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Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel.© 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.

Spodosols: Soils with Subsoil Accumulations of Humus and Sesquioxides

The “white earths,” the Spodosols, contrast sharply with the “black earths,” the Mollisols. Spodosols encompass many of the soils called Podzols, a term from the Russian pod (beneath) and zol (ash), or “ashy underneath,” referring to the light-colored E horizon (Bullock and Clayden 1980). The term “Spodosol” (wood ash) focuses attention on the spodic horizon, a subsurface horizon of illuvial accumulated organic matter complexed with aluminum, with or without iron. Spodosols are among the most eye-catching and photogenic of soils (Figure 17.1, Figure 17.2).

SettingThe major setting for Spodosols is the humid boreal climatic zone (microthermal snow-forest climate [Trewartha 1970]) where the natural vegetation was, and in many areas still is, needleleaf trees (Küchler 1970). Seasonally, rain and snowmelt water percolate through the solum. But the presence of 6 million acres (about 3 million hectares) of Spodosols in Florida (Brasfield et al. 1983) and 1.4% by area of Spodosols in the tropical lowlands of South America (Cochrane et al. 1985), counters the first impression that Spodosols are strictly boreal (and correspondingly alpine) and therefore essentially zonal (in the sense of Sibirtsev [1901]). This diversity of distribution of Spodosols indicates that coarse-textured initial material (sand, loamy sand, sandy loam) is a second common, although not universal, characteristic of the setting. A third factor is presence of vegetation that can supply mobile and sesquioxide-mobilizing organic compounds. Reports from laboratory experiments that percolates from leaves from many kinds of trees readily form miniature color “sola” in columns of sand would lead us to expect Spodosols to dominate the lands of the earth. Actually, these soils are prominent in only about 2.5% of the land area (Mokma 2006), and their extent is diminishing under the impact of human disturbance by fire, logging, cultivation, and pasturing. Hans and Jean Jenny are among persons who have established a preserve for the Blacklock soils (sandy, mixed, isomesic, ortstein, shallow Typic Duraquods) in California, lest the soil and its pigmy forest ecosystem become extinct (Jenny 1980).

Spodosols are regionally extensive near the Great Lakes of North America, in New England, eastern and western Canada, southeastern Alaska, Scandinavia, western Europe, and northwestern Russia. They occur more locally on every other continent,

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362 Soil Genesis and Classification

Figure 17.1. Profile of the Myakka (now classified as sandy, siliceous, hyperthermic Aeric Alaquods) soils in Florida. Scale is meters and feet.

Figure 17.2. A Haplorthod in the boreal forest of southwest Sweden formed in loess over gneissic till. The pedon is about 1 m deep. For color detail, please see color plate section.


17 / Spodosols: Soils with Subsoil Accumulations of Humus and Sesquioxides 363

except Antarctica, including temperate zones in the southern Andes, Tierra del Fuego, New Zealand, and Australia, and subtropical to tropical zones in Florida, Malaysia, southern Borneo, Angola, Zambia, and the tropical lowlands of South America. Favorable vegetation that supplies persistent organic compounds essential to the formation of the spodic horizon include hemlock (Tsuga) and other conifers (Pinus, Agathis, Cupressus sp.), deciduous trees (Quercus, Fagus, Betula, Populus sp.), heather (Calluna vulgaris) and other plants of the family Ericaceae, alpine grasses and sedges, tropical rain forest, savanna, and stands of palms (Bullock and Clayden 1980).

Within a given landscape, distribution of bodies of Spodosols may be fairly continuous, or it may be spotty. Where these soils effectively blanket a terrain (though with marked variability in thickness and continuity of individual horizons), a toposequence may include well to excessively drained (udic; rarely, ustic) bodies of Orthods and Humods in upland positions; and bodies of poorly drained (aquic) Aquods where pans perch the water table locally and where, in depressions, the seasonal stand of the water table is high. Scattered bodies of Spodosols reflect patchiness of (1) podzol-promoting trees (the kauri tree, Agathis australis, in New Zealand) (Wells and Northey 1985) or heather (Calluna sp. in western Europe); (2) bodies of coarse-textured geologic material in a matrix of finer-textured material; and (3) sites of local cool, moist microclimate (such as frost-prone depressions) or of a high water table.

