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.
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:
- 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.
- If the surface layer is mesic or humic, the organic material must extend to a depth of at least 40 cm.
- 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.
- 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.
- 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
- Forty centimetres or more of folic materials if directly overlying mineral soil or peat materials.
- Greater than 10 cm of folic materials if directly overlying a lithic contact or fragmental materials.
- 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:
- The thickness and the organic C content of organic-rich surface horizons in the case of soils with O horizons.
- The thickness of the folic material for soils with L, F, and H horizons.
- 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 |
| Fibric middle tier | Mesic middle tier | Humic middle tier | Folic materials, rarely saturated with water | |
Fibrisol
Soils of this great group are composed largely of relatively undecomposed fibric organic material. Fibric material is usually classified on the von Post scale of decomposition as classes 1-4. Fibrisols occur extensively in Canada, particularly in peat deposits dominated by sphagnum mosses.
Fibrisols have a dominantly fibric middle tier, or middle and surface tiers if a terric, lithic, or hydric contact occurs in the middle tier. Fibric material is the least decomposed type of organic material. It contains large amounts of well-preserved fiber that is retained on a 100-mesh sieve (0.15 mm) and can be identified as to botanical origin. A fibric horizon has 40% or more of rubbed fiber by volume and a pyrophosphate index of 5 or more (see Chapter 2, Organic horizons, Of). If the rubbed fiber volume is 75% or more, the pyrophosphate criterion does not apply. Dominantly fibric means that fibric material is the most abundant type of organic material. If both fibric and mesic layers occur in the middle tier, the tier is dominantly fibric if more than half of its thickness is composed of fibric material. If fibric, mesic, and humic layers are present in the middle tier, it is dominantly fibric if the thickness of fibric layers is greater than that of either mesic or humic layers. Subdominant in the following definitions means next in abundance to the dominant material but the layer must not be less than 12 cm in thickness if sharply contrasting (Of vs. Oh), or 25 cm in thickness if not sharply contrasting (Om vs. Of or Oh).
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:
- If present, the middle and bottom tiers are dominantly fibric. A lithic contact may occur.
- 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.
- The middle tier lacks terric, hydric, cumulic, and limnic layers.
Common horizon sequence: Of or Om, Of, Om, Of
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 mesic layer (thicker than 25 cm) in the middle or bottom tier. The control section lacks terric, hydric, cumulic, or limnic layers.
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.
Common horizon sequence: Of or Om, Of, Oco
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 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. Limnic materials include both organic and inorganic materials either deposited in water by precipitation or by the action of aquatic organisms such as algae and including diatoms, or derived from underwater and floating aquatic plants subsequently modified by aquatic animals. Except for some of the coprogenous earths, most of these limnic materials are inorganic. Diatomaceous earth is highly siliceous and marl is mainly CaCO3. Limnic Fibrisols may have mesic, humic, or cumulic layers but do not have terric or hydric layers.
Common horizon sequence: Of or Om, Of, C, Of
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 cumulic layer beneath the surface tier. They may have mesic or humic 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.
Common horizon sequence: Of or Om, Of, C
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 terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier. They may also have cumulic or limnic layers but do not have mesic, humic, or hydric layers within the control section.
Common horizon sequence: Of or Om, Of, Om, C
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 both a terric layer beneath the surface tier and a subdominant mesic layer thicker than 25 cm in the control section. They may also have cumulic or limnic layers but lack humic and hydric layers.
Common horizon sequence: Of or Om, Of, Oh, C
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 both a terric layer beneath the surface tier and a subdominant humic layer thicker than 12 cm in the control section. They may also have mesic, cumulic, or limnic layers but lack a hydric layer.
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.
Mesisol
Soils of this great group are at a stage of decomposition intermediate between Fibrisols and Humisols. Mesisols have a dominantly mesic middle tier or middle and surface tiers if a terric, lithic, or hydric contact occurs in the middle tier. A mesic layer is an organic layer that fails to meet the requirements of either a fibric or a humic layer. Thus it has a rubbed fiber content ranging from 10% to less than 40% by volume and has a pyrophosphate index of >3 and <5. Mesic material is usually classified on the van Post scale of decomposition as class 5 or 6.
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:
- If present, the middle and bottom tiers are dominantly mesic. A lithic contact may occur.
- These soils do not have terric, hydric, cumulic, or limnic layers within the middle tier.
- 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.
Common horizon sequence: Of, Om or Oh, Om, Of, 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 subdominant fibric layer thicker than 25 cm in the middle or bottom tiers. These soils do not have a subdominant humic layer thicker than 25 cm.
Common horizon sequence: Of, Om or Oh, Om, Oh, 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 subdominant humic layer thicker than 25 cm in the middle or bottom tiers. They may also have a subdominant fibric layer.
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.
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 cumulic layer beneath the surface tier. They may also have fibric or humic 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.
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.
Common horizon sequence: Of, Om or Oh, Om , Of, 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 both a terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier and a subdominant fibric layer thicker than 25 cm in the control section. These soils may also have cumulic and limnic layers but lack humic and hydric layers.
