Orders: 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.
