LFS:SoilWeb/Soil Classification/Soil Horizons

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 colour, 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.

Mineral Horizons

A Horizon: Mineral horizon, near the soil surface characterized by (i) eluviation of materials in solution, or (ii) accumulation of organic matter, or both. This is usually the most fertile part of soil profile.

• Ah - Enrichment with organic matter.

• Ap - Horizon that has been mechanically disturbed by plowing, logging or human habitation.

• Ae - Eluvial horizon with losses of clay, organic matter, Fe and Al oxides.

• Ahe - Enriched by organic matter but eluviated.

• Aej - Eluvial horizon that is thin, discontinuous or only slightly noticeable.

B Horizon: Mineral horizon characterized by illuviation (enrichment) of clay, organic matter, and iron and aluminum oxides or by in situ weathering.

• Bm - Modified horizon only slightly developed by hydrolysis, oxidation or solution that is different from adjacent horizons in color, structure, or both.

• Bt - Enriched with silicate clay, usually from Ae horizon above.

• Bf - Enriched with Fe and Al oxides.

• Bh - Enriched with organic matter.

• Bca - Secondary carbonate enrichment from horizons above, horizon > 10 cm thick.

• Bmk - Slight development and calcium carbonates present (originating from parent material).

• Bn - Presence of a high % of Na ions, with distinctive prismatic or columnar structure.

• Bnt - Na ions and illuvial silicate clay.

• Bg - Gleyed horizon with grey color, mottling, or both as a result of permanent or periodic intense anaerobic conditions.

• AB - Transition between A and B, where A dominates.

• BA - Transition between B and A, where B dominates.

C Horizon: Mineral horizon characterized by little or no alteration through the soil-forming processes operating in the A and B horizons. The C horizon usually represents the parent material from which the A and B horizons have formed.

• C - No particular properties.

• Ck - Presence of carbonates (CaCO₃).

• Cs - Detectable soluble salts (NaCl, Na₂SO₄).

• Csk - Carbonates and soluble salts.

• Csa - Secondary enrichment of salts more soluble than Ca and Mg (generally Na).

• Cg - Gleyed horizon.

• Cy - Horizon affected by cryoturbation.

• Cz - A perennially frozen layer.

• BC - Transition between B and C where B dominates.

Other Lower-Case Designations (in mineral horizons)

The lowercase suffixes are used to describe the dominant soil forming process in a horizon (see next page). Please note that detailed description of lowercase suffixes is provided in the “The Canadian System of Soil Classification” (1998).

• b - buried soil horizon

• c - cemented irreversibly

• ca – horizon of secondary carbonate enrichment in which the concentration of CaCO3 exceeds that in the enriched parent material

• e - alluvial horizon, showing losses (eluviation) of clay, organic matter, and Fe and Al oxides. It is used with an A horizon (Ae)

• f - enrichment with Fe and Al oxides. It is used with B alone (Bf), with B and h (Bfh), with B and g (Bfg), and with other suffixes

• g - gleyed horizon of gray color or mottling or both as a result of permanent or periodic intense anaerobic conditions. It is used with A and e (Aeg), with B alone (Bg), with B and f (Bfg, *

Bgf), with B, h, and f (Bhfg), with B and t (Btg), with C alone (Cg), with C and k (Ckg), and several others (Ccag, Csg, Csag).

• h - enrichment with organic matter. It is used with A alone (Ah), or with A and e (Ahe), with B alone (Bh), or with B and f (Bhf or Bfh).

• j – denotes a ‘juvenile’ horizon. It is used as a modifier of suffixes e, f, g, n, and t to denote an expression of, but failure to meet, the specified limits of the suffix it modifies. For

example, Aej is an eluvial horizon that is thin, discontinuous or slightly noticeable.

• k – denotes the presence of carbonates, as indicated by visible effervescence when dilute Hal is added. Most often it is used with B and m (Bmk), or C alone (Ck), and occasionally with A and h (Ahk) or A and p (Apk), or organic horizons (Ofk, Omk).

