Mesisol (M)
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 4070 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.
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:
- 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.
Fibric Mesisol (FI.M)
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.
Humic Mesisol (HU.M)
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.
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.
Cumulic Mesisol (CU.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 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.
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.
Terric Fibric Mesisol (TFI.M)
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.
Terric Humic Mesisol (THU.M)
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.
Hydric Mesisol (HY.M)
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.
Note: Diagnostic horizons are underlined.