Thickness of the solum of Spodosols varies widely. Arctic Spodosols (Gelods and Cryods) may be no thicker than 10 cm (Buurman 1984). In contrast, a Spodosol in North Carolina had a Bh horizon 9-m thick (Daniels et al. 1975). In Spodosols with a sandy particle size class, the upper boundary of the spodic horizon occurs within 200 cm of the mineral soil surface. These soils are often associated with Quartzipsamments, under which spodic horizons may lie at a depth of several meters (Andriesse 1969, 1970).

The Spodosol of North Carolina just mentioned is estimated to be more than 25,000 years old, judging by the amount of organic matter translocated down-profile annually. The Blacklock soils of California have a cemented spodic horizon (ortstein, with accessory silica cementation) and formed in deposits that are probably hundreds of thousands to a million years old (Jenny 1980), although the age of the spodic horizon itself is not certain. In glaciated terrains, Spodosols are relatively young: about 10,000 years in southwestern Sweden (Olsson and Melkerud 1989); about 2,000 to 11,300 years in Finland (Mokma et al. 2004); 3,000 to 8,000 years in Michigan (Franzmeier and Whiteside 1963a); and more than 500 years under Tsuga canadensis in Wisconsin (Hole 1975).

Pedogenic ProcessesThe dominant processes in most Spodosols are the mobilization and eluviation of aluminum and iron from O, A, and E horizons and the immobilization of these metals in short-range-order complexes with organic matter, and in some cases silica, in the



B horizon (Mokma and Evans 2000). Collectively, these processes are known as podzolization, clearly a bundle of processes that bring about translocation of aluminum and iron under the influence of organic compounds and protons (Ponomareva 1964; Bloomfield 1954; Hallsworth et al. 1953), and, possibly, silica (Lundström et al. 2000). The process is driven largely by the production of organic acids from the decomposition of plant materials deposited on the soil surface by littering. Leaching of any carbonates present and significant replacement of the exchangeable cations Ca2+, Mg2+, K+, and Na+ by H+ and Al3+ in the A horizon are prerequisite to mobilization of organic matter and sesquioxides. Malcolm and McCracken (1968) reported that a major source of mobile organic matter is tree canopy drip.

DeConinck (1980) illustrated in detail the nature and behavior of organic compounds that dominate processes of mobilization, migration, and accumulation of materials in profiles of Spodosols and other soils, particularly in cool, temperate regions. Figure 17.3 is a conceptual diagram of three kinds of Spodosols. The domain labeled O represents decomposing plant and animal remains on the forest floor, in root channels, and in animal borrows. The soil profile in column number 1 developed in the presence of supplies of Al3+ and Fe3+ abundant enough to neutralize the negatively charged organic acids by chemisorption to form bound organometallic compounds. More clay is present than is usual in Spodosols. A pronounced electrical double layer does not develop at the surfaces of organic materials, and repulsion between particles of them is not significant. The organic matter is immobilized in the O and A horizons (ochric and umbric epipedons) as a mixture of complexes and chelates, along with fragments of fresh tissues. The immobilized material does not move downward. Excess Al3+ and Fe3+ ions precipitate as hydroxides and oxyhydroxides or, if silica is present in solution, as aluminosilicates (for example, allophane or imogolite). Some of the organic matter becomes bound to the surfaces of silicate clay particles in the granular peds of the A horizon. No E horizon forms because the grains of light-colored minerals do not lose their dark coatings. Under these circumstances,

Figure 17.3. Conceptual diagram of three varieties of profiles of Spodosols differentiated by behavior of organic compounds in response to their environments (see the text).



spodic horizons are only weakly developed because the organic and inorganic short-range-order compounds mostly accumulate in the surface horizons, and soil properties are very similar to those of Andisols. (See Chapter 9.) The minimal spodic horizon is loose, contains iron and aluminum as free oxyhydroxides (or as Al-interlayers in clay minerals), and is transitional in form to a cambic horizon.

Profiles in columns 2a and 2b of Figure 17.3 develop in the absence of an abundant supply of Al3+ and Fe3+ ions. Parent materials are low in contents of aluminum, iron, and clay. Because the negative charge of the acidic organic compounds is not balanced by sorption of aluminum or iron, the hydrophilic organometallic compounds repel each other, disperse, and migrate. The loose, “pellet” structure in spodic horizons (profile 1 and 2a, Figure 17.3) may form by flaking of cracked illuvial humic coatings from surfaces of sand grains (Flach 1960), excretion by fauna (Robin and DeConinck 1978), or adsorption of soluble organic complexes onto surfaces of particles of clay and amorphous hydroxides (Bruckert and Selino 1978). Decay of roots and microorganisms in the B horizon is a source of organic matter in the subsoil.

