SOIL HORIZONS AND OTHER LAYERS
The definitions of classes in the Canadian system are based mainly on the kinds, degrees of development, and the sequence of soil horizons and other layers in pedons. Therefore, the clear definition and designation of soil horizons and other layers are basic to soil classification. A soil horizon is a layer of mineral or organic soil or soil material approximately parallel to the land surface that has characteristics altered by processes of soil formation.
It differs from adjacent horizons in properties such as color, structure, texture, and consistence, and in chemical, biological, and mineralogical composition. The other layers are either nonsoil layers such as rock and water or layers of unconsolidated material considered to be unaffected by soil-forming processes. For the sake of brevity these other layers are referred to simply as layers, but it is recognized that soil horizons are also layers In previous editions of this publication and in the Glossary of Terms in Soil Science organic materials are designated as layers and not horizons.
The major mineral horizons are A, B, and C.
The major organic horizons are L, F, and H, which are mainly forest litter at various stages of decomposition, and O, which is derived mainly from bogs, marsh, or swamp vegetation. Subdivisions of horizons are labeled by adding lower case suffixes to some of the major horizon symbols as with Ah or Re. Well-developed horizons are readily identified in the field. However, in cases of weak expression or of borderline properties, as between Ah and H, laboratory determinations are necessary before horizons can be designated positively. Many of the laboratory methods required are outlined in a manual prepared by a subcommittee of CSSC (McKeague 1976). Some other methods pertaining to organic horizons are outlined near the end of this chapter.
The layers defined are R, rock; W, water; and IIC or other nonconforming, unconsolidated mineral layers, IIIC, etc. below the control section that are unaffected by soil-forming processes. Theoretically a IIC affected by soil-forming processes is a horizon; for example a IICca is a horizon. In practice, it is usually difficult to determine the lower boundary of soil material affected by soil-forming processes.
Thus the following are considered as horizons: C(IC), any unconforming layer within the control section, and any unconforming layer below the control section that has been affected by pedogenic processes (e.g. IIBc, IIIBtj). Unconforming layers below the control section that do not appear to have been affected by pedogenic processes are considered as layers. The tiers of Organic soils are also considered as layers.
Mineral horizons contain 17% or less organic C (about 30% organic matter) by weight.
A This is a mineral horizon formed at or near the surface in the zone of leaching or eluviation of materials in solution or suspension, or of maximum in situ accumulation of organic matter or both. The accumulation of organic matter is usually expressed morphologically by a darkening of the surface soil (Ah), and conversely the removal of organic matter is usually expressed by a lightening of the soil color usually in the upper part of the solum (Ae)· The removal of clay from the upper part of the solum (Ae) is expressed by a coarser soil texture relative to the underlying subsoil layers. The removal of iron is indicated usually by a paler or less red soil color in the upper part of the solum (Ae) relative to the lower part of the subsoil.
B This is a mineral horizon characterized by enrichment in organic matter, sesquioxides, or clay, or by the development of soil structure; or by a change of color denoting hydrolysis, reduction, or oxidation. The accumulation in B horizons of organic matter (Bh) is evidenced usually by dark colors relative to the C horizon. Clay accumulation is indicated by finer soil textures and by clay cutans coating peds and lining pores (Bt). Soil structure developed in B horizons includes prismatic or columnar units with coatings or stainings and significant amounts of exchangeable sodium (Bn) and other changes of structure (Bm) from that of the parent material. Color changes include relatively uniform browning due to oxidation of iron (Bm), and mottling and gleying of structurally altered material associated with periodic reduction (Bg)·
C This is a mineral horizon comparatively unaffected by the pedogenic processes operative in A and B, (C), except the process of gleying (Cg), and the accumulation of calcium and magnesium carbonates (Cca) and more soluble salts (Cs, Csa). Marl, diatomaceous earth, and rock no harder than 3 on Mohs' scale are considered to be C horizons.
R This is a consolidated bedrock layer that is too hard to break with the hands (>3 on Mohs' scale) or to dig with a spade when moist and does not meet the requirements ·of a C horizon. The boundary between the R layer and any overlying unconsolidated material is called a lithic contact.
