Soil taxonomy in Canada is based on properties of the soil.
Therefore, there is a need to define soil (as opposed to
nonsoil) and the unit of soil being classified. These and
the equally basic definitions of soil horizons are given in
this chapter.
Bodies of soil and nonsoil occur as a continuum at the surface
of the earth. They merge into one another often at
imperceptible though arbitrarily defined boundaries. Soil
is defined herein as the naturally occurring, unconsolidated
mineral or organic material at least 10 cm thick that occurs
at the earth's surface and is capable of supporting plant
growth. In this definition "naturally occurring"
includes disturbance of the surface by activities of man such
as cultivation and logging but not displaced materials such as
gravel dumps and mine spoils. Unconsolidated material includes
material compacted or cemented by soil-forming processes. Soil
extends from the earth's surface through the genetic horizons,
if present, into the underlying material to the depth of the
control section. Soil may have water covering its surface to
a depth of 60 cm or less either at low tide in coastal areas
or during the driest part of the year in areas inland. A soil
covered by a veneer of new material at least 50 cm thick is
considered to be a buried soil. Soil development involves
climatic factors and organisms, as conditioned by relief and
hence water regime, acting through time on geological
materials and thus modifying the properties of the parent
material.
Nonsoil is the aggregate of surficial materials that do not
meet the preceding definition of soil. It includes soil
materials displaced by unnatural processes such as dumps
of earth fill, unconsolidated mineral or organic material
thinner than 10 cm overlying bedrock, exposed bedrock, and
unconsolidated material covered by more than 60 cm of water
throughout the year. Nonsoil also includes organic material
thinner than 40 cm overlying water.
The definitions reflect the fact that bodies of soil and
nonsoil have a continuum of properties. For example, the
thickness of soil material overlying bedrock might range
from 1 m at the base of a slope to 20 cm at midslope and
gradually thin out to exposed bedrock at the top. The exposed
bedrock is nonsoil, but the thickness of unconsolidated
material over bedrock that should qualify as soil is not
obvious. To avoid ambiguity and permit uniformity of
classification, an arbitrary depth limit of 10 cm is used.
Similarly, bodies of periodically submerged soil merge into
bodies of water in the natural landscape.
The pedon as defined by pedologists of the United States
(Soil Survey Staff 1975) serves as the basic unit of soil
in the Canadian classification system. It is the smallest,
three-dimensional unit at the surface of the earth that is
considered as a soil. Its lateral dimensions are 1 m if
ordered variation in genetic horizons can be sampled within
that distance or if these horizons are few and faintly
expressed. If horizons are cyclical or intermittent and are
repeated in a lateral distance of 2-7 m, the lateral dimensions
of the pedon are half the cycle (1-3.5 m). The vertical
dimension of the pedon is to the depth of the control
section. A contiguous group of similar pedons is called a
polypedon, which is indicated in Soil Taxonomy (Soil
Survey Staff 1975) as a unit of classification.
The pedon concept applies to the classification of all soils,
but its relevance to soils having cyclic variation, such as
Turbic Cryosols, is most apparent. Examples of pedons are
illustrated (Figs.
1,
2,
and
3).
In
Figure 1
the profiles
beneath the nonsorted circles and the intercircle material
differ markedly. However, the variation is cyclic and occurs
repeatedly over the landscape. In the case of circles, the
patterned ground unit is delimited by the trough that forms
the perimeter of the circle. The diameter of the circle is
measured from the midpoint of the trough on one side of the
circle to its midpoint on the other side. If this diameter
is no greater than 2 m, the full cycle forms the pedon
(Fig. 1).
