Chapter 2: Soil, Pedon, Control Section, and Soil Horizons (continued)
Soil Horizons and Other Layers
The definitions of taxa in the Canadian system are based mainly on the kinds, degree 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 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, or 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 (Canada Department of Agriculture 1976) 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 wetland vegetation. Subdivisions of horizons are labeled by adding lower-case suffixes to some of the major horizon symbols as with Ah or Ae. 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 publication sponsored by the Canadian Society of Soil Science (Carter 1993). 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 and layers
Mineral horizons contain 17% or less organic C (about 30% organic matter) by weight.
A - This mineral horizon forms 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 accumulated organic matter is usually expressed morphologically by a darkening of the surface soil (Ah). 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 mineral horizon is 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. In B horizons, accumulated 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 mineral horizon is comparatively unaffected by the pedogenic processes operating in A and B horizons, 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 with a hardness ≤3 on Mohs' scale are considered to be C horizons.
R - This consolidated bedrock layer is too hard to break with the hands (>3 on Mohs' scale) or to dig with a spade when moist. It 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 layer of water may occur in Gleysolic, Organic, or Cryosolic soils. Hydric layers in Organic soils are a kind of W layer as is segregated ice formation in Cryosolic soils.
b - A buried soil horizon.
c - A cemented (irreversible) pedogenic horizon. Ortstein, placic and duric horizons of Podzolic soils, and a layer cemented by CaCO3 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 CaCO3 equivalent exceeds that of the parent material by at least 5% if the CaCO3 equivalent is less than 15% (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 or Ahe).
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% or more 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 %. 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. The 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, indicating permanent or periodic intense reduction. Chromas of the matrix are generally 1 or less. It 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 of 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 Bm horizon but has colors indicating poor drainage and periodic reduction. It includes horizons occurring between A and C horizons in which the main features are as follows:
(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 10Y, 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. Pyrophosphate-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 - This A horizon, enriched with organic matter, has a color value at least one unit lower than the underlying horizon or 0.5% more organic C than the IC or both. It contains 17% or less organic C by weight.
- Ahe - This Ah horizon 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 Ae.
- 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 3 or less when moist.
- Bhf - Defined under f.
j - A modifier of suffixes e, f, g, n, t, and v. It is used 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 horizons 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 - In these horizons the development of solonetzic B properties is evident but insufficient to meet the limits for Bn or Bnt.
- Bvj - In this horizon argillipedoturbation is evident but the disruption of other horizons is insufficient to severely alter them.
k - Denotes the presence of carbonate as indicated by visible effervescence when dilute HCI is added. It is used mostly with B and m (Bmk) or C (Ck) and occasionally with Ah or Ap (Ahk, Apk), or organic layers (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 the following properties:
- Evidence of alteration in one of the following forms
- Higher chromas and redder hues than the underlying horizons.
- Removal of carbonates either partially (Bmk) or completely (Bm).
- A change in structure from that of the original material.
- Illuviation, if evident, too slight to meet the requirements of a Bt or a podzolic B.
- Some weatherable minerals.
- No cementation or induration and lacks a brittle consistence when moist.
This suffix can be used as Bm, Bmj, Bmk, and Bms.
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 Bn 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.
ss - Denotes the presence of several (more than two) slickensides. It is used with B alone (Bss), with B and other lower case suffixes (Bssk, Bssgj, Bsskgj, Btss, etc.), with C alone (Css), with C and other lower case suffixes (Ckss, Ckssgj, etc.), with AC (ACss) or with BC (BCss). Slickensides are shear surfaces, with an aerial extent of at least 4 cm2, that form when one soil mass moves over another. They commonly display unidirectional grooves parallel to the direction of movement and often occur at an angle of 20-60 degrees from the horizontal. Slickensides often intersect, resulting in the formation of wedge shaped aggregates that commonly occur in these soils.
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:
- If any part of an eluvial horizon remains and there is no lithologic discontinuity between it and the Bt horizon, the Bt horizon contains more total clay than the eluvial horizon as follows:
- 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., Ae 10% clay; Bt minimum 13% clay).
- 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., Ae 25% clay; Bt at least 30% clay).
- 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., Ae 50% clay; Bt at least 58% clay).