W This is a layer of water in Gleysolic, Organic, or Cryosolic soils. Hydric layers in Organic soils are a kind of W layer.
b-A buried soil horizon.
c -- A cemented (irreversible) pedogenic horizon. Ortstein, placic, and duric horizons of Podzolic soils, and a layer cemented by CaC03 are examples.
ca - A horizon of secondary carbonate enrichment in which the concentration of lime exceeds that in the unenriched parent material. It is more than 10 cm thick, and its CaCO, equivalent exceeds that of the parent material by at least 5% if the CaCO, equivalent is less than 1 (13% vs 8%), or by at least 1/3 if the CaCO3 equivalent of the horizon is 15% or more (28% vs 21%). If no IC is present, this horizon is more than 10 cm thick and contains more than 5% by volume of secondary carbonates in concretions or in soft, powdery forms, cc -- Cemented (irreversible) pedogenic concretions.
e -- A horizon characterized by the eluviation of clay, Fe, Al, or organic matter alone or in combination. When dry, it is usually higher in (color value by one or more units than an underlying B horizon. It is used with A (Ae).
f-A horizon enriched with amorphous material, principally Al and Fe combined with organic matter. It must have a hue of 7.5YR or redder, or its hue must be 10YR near the upper boundary and become yellower with depth. When moist the chroma is higher than 3 or the value is 3 or less. It contains at least 0.6% pyrophosphate-extractable Al + Fe in textures finer than sand and 0.4% in sands (coarse sand, sand, fine sand, and very fine sand). The ratio of pyrophosphate-extractable Al + Fe to clay (< 0.002 mm) is more than 0.05 and organic C exceeds 0.5 ·o . Pyrophosphate- extractable Fe is at least 0.3%, or the ratio of organic C to pyrophosphate-extractable -Fe is less than 20, or both are true. It is used with B alone (Bf), with B and h (Bhf), with B and g (Bfg), and with other suffixes. These criteria do not apply to Bgf horizons. ?'he following f horizons are differentiated on the basis of the organic C content:
Bf - 0.5-5% organic C
Bhf --- more than 5% organic C
No minimum thickness is specified for a BF or a Bhf horizon. Thin Bf and Bhf horizons do not qualify as podzolic B horizons as defined later in this chapter. Some Ah and Ap horizons contain sufficient pyrophosphate-extractable Al + Fe to satisfy this criterion of f but are designated Ah or Ap.
g --- A horizon characterized by gray colors, or prominent mottling, or both, indicative of permanent or periodic intense reduction. Chromas of the matrix are generally :1 or less. T-t is used with A and e (Aeg); B alone (Bg); B and f (Bfg, Bgf); B, h, and f (Bhfg); B and t (Btg) C alone (Cg); C and k (Ckg); and several others. In some reddish parent materials matrix colors o reddish hues and high chromas may persist despite long periods of reduction. In these soils, horizons are designated as g if there is gray mottling or marked bleaching on ped faces or along cracks.
Aeg -- 'This horizon must meet the definitions of A, e, and g.
Bg This horizon is analogous to a Em horizon but has colors indicative of poor drainage and periodic reduction. It includes horizons occurring between A and C horizons in which the main features are: (i) Colors of low chroma, that is: chromas of -1 or less, without mottles on ped surfaces or in the matrix if peds are lacking; or chromas of 2 or less in hues of 10YR or redder, on ped surfaces or in the matrix if peds are lacking, accompanied by more prominent mottles than those in the C horizon; or hues bluer than lOY, with or without mottles on ped surfaces or in the matrix if peds are lacking. (ii) Colors indicated in (i) and a change in structure from that of the C horizon. (iii) Colors indicated in (i) and illuviation of clay too slight to meet the requirements of Bt, or an accumulation of iron oxide too slight to meet the limits of Bgf. (iv) Colors indicated in (i) and the removal of carbonates. Bg horizons occur in some Orthic Humic Gleysols and some Orthic Gleysols.
Bfg, Bhfg, Btg, and others -- When used in any of these combinations the limits set for f, hf, t, and others must be met.
Bgf -- The dithionite-extractable Fe of this horizon exceeds that of the IC by 1% or more. Pyre phosphate-extractable Al + Fe is less than the minimum limit specified for f horizons. This horizon occurs in Fera Gleysols and Fera Humic Gleysols and possibly below the Bfg of gleyed Podzols. It is distinguished from the Bfg of gleyed Podzols on the basis of the extractability of the Fe and Al. The Fe in the Bgf horizon is thought to have accumulated as a result of the oxidation of ferrous iron. The iron oxide formed is not associated intimately with organic matter or with Al and is sometimes crystalline. The Bgf horizons are usually prominently mottled, more than half of the soil material occurs as mottles of high chroma.
Cg, Ckg, Ccag, Csg, Csag -- When g is used with C alone, or with C and one of the lowercase suffixes k, ca, s, or sa the horizon must meet the definition for C and for the particular suffix as well as for g.
h -- A horizon enriched with organic matter. It is used with A alone (Ah), or with A and e (Ahe), or with B alone (Bh), or with -B and f (Bhf).
Ah -- A horizon enriched with organic matter, it has a color value at least one unit lower than the underlying horizon or 0.5·0 more organic C than the IC or both. It contains less than 17% organic C by weight.
Ahe -- An Ah horizon that has undergone eluviation as evidenced, under natural conditions, by streaks and splotches of different shades of gray and often by platy structure. It may be overlain by a dark-colored Ah and underlain by a light-colored Re.