All the variability within the pedon is included
in the concept of the taxonomic class used from order to
series. In this case classification would be based upon the
properties of the intercircle material as it is dominant in
extent. If the circles were further apart such that a full
cycle was 2-7 m in diameter, the pedon would include half a
cycle. Thus it would extend from the midpoint of a circle to
the midpoint of the trough or intercircle material. If the
circles were still further apart, such that the lateral
dimension of the cycle was greater than 7 m, two pedons would
be identified. Since this is common with ice-wedge polygons,
one pedon associated with the ice-wedge polygon trench or
trough and the other associated with the central part of
the ice-wedge polygon or circle would be identified.
Figure 1: Pedon of Orthic Turbic
Cryosol in area of nonsorted circles
Cyclic variation involving tonguing of Ah horizon material
into the IICk horizon (see Soil Horizons and Other Layers)
is shown in
Figure 2.
The full cycle has lateral dimensions
of about 1 m or less so that the pedon includes a full cycle.
All the variability in thickness of the Ah is included in the
concept of the soil series.
Figure 2: Pedon of Gleyed Vertic
Black Chernozem with tonguing Ah horizon
The pedon is half of the cycle in the example of hummocky
terrain resulting from the blowdown of trees shown in
Figure 3.
In such cases the hummocks are usually not
regularly distributed and the dimensions of the pedon may
vary appreciably over short distances.
Figure 3: Pedon of Orthic Humo-Ferric
Podzol, turbic phase, in hummocky terrain due to blowdown
of trees
The control section is the vertical section of soil upon
which classification is based. It is necessary to provide a
uniform basis for soil classification. In general, pedons
should be sampled at least to the depth of the control section.
The properties of the material beneath the control section are
important for many interpretive purposes. Therefore, the
underlying material should be examined and its properties
recorded whenever possible.
For mineral soils in general, the control section extends
from the mineral surface either to 25 cm below the upper
boundary of the C, IIC, or permafrost table, or to a depth
of 2 m, whichever is less. Exceptions are as follows:
- If the upper boundary of the C or IIC is less than 75 cm
from the mineral surface, the control section extends
to a depth of 1 m.
- If bedrock occurs at a depth of 10 cm or more but less
than 1 m, the control section extends from the surface
to the lithic contact.
- If permafrost occurs at a depth of less than 1 m and
the soil does not show evidence of cryoturbation
(Static Cryosol), the control section extends to a
depth of 1 m.
A. Organic Order
The control section for Fibrisols, Mesisols, and Humisols
extends from the surface either to a depth of 1.6 m or to a
lithic contact. It is divided into tiers, which are used in
classification. The tiers are layers based upon arbitrary
depth criteria.
The surface tier is 40 cm thick exclusive of loose litter,
crowns of sedges and reeds, or living mosses. Mineral soil
on the surface of the profile is part of the surface tier,
which is used to name the soil family. Shallow lithic organic
soils may have only a surface tier.
The middle tier is 80 cm thick. It establishes the great
group classification if no terric, lithic, or hydric substratum
is present. Otherwise the dominant kind of organic material in
this and the surface tier establishes the great group
classification. The nature of the subdominant organic
material in the middle or bottom tier assists in establishing
the subgroup classification.
The bottom tier is 40 cm thick. The material in this tier
establishes in whole or in part the subgroup classification.
The control section for Folisols is the same as that used for
mineral soils. These soils must have more than 40 cm of folic
materials if they overlie mineral soils or peat materials, or
at least 10 cm if they overlie a lithic contact or fragmental
materials.
B. Organic Cryosol Great Group
The control section for Organic Cryosols extends to a
depth of 1 m or to a lithic contact. No tiers are defined.
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 11 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 contain 17% or less organic C (about 30%
organic matter) by weight.
| 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. |
| 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). |
| 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 Mohsí scale are
considered to be C horizons. |
| This consolidated bedrock layer is too hard to break
with the hands (>3 on Mohsí 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. |
| 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. |
| A buried soil horizon. |
| A cemented (irreversible) pedogenic
horizon. Ortstein, placic and duric horizons of
Podzolic soils, and a layer cemented by
CaCO3 are examples. |
| 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. |
| Cemented (irreversible) pedogenic
concretions. |
| 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). |
| 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:
- 0.5-5% organic C.
