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Soil Classification

3. Soil Horizons & Layers

3.1 Soil Horizon 

3.2 Forest humus forms  

3.1 Soil Horizons

Soil Horizon - A soil horizon is a layer of soil or soil material that lies approximately parallel to the land surface.  It differs from adjacent genetically related layers in properties such as color, structure, texture, consistence, and chemical, biological, and mineralogical composition. The presence or absence of certain diagnostic soil horizons determines the place of a soil in the classification system. 

Click on the Horizon Name to
 Review Detailed information

Illustration of soil horizons within a soil profile

Source: Leslie Dampier

Additional Horizons 

LFH horizons

Other horizons  

  LFH horizons

These are organic horizons, developed under relatively well drained conditions that have >0.17 kg org. C/kg dry soil (or  >17%C,  or >30% org. matter by weight).  

L - organic horizon characterized by accumulation of organic matter derived mainly from litter residues (leaves, twigs, and woody material). Original structures are easily recognizable.

F - organic horizon characterized by accumulation of partly decomposed (e.g., occupation by filamentous fungi) organic matter derived mainly leaves, twigs, and woody material. Some of the original structures are difficult to recognize.

H - organic horizon characterized by accumulation of decomposed organic matter in which the organic structures are indiscernible.

Other horizons

R – Rock

W – Water (found in Cryosols, Gleysols or Organic soils)

E – Mineral horizon, used in the U.S. (but not the Canadian) Soil Classification System, in which the main feature is loss of clay, Fe and Al oxides, or both, leaving a combination of sand and silt particles of quartz (or other resistant material)

Other Lower-Case Designations (in mineral horizon)

b - buried soil horizon

c - cemented irreversibly

ss – presence of several (more than two) slickensides.

v – horizon affected by agrillipedoturbation as manifested by disruption and mixing caused by shrinking and swelling of soil mass.  

Formation of Various Soil Horizons

Click on the image to view an slide show of soil profile development over time.

 

RELATED LINK: 
Canadian System of Soil Classification (3rd edition)

 

 

3.2 Forest humus forms  

The physical activity of worms and/or arthropods near the bottom of organic layers in the soil profile leads to mixing of organic and mineral material. This results in the formation of an Ah horizon and may prevent formation of an H (or in some cases both F and H) horizon. Organic matter accumulation is inversely related to decomposition rate. Factors such as abundance and activity of soil organisms tend to produce a characteristic organic profile morphology or “forest humus form”. Forest humus form classification is based on the sequence, properties, and inferred origin of organic horizons (usually just the H horizon) and the Ah horizon (if one is present). Several systems of forest humus classification have been proposed, often causing confusion by adopting and redefining established terminology. Despite the differences, all these classification systems tend to put forest humus forms into three broad categories:

(1) mull, 

(2) moder (or called “duff mull” in the U.S.), and

(3) mor.  

Forest Humus Forms: Examples of mor, moder and mull forest humus forms
Image Source: : Centre for Teaching, Learning and Technology, UBC

One of the systems commonly used in Canada is that of Bernier (Descriptive outline of forest humus-form classification. Proceeding 7th Meeting National Soil Survey Committee of Canada , Edmonton , 1968). Major taxa in the system may be distinguished by using the table below, in which + signifies present, - signifies absent, and ± signifies either present or absent.

 

 

 

 

Properties of Ah, if Ah is present

Major taxa

H

Ah

Clay-humus complex

Transition to overlying horizon

Faunal origin

Mull

-

+

+

gradual

+ (lumbricid worms, other   animals)

Moder

±

+

-

Not abrupt

+ (arthropods)

Mor

±

±

-

Very abrupt

-

The “new BC system” of forest humus form classification was proposed by Green et al. (1993) as a modification of the system by Klinka et al. (1981). In the taxonomic hierarchy of the new system, two levels are recognized: orders (mull, moder, and mor are differentiated by the type of F horizon and the relative prominence of Ah horizon) and groups (16 groups of this system reflect differences in the nature and rate of decomposition processes).

RELATED LINK: 
BC Ministry of Forests
Field Manual for Describing Terrestrial Ecosystems (1999), Land Management Handbook 25
  • The system proposed by Green et al. (1993) can be found at the web page  as a part of the Field Manual for Describing Terrestrial Ecosystems (1999), Land Management Handbook 25, produced by the BC Min. of Forests.  

 

Faculty of Land and Food Systems
THE UNIVERSITY OF BRITISH COLUMBIA