Bh - This horizon contains more than 1% organic C, less than 0.3% pyrophosphate-extractable Fe, and has a ratio of organic C to pyrophosphate-extractable Fe of 20 or more. Generally the color value and chroma are less than 3 when moist.
Bhf -- Defined under f.
j This 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. It must be placed to the right and adjacent to the suffix it modifies. For example, Bfgj means a Bf horizon with a weak expression of gleying, Bfjgj means a B horizon with weak expression of both f and g features.
Aej -- It denotes an eluvial horizon that is thin, discontinuous, or slightly discernible.
Btj -- It is a horizon with some illuviation of clay but not enough to meet the limits of Bt.
Btgj, Bmgj These are horizons that are mottled but do not meet the criteria of Bg.
Bfj It is a horizon with some accumulation of pyrophosphate-extractable Al + Fe but not enough to meet the limits of Bf. In addition, the color of this horizon may not meet the color criteria set for Bf.
Btnj or Bnj - These are horizons in which the development of solonetzic B properties is evident but insufficient to meet the limits for En or Bnt.
k - Denotes the presence of carbonate as indicated by visible effervescence when dilute HC1 is added. It is used mostly with B and m (Bmk) or C (Ck) and occasionally with Ah or Ap (Ahk, Apk), or organic horizons (Ofk, Omk).
m - A horizon slightly altered by hydrolysis, oxidation, or solution, or all three to give a change in color or structure, or both. It has:
a. Higher chromas and redder hues than the underlying horizons.
b. Removal of carbonates either partially (Bmk) or completely (Bm).
c. A change in structure from that of the original material.
n -- A horizon in which the ratio of exchangeable Ca to exchangeable Na is 10 or less. It must also have the following distinctive morphological characteristics: prismatic or columnar structure, dark coatings on ped surfaces, and hard to very hard consistence when dry. It is used with B as En or Bnt.
p-A horizon disturbed by man's activities such as cultivation, logging, and habitation. It is used with A and O.
s - A horizon with salts, including gypsum, which may be detected as crystals or veins, as surface crusts of salt crystals, by depressed crop growth, or by the presence of salt-tolerant plants. It is commonly used with C and k (Csk), but can be used with any horizon or combination of horizon and lowercase suffix.
sa - A horizon with secondary enrichment of salts more soluble than Ca and Mg carbonates; the concentration of salts exceeds that in the unenriched parent material. The horizon is at least 10 cm thick. The conductivity of the saturation extract must be at least 4 mS/cm and exceed that of the C horizon by at least one-third. (The unit mho has been replaced by siemens [S].) t -- An illuvial horizon enriched with silicate clay. It is used with B alone (Bt), with B and g (Btg), with B and n (Bnt), etc.
Bt - A Bt horizon is one that contains illuvial layer-lattice clays. It forms below an eluvial horizon but may occur at the surface of a soil that has been partially truncated. It usually has a higher ratio of fine clay to total clay than the IC. It has the following properties :
a. If any part of the eluvial horizon has less than 15% total clay in the fine earth fraction (<2 mm), the Bt horizon must contain at least 3% more clay, e.g. Re 10% clay; Bt minimum 13% clay.
b. If the eluvial horizon has more than 15% and less than 40% total clay in the fine earth fraction, the ratio of the clay in the Bt horizon to that in the eluvial horizon must be 1.2 or more, e.g. Re 25% clay; Bt at least 30% clay.
c. If the eluvial horizon has more than 40% total clay in the fine earth fraction, the Bt horizon must contain at least 8% more clay, e.g. Re 50·0 clay; Bt at least 58% clay.
u -- A horizon that is markedly disrupted by physical or faunal processes other than cryoturbation. Evidence of marked disruption such as the inclusion of material from other horizons or the absence of the horizon must be evident in at least half of the cross section of the pedon. Such turbation can result from a blowdown of trees, mass movement of soil on slopes, and burrowing animals. The u can be used with any horizon or subhorizon with the exception of A or B alone; e.g. Aeu, Bfu, BCu.
x - A horizon of fragipan character. A fragipan is a loamy subsurface horizon of high bulk density and very low organic matter content. When dry, it has a hard consistence and seems to be cemented. When moist, it has moderate to weak brittleness. It frequently has bleached fracture planes and is overlain by a friable B horizon. Air-dry clods of fragic horizons slake in water.
y -- A horizon affected by cryoturbation as manifested by disrupted and broken horizons, incorporation of materials from other horizons, and mechanical sorting in at least half of the cross section of the pedon. It is used with A, B, and C alone or in combination with other subscripts, e.g. Ahy, Ahgy, Bmy, Cy, Cgy, Cygj.
z -- A frozen layer. It may be used with any horizon or layer, e.g. Ohz, Bmz, Cz, Wz.
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