- 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. |
| 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.
- This horizon must meet the definitions of A, e,
and g.
- 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:
- 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.
- Colors indicated in (i) and a change in structure
from that of the C horizon.
- 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.
- Colors indicated in (i) and the removal of
carbonates. Bg horizons occur in some Orthic Humic
Gleysols and some Orthic Gleysols.
- When used in any of these combinations, the limits
set for f, hf, t, and others must be met.
- 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 13 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.
- 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. |
| 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).
- 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.
- 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.
- This horizon contains more than 1% organic C, less
than 0.3% pyrophosphateextractable 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.
- Defined under f. |
| 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.
- It denotes an eluvial horizon that is thin,
discontinuous, or slightly discernible.
- It is a horizon with some illuviation of clay but
not enough to meet the limits of Bt.
- These horizons are mottled but do not meet the
criteria of Bg.
- 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.
- In these horizons the development of solonetzic B
properties is evident but insufficient to meet the
limits for Bn or Bnt.
- In this horizon argillipedoturbation is evident
but the disruption of other horizons is insufficient
to severely alter them. |
| Denotes the presence of carbonate as
indicated by visible effervescence when dilute HCl
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). |
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.
|
| 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. |
| A horizon disturbed by manís activities
such as cultivation, logging, and habitation. It is
used with A and O. |
| 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. |
| 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. |
| 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 20ñ60 degrees from the
horizontal. Slickensides often intersect, resulting
in the formation of wedge shaped aggregates that
commonly occur in these soils. |
| 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.
- 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.
- Defined under j and g. |
| 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. |
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.
|
| A horizon of fragipan character. See
definition of fragipan. |
| 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. |
| A frozen layer. It may be used with any
horizon or layer, e.g., Ohz, Bmz, Cz, Wz. |
- 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 singlegrained 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 0°C 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.
- 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 40ñ80 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.
- 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.
- 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.
- 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.
- 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.
Not all Bh, Bhf, and Bf horizons are podzolic B horizons
because a podzolic B has a thickness requirement whereas Bh,
Bhf, and Bf horizons do not.
- 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.
- See definition of "v"
- 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.
- 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 17 these horizons
are recognized, the O horizons (peat materials) and the L, F,
and H horizons (folic materials).
- This organic horizon is developed mainly from mosses,
rushes, and woody materials. It is divided into the following
subhorizons:
- 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.
- 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.
- 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.
- 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.
- 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.
- This organic horizon is characterized by an accumulation
of organic matter in which the original structures are easily
discernible.
- 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.
- 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.
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).
| |
| 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
CaCO3 precipitated in water. It has a moist
color value of 6±1 and effervesces with dilute HCl. 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.
See methods 2.81 and 2.82 in Manual on Soil Sampling and
Methods of Analysis (McKeague 1978).
Place 1 g 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.
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:
- Undecomposed; plant structure unaltered; yields only clear
water colored light yellow-brown.
- Almost undecomposed; plant structure distinct; yields
only clear water colored light yellow-brown.
- Very weakly decomposed; plant structure distinct; yields
distinctly turbid brown water, no peat substance passes
between the fingers, residue not mushy.
- Weakly decomposed; plant structure distinct; yields
strongly turbid water, no peat substance escapes between
the fingers, residue rather mushy.
- Moderately decomposed; plant structure clear but becoming
indistinct; yields much turbid brown water, some peat
escapes between the fingers, residue very mushy.
- 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.
- Strongly decomposed; plant structure indistinct but
recognizable; about half the peat escapes between the
fingers.
- 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.
- Almost completely decomposed; plant structure almost
unrecognizable; nearly all the peat escapes between
the fingers.
- Completely decomposed; plant structure unrecognizable;
all the peat escapes between the fingers.
- 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.
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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