Geoderma 209–210 (2013) 153–160
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Geoderma
journal homepage: www.elsevier.com/locate/geoderma
Distribution and classification of soils with clay-enriched horizons in the USA
J.G. Bockheim ⁎, A.E. Hartemink
Department of Soil Science, University of Wisconsin, Madison, WI 53706-1299, USA
a r t i c l e
i n f o
Article history:
Received 3 August 2012
Received in revised form 6 June 2013
Accepted 15 June 2013
Available online xxxx
Keywords:
Argillic horizon
Natric horizon
Kandic horizon
Alfisols
Ultisols
Argids
a b s t r a c t
In Soil Taxonomy three diagnostic subsurface horizons reflect clay enrichment: the argillic, kandic, and natric
horizons. Clay illuviation is recognized in Soil Taxonomy at some level in 10 of the 12 orders, including the
order (Alfisols, Ultisols), suborder (Aridisols), great group (Aridisols, Gelisols, Mollisols, Oxisols, Vertisols),
and subgroup (Andisols, Aridisols, Inceptisols, Mollisols, Oxisols, Spodosols). Forty-four percent of the soil
series in the USA contain taxonomically defined clay-enriched horizons. However, many other soils contain
Bt horizons that do not qualify as an argillic or related horizons. Several soil-forming factors are important
in their development, including udic and ustic soil climates, lithological discontinuities, parent materials
enriched in carbonate-free clays and coarse fragments, well-drained conditions, backslopes rather than
eroding shoulders, and a time interval of N 2000 yr or more. The genesis of argillic, kandic, and natric horizons
is also dependent on electrolyte concentration, the amount and distribution of precipitation, clay charge, and
microfabric.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Nearly all classification systems recognize clay-enriched subsoils
at a high hierarchical level. Some of the most productive soils in the
World for food and fiber production have clay-enriched horizons.
Clay-enriched horizons are important for the nutrient status of soils,
water retention, and geomorphic stability (Hopkins and Franzen,
2003). In Soil Taxonomy (ST) (Soil Survey Staff, 2010), Alfisols and
Ultisols are defined on the basis of clay-enriched horizons and many
Aridisols and Mollisols have clay-enriched subsoils. Argillic and
related horizons have been particularly important in soil stratigraphy,
relative dating, pedodiversity studies, and climate-change research
(Eghbal and Southard, 1993; Frazmeier et al., 1985; Holliday and
Rawling, 2006; Karlstrom, 2000; Karlstrom et al., 2008; Kemp et al.,
1998; Othberg et al., 1997; Wilson et al., 2010).
Studies of clay-enriched horizons have been conducted in many
countries and regions, such as Russia (Fridland, 1958; Rode, 1964), the
United Kingdom (e.g., Avery, 1983), Eastern Europe (Bronger, 1991),
Australia (Walker and Chittleborough, 1986), Canada (Lavkulich and
Arocena, 2011), Argentina (Blanco and Stoops, 2007), and Iran
(Khademi and Mermut, 2003; Khormali et al., 2003, 2012). Birkeland
(1999) reviewed the genesis of soils with argillic and related horizons,
focusing on field and laboratory data, thin-section and scanning electron
microscope (SEM) analysis, and mass-balance studies. In summary,
clay-enriched subsoils are the result of translocation, in situ formation,
and relative loss of clay from the topsoil.
⁎ Corresponding author. Tel.: +1 6082635903; fax: +1 6082652595.
E-mail address: [email protected] (J.G. Bockheim).
0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.geoderma.2013.06.009
The objectives of this study are to (i) identify the soil taxa in
ST featuring clay enrichment; (ii) show the distribution of clayenriched soils in the USA with clay enrichment; (iii) discuss the
relative importance of the soil-forming factors on the development of
clay-enriched horizons; and (iv) compare and contrast the pedogenetic
processes involved in forming argillic, kandic, and natric horizons.
2. Historical overview of clay-enriched horizons
That fine soil particles moved through the soil profile was recognized as early as the late 1800s (King, 1895; Sibirtsev, 1900). The
importance of clay was stressed by Hilgard (1906), who reviewed
the physico-chemical properties in relation to soil development and
plant growth. At that time, no size boundary for these fine particles
was set and the fine soil particles were often referred to as colloids.
It was probably at the First International Congress of Soil Science in
1927 that the size limit for clay was set at 2 μm.
Merrill (1906) observed that in soils of humid regions colloidal
particles became partially diffused in rainwater, percolated through
the soil, and accumulated in the subsoil. He found that almost without
exception the subsoils of humid regions have much more clay than
the corresponding surface soils. As a result the subsoils are more
compact, heavier and less permeable. He also observed that clay
eluviation was sometimes accompanied by CaCO3 leaching which
could result in the formation of a hardpan. Merrill (1906) distinguished between soils of the humid regions where clay eluviation
takes place and soils of the drier regions where such processes are
absent. This climatic distinction on percolating water and its effect
on movement of soil particles were further developed by C.F. Marbut
and resulted in the distinction between pedalfers and pedocals
154
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
(Marbut, 1927). Wolfanger (1930) described pedalfers and named A
the horizon of maximum extraction and B the horizon of concentration. He wrote: “The extraction and concentration are brought about
in part through eluviation (the mechanical transfer of material), in
part by transfer through solution and reprecipitation (chemically)
and in part by both processes. Fine grained materials, clay and silt,
are mechanically transferred from the upper to the lower horizons.”
