Soils of the Organic order are the dominant wetland soils found in forested regions of Canada. Their greatest extent is in the low-lying landscapes of the Hudson Plains ecozone but they occur in wetlands throughout the boreal forest. In the Prairie provinces they are commonly found immediately south of the contact between the sedimentary rocks of the southern Prairies and the Canadian Shield, whereas in Ontario and parts of Northern Quebec and Newfoundland and Labrador they occur on the Canadian Shield itself. Organic soils in these regions are commonly referred to as peats, bog or fen soils, or mucks. (Patterned Fen)
A second group of organic soils occurs in non-wetland positions in upland sites where leaf litter accumulates. In coastal B.C. they are found in the coastal rain forests where precipitation levels can reach 3000 mm per year, and great thicknesses of leaf litter and woody forest material accumulates. In other forested regions they are found where a layer of leaf litter directly overlies the bedrock surface.
The wetland variants of the Organic soils are associated with landscape positions where water accumulates and saturates the soil. In some sites the lack of an integrated river network causes runoff water accumulation in topographic low points. In other sites the groundwater table may be located at or near the mineral soil surface, and the soil profile is saturated with water for all or most of the year. The loss of dissolved oxygen in this stagnant water causes the soil to become anaerobic or anoxic. Ultimately the decomposition of any organic material added to the soil (both through roots and above-ground plant materials) becomes very slow or ceases altogether due to the inability of microorganisms to function in the anoxic conditions, and build-up of unaltered or partially decomposed organic matter begins at the soil surface. (Fibric Material) The thickness of the organic layer increases with time, and this process of organic matter accumulation (or paludification) is responsible for formation of Organic soils.
The degree of decomposition of the organic material in wetlands differs between sites and depends on the botanical origin of the wetland plant species, temperature, and the duration of water saturation and chemical composition of the water. Fibric material has undergone relatively little decomposition and most of the plant material can be readily identified as to its botanical origin. (Fibric Material) The other extreme is humic material, which is greatly decomposed and unrecognizable in origin. Mesic organic materials are intermediate between these two extremes. In the field these three stages are classified using the von Post scale of decomposition.
The master horizon for the wetland Organic soils is the O horizon. The three decomposition stages are assigned Of, Om, or Oh designations depending on the degree of decomposition of the organic material. The organic horizons must achieve a minimum thickness to be classified into the Organic order. (Depth Relationships) If the Organic horizons are less than these critical thicknesses, they are considered peaty phases of the Gleysolic order. If permafrost occurs at a depth less than 1 m from the organic soil surface, the soils are classified into the Cryosolic Order.
The upland versions of the Organic order are composed of leaf litter and other woody debris, which are termed folic materials. These organic horizons are assigned an L, F, or H designation, depending again on the degree of decomposition.
Organic Great Groups
The placement of wetland organic soils into the three main great groups for these soils depends on depth relationships of the organic layers within the control section. (Depth Relationships) For organic soils the control section extends from the surface to a depth of 1.6 m or to a contact with mineral soil or bedrock if this occurs within 1.6 m of the surface. If the full 1.6 m extent is present the control section is split into three tiers:
Surface tier
The surface tier is 40 cm thick excluding any loose litter, crowns of sedges and reeds, or living mosses. Shallow organic soils over mineral soil or bedrock may have only a surface tier.
Middle tier
The middle tier is 80 cm thick. It establishes the great group classification if no mineral soil, bedrock, or water substratum is present. The dominant kind of organic material (Of, Om, Oh) in this and the surface tier establishes the great group classification. The nature of the subdominant organic material in the middle or bottom tier assists in establishing the subgroup classification.
Bottom tier
The bottom tier is 40 cm thick. The material in this tier establishes in whole or in part the subgroup classification.