Common horizon sequence: Of, Om or Oh, Om, Oh, 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 both a terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier and a subdominant humic layer thicker than 25 cm within the control section. These soils may also have fibric, cumulic, or limnic layers but lack a hydric layer.
Common horizon sequence: Of, Om or Oh, Om, W
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 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). These soils may also have fibric, humic, cumulic, terric, or limnic layers.
Humisol
Soils of this great group are at the most advanced stage of decomposition of the great groups of Organic soils. Most of the material is humified with few recognizable fibers. Humisols have a dominantly humic middle tier or middle and surface tiers if a terric, lithic, or hydric contact occurs in the middle tier. A humic layer is an organic layer having less than 10% rubbed fiber by volume and a pyrophosphate index of 3 or less. It has a higher bulk density, usually >0.195 Mg m-3, and a lower water-holding capacity than fibric or mesic layers. Humic material usually is classified on the von Post scale of decomposition as class 7 or higher and rarely in class 6.
Only minor areas of Humisols are known to occur in Canada.
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:
- The middle and bottom tiers, if present, are dominantly humic. A lithic contact may occur.
- They do not have terric, hydric, cumulic, or limnic layers within the middle tier.
- 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).
Common horizon sequence: Om or Oh, Oh, Of, Oh
Soils of this subgroup have the general properties specified for the Organic order and Humisol great group. They differ from Typic Humisols by having a subdominant fibric layer thicker than 12 cm in the middle or bottom tiers. They may also have a subdominant mesic layer.
Common horizon sequence: Om or Oh, Oh, Om, 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 subdominant mesic layer thicker than 25 cm in the middle or bottom tiers. They lack a subdominant fibric layer below the surface tier.
Common horizon sequence: Om or Oh, Oh, Oco, 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 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. They may also have fibric, mesic and cumulic layers but do not have terric or hydric layers.
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.
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 terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier. They may also have cumulic or limnic layers but do not have fibric, mesic or hydric layers within the control section.
Common horizon sequence: Of or Oh, Oh, Of, 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 both a terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier and a subdominant fibric layer thicker than 12 cm within the control section. They may also have mesic, cumulic or limnic layers but lack a hydric layer.
Common horizon sequence: Om or Oh, Oh, Om, 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 both a terric layer (an unconsolidated mineral layer at least 30 cm thick) beneath the surface tier and a subdominant mesic layer thicker than 25 cm within the control section. These soils may also have cumulic or limnic layers but lack a subdominant fibric or hydric layer.
Common horizon sequence: Om or Oh, Oh, W
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 hydric layer. These soils may also have fibric, mesic, humic, cumulic, terric, or limnic layers. A hydric layer is a layer of water that extends from a depth of not less than 40 cm to a depth of more than 160 cm.
Folisol
Soils of the Folisol great group are composed of upland organic (folic) materials, generally of forest origin, that are either 40 cm or more in thickness, or are at least 10 cm thick if overlying bedrock or fragmental material. Deep Folisols (greater than 40 cm of folic material) occur frequently in cool, moist, and humid forest ecosystems, particularly on the West Coast of Canada. They also develop in northern regions where soil temperatures are low, but the soil is without permafrost. Shallow Folisols are found throughout Canada and commonly occur on upper slope shedding positions over bedrock and on, or incorporated in, fragmental or skeletal material.
Folic materials are formed under ecosystems different from those of peat materials. Folic materials are the product of upland ecosystem development, whereas peat materials are the product of wetland development.
Folisols are well to imperfectly drained, although they may become saturated after rainfall or snowmelt. They contain organic C at a level of >17% (about 30% or more Organic matter) by weight in diagnostic horizons. Folic materials qualify as Folisols if they meet the following criteria:
- Folic material is 40 cm or more in depth; or
- Folic material is 10 cm or more in depth if directly overlying a lithic contact.or fragmental material, or if occupying voids in fragmental or skeletal material; or
- Folic material is more than twice the thickness of a mineral soil layer if the mineral layer is less than 20 cm thick.
Folic materials containing permafrost at depths of 1 m or less are classified as Cryosolic soils.
The Folisol great group is divided into four subgroups, based on the degree of decomposition of the folic material (as distinguished by the diagnostic F and H soil horizons) or on the type of organic materials in the control section, or on both. Layers or pockets of decaying wood may be designated as an F or an H
Common horizon sequence: L, F, H, O, R, (M[1])
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.
Common horizon sequence: L, F, H, O, R, (M[2])
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.
Common horizon sequence: L, F, H, R, (M[3])
Soils of this subgroup are dominated by F or H horizons, which are composed primarily of moderately to well-decomposed woody materials. These materials occupy more than 30% of the surface area of the F and H horizons. The decaying wood that makes up the F and H horizons generally consists of trees that have been blown down. The destruction of trees is either a continuing process in unevenly aged forests or occurs at periodic intervals as a result of major storms, which create evenly aged forest stands. Lignic Folisols also occur in a second-growth forest as a result of logging.
Common horizon sequence: L, F, H, O, R, (M[4])
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.