- A Bt horizon must be at least 5 cm thick. In some sandy soils where clay accumulation occurs in the lamellae, the total thickness of the lamellae should be more than 10 cm in the upper 1.5 m of the profile.
- In massive soils the Bt horizon should have oriented clay in some pores and also as bridges between the sand grains.
- If peds are present, a Bt horizon has clay skins on some of the vertical and horizontal ped surfaces and in the fine pores or has illuvial oriented clays in 1% or more of the cross section as viewed in thin section.
- If a soil shows a lithologic discontinuity between the eluvial horizon and the Bt horizon, or if only a plow layer overlies the Bt horizon, the Bt horizon need show only clay skins in some part, either in some fine pores or on some vertical and horizontal ped surfaces. Thin sections should show that the horizon has about 1% or more of oriented clay bodies.
- If any part of an eluvial horizon remains and there is no lithologic discontinuity between it and the Bt horizon, the Bt horizon contains more total clay than the eluvial horizon as follows:
- Btj and Btg - Defined under j and g.
u - A horizon that is markedly disrupted by physical or faunal processes other than cryoturbation or argillipedoturbation caused by Vertisolic processes. 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.
v - A horizon affected by argillipedoturbation, as manifested by disruption and mixing caused by shrinking and swelling of the soil mass. It is characterized by the presence of the following:
- Irregular shaped, randomly oriented, intrusions of displaced materials within the solum.
- Vertical cracks, often containing sloughed-in surface materials. The disruption within this horizon is strong enough to prevent the development of horizons diagnostic of other orders, or if these horizons are present they are disrupted to the extent that they are no longer continuous and their orientation has been severely changed. It is used with B or BC horizons alone or in combination with other suffixes; e.g., Bv, Bvk, Bvg, Bvgj, BCvj, etc.
x - A horizon of fragipan character. See definition of fragipan.
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.
Named diagnostic horizons and layers of mineral soils
Chernozemic A - This A horizon has all the following characteristics:
- It is at least 10 cm thick or is thick and dark enough to provide 10 cm of surface material that meets the color criteria given in 2 and 3.
- It has a color value darker than 5.5 dry and 3.5 moist, and its chroma is less than 3.5 moist.
- It has a color value at least one Munsell unit darker than that of the IC horizon.
- It contains 1-17% organic C and its C:N ratio is less than 17.
- Characteristically it has neither massive structure and hard consistence nor single- grained structure, when dry.
- It has a base saturation (neutral salt) of more than 80% and Ca is the dominant exchangeable cation.
- It is restricted to soils having a mean annual soil temperature of 0oC or higher and a soil moisture regime subclass drier than humid. Usually chernozemic A horizons are associated with well to imperfectly drained soils having cold semiarid to subhumid soil climates.
Duric horizon - This strongly cemented horizon does not satisfy the criteria of a podzolic B horizon. Usually it has an abrupt upper boundary to an overlying podzolic B or to a Bm horizon and a diffuse lower boundary more than 50 cm below. Cementation is usually strongest near the upper boundary, which occurs commonly at a depth of 40-80 cm from the mineral surface. The color of the duric horizon usually differs little from that of the moderately coarse textured to coarse textured parent material, and the structure is usually massive or very coarse platy. Air-dry clods of duric horizons do not slake when immersed in water, and moist clods at least 3 cm thick usually cannot be broken in the hands.
Fragipan - 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.
Ortstein - This strongly cemented horizon (Bhc, Bhfc, or Bfc) is at least 3 cm thick and occurs in more than one-third of the exposed face of the pedon. Ortstein horizons are generally reddish brown to very dark reddish brown.
Placic horizon - This horizon is a thin layer (commonly 5 mm or less in thickness) or a series of thin layers that are irregular or involuted, hard, impervious, often vitreous, and dark reddish brown to black. Placic horizons may be cemented by Fe, Al-organic complexes (Bhfc or Bfc), hydrated Fe oxides (Bgfc), or a mixture of Fe and Mn oxides.
Podzolic B horizon - This diagnostic horizon is defined by morphological and chemical properties.
- It is at least 10 cm thick.
- The moist crushed color is either black, or the hue is 7.5YR or redder or 10YR near the upper boundary and becomes yellower with depth. The chroma is higher than 3 or the value is 3 or less.