Robinson (1932) distinguished between two types of eluviaton:
mechanical eluviation in which, apart from any chemical differentiation, the finer fractions of the mineral portion of the soil are washed
down to lower levels, and chemical eluviation in which decomposition occurs and certain products thus liberated are translocated in
true or colloidal solution to be deposited in other horizons. Mechanical eluviation results in the development of a texture profile characterized by a light textured A horizon and a heavy textured B horizon
enriched by the finer material from the A horizon, and such soils are
common in the southeastern US (Robinson, 1932).
One of the first descriptions of the argillic horizon was by Joffe
(1936). He also considered the B as a horizon that is gaining instead
of losing as with the A horizon. The B horizon is therefore known as
the horizon of illuviation (washing in) or horizon of accumulation.
Joffe recognized that the fine particles were mechanically carried
from the A to the B horizon and that it will result in a more compact
horizon. The B was named an illuvial horizon and Joffe also postulated
the idea of new clay formations in the B horizon which enhances the
differences in clay content between the A and the B horizon.
The eluvial and illuvial horizon model was well-developed in the first
half of the 20th century. The migration processes were well-understood
and the concepts were integrated in the classification of horizons and in
the classification of the whole soil profile. The Bt horizon (t for ton,
German for clay) is now integrated in most soil and horizon classification
systems. The French developed the concept of the argillic horizon and
the formation of coatings (Duchaufour, 1998). Main characteristic of
the B horizon are coatings formed of fine colloidal particles deposited
and these have been termed cutans. Cutans can be amorphous
organomineral complexes termed organans or sesquioxide complexes
termed sesquans or cutans can be formed from crystalline clay minerals
laid down in parallel orientation in which they are called argillans. Such
argillans characterize the Bt horizon of argillic soils (Duchaufour, 1998).
An overview of the different horizons in Soil Taxonomy (2010) and their
approximate conceptual history is given in Table 1.
In the literature, there is some confusion in distinguishing between
the Bt horizon and diagnostic subsurface horizons featuring clay
enrichment. A Bt horizon “indicates an accumulation of silicate clay
that either has formed within a horizon or has been moved into the
horizon by illuviation, or both” (Soil Survey Staff, 2010, p. 318). The
definition further states: “at least part of the horizon should show
evidence of clay accumulation either as coating on surfaces of peds
or in pores, as lamellae, or as bridges between mineral grains.” However, not all Bt horizons meet the thickness or depth-distribution of
clay requirements of diagnostic subsurface horizons with clay enrichment (see below).
In Soil Taxonomy (ST), the argillic horizon is a subsurface horizon that
contains “a significantly higher percentage of phyllosilicate clay than the
overlying soil material” and “shows evidence of clay illuviation” (p. 10).
The thickness requirement ranges between 7.5 and 15 cm, depending
on the particle-size class. There must be evidence of clay illuviation in
at least one of the following forms: (i) oriented clay bridging sand grains,
(ii) clay films lining pores, (iii) clay films on both vertical and horizontal
surfaces of peds, or (iv) thin sections with oriented clay bodies that comprise more than 1% of the section. In addition to a thickness requirement
and evidence for clay illuviation, the argillic horizon must have a greater
amount of clay than an overlying eluvial horizon; the amount of clay depends on the clay content of the eluvial horizon and ranges from at least
3% (absolute) for eluvial horizons with b15% clay to at least 8% (absolute)
for eluvial horizons with N 40% clay.
The kandic horizon is a subsurface horizon defined in ST on the
basis of its thickness (minimum of 15 to 30 cm, depending on soil
depth), the depth interval at which the clay increases from an overlying eluvial horizon (50 to 200 cm), the amount of clay increase from
an overlying eluvial horizon, an apparent cation-exchange capacity
(CEC) of b 16 cmol(+)/kg clay (by 1 M NH4OAc, pH 7), and an apparent effective CEC of b 12 cmol(+)/kg clay (sum of bases extracted
with 1 M NH4OAc, pH 7, plus 1 M KCl-extractable Al). The amount
of clay increase ranges from 4% (absolute) for eluvial horizons with
b20% clay to at least 8% (absolute) for eluvial horizons with N40%
clay. It is noteworthy that the kandic horizon does not require
evidence for clay illuviation.
In ST the natric horizon is comparable to the argillic horizon except
that it shows evidence of accelerated clay illuviation by the dispersive
properties of Na. The natric horizon has a thickness requirement (7.5
to 15 cm) and evidence for clay illuviation and a clay increase from an
overlying eluvial horizon that are comparable to the argillic horizon.
In addition, the natric horizon must have either a columnar or a prismatic structure in some part and an exchangeable Na percentage of
15% or more.
In addition to diagnostic subsurface horizons, there are two diagnostic soil characteristics that reflect clay movement: (i) abrupt
textural change and (ii) lamellae. An abrupt textural change is “a
specific kind of change that may occur between an ochric or an
albic horizon and an argillic horizon” (Soil Survey Staff, 2010, p. 15)
and is characterized by a considerable increase in clay content within
a very short vertical distance. In Australia such soils are called
“duplex” and “texture-contrast” soils. A lamella is defined as “an
Table 1
Soil textural horizons and their approximate history and current definition in Soil Taxonomy.
