B.Histosols

The central concept of Histosols is that of soils forming in organic soil materials. The general rule is that a soil without permafrost is classified as a Histosol if half or more of the upper 80 cm is organic. A soil is also classified as a Histosol if the organic materials rest on rock or fill or partially fill voids in fragmental, cindery, or pumiceous materials. If the bulk density is very low, less than 0.1, three-fourths or more of the upper 80 cm must be organic. 

Definition of Histosols and Limits Between Histosols and Soils of Other Orders 

Histosols are soils that:

1. Do not have either of the following:

                a. Permafrost within 100 cm of the soil surface; or

                b. Gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface; and

2. Do not have andic soil properties in 60 percent or more of the thickness between the soil surface and either a depth of 60 cm or a densic, lithic, or paralithic contact or duripan if shallower; and

3. Have organic soil materials that meet one or more of the following:

                a. Overlie cindery, fragmental, or pumiceous materials and/or fill their interstices and directly below these materials have either a densic, lithic, or paralithic contact; or

                b. When added with the underlying cindery, fragmental, or pumiceous materials, total 40 cm or more between the soil surface and a depth of 50 cm; or

                c. Constitute two-thirds or more of the total thickness of the soil to a densic, lithic, or paralithic contact and have no mineral horizons or have mineral horizons with a total thickness of 10 cm or less; or

                d. Are saturated with water for 30 or more cumulative

days during normal years (or are artificially drained), have an upper boundary within 40 cm of the soil surface, and have a total thickness of either:

                                (1) 60 cm or more if three-fourths or more of their volume consists of moss fibers or if their bulk density, moist, is less than 0.1 g/cm3; or

                                (2) 40 cm or more if they consist either of sapric or hemic materials, or of fibric materials with less than three-fourths (by volume) moss fibers and a bulk density, moist, of 0.1 g/cm3 or more. 

Limits Between Histosols and Soils of Other Orders 

The definition of Histosols must provide criteria that separate Histosols from all other orders. The aggregate of these criteria defines the limits of Histosols in relation to all other known soils.

1. Unlike Gelisols, Histosols do not have:

                a. Permafrost within 100 cm of the soil surface; or

                b. Gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface;

2. Unlike Andisols, Histosols do not have andic soil properties in 60 percent or more of the thickness between the soil surface and either a depth of 60 cm, or a densic, lithic, or paralithic contact or duripan if shallower;

3. Unlike all other mineral soil orders, Histosols have organic soil materials that meet one or more of the following:

                a. Overlie cindery, fragmental, or pumiceous materials and/or fill their interstices1 and directly below these materials have either a densic, lithic, or paralithic contact; or

                b. When added with the underlying cindery, fragmental, or pumiceous materials, total 40 cm or more between the soil surface and a depth of 50 cm; or

                c. Constitute two-thirds or more of the total thickness of the soil to a densic, lithic, or paralithic contact and have no mineral horizons or have mineral horizons with a total thickness of 10 cm or less; or

                d. Constitute two-thirds or more of the total thickness of the soil from the soil surface to 25 cm below the upper boundary of permafrost and have no mineral horizons or have mineral horizons with a total thickness of 10 cm or less within that depth; or

                e. Are saturated with water for 30 or more cumulative days during normal years (or are artificially drained), have an upper boundary within 40 cm of the soil surface, and have a total thickness of either:

                                (1) 60 cm or more if three-fourths or more of their volume consists of moss fibers or if their bulk density, moist, is less than 0.1 g/cm3; or

                                (2) 40 cm or more if they consist either of sapric or hemic materials, or of fibric materials with less than three-fourths (by volume) moss fibers and a bulk density, moist, of 0.1 g/cm3 or more. 

Key to Suborders 

BA. Histosols that are saturated with water for less than 30 cumulative days during normal years (and are not artificially drained). Folists, p. 478 

BB. Other Histosols that:

1. Have more thickness of fibric soil materials than any other kind of organic soil material either:

                a. In the organic parts of the subsurface tier if there is no continuous mineral layer 40 cm or more thick that has its upper boundary within the subsurface tier; or

                b. In the combined thickness of the organic parts of the surface and subsurface tiers if there is a continuous mineral layer 40 cm or more thick that has its upper boundary within the subsurface tier; and

2. Do not have a sulfuric horizon that has its upper boundary within 50 cm of the soil surface; and

3. Do not have sulfidic materials within 100 cm of the soil surface. Fibrists, p. 474 

BC. Other Histosols that have more thickness of sapric soil materials than any other kind of organic soil materials either:

1. In the organic parts of the subsurface tier if there is no continuous mineral layer 40 cm or more thick that has its upper boundary within the subsurface tier; or

2. In the combined thickness of the organic parts of the surface and subsurface tiers if there is a continuous mineral layer 40 cm or more thick that has its upper boundary within the subsurface tier. Saprists, p. 484 

BD. Other Histosols. Hemists, p. 480

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