- The accumulation of amorphous material is indicated by brown to black coatings on some mineral grains or brown to black microaggregates. Also there is a silty feel when the material is rubbed wet, unless it is cemented.
Two kinds of podzolic B horizons are differentiated chemically.
- Very low Fe. Such a podzolic B horizon (Bh) must be at least 10 cm thick and have more than 1% organic C, less than 0.3% pyrophosphate-extractable Fe, and a ratio of organic C to pyrophosphate-extractable Fe of 20 or more.
- Contains appreciable Fe as well as Al. Such a podzolic B horizon (Bf or Bhf) must be at least 10 cm thick and have an organic C content of more than 0.5%. It contains 0.6% or more pyrophosphate-extractable Al+Fe in textures finer than sand and 0.4% or more in sands (coarse sand to very fine sand). The ratio of pyrophosphate-extractable Al+Fe to clay (<2 µm) is more than 0.05. 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.
Solonetzic B horizon - The term includes both Bn and Bnt horizons. These horizons have prismatic or columnar primary structure that breaks to blocky secondary structure; both structural units have hard to extremely hard consistence when dry. The ratio of exchangeable Ca to Na is 10 or less.
Vertic horizon - See definition of "v".
Lithic layer - This layer is consolidated bedrock (R) within the control section below a depth of 10 cm. The upper surface of a lithic layer is a lithic contact.
Mull - This form of zoogenous, forest humus consists of an intimate mixture of well-humified organic matter and mineral soil with crumb or granular structure that makes a gradual transition to the horizon underneath. Because of the activity of burrowing microfauna, (mostly earthworms), partly decomposed organic debris does not accumulate as a distinct layer (F layer) as in mor and moder. The organic matter content is usually 5-25% and the C:N ratio 12-18. It is a kind of Ah horizon.
Organic horizons occur in Organic soils and commonly at the surface of mineral soils. They may occur at any depth beneath the surface in buried soils or overlying geologic deposits. They contain more than 17% organic C (about 30% or more organic matter) by weight. Two groups of these horizons are recognized, the O horizons (peat materials) and the L, F, and H horizons (folic materials).
O - This organic horizon is developed mainly from mosses, rushes, and woody materials. It is divided into the following subhorizons:
- Of - This O horizon consists largely of fibric materials that are readily identifiable as to botanical origin. A fibric horizon (Of) has 40% or more of rubbed fiber by volume and a pyrophosphate index of 5 or more. If the rubbed fiber volume is 75% or more, the pyrophosphate criterion does not apply. Fiber is defined as the organic material retained on a 100-mesh sieve (0.15 mm), except for wood fragments that cannot be crushed in the hand and are larger than 2 cm in the smallest dimension. Rubbed fiber is the fiber that remains after rubbing a sample of the layer about 10 times between the thumb and forefinger. Fibric material usually is classified on the von Post scale of decomposition as class 1 to class 4. Three kinds of fibric horizons are named. Fennic horizons are derived from rushes, reeds, and sedges. Silvic horizons are derived from wood, moss with less than 75% of the volume being Sphagnum spp., and other herbaceous plants. Sphagnic horizons are derived from sphagnum mosses.
- Om - This O horizon consists of mesic material, which is at a stage of decomposition intermediate between fibric and humic materials. The material is partly altered both physically and biochemically. It does not meet the requirements of either a fibric or a humic horizon, has a rubbed fiber content ranging from 10% to less than 40%, and has a pyrophosphate index of >3 and <5. Mesic material usually is classified on the von Post scale of decomposition as class 5 or 6.
- Oh - This O horizon consists of humic material, which is at an advanced stage of decomposition. The horizon has the lowest amount of fiber, the highest bulk density, and the lowest saturated water-holding capacity of the O horizons. It is very stable and changes little physically or chemically with time unless it is drained. The rubbed fiber content is less than 10% by volume and the pyrophosphate index is 3 or less. Humic material usually is classified on the von Post scale of decomposition as class 7 or higher and rarely as class 6.
The methods of determining the properties of fibric, mesic, and humic materials are outlined later in this chapter.
- Oco - This material is coprogenous earth, which is a limnic material that occurs in some Organic soils. It is deposited in water by aquatic organisms such as algae or derived from underwater and floating aquatic plants subsequently modified by aquatic animals.