Horizon
History
Current definition in Soil Taxonomy (abridged)
Bt
Part of the B2 horizon (“zone of accumulation” or “zone of compaction”) until
1951. The term was included in the 1951 edition of the Soil Survey Manual.
However, it does not appear to have been used widely in Europe (Kubiëna, 1950);
in the US Baur and Lyford (1957) used “t” to designate clay accumulation in some
New England soils. Once the 7th Approximation (Soil Survey Staff, 1960) was
published, the term experienced widespread use.
Included in the 7th Approximation in 1960 (Soil Survey Staff, 1960). However,
it doesn't appear to have been used in ASA-SSSA-CSSA publications until 1964,
when Harpstead and Rust (1964) used the term for some Alfisols in Minnesota,
USA. “The Supplement to Soil Classification” was added in 1967, and the term
became widely used shortly thereafter.
Included in the 7th Approximation in 1960 (Soil Survey Staff, 1960). However,
it does not appear to have been used in ASA-SSSA-CSSA publications until 1974,
when Sharma et al. (1974) used the term for a Natraqualf in Illinois (USA).
An accumulation of silicate clay that has formed within a horizon
or and has subsequently has been translocated within the horizon
or has been moved into the horizon by illuviation, or both. Evidence
of clay accumulation by coatings on ped, lamellae, or as bridges
between mineral grains.
Argillic
Natric
Kandic
Introduced in Soil Taxonomy between 1985 and 1987 and first appeared in
the 3rd edition of Keys to Soil Taxonomy.
A subsurface horizon with a significantly higher percentage of phyllosilicate
clay than the overlying soil material. It shows evidence of clay illuviation.
An illuvial horizon that is normally present in the subsurface and has a
significantly higher percentage of silicate clay than the overlying horizons.
Evidence of clay illuviation that has been accelerated by the dispersive
properties of sodium.
Subsurface horizon that is dominated by low activity clays and underlying a
coarse textured surface horizon.
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
illuvial horizon less than 7.5 cm thick … which contains an accumulation of oriented silicate clay on or bridging sand and silt grains…”
and has more silicate clay than the overlying eluvial horizon (Soil
Survey Staff, 2010, p. 18). Lamellae are often formed in dunes when
the sand contains small amounts of clay, but are not restricted to
these conditions.
In the World Reference Base for Soil Resources (IUSS Working
Group WRB, 2006), clay illuviation is recognized in the argic horizon,
which is a subsurface horizon with distinctly higher clay content
than the overlying horizon. The textural differences may be caused
by illuviation, neoformation of clay in the subsoil, destruction and
erosion of clay in the topsoil, upward movement of coarse particles,
biological activity, or a combination of these processes. Diagnostic
criteria include a texture of loamy sand or finer and 8% or more clay
in the fine earth fraction, but this depends on the clay content of
the overlying horizon and the thickness of the soil. Textural differentiation is the main feature for recognition of argic horizons and the
illuvial clay may be observed using a hand-lens if clay skins occur
on ped surfaces, in fissures, in pores and in channels. Illuvial argic
horizon should show clay skins on at least 5% of the ped faces and
in the pores. According to WRB the best identification for an argic
horizon is by thin sections. Argic horizons are normally found below
eluvial horizons from which clay and Fe have been removed. Some
clay-increase horizons may have the properties that characterize the
ferralic horizon, i.e. a low CEC and effective CEC, a low content of
water-dispersible clay and a low content of weatherable minerals.
In WRB, argic horizons lack the sodium saturation characteristics of
the natric horizon. Argic horizons that occur in cool and moist, freely
drained soils of high plateaus and mountains in tropical and subtropical regions are often found in association with sombric horizons
(IUSS Working Group WRB, 2006). The nitic horizon is a special
type of argic horizon and also the natric horizon may have increased
clay content. Reference Soil Groups that may have an argic horizon in
WRB are Albeluvisols, Alisols, Acrisols, Luvisols, Lixisols, Chernozems,
Kastanozems, and Phaeozems (IUSS Working Group WRB, 2006).
3. Methods and materials
The diagnostic subsurface horizons and soil characteristics of clayenriched horizons were examined for each soil order within ST at
different taxonomic levels. Using the “Soil Classification Database”
and “Official Soil Descriptions” functions (http://soils.usda.gov/
technical/classification), a list was prepared of soil series showing
clay enrichment.
A map of soils containing taxonomically recognized clay-enriched
horizons was prepared using the July 5, 2006 version of the Digital General Soil Map of the U.S. published by the U.S. Department of Agriculture,
Natural Resources Conservation Service. This dataset consists of general
soil association units created by generalizing more detailed soil survey
maps. Since the taxonomic nomenclature for a map unit is recorded at
the component level and a map unit is typically composed of one or
more components, aggregation is needed to reduce a set of component
attribute values to a single value that will represent the map unit as
a whole. For taxonomic order, suborder and great group distribution maps, data were aggregated to the map-unit level using the
“dominant-component-aggregation” approach. This approach returns
the attribute value associated with the component with the highest percent composition in the map unit, which may or may not represent the
dominant condition throughout the map unit. For taxonomic subgroup
distribution maps, data were aggregated to the map-unit level using
the “presence method;” that is, if any component attribute matched
the taxonomic subgroup of interest then that map unit would be
shown on the map regardless of its map unit composition.
Using Thomson-Reuters Web of Knowledge, 311 publications
were examined on Bt horizons, 253 on argillic horizons, 28 on natric
horizons, and 13 on kandic horizons over the past ca. 50 years.
