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Soil formation: horizons

The action and interaction of soil-forming processes as influences by soil-forming factors gives rise to distinct soil horizons, These layers are assigned distinctive alphabetic symbols as a form of shorthand for their characteristics. The horizon description system begins by splitting soil horizons into two distinct groups: organic and mineral horizons. Organic horizons are those that contain 17% or more organic carbon; mineral horizons have less than 17% organic carbon.

The major symbols used in describing mineral soil layers in Canada are shown in the following tables. The assignment of mineral soil layers to each horizon is done by comparing the properties of the horizons in the field to a list of distinctive characteristics, called diagnostic properties.

Table 1: Basic description of mineral soil horizons in the Canadian System of Soil Classification (Agriculture Canada Expert System on Soil Survey, 1987).

Master HorizonSuffixesBasic Description
A Mineral horizon formed at or near the soil surface
 AhAccumulation of soil organic matter (SOM)
 AeRemoval of clay, SOM, iron, or aluminum
B Horizon formed by accumulation of material removed from Ae horizon or by alteration of the parent material
 BhAccumulation of SOM
 BfAccumulation of iron and/or aluminum
 BssPresence of slickensides (smooth clay coating caused by stress in high clay soils)
 BvVertic horizon caused by turbation (mixing) of material in high clay soils
 BtAccumulation of clay
 BnStrong soil structure and sodium accumulation
 BgMottling and gleying due to water saturation
 BmSlight colour or structural changes from the parent material
CCHorizon with little evidence of pedogenic activity
 CcaAccumulation of Ca and Mg carbonates
 CsAccumulation of soluble salts
 CkPresence of original Ca and Mg carbonates
 CssPresence of slickensides
 CgMottling and gleying due to water saturation
R Consolidated bedrock
W Water layer

Table 2: Basic description of organic soil horizons as classified in the Canadian System of Soil Classification. Reference as in Table 1.

Master HorizonSuffixesBasic Description
O An organic horizon developed mainly from bog vegetation; it is more commonly called peat. These materials are usually water saturated.
 OfComposed of fibrous materials of readily recognizable origin
 OmOrganic materials in an intermediate (or mesic) stage of decomposition; some have a recognizable form, but the remainder is highly decomposed
 OhOrganic material which is highly decomposed (in a humic state); the origin of the material is unrecognizable
L, F, H Organic materials that occur from the accumulation of leaves, twigs and woody materials and which overlies a mineral soil; commonly found in well to imperfectly drained forest environments.
 LLeaf litter, readily recognizable
 FPartially decomposed leaf and twig material (folic material)
 HHumic material; decomposed organic materials with no original structures evident

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