L, F, and H - These organic horizons developed primarily from the accumulation of leaves, twigs, and woody materials with or without a minor component of mosses. They are normally associated with upland forested soils with imperfect drainage or drier.
- L - This organic horizon is characterized by an accumulation of organic matter in which the original structures are easily discernible.
- F - This organic horizon is characterized by an accumulation of partly decomposed organic matter. Some of the original structures are difficult to recognize. The material may be partly comminuted by soil fauna as in moder, or it may be a partly decomposed mat permeated by fungal hyphae as in mor.
- H - This organic horizon is characterized by an accumulation of decomposed organic matter in which the original structures are indiscernible. This horizon differs from the F by having greater humification due chiefly to the action of organisms. It is frequently intermixed with mineral grains, especially near the junction with a mineral horizon.
Named layers and materials of Organic soils
Fibric, mesic, and humic materials were defined under Of, Om, and Oh. Some typical physical properties of fibric, mesic, and humic materials are listed below (Boelter 1969).
|Fibric material||Mesic material||Humic material|
|bulk density (Mg m-3)||<0.075||0.075-0.195||>0.195|
|total porosity (% vol)||>90||90-85||<85|
|0.01 M Pa H2O content (% vol)||<48||48-70||>70|
|hydraulic conductivity (cm hr-1)||>6||6-0.1||<0.1|
Limnic layer - This is a layer or layers, 5 cm or more thick, of coprogenous earth (sedimentary peat), diatomaceous earth, or marl. Except for some of the coprogenous earths containing more than 30% organic matter, most of these limnic materials are inorganic.
Coprogenous earth is composed of aquatic plant debris modified by aquatic animals. It makes slightly viscous water suspensions and is slightly plastic but not sticky. The material shrinks upon drying to form clods that are difficult to rewet and commonly crack along horizontal planes. It has very few or no plant fragments recognizable to the naked eye, a pyrophosphate index of 5 or more, and a dry color value of less than 5. The cation exchange capacity (CEC) is less than 240 cmol kg-1. It is designated Oco in horizon descriptions.
Diatomaceous earth is composed mainly of the siliceous shells of diatoms. It has a matrix color value of 4±1, if not previously dried, that changes on drying to the permanent, light gray or whitish color of diatoms. The diatom shells can be identified by microscopic (440 x) examination. Diatomaceous earth has a pyrophosphate index of 5 or more. It is frequently more nearly mineral than organic in composition. It is designated C in horizon descriptions.
Marl is composed of the shells of aquatic animals and CaC03 precipitated in water. It has a moist color value of 6±1 and effervesces with dilute HCI. The color of the matrix usually does not change on drying. Marl contains too little organic matter to coat the carbonate particles. It is designated Ck in horizon descriptions.
Cumulic layer-This is a layer or layers of mineral material in Organic soils. Either the combined thickness of the mineral layers is more than 5 cm or a single mineral layer 5-30 cm thick occurs. One continuous mineral layer more than 30 cm thick in the middle or bottom tier is a terric layer.
Terric layer-This is an unconsolidated mineral substratum not underlain by organic matter, or one continuous unconsolidated mineral layer (with 17% or less organic C) more than 30 cm thick in the middle or bottom tiers underlain by organic matter, within a depth of 1.6 m from the surface.
Lithic layer-This is a consolidated mineral layer (bedrock) occurring within 10-160 cm of the surface of Organic soils.
Hydric layer-This is a layer of water that extends from a depth of not less than 40 cm from the organic surface to a depth of more than 1.6 m.
Tests for distinguishing organic layers
Unrubbed and rubbed fiber See methods 2.81 and 2.82 in Manual on Soil Sampling and Methods of Analysis (McKeague 1978).