155
These publications were used to prepare tables summarizing the
role of soil-forming factors and the pedogenetic processes involved
in development of clay-enriched horizons.
4. Results
4.1. Soil taxa containing taxonomic clay enrichment
Clay illuviation is recognized in ST in 10 of the 12 orders (Table 2).
Clay illuviation does not occur in Histosols or in Entisols. Two of the
orders, Alfisols and Ultisols, require an argillic or kandic horizon
(or natric horizon, in the case of some Alfisols). In Aridisols, clay
enrichment is recognized at the suborder level (Argids) and at the
great-group (Argi-, Natri-) and subgroup (Argic, Natric, Ustalfic)
levels. In Mollisols argillic and natric horizons are recognized at the
great-group level (Argi-, Natr-) and at the subgroup level (Argic,
Natric). Soils of the Argiorthels great group in Gelisols contain an
argillic horizon. In Oxisols soils of the Kandiudox and Kandiustox
great groups and in Kandiudalfic subgroups contain kandic horizons.
There is one great group, the Natraquerts within the Vertisols,
which contains a natric horizon. Spodosols may contain argillic or
kandic-like horizons or lamellae, which are recognized only at the
subgroup level (Alfic, Ultic, Lamellic, etc.). Andisols contain Alfic and
Ultic subgroups (and occasionally Alfic Humic subgroups) within
nine great-groups. Clay illuviation also is recognized in Inceptisols
at the subgroup level (Lamellic Eutrudepts, Lamellic Haplustepts).
However, the lamellae are considered part of the cambic horizon
and not the argillic horizon.
4.2. Distribution of soils with clay enrichment
All of the Alfisols and Ultisols contain evidence of clay enrichment;
these soils contain 3984 and 1330 soil series and cover 1.2 million
and 0.82 million km2 of the USA, respectively (Table 3). The distribution of these soils in the USA is shown in Fig. 1. Nearly half of the soil
series in the Mollisol and Aridisol orders contain diagnostic horizons
or characteristics of clay enrichment. They comprise a large number
of soil series, 3534 and 1410, respectively, in the USA. Within the
Spodosols, there is a high number (161) and proportion (22%) of
soil series with an argillic or kandic-like horizon. Andisols contain
109 soil series with an argillic horizon, which constitutes 11% of the
total soil series in that order and an area of about 17,150 km2. The
remaining soil orders, Inceptisols, Oxisols, and Vertisols, each contain
eight soil series or less and collectively comprise about 4275 km2 in
the USA. A total of 44% of the soil series in the USA contain taxonomically defined soil horizons or characteristics reflecting clay illuviation. These soils account for an area of 4.5 million km2, which is 56%
of the area of conterminous USA.
5. Discussion
5.1. Soil-forming factors and clay-enriched horizons
Soils with argillic horizons occur in areas with pergelic, cryic, frigid, mesic, thermic, and hyperthermic soil-temperature regimes and in
areas with aquic, udic, ustic, xeric, and aridic soil-moisture regimes
(Table 4). However, Nettleton et al. (1975) suggested that the
clay-enriched horizon in many aridic environments was below the
current wetting zone; it may have been inherited from a moister
environment during pluvial periods (Eghbal and Southard, 1993;
Gile and Grossman, 1968; Khademi and Mermut, 2003; Khormali et
al., 2003). Moreover, studies examining argillic-horizon formation
across regional environmental gradients show that clay illuviation is
stronger in the humid portion than in the dry portion of the landscape
(Gunal and Ransom, 2006; Khormali et al., 2012; Rabenhorst and
Wilding, 1986a, 1986b). Along an elevational gradient in Nevada,
156
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
Table 2
Soil taxa with argillic, kandic, natric, and agric horizons.
Order
Alfisols
Andisols
Aridisols
Suborders Great groups
All
Cryands
Udands
Udands
Udands
Udands
Ustands
Vitrands
Xerands
All
Vitricryands
Fulvudands
Hapludands
Hydrudands
Melanudands
Haplustands
Udivitrands
Haploxerands
Xerands
Vitrixerands
Argids
Calcids
All
Petrocalcids
Cryids
Durids
Durids
Gypsids
Gypsids
Entisols
None
Gelisols
Orthels
Histosols
None
Inceptisols Udepts
Ustepts
Mollisols
Albolls
Albolls
Aquolls
Aquolls
Aquolls
Aquolls
Cryolls
Cryolls
Cryolls
Cryolls
Udolls
Udolls
Ustolls
Ustolls
Ustolls
Ustolls
Xerolls
Xerolls
Oxisols
Spodosols
Ultisols
Vertisols
a
Argicryids
Argidurids
Natridurids
Argigypsids
Natrigypsids
None
Argiorthels
None
Eutrudepts
Haplustepts
Argialbolls
Natralbolls
Argiaquolls
Cryaquolls
Duraquolls
Natraquolls
Argicryolls
Duricryolls
Natricryolls
Palecryolls
Argiudolls
Natrudolls
Argiustolls
Durustolls
Natrustolls
Paleustolls
Argixerolls
Durixerolls
Xerolls
Haploxerolls
Xerolls
Xerolls
Udox
Udox
Ustox