Ugolini et al. (1977) found direct evidence from lysimeter studies of a Spodosol pedon of the migration of organic matter particles (0.5–1.5 µm in diameter) in the solum and of mineral particles (2–22 µm in diameter) below that. An E horizon usually forms. Below the E horizon, one of two kinds of spodic horizons may develop that have relatively high organic matter contents. In profile 2a of Figure 17.3, biologic activity keeps up with the influx of eluvial organic matter, which is converted into pellets, pedo-tubules, and porous, polymorphic (discontinuous mass with variable color and density) aggregates. Silicate mineral particles are mixed with these. Under broadleaf forest, this spodic horizon is friable to loose, high in content of relatively young humins and humic acids that can aggregate, and is well supplied with roots. It remains hydrophilic. The mean 14C-residence time of organic matter is about 200 years, indicating rapid turnover. In profile 2b, under coniferous forest, the rate of eluviation of organic matter exceeds the capacity of biologic agents to rework it. Organometallic compounds accumulate as monomorphic (uniform mass) coatings (organs) that can cement the horizon (silica and aluminosilicates may be involved as well), which passes in the process from a nodular condition to ortstein (DeConinck and Righi 1983). During dehydration and shrinkage, the coatings develop minute polygonal cracks (that is, the cracked coatings, as part of the definition of spodic materials). Cementation leaves little or no room for roots, which form a mat above the cemented horizon. The horizon becomes somewhat hydrophobic. Simple, relatively old fulvic acids predominate. Mean residence time of organic matter is 10 times that in the spodic horizon of column 2a. A cemented placic horizon under anaerobic conditions in Aquods may be the result of oxygen exudation by roots in saturated soil or relict from an earlier lower stand of the water table.

Immobilization of the organometallic complexes in the spodic horizon occurs by a number of possible mechanisms (McKeague et al. 1983), but a critical factor seems to be the ratio of carbon to metal ions. When the ratio is high, the compounds are dispersed and mobile. As the ratio decreases (due to additional complexation of metal



ions during transport; sorption by sesquioxides, existing organometallic complexes, or allophane/imogolite; or decomposition of parts of the organic component), a critical value is reached at which the complex is immobilized by precipitation or flocculation.

In addition to aluminum and iron, monovalent cations and most Ca2+ and Mg2+ ions are leached down into underlying horizons or to groundwater, along with silica (Singer and Ugolini 1974; Zabowski and Ugolini 1990), from all three kinds of profiles (Duchaufour and Souchier 1978). The translocation of silica provides a mechanism by which allophane or imogolite forms in the spodic horizon (Dahlgren and Ugolini 1989). Farmer et al. (1980) and Johnson and McBride (1989) proposed that these aluminosilicates might form in spodic horizons via transport of a hydroxyaluminum-orthosilicate complex (proto-imogolite) in solution. The capacity of some spodic horizons to sorb dissolved organic carbon may be due to the presence of imogolite (Dahlgren and Marrett 1991).

An example of the profile distributions of aluminum, iron, and carbon is shown in Figure 17.4, along with the distribution of the optical density of the oxalate extract (ODOE). Ammonium oxalate is used to dissolve short-range-order organometallic and aluminosilicate compounds (the oxalate forms strong complexes with the aluminum and iron). The carbon reported is the amount soluble in sodium pyrophosphate, which dissolves fairly labile organic matter and organometallic complexes. Generally speaking, the iron and carbon distributions parallel each other in both the well-drained and poorly drained soils, suggesting that they probably occur in complex. The aluminum content in both soils peaks at a greater depth than either the iron or carbon, suggesting the possibility that the aluminum is not in complex and may have been translocated as an inorganic ion. The ODOE parallels quite closely the carbon and iron distributions. Note that the poorly drained soil contains more total aluminum, iron, and

00

–20

–40

–60 AloFeoCpODOE

Dep

th, c

m

–80

1 2

Typic Haplorthod Oxyaquic Haplorthod

3

% element or ODOE

0 1 2 3

Figure 17.4. Distribution of oxalate extractable aluminum (Alo), iron (Feo), sodium pyrophosphate soluble carbon (Cp), and the optical density of the oxalate extract (ODOE) in a well-drained (Typic Haplorthod) soil and a poorly drained (Oxyaquic Haplorthod) soil formed from loess and till in southwestern Sweden. (After Southard 1994)



carbon (compare the areas to the left of each depth distribution) and that the iron and carbon accumulate at shallower depths than in the well-drained soil.