Pyrophosphate index Place 1g of sodium pyrophosphate in a small plastic, screw-topped container, add 4 ml of water and stir. With a syringe measure a 5 cm3 sample of moist organic material as in method 2.81 and place it in the plastic container, stir, and let stand overnight. Mix the sample thoroughly the next day. Using tweezers insert one end of a strip of chromatographic paper about 5 cm long vertically into the suspension. With the screw top in place to avoid evaporation, let the paper strip stand in the suspension until it is wetted to the top. Remove the paper strip with tweezers, cut off and discard the soiled end, and blot the remainder of the strip on absorbent paper. Read the value and chroma of the strip using good illumination and viewing the strip through the holes in the Munsell chart. The pyrophosphate index is the difference between the Munsell value and chroma of the strip.
von Post scale of decomposition
In this field test squeeze a sample of the organic material within the closed hand. Observe the color of the solution that is expressed between the fingers, the nature of the fibers, and the proportion of the original sample that remains in the hand. Ten classes are defined as follows:
1 - Undecomposed; plant structure unaltered; yields only clear water colored light yellow-brown.
2 - Almost undecomposed; plant structure distinct; yields only clear water colored light yellow-brown.
3 - Very weakly decomposed; plant structure distinct; yields distinctly turbid brown water, no peat substance passes between the fingers, residue not mushy.
4 - Weakly decomposed; plant structure distinct; yields strongly turbid water, no peat substance escapes between the fingers, residue rather mushy.
5 - Moderately decomposed; plant structure clear but becoming indistinct; yields much turbid brown water, some peat escapes between the fingers, residue very mushy.
6 - Strongly decomposed; plant structure somewhat indistinct but clearer in the squeezed residue than in the undisturbed peat; about one-third of the peat escapes between the fingers, residue strongly mushy.
7 - Strongly decomposed; plant structure indistinct but recognizable; about half the peat escapes between the fingers.
8 - Very strongly decomposed; plant structure very indistinct; about two-thirds of the peat escapes between the fingers, residue almost entirely resistant remnants such as root fibers and wood.
9 - Almost completely decomposed; plant structure almost unrecognizable; nearly all the peat escapes between the fingers.
10 - Completely decomposed; plant structure unrecognizable; all the peat escapes between the fingers.
Rules concerning horizon and layer designations
- Do not use the uppercase letters A, B, and O singly for horizons in pedon descriptions, but accompany them by a lowercase suffix (e.g., Ah, Bf, or Om) indicating the estimated nature of the modification of the horizon from the parent material. The horizon and layer designations L, F, H, R, and W may be used alone, and the horizon designation C may be used alone except when the material is affected by reducing conditions (Cg), cementation (Cc), salinity (Cs or Csa), CaCO3, (Ck or Cca), or permafrost (Cz).
- Unless otherwise specified, additional lowercase suffixes indicate a feature or features in addition to those characteristic of the defined main horizon. For example, the symbol Btg indicates that in addition to illuvial clay in the B horizon there is evidence of strong gleying. Some combinations such as Bmj are not used. In some cases, such as Bgf and Bhf, the combination of suffixes has a specific meaning that differs from the sum of the two suffixes used singly.
- All horizons except A and B, and B and A may be vertically subdivided by consecutive numeral suffixes. The uppermost subdivision is indicated by the numeral 1; each successive subdivision with depth is indicated by the next numeral. This convention is followed regardless of whether or not the horizon subdivisions are interrupted by a horizon of a different character. For example, an acceptable subdivision of horizons would be Ae1, Bf, Ae2, Bt1, Bt2, C1, C2. In some instances it may be useful for sampling purposes to subdivide a single horizon, for example, Bm1-1, Bm1-2, Bm1-3.
- Roman numerals are prefixed to the contrasting master horizon or layer designation (A, B, C) to indicate lithological discontinuities either within or below the solum. The first, or uppermost, material is not numbered, because the Roman numeral I is understood; the second contrasting material is designated II, and the others are numbered consecutively, with depth. Thus, for example, a sequence from the surface downward might be Ah, Bm, IIBm, IICa, IICk, IIICk.
Lithological discontinuity is due to a different mode of deposition, indicated by strongly contrasting textures (differing by two textural classes), or to a different mineralogical composition, indicating a difference in the material from which the horizons have formed. These contrasting materials have resulted form geologic deposition rather than pedogenic processes.
A change in the clay content associated with a Bt horizon (textural B) does not indicate a difference in parent material. The appearance of gravel, or a change in the ratio between the various sand separates, normally suggests a difference in parent materials. A different Roman numeral would not normally be needed for a buried soil, because the symbol (b) would be used. A stone line usually indicates the need for another Roman numeral. The material above the stone line is presumed to be transported. If transport was by wind or water, it is likely that during movement, material was sorted according to size.