Ustox
Aquods
Natrixerolls
Palexerolls
Eutrudox
Kandiudox
Eutrustox
Kandiustox
Alaquods
Aquods
Aquods
Orthods
Orthods
Endoaquods
Epiaquods
Alorthods
Fragiorthods
Orthods
Haplorthods
All
Aquerts
All
Natraquerts
Subgroupsa
All
Alfic (12), Ultic (3)
Ultic
Ultic
Ultic
Ultic
Alfic, Ultic
Alfic (45), Ultic (2)
Alfic (0), Alfic Humic (5),
Ultic (0)
Alfic (29), Alfic Humic (13),
Ultic (0)
All
Argic (22), Natric (0),
Ustalfic (25)
All
All
All
All
All
None
All
None
Lamellic
Lamellic
All
All
All
Argic
Argic (2), Natric (4)
All
All
Argic
All
All
All
All
All
Natric
All
All
All
Argidic (8), Paleargidic (3),
Abruptic Argiduridic (13),
Argic (0)
Lithic Ultic (30), Cumulic Ultic
(19), Ultic (55), Aquultic (6),
Entic Ultic (12), Pachic Ultic (34)
All
All
Kandiudalfic
All
Kandiustalfic
All
Alfic Arenic (2), Arenic Ultic (2),
Alfic (6), Ultic (12)
Argic
Alfic (17), Ultic (2)
Alfic, Ultic, Arenic Ultic
Alfic (9), Alfic Oxyaquic (10),
Ultic (1)
Aqualfic (3), Alfic Oxyaquic (30),
Alfic (36), Lamellic (9),
Lamellic Oxyaquic (2), Ultic (1),
Oxyaquic Ultic (2)
All
Typic
Table 3
Proportion of soil series with argillic and related horizons within each order in the USA.
No.
series
3984
15
0
0
0
0
0
47
5
42
Order
Series with argillic
Total series
Proportion with argillic (%)
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Total
3984
109
1410
0
0
0
8
3534
7
161
1330
1
10,544
3984
1020
2826
2809
71
313
3054
7532
60
746
1330
482
24,227
100
11
50
0
0
0
0
47
12
22
100
0
44
1104
47
5
197
31
18
8
0
0
0
4
4
75
0
119
5
6
15
394
1
1
34
343
16
851
1
63
98
1164
24
156
23
133
0
3
3
1
22
17
19
0
20
83
1330
9
10,540
Number of soil series within a subgroup given in parentheses.
Parent material is a critical factor influencing clay illuviation
(Table 4). Argillic horizons tend to form more readily where water
is arrested at the contact between two lithological units (Bockheim,
2003; Cabrera_Martinez et al., 1989; Ogg and Baker, 1999; Shaw et
al., 2004), in stratified materials (Hopkins and Franzen, 2003), or
where a lithic or a paralithic layer is present (Aide et al., 2006;
Blanco and Stoops, 2007; Bruckert and Bekkary, 1992). Soils with
abundant coarse fragments often contain deeper argillic horizons,
especially in Ustolls, Ustalfs, and Argids (Gile and Grossman, 1968;
Rabenhorst and Wilding, 1986a, 1986b), possibly because water
containing suspended silicate clays is able to move downward more
readily in the profile. Bruckert and Bekkary (1992) described this as
the “rock effect.” There may be a direct correlation between the clay
content of the argillic horizon and the clay content of the soil parent
material (Frazmeier et al., 1985; Gile and Grossman, 1968; Smith
and Wilding, 1972). Although loess rejuvenation favors the development of argillic horizons (Mubiru and Karathanasis, 1994), calcareous
dust (Elliott and Drohan, 2009; Gile and Grossman, 1968) and calcareous parent materials (Smeck et al., 1968) may inhibit clay illuviation.
High exchangeable Na percentages may favor clay illuviation by
dispersing clays and enabling the formation of natric horizons
(Alexander and Nettleton, 1977). Several of the larger areas not
containing an argillic horizon in Fig. 1 occur in areas with thin drift
over granitic bedrock, including New England, the upper Great
Lakes region, and the Sierra Nevada Range.
In humid environments, argillic horizons are more strongly developed in well-drained soils than in soils with restricted drainage
(Cremeens and Mokma, 1986; Hopkins and Franzen, 2003) (Table 4).
In Vertisols under a thermic soil temperature regime, argillic horizons
are more strongly developed in micro-lows (Sobecki and Wilding,
1983). Argillic horizons are more strongly developed on backslopes
than on actively eroding shoulders (Olson et al., 2005; Wilson et al.,
2010; Young and Hammer, 2000).
In humid environments, argillic horizons require about 12,000 yr to
form (Table 4). However, in soils with ustic or aridic soil-moisture regimes, the argillic horizon develops in 9000 yr (Karlstrom, 2000;
Nettleton et al., 1975; Southard and Southard, 1985). Alexander and
Nettleton (1977) reported a Natrargid forming in as little as 6600 yr
in Nevada. Kandic horizons may require 1–2 million yr to form
(Alexander, 2010). However, Bt horizons may develop in as few as
2100 yr (Cremeens, 1995). These isolated studies do not necessarily
imply that argillic horizons form more rapidly in soils with a ustic or
aridic soil-moisture regime than in those with a udic regime (Gunal
and Ransom, 2006; Khormali et al., 2012; Rabenhorst and Wilding,
1986a, 1986b).
5.2. Genesis of clay-enriched horizons
argillic horizons formed more readily in udic and aridic–udic
soil-moisture regimes than in an aridic soil-moisture regime (Elliott
and Drohan, 2009).