• m - modified horizon of slight development by hydrolysis, oxidation, and/or solution. It has a different colour and/or structure than the underlying horizon. It can be used as Bm, Bmgj,

Bmk, and Bms.

• n - presence of a high % of Na ions (ratio of exchangeable Ca to exchangeable Na is 10 or less). This leads to distinctive prismatic or columnar structure, dark coatings on aggregate

surfaces, and hard to very hard consistency when dry. It is used with B alone (Bn), or B and t (Bnt).

• p – a horizon disturbed by human’s activities, such as plowing, cultivation, logging, habitation, etc. It is used with A and O.

• s – accumulation of soluble salts including gypsum (CaSO4), which may be detected as crystals, veins, surface crusts, or by depressed plant growth, or by the presence of salt-tolerant plant species. It is commonly used with C and k (Csk) but can be used with any other horizon.

• sa- secondary enrichment of salts more soluble than Ca and Mg carbonates (generally Na). The concentration of salts exceeds that in the unenriched parent material.

• ss – presence of several (more than two) slickensides (smooth shear clayey surfaces).

• t – an illuvial horizon enriched with silicate clay (usually from Ae horizon above). It is used with B alone (Bt), with B and g (Btg), with B and n (Bnt), etc.

• v – horizon affected by argillipedoturbation (disruption and mixing of soil as a result of shrinking and swelling of clays).

• y - horizon affected by cryoturbation (mixing caused by freeze/thaw action) as manifested by disrupted and broken horizons and incorporation of material from other horizons. It is used

with A, B, and C alone or in combination with other lowercase suffixes, e.g. Ahy, Ahgy, Bmy, Cy, Cgy, Cygj, etc.

• z – a frozen layer. It may be used with any horizon or layer, e.g. Ohz, Bmz, Cz, Wz.

Organic Horizons

Organic horizons occur in Organic soils or they may be present at the surface of mineral soils. These horizons contain more than 17% organic C (or more than 30% of organic matter) by weight. There are two groups of organic horizons – the O horizons (peat materials) and the L, F, and H horizons (folic materials).

O Horizon: Organic horizon developed mainly from mosses, rushes, and woody materials. This horizon is developed under poorly drained conditions. It can be classified as either:

• Of - Least decomposed organic layer, containing large amounts of well-preserved fibre, and called the fibric layer.

• Om - An intermediately decomposed organic layer containing less fibre than an Of layer and called the mesic layer.

• Oh - The most decomposed organic layer, containing only small amounts of raw fibre and called the humic layer.

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

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

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

• H - Organic horizon characterized by the 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).

RELATED LINKS: More information on soil horizons can be found in the Canadian System of Soil Classification (3rd edition). Forest Floor SoilWeb

Forest Humus Forms

The term forest floor is used by many forest scientists and practitioners in North America to designate humus forms. Humus forms refer to organic materials that have formed at and near the mineral soil surface and can include both humus and unhumified material. A humus form profile is the sequence of organic and mineral horizons in a humus form.

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.

One of the systems commonly used in Canada is that of Bernier (Descriptive outline of forest humus-form classification. Proceedings of the 7th Meeting of the 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 present)	Properties of Ah (if 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 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 the Ah horizon.
• Groups: 16 groups of this system reflect differences in the nature and rate of decomposition processes.

RELATED LINK: Field Manual for Describing Terrestrial Ecosystems, BC Ministry of Forests Forest Floor Classification

Humus forms are closely related to overall site quality, with the succession of mor, moder, and mull associated with increasing soil biodiversity, increasing soil fertility, and improved productivity. As habitat for the soil organisms that decompose organic matter, soil humus forms act as both a sink and source of nutrients. Humus forms could be used as early indicators of ecosystem changes due to pollution, climate change, or management practices.