Having emphasized the role of organometallic complex migration in the formation of Spodosols, it is important to recognize that other processes may be operating (Van Rompaey et al. 2007). McKeague (1981) recommended study of soils in the field, where many phenomena may be explainable in other terms (or perhaps not at all yet), as illustrated by the following examples:

1. Al3+ and Fe3+ ions are released at any depth by weathering of mineral grains.2. A thin layer (<5 mm) placic horizon may contain inexplicably higher proportions

of iron and lower proportions of aluminum than does the host spodic horizon.3. Placic horizons also occur in Inceptisols and Histosols (DeConinck and Righi

1983; Buurman and Van Reeuwijk 1984).4. Changing conditions and processes are indicated by two-generation cutans on

sand grains in some ortsteins: an inner cutan composed of polymorphic complexes plus silt and clay, and an outer monomorphic (unlayered) organ.

5. E horizons are present in a variety of soil orders, which must mean that an array of processes is involved in their formation.

6. Much reactive Al3+ in the soil solution (Nilsson and Berkoist 1983) and in lower spodic subhorizons is not in organic complexes (Van Rompaey et al. 2007) and may have migrated in an inorganic complex (Childs et al. 1983; also see Figure 17.4), or perhaps a soluble hydroxyaluminum-orthosilicate complex (proto-imogolite) (Farmer 1981).

7. Microorganisms of the Metallogenium-Siderococcus group decompose organs but still fix Al3+ by forming Al(OH)

3 and complexes of Al with fulvic and other

acids on fungal hyphae (Aristovakaya 1981). The role of iron-reducing bacteria in mobilizing iron is well known from studies of rice paddy soils and may be important in Aquods.

8. Burial of dark soil horizons, as in certain tundra areas and in tree-tip mounds in bodies of Spodosols, is an alternative process to migration of organic matter.

Thus, a large number of processes may contribute to Spodosol formation, and the relative importance of any one process may not always be clear.

A common phenomenon in Spodosol-dominated landscapes is treethrow, caused by the uprooting, generally by wind, of tall trees with shallow root systems and rigid trunks. Soil materials adhering to the tree roots are lifted and transported, producing tree-tip mounds with adjacent pits. The degree of horizon mixing is largely a function of steepness of the slope, and on steep slopes, the soil material may be completely overturned (Schaetzl 1986; Schaetzl et al. 1989). After disappearance of the tip-tree by decomposition, the mound and pit may have a relief of 1 m and length of 3 m. Growth of moss and other ground cover may prevent erosion of the mounds. Veneman et al. (1984) and Schaetzl (1986) found that the formation of E horizons was much more intensive in the pits than on the mounds, due to greater accumulation of litter



and water. A major implication of treethrow is that, on the landscape scale, upper horizons of Spodosols (O, A, E, and upper B) may be very dynamic and complicate the interpretation of the rate of formation of classic Spodosol morphology.

Shifting ecotones may enhance or diminish the podzolization process (Hartshorn et al 2003). In south central Alaska forests have encroached on grasslands, convert-ing Cryands to Cryods (Rieger and DeMent 1965). Expansion of bogs by lateral growth of sphagnum may bury Spodosols under Histosols in the taiga of Canada and southeastern Alaska. Three centuries of harvesting hay in sparsely stable-manure-fertilized alpine meadows converted Spodosols to “Brown” soils (Inceptisols) in parts of Switzerland (Bouma et al. 1969). Invasion of a Calluna-Empetrum heath by oak (Quercus robur) in Denmark caused a nearly complete conversion of Haplorthod O and E horizons into A horizons within 70 years. This conversion was accompa-nied by lower production of humic and fulvic acids, a less acidic A horizon, and higher rates of cellulose decomposition under oak (Nielsen et al. 1987a, 1987b). Deforestation and conversion to grass/shrub vegetation can reduce the supply of organic materials available for organometallic complex formation and result in a decrease of spodic characteristics, especially organic C contents, in spodic B hori-zons (Barret and Schaetzl 1998). Hole (1975) showed that the degradation of the spodic horizon is quite rapid following tree removal, with an estimated half-life of about a century.

