earth (sedimentary peat), diatomaceous earth, or marl. Also they may have fibric, humic, and cumulic layers but do not have terric or hydric layers.

Terric Mesisol (T.M)
Common horizon sequence: Of, Om or Oh, Om, C, Om. Soils of this subgroup have the general properties specified for the Organic order and the Mesisol great group. They differ from Typic Mesisols by having a terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier. They may also have cumulic or limnic layers, but they do not have fibric, humic, or hydric layers within the control section.

Typic Humisol (TY.H)
Common horizon sequence: Om or Oh, Oh. Soils of this subgroup have the general properties specified for the Organic order and the Humisol great group. They are composed dominantly of well-decomposed organic materials. They are identified by the following properties: 
1.	The middle and bottom tiers, if present, are dominantly humic. A lithic contact may occur.
2.	They do not have terric, hydric, cumulic, or limnic layers within the middle tier.
3.	They have neither subdominant fibric layers with a total thickness greater than 12 cm nor subdominant mesic layers with a total thickness greater than 25 cm in the middle or bottom tiers (or in the middle and surface tiers if a lithic contact occurs in the middle tier).

Cumulic Humisol (CU.H)
Common horizon sequence: Om or Oh, Oh, C, Oh. Soils of this subgroup have the general properties specified for the Organic order and the Humisol great group. They differ from Typic Humisols by having a cumulic layer beneath the surface tier. Also they may have fibric or mesic layers but lack terric, hydric, and limnic layers. A cumulic layer consists either of multiple layers of mineral material (alluvium) that together are more than 5 cm thick, or of one layer 5-30 cm thick.
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Figure 36 Diagrammatic horizon pattern of some subgroups of the Folisol great group.

Figure 36 is a Diagrammatic horizon pattern of some subgroups of the Folisol great group.
Hemic Folisol (HE.FO)
Common horizon sequence: L, F, H, O, R, (M).  Soils of this subgroup are composed dominantly of the moderately decomposed F horizon in the control section and may have subdominant H and O horizons, each less than 10 cm thick. They commonly have a lithic contact or fragmental layers but meet the requirements of the Folisol great group. The F horizon consists of partly decomposed folic material generally derived from mosses, leaves, twigs, reproductive structures, and woody materials containing numerous live and dead roots.
Hemic Folisols usually occur on upper slope shedding positions and commonly consist of shallow folic material over bedrock or fragmental material, or the folic materials may occupy voids in fragmental material. There may be a thin layer of mineral soil separating the folic horizon from bedrock or from the fragmental material.

Humic Folisol (HU.FO)
Common horizon sequence: L, F, H, O, R, (M). Soils of this subgroup are composed dominantly of the well-decomposed H horizon in the control section and may have subdominant F and O horizons each less than 10 cm thick. A lithic contact, fragmental, or mineral layers may be common in the control section, but the soils meet the requirements for the Folisol great group. 
Humic Folisols occur most frequently in cool, moist, humid forest ecosystems. Although they occur in many landscape positions, they commonly develop on lower slopes and in valley bottoms. Rooting channels and other voids are common in these soils.

Histic Folisol (HI.FO)
Common horizon sequence: L, F, H, O, R, (M). Soils of this subgroup are dominated by F or H horizons and are directly underlain by a significant (greater than 10 cm) O horizon. Generally, saturation or high water tables (resulting from drainage impediment caused by mineral horizon cementation or localized bedrock configuration) initially encouraged the production of peat. Peat development subsequently became deep enough to produce surface conditions suitable for forest encroachment and Folisol development.
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Soils of the Organic order are composed largely of organic materials. They include most of the soils commonly known as peat, muck, or bog and fen soils. Most Organic soils are saturated with water for prolonged periods. These soils occur widely in poorly and very poorly drained depressions and level areas in regions of subhumid to perhumid climate and are derived from vegetation that grows in such sites. However, one group of Organic soils (Folisols) consists of upland (folic) organic materials, generally of forest origin. These Folisols are well to imperfectly drained, although they may become saturated after rainfall or snowmelt.