All O horizons, which have developed from peat materials in a wetland environment, are considered to have resulted from only one mode of deposition. The same principle applies to L, F, and H horizons, which have developed from folic materials in a dominantly forest system. These horizons (O, L, F, and H) should not be designated as contrasting, even if they differ in botanical composition or degree of decomposition.
In some cases it is not necessary to use Roman numerals to show strongly contrasting horizons, for example if the horizon symbol already indicates the difference. Roman numerals are not required if the soil is composed of peat materials overlain by folic materials and underlain by mineral soil (L, F, Om, Oh, C) or if a mineral soil has a folic or peaty surface layer (L, F, Bm, BC, C; or Om, Ahg, Cg).
- For transitional horizons uppercase letters are used as follows:
If the transition is gradual, use AB, BC, etc.
If the horizons are interfingered in the transitional zone, use A and B, B and C, etc.
The dominance of horizons in the transitional zone may be shown by order, AB or BA, etc. Lower case suffixes may also be added in some instances, e.g., ABg, ABgj, etc.
- The designations for diagnostic horizons must be given in the sequence shown in the horizon definitions, e.g., Ahe not Aeh.
- Where j is used, the suffix or suffixes that it modifies are written after other horizon suffixes, e.g., Btnj, Bntj, Bfjtj, Bfcjgj.
Although definitions are given for all horizon symbols, all possible combinations of horizon designations have not been covered and all horizons having the same designation do not have identical properties. Therefore horizon descriptions are necessary.
Need for precise definitions of horizons and layers
In many cases the definitions of soil horizons may seem almost pedantically specific. For example, the suffix "t" indicates a horizon enriched with silicate clay. However, a Bt horizon must have a clay content exceeding that of the overlying eluvial horizon by specified amounts depending upon texture. For example, if the clay content of the Ae is 10%, that of the Bt must be 13% or more; if the clay content of the Ae is 40%, that of the Bt must be 48% or more. Also a Bt horizon must have a thickness that meets specified limits and clay skins on ped surfaces or oriented clay in pores.
Some B horizons that are slightly enriched with silicate clay are not Bt horizons. For example, two pedons X and Y have clay contents as follows: X: Ae-20%, B-22%, C-21%; Y: Ae-20%, B-25%, C-21%. If there is no parent material discontinuity in either pedon and both have B horizons more than 5 cm thick with clay skins on ped surfaces, the B horizon of pedon Y is a Bt, but that of pedon X is not. The two pedons would probably be closely similar if they were derived from similar materials in the same area, but they would be classified in different orders Luvisolic and Brunisolic) because one has a Bt horizon and the other does not. Yet the difference in the clay contents of the B horizons is only 3% and it could result from an analytical error. If the descriptions of the pedons indicated no difference in the development of B horizons, the particle size data would be checked. In most cases, clay skins would be thicker and more continuous in the B horizon of pedon Y than in that of pedon X.
From the point of view of the soil surveyor in the area, pedons X and Y are closely similar soils that belong in the same class even at the series level and certainly at the order level. However, for the soil taxonomist concerned with ordering the information on the whole population of soils in the country the classification of pedons X and Y in different orders is inevitable for two reasons. Soils have a continuum of properties, and specific limits are essential if soil taxonomy is to be applied in a uniform manner by users of the system. The classification of pedons X and Y in different orders does not imply that the use interpretations must be different nor that the pedons must be separated and delineated in mapping. This depends on the pattern of distribution of pedons X and Y and the scale of mapping. The indication that pedon X does not have a Bt horizon and that pedon Y does simply informs pedologists that the two B horizons have properties such that they are on opposite sides of the artificial line through the continuum of properties indicating the development of a horizon enriched in silicate clay. The alternatives of vague specifications of limits of diagnostic horizons or of relying on individual judgments lead to chaos in the ordering of soil information throughout the country.
Specific horizon definitions are based on a generalization of properties of soil horizons that are known to be representative of the main soil classes and reflect the kinds and degrees of soil development. Whenever possible, the specifications are based on observable or easily measurable properties. These horizon definitions are modified as the knowledge of soils increases and as concepts change. Because of the lack of sufficient knowledge, some soil horizons may not be defined adequately.
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