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.
|Hydrophytic Vegetation||Upland Organic Material|
|Diagnostic horizons or layers are underlined|
|Fibric middle tier||Mesic middle tier||Humic middle tier||Folic materials, rarely saturated with water|
|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|