Distribution and classification of soils with clay-enriched horizons in the USA

Geoderma 209–210 (2013) 153–160

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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

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Article history: Received 3 August 2012 Received in revised form 6 June 2013 Accepted 15 June 2013 Available online 12 July 2013 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

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(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.

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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.

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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,

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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

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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

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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 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

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

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;

Bruckert and Bekkary (1992) Elliott and Drohan (2009) Gile and Grossman (1968)

Smith and Wilding (1972)

Hopkins and Franzen (2003) Sobecki and Wilding (1983) Wilson et al. (2010) Young and Hammer (2000) Olson et al. (2005) Cremeens and Mokma (1986)

Berg (1984) Karlstrom (2000) 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,

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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. References Aide, M.T., Dunn, D., Stevens, G., 2006. Fragiudults genesis involving multiple parent materials in the eastern Ozarks of Missouri. Soil Science 171, 483–491. Alexander, E.B., 2010. Old Neogene summer-dry soils with ultramafic parent materials. Geoderma 159, 2–8. Alexander, E.B., Nettleton, W.D., 1977. Post-Mazama natrargids in Dixie Valley, Nevada. Soil Science Society of America Journal 41, 1210–1212.

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