Organic soils contain more than 17% organic C (30% or more organic matter) by weight and meet the following specifications.

For organic materials (O) that are commonly saturated with water and consist mainly of mosses, sedges, or other hydrophytic vegetation the specifications are as follows:

  1. If the surface layer consists of fibric organic material with or without mesic or humic Op horizons thinner than 15 cm, the organic material must extend to a depth of at least 60 cm.
  2. If the surface layer is mesic or humic, the organic material must extend to a depth of at least 40 cm.
  3. If a lithic contact occurs at a depth shallower than 40 cm, the organic material must extend to a depth of at least 10 cm. Mineral material less than 10 cm thick may overlie the lithic contact, but the organic material must be more than twice the thickness of the mineral layer.
  4. The organic soil may have a mineral layer thinner than 40 cm on the surface provided that the underlying organic material is at least 40 cm thick.
  5. Mineral layers thinner than 40 cm that begin within a depth of 40 cm from the surface may occur within an Organic soil. A mineral layer or layers with a combined thickness of less than 40 cm may occur within the upper 80 cm.

For folic materials (L, F. and H) not usually saturated with water there must be

  1. Forty centimetres or more of folic materials if directly overlying mineral soil or peat materials.
  2. Greater than 10 cm of folic materials if directly overlying a lithic contact or fragmental materials.
  3. More than twice the thickness of a mineral soil layer if the mineral layer is less than 20 cm thick.

The control section (160 cm) for Fibrisol, Mesisol, and Humisol great groups is divided into three tiers: surface (0-40 cm); middle (40-120 cm); and bottom (120-160 cm) (see Chapter 2 for detailed definitions). Classification at the great group level is based primarily on properties of the middle tier.

Distinguishing Organic Soils from Soils of Other Orders

Many soils of other orders may have organic horizons at the surface. The distinction between Organic soils and soils of other orders is based on the following:

  1. The thickness and the organic C content of organic-rich surface horizons in the case of soils with O horizons.
  2. The thickness of the folic material for soils with L, F, and H horizons.
  3. The depth to permafrost; organic materials having permafrost at depths of 1 m or less are classified as Cryosolic soils.

Organic soils are divided into four great groups as indicated in the Organic order chart. Three of these represent Organic soils formed in hydrophytic vegetation and are separated on the basis of degree of decomposition of the organic material. These soils are commonly saturated with water throughout the year. The fourth represents organic soils formed in upland (folic) organic materials and are soils that are only briefly saturated with water.

Subgroups are based upon the kinds and sequences of horizons.

Organic Order
Hydrophytic Vegetation Upland Organic Material
Fibrisol Mesisol Humisol Folisol
Diagnostic horizons or layers are underlined
Fibric middle tier Mesic middle tier Humic middle tier Folic materials, rarely saturated with water


Great Group Subgroup
Fibrisol Typic Fibrisol TY.F
Mesic Fibrisol ME.F
Humic Fibrisol HU.F
Limnic Fibrisol LM.F
Cumulic Fibrisol CU.F
Terric Fibrisol T.F
Terric Mesic Fibrisol TME.F
Terric Humic Fibrisol THU.F
Hydric Fibrisol HY.F
Mesisol Typic Mesisol TY.M
Fibric Mesisol FI.M
Humic Mesisol HU.M
Limnic Mesisol LM.M
Cumulic Mesisol CU.M
Terric Mesisol T.M
Terric Fibric Mesisol TFI.M
Terric Humic Mesisol THU.M
Hydric Mesisol HY.M
Humisol Typic Humisol TY.H
Fibric Humisol FI.H
Mesic Humisol ME.H
Limnic Humisol LM.H
Cumulic Humisol CU.H
Terric Humisol T.H
Terric Fibric Humisol TFI.H
Terric Mesic Humisol TME.H
Hydric Humisol HY.H
Folisol Hemic Folisol HE.FO
Humic Folisol HU.FO
Lignic Folisol LI.FO
Histic Folisol HI.FO

Source: The Canadian System of Soil Classification (Third Edition)