The evidence for clay enrichment in argillic and related horizons
includes (i) clay skins or cutans on horizontal and vertical ped
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
157
Fig. 1. Distribution of soils with argillic, natric, and kandic horizons in conterminous USA.
faces, (ii) clay bridging sand grains, (iii) clay lining pores, (iv) an increase in clay from an overlying eluvial horizon that does not directly
reflect stratification or a lithologic discontinuity, (v) lamellae, (vi) a
wider fine clay:total clay ratio than in an overlying horizon,
(vii) thin sections with oriented clay bodies that are more than 1%
of the section, and (viii) a high coefficient of linear extensibility
(COLE) which enables shrinking and swelling clays (Soil Survey
Staff, 2010).
Birkeland (1999) recognized four processes that account for clay enrichment in argillic and related horizons: (i) translocation of clay from
eluvial to illuvial horizons; (ii) translocation of clay contributed by aeolian processes to illuvial horizons; (iii) weathering of silt-size or coarser
particles into clay-size material in situ; and (iv) synthesis of clays from
the soil solution, i.e., neoformation. Two addition processes include parent material stratification and preferential erosion of fine particles from
landform and ped surfaces (Walker and Chittleborough, 1986). There
are three mechanisms involved in formation of clay-enriched horizons,
including (i) dispersion, (ii) translocation, and (iii) accumulation
(Eswaran and Sys, 1979).
The argillic horizon requires decalcification in order for the clay
particles to become dispersed. A sufficient amount of water is
required to move the clay from the eluvial to the illuvial horizon.
For this reason, the argillic horizon is generally found in soils with
aquic, udic, ustic or xeric soil-moisture regimes (Table 5). Argillic
horizons are common in Aridisols, but most investigators in these regions attribute them to a previous moister climate (Table 4). Argillic
horizons commonly contain high-activity clays such as the smectites.
Members of this mineral group are readily broken down into fine
clays, which can be readily translocated through the soil; they have
a high COLE. The argillic horizon is manifested by a clay maximum
or “bulge” when the depth-distribution of clay is examined. Argillans
are common in argillic horizons except in those of aridic environments, where the clay skins may be destroyed by shrinking and
swelling (Gile and Grossman, 1968; Khormali et al., 2003; Nettleton
et al., 1969). Similarly, the upper part of argillic horizons in southeastern USA often lacks evidence of translocation because argillans
are destroyed by weathering and the clay that is released forms
clay films in the lower Bt and BC horizons (Brook and van
Schuylenborgh, 1975). Argillic horizons commonly show an increase
in the ratio of fine clay to total clay from an overlying eluvial horizon.
Although individual clay lamella do not qualify as argillic horizons,
they do qualify as argillic if the cumulative thickness is N15 cm
(e.g., Holliday and Rawling, 2006; Torrent et al., 1980).
Clay illuviation has been successfully reproduced in the laboratory. Bond (1986) created illuvial bands in a laboratory column of
sand, hypothesizing that band formation resulted from dispersion of
clay in the sand and its subsequent deposition, which was triggered
by layers of small pores within the sand column and/or by exceeding
the maximum possible suspension concentration. Gombeer and
D'Hoore (1971) induced migration of clay in the laboratory, reporting
that clay movement was dependent on soil/water dispersion ratio,
colloid stability, and “electrophoretic mobility”. Mel'nikova and
Kovenya (1971) used clay mineral particles irradiated by thermal
neutrons in a reactor to study the effects of chemical and physical
soil properties on clay illuviation. Large amounts of the irradiated
clay were translocated with a weakly acidic solution without destruction of eluvial horizons in podzols. The rate of clay translocation was
dependent on the density and sorption capacity of clay minerals and
was greater in the E horizon than in the B and C horizons. As the pH of
the leaching solution increased, so did the mobility of the particles,
which was attributed to an increase of the electrokinetic potential.
Gagarina and Tsyplenkov (1974) used open-top chambers containing
disturbed soil to study clay illuviation in the forest-steppe zone of Russia.
Clays became mobile 10 years after the beginning of the experiment
following dissolution of “microcryptogranular” carbonates. During
movement, clays filled all the cracks and fine pores within aggregates
158
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
Table 4
Relation of soil-forming factors and argillic horizons.