The three great groups for the wetland organic soils are (text modified from Canadian System of Soil Classification, 3rd. Ed.):
Fibrisol (Typic Fibrisol)
The middle tier is dominated by Of material. This O horizon consists largely of fibric materials that are readily identifiable as to botanical origin. A fibric horizon (Of) has 40% or more of rubbed fiber by volume. Fiber is defined as the organic material retained on a 100-mesh sieve (0.15 mm), except for wood fragments that cannot be crushed in the hand and are larger than 2 cm in the smallest dimension. Rubbed fiber is the fiber that remains after rubbing a sample of the layer about 10 times between the thumb and forefinger. Fibric material usually is classified on the van Post scale of decomposition as class 1 to class 4. Three kinds of fibric horizons are named: 1) Fennic horizons are derived from rushes, reeds, and sedges; 2) Silvic horizons are derived from wood, moss with less than 75% of the volume composed of sphagnum spp., and other herbaceous plants; and 3) Sphagnic horizons are derived from sphagnum mosses. (Sphanum)
Mesisol (Humic Mesisol)
The middle tier is dominated by Om material. This O horizon consists of mesic material, which is at a stage of decomposition intermediate between fibric and humic materials. The material is partly altered both physically and biochemically. It does not meet the requirements of either a fibric or a humic horizon, has a rubbed fiber content ranging from 10% to less than 40%. Mesic material usually is classified on the van Post scale of decomposition as class 5 or 6.
Humisol
The middle tier is dominated by Om material. This O horizon consists of humic material, which is at an advanced stage of decomposition. The horizon has the lowest amount of fiber and is very stable and changes little physically or chemically with time unless it is drained. Humic material usually is classified on the van Post scale of decomposition as class 7 or higher and rarely as class 6.
Folisol
For upland organic soils the control section is the same as that used for mineral soils. These soils must have more than 40 cm of folic materials if they overlie mineral soils or peat materials, or at least 10 cm if they overlie bedrock or fragmental materials.
Organic Subgroups
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Subgroup
|
Great Group
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|||
|
Fibrisol
|
Mesisol
|
Humisol
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Folisol
|
|
|
Typic
|
X
|
X
|
X
|
Not applicable
|
| Fibric |
Not applicable
|
X
|
X
|
Not applicable
|
|
Mesic
|
X
|
Not applicable
|
X
|
Not applicable
|
|
Humic
|
X
|
X
|
Not applicable
|
X
|
|
Limnic
|
X
|
X
|
X
|
Not applicable
|
|
Cumulic
|
X
|
X
|
X
|
Not applicable
|
|
Terric
|
X
|
X
|
X
|
Not applicable
|
|
Terric Fibric
|
Not applicable
|
X
|
X
|
Not applicable
|
|
Terric Mesic
|
X
|
Not applicable
|
X
|
Not applicable
|
|
Terric Humic
|
X
|
X
|
Not applicable
|
Not applicable
|
|
Hydric
|
X
|
X
|
X
|
Not applicable
|
|
Hemic
|
Not applicable
|
Not applicable
|
Not applicable
|
X
|
|
Lignic
|
Not applicable
|
Not applicable
|
Not applicable
|
X
|
|
Histic
|
Not applicable
|
Not applicable
|
Not applicable
|
X
|
Typic subgroup
Soils of the Typic subgroup do not have any of the horizons discussed below.
Fibric subgroup
Soils of the Fibric subgroup have a subdominant fibric layer thicker than 25 cm in the middle or bottom tiers.
Mesic subgroup
These soils have a subdominant mesic layer thicker than 25 cm in the middle or bottom tiers.
Humic subgroup
These soils have a subdominant humic layer thicker than 25 cm in the middle or bottom tiers.
Terric subgroup (Terric Mesisol 2)
Soils of these subgroups have a contact with a mineral horizon within the depths specified by the depth relationship diagram. The Terric prefix can also be used before Fibric, Mesic, or Humic suffixes for soils that meet both sets of criteria.
Limnic subgroup
These soils have a limnic layer beneath the surface tier.
Cumulic subgroup
These soils have either multiple layers of mineral material (pond or alluvial sediments) that together are more than 5 cm thick or on layers 5 to 30 cm thick.
Hydric subgroup
These soils have a layer of water that extends from a depth of not less than 40 cm to a depth of more than 1.6 m.
The final four subgroups are only found in the Folisol great group.
Hemic subgroup
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.
Humic subgroup
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.
Lignic subgroup
Soils of this subgroup are dominated by F or H horizons 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 fallen trees. (Organic Vegetation)
Histic subgroup
Soils of this subgroup are dominated by F or H horizons and are directly underlain by a O horizon greater than 10 cm.