Area
Soil taxa
Role of soil-forming factor
Citation
Climate
Conterminous USA
Conterminous USA
All with argillic
All with argillic
NRCS
NRCS
Mojave desert, CA
KS
Typic Haplargids
Paleudolls, Paleustolls
NV
NM
Edwards Plateau, TX
Aridic Argiustolls
Haplargids, Paleargids
Calciustolls, Haplustalfs
Argillic occurs in aquic, udic, ustic, xeric, aridic smrs
Argillic occurs in pergelic, cryic, frigid, mesic, thermic,
hyperthermic strs
Argillic relict from moister mid-Pleistocene climate
Argillic more developed where MAP is 750–1100 mm/yr
than 400–500 mm/yr
Argillic forming today in aridic–udic, high montane environment
Prominent argillic horizons occur only in Pleistocene-age soils
Argillic more strongly developed in humid eastern than dry western
Iran
Haploxeralfs, Haplustalfs, Argixerolls, Argiustolls
Iran
Iran
CA, NV, AZ
Argids
Hapludalfs, Haploxeralfs, Argixerolls
Haplargids, Paleargids
Parent material
Ozarks, MO
Northern WI, MI
Pampas, Argentina
Fragiudults
Alfic Haplorthods, Alfic Oxyaquic Haplorthods
Petrocalcic Paleudolls
France
Typic/Glossic Hapludalfs/Fragiudalfs
NV
NM
Aridic Argiustolls
Haplargids, Paleargids
Coarse fragment content of parent material
ND
Hapludolls
Edwards Plateau, TX
Calciustolls, Haplustalfs
KY
CA, NV, AZ
VA
GA
OH
FL coastal plain
IN
Hapludults, Paleudults, Paleudalfs
Haplargids, Paleargids
Hapludalfs, Paleudalfs
Kandiudults, Paleudults
Hapludalfs
Hapludults, Paleudults
[not provided]
MI, OH
Epiaqualfs, Hapludalfs
Relief
ND
Hapludolls
TX coast prairie
Southern IL
MO
Calciaquolls, Argiaquolls,
Vermaqualfs, Glossaqualfs
Hapludalfs, Epiaqualfs
[Not provided]
IL
Hapludalfs
MI
Hapludalfs
Time
IL
MT
[Not provided]
Paleudolls, Paleustolls
WV
CA, NV, AZ
ID
UT
NV
Typic Hapludults
Haplargids, Paleargids
[not provided]
Haplargids
Natrargids
Some argillic horizons formed during a time when the
climate was less arid
Paleo-argillic horizon formed during moister period
Argillic more strongly developed in udic than xeric SMR
Clay maxima below modern wetting front
Eghbal and Southard (1993)
Gunal and Ransom (2006)
Elliott and Drohan (2009)
Gile and Grossman (1968)
Rabenhorst and Wilding
(1986a, 1986b)
Khormali et al. (2003)
Khademi and Mermut (2003)
Khormali et al. (2012)
Nettleton et al. (1975)
Argillic forms with lithologic discontinuity (loess/residuum)
Stronger argillic with lithologic discontinuity (outwash/till)
Argillic forms with lithologic discontinuity (loess/tosca
(paleo-calcrete))
Stronger argillic with lithologic discontinuity (loess/bedrock);
“rock effect”
Modern calcarous dust inhibits argilluviation
Morphology of argillic influenced by authigenic carbonate,
clay content and
Aide et al. (2006)
Bockheim (2003)
Blanco and Stoops (2007)
Stronger argillic with stratified parent materials
Argillic more strongly developed in soils with abundant
coarse fragments
Loess rejuvenation increases argilluvation and mineral weathering
Argillic forms more readily in alluvium than dune sands
Argilluviation enhanced by lithologic discontinuities
The base of the argillic is controlled by lithologic discontinuities
Argillic forms more readily with low caco3 in parent materials
Clay translocation enhanced by lithologic discontinuities
Clay content of argillic strongly related to clay content of
parent material
Clay content of argillic strongly related to clay content of
parent material
Hopkins and Franzen (2003)
Rabenhorst and Wilding
(1986a, 1986b)
Mubiru and Karathanasis (1994)
Nettleton et al. (1975)
Ogg and Baker (1999)
Shaw et al. (2004)
Smeck et al. (1968)
Cabrera_Martinez et al. (1989)
Frazmeier et al. (1985)
Argillic horizons more strongly developed under well
drained conditions
Argillans more strongly developed in micro-lows
Hopkins and Franzen (2003)
Bruckert and Bekkary (1992)
Elliott and Drohan (2009)
Gile and Grossman (1968)
Smith and Wilding (1972)
Sobecki and Wilding (1983)
Argillic best developed on sideslope and headslope
Argillic more strongly developed on backslopes than
ridges & shoulders
Argillic more developed on stable ridge crests & depressions
than shoulders
Argillic more developed and deeper in well drained areas
Wilson et al. (2010)
Young and Hammer (2000)
Clay-rich lamellae form in 3500 yr
Argillic forms in early Wisconsin-late Illinoian soils & not
late Wisconsin soils
Argillic-like horizon on cotiga mound in 2100 yr
Argillic occurs only in soils N12,000 yr old
Argillic occurs only in soils N14,500 yr old
Argillic occurs in soils 9000 yr old
Natric occurs in soils b6600 yr old
Berg (1984)
Karlstrom (2000)
that formed following the leaching of carbonates. With time the clays
became more strongly aggregated due to increased orientation of
the clay particles. Circular, striated and fibrous forms of orientation
predominated. By filling large cracks and pores in the aggregates, the
clays formed encrusted or conchoidal segregations that were characteristically stratified.
Mass-balance studies show that only part of the clay in the
argillic horizon of humid soils originated from translocation out
of an eluvial horizon (Rostad et al., 1976; Smeck et al., 1968;
Olson et al. (2005)
Cremeens and Mokma (1986)
Cremeens (1995)
Nettleton et al. (1975)
Othberg et al. (1997)
Southard and Southard (1985)
Alexander and Nettleton (1977)
Smith and Wilding, 1972). Synthesis of clays from the soil solution
or suspension is an important source of the clay, as well as
weathering in situ. In arid regions, a large portion of the clay in
the argillic horizon may have been contributed by dust deposition
(Alexander and Nettleton, 1977; Elliott and Drohan, 2009).
The kandic horizon was introduced into ST to provide an intermediary between the argillic and oxic horizons with regard to
low-activity clays, primarily as a solution for keeping soils of the
southeastern USA from classifying as Oxisols (Buol and Eswaran,
J.G. Bockheim, A.E. Hartemink / Geoderma 209–210 (2013) 153–160
159
Table 5
Genesis of argillic, natric and kandic horizons.
Horizon
Argillic
Natric
Kandic
Mechanism
Dispersion
Dispersant
Sodium adsorption ratio
Translocation
Soil moisture regime
Decalcification
Low
Dispersion by abundant Na
High
Dispersion by organic matter, oxyhydroxides
Low
Aquic, udic, ustic, xeric,
Aridica
Aquic, ustic, xeric, aridica
Aquic, udic, ustic, xeric
High
Mixed, smectitic
High
Mixed, smectitic
Low
Kaolinitic, siliceous, sesquic, ferritic, ferruginous
“Bulge” Bt
Common
Increase
“Bulge” Bt
Few or thin
Slight increase
“Bulge” persists in C
Few or thin
Slight or no increase
High
Subangular blocky
Argillasepic/silasepic
Open/close porphyric
Low
XXX
XX (XXX in aridic)
XX
XX
X
X
XXX
XX
Accumulation
Clay activity
Common mineralogy class
Evidence
Depth-distribution of clay
Argillans
Fine clay/total clay ratio
Microfabric
Microstructure
Plasma fabric
Related distribution
Coefficient of linear extensibility
Other
Relative importance
Translocation from eluvial
Dust deposition & translocation
Weathering in situ
Neoformation
a
Granular
Skel-masepic, Ma-skelsepic
Porphyroskelic
High
Lamellae
XX
XX (XXX in aridic)
XX
XXX
Paleo-argillic; formed in moister SMR.
1988). The introduction of the kandic horizon addressed the issue
of clay-enriched horizons with a clay “bulge,” few or no argillans,
and a lack of or slight increase in the ratio of fine clay to total
clay from an overlying eluvial horizon (Table 5). In contrast to
the argillic horizon, the kandic horizon contains low-activity
clays, and generally has a kaolinitic, siliceous, sesquic, ferritic, or
ferruginous soil-mineral class. Much of the clay in the kandic
horizon has originated from “clay decomposition,” or weathering
in situ (Eswaran and Sys, 1979; Okusami et al., 1997; Shaw et al.,
2004).
The natric horizon is a type of argillic horizon that is dispersed by
abundant sodium; therefore, it has a high sodium–adsorption ratio
(SAR). The clay-activity tends to be high and dominant mineral
classes are mixed or smectitic (Table 5). Although soils with a natric
horizon show a distinct clay accumulation in the Bt or Btn horizon,
there generally are few argillans because they are destroyed by
shrinking and swelling (Alexander and Nettleton, 1977; Nettleton
et al., 1969). Soils with a natric horizon often have a high COLE. The
dominant source of the clay in natric horizons is from weathering
in situ (Nettleton et al., 1969), although dust deposition can be a
major source (Alexander and Nettleton, 1977; Elliott and Drohan,
2009).
The genesis of clay-enriched horizons is complicated. In his discussion of the origin of texture-contrast soils, Phillips (2001) states:
“Multiple causality is likely, and attempts to apply any single explanation to a county-size area (and sometimes to a pedon) are not likely
to be successful. The implication is not that pedologists should abandon
the search for generalizations, but that the context in which laws and
generalizations are developed needs rethinking. Explanatory constructs
should be formulated not with the notion that a single explanation is
likely to be applicable to most soils, but with the idea that multiple causality and polygenesis are likely, and that location-specific characteristics cannot be ignored” (p. 347). In Australia about 20% of the soils
have pronounced differences in texture between the A and B horizons,
that were envisaged as progressing from an initial translocation of the
clay inherited from parent materials to intensive weathering and size
reduction of clay particles in response to strong seasonal fluctuations
in soil moisture (Walker and Chittleborough, 1986).
6. Conclusions
• There are three diagnostic subsurface horizons in ST that are defined on the basis of clay illuviation of silicate clays: (i) the argillic
horizon, (ii) the kandic horizon, and (iii) the natric horizon. In addition to diagnostic subsurface horizons, there are two diagnostic soil
characteristics that are based on clay movement: (i) abrupt textural
change and (ii) lamellae.
• The analysis suggests that clay illuviation is recognized in ST at
some level in 10 of the 12 orders, including order (Alfisols, Ultisols),
suborder (Aridisols), great group (Aridisols, Gelisols, Mollisols,
Oxisols, Vertisols), and subgroup (Andisols, Aridisols, Inceptisols,
Mollisols, Oxisols, Spodosols).
• Forty-four percent of the soil series in the USA contain taxonomically defined argillic, nitric, or kandic horizons. Other soils contain a Bt
horizon so that more than half of the soils of the country feature
clay illuviation.
• All of the soil-forming factors play an important role in processes
leading to the development of horizons of clay enrichment.
• The genesis of argillic, kandic, and natric horizons is strongly dependent on electrolyte concentration, the amount and distribution of
precipitation, clay charge, and microfabric.
Acknowledgments
The authors are grateful to the professional soil surveyors and
scientists of the USDA NRCS, the laboratory technicians, and the
information technologists that have made the data used in this
study generously available to the public. Fig. 1 was drafted using
SSURGO data by NRCS MLRA GIS Specialist, Adolfo Diaz.
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