Uses of SpodosolsSpodosols are used for forestry, pasture, hay, and cultivated crops. The soilscape in Figure 17.5 is covered for the most part with conifer-hardwood forest. Some areas in such ecosystems are devoted to recreation and others to farming (potatoes, corn silage, hay, apples). In the north central region of the United States, crop rotations on Spodosols include corn silage, oats, rye, potatoes, red clover, rutabagas, flax, strawberries, and raspberries. Commercial blueberries are grown on these soils on the coastal plain of North Carolina, in New England, and in the upper Midwest states. Fertilization and cultivation raise the nutrient levels of these soils and lead to soil compaction, to mixing of O and E horizons, and to some degradation of spodic horizons by aeration and leaching, particularly in irrigated potato fields. On the other hand, the heath lands and associated Spodosols of northern Europe persist mainly due to human activities of burning, cutting, and grazing of the heather. These activities prevent the colonization of the heath by forests (Nielsen et al. 1987a).

Spodosols and associated Quartzipsamments (in which depth to the spodic horizon exceeds 2 m) in humid tropical lowlands present problems unique to such environments. Being almost devoid of nutrients below the litter, these soils are slow to support reforestation wherever the surface horizons have been destroyed. Tropical savannas may result. The sandy E horizons of these soils are often excavated as a source of road construction material, especially in areas where gravel, rock, and plinthite materials are not available.



DeConinck (1980) recognized four classes of spodic horizons important to soil management: friable, cemented, nodular, and placic. The first is best suited to plant growth because of favorable porosity, low resistance to root penetration, and good hydraulic conductivity. The three kinds of hardened horizons may have porosities higher than those of C horizons but are resistant to penetration by roots and are barriers to upward movement of soil water. Drekker et al. (1984) found temporary ponding in slight depressions on the forest floor on Typic Haplorthods caused by crusting (plugging of soil pores by fine organic debris) of soil, and saturated conductivity (K

sat) was as low as 1 cm d−1 (per day). The cemented Bh with a K

sat of

8 cm d−1 had an actual flow rate of 32 cm d−1 because of a hydraulic head gradient across the spodic horizon of 3 cm cm−1. Lambert and Hole (1971) reported that an

Colton

Waskish

Waskish

Gravelpit

Adams

Adams

OrganicGlacial outwash

sands Croghan

Naumburg

Chocorua

Madawaska

Glacial o

utwash

-sands

Organic

Glacial o

utwash

-sands a

nd gravels

Glacial outwash-sands and gravels

Vassalboro

Figure 17.5. Typical pattern of soils and underlying materials in the Adams-Colton association in York County, Maine (Flewelling and Lisante 1982). The Spodosols are Adams (sandy, isotic, frigid Typic Haplorthods), Colton (sandy-skeletal, isotic, frigid Typic Haplorthods), Croghan (sandy, isotic, frigid Aquic Haplorthods), Madawaska (coarse-loamy over sandy or sandy-skeletal, isotic, frigid Aquic Haplorthods), Naumburg (sandy, isotic, frigid Typic Endoaquods). Geographically associated Histosols are Chocorua (sandy or sandy-skeletal, mixed, dysic, frigid Terric Haplohemists), Vassalboro (dysic, frigid Typic Haplofibrists), Waskish (dysic, frigid Typic Sphagnofibrists).



ortstein spodic horizon that impeded root penetration had low hydraulic conductivity during dry periods (0.072 cm d−1 at −70 millibars tension), which increased under more moist conditions to 98.5 cm d−1 at 30 millibars tension. Deep plowing of cemented Spodosols usually improves plant growth by reducing the effects of seasonal perched water tables and resistance to root penetration, although blueberry vegetation seems to be particularly effective at promoting spodic horizon formation, and has been shown to facilitate the re-cementation of ortstein layers in Michigan Spodosols that have been deep tilled to break up the ortstein (Bronick et al. 2004).

Classification of SpodosolsThe shift in emphasis from albic horizon (E) to spodic horizon (often with Bh, Bs, or Bhs horizon designations) has the advantages of (1) basing classification on a subsoil horizon less subject to alteration or removal through human activity than is the surface soil; and (2) reducing diversity of soils in the order by eliminating from it many “podzolic” soils (soils with albic horizons) of earlier classifications. On the other hand, this shift has made the separation of Andisols from Spodosols more difficult, because short-range-order aluminosilicates and organometallic complexes occur in the B horizons of soils of both orders (Nettleton et al. 1986) and causes even some rather strongly developed Podzols of other classification systems to be classified in orders other than Spodosols, particularly Entisols and Inceptisols (Mokma et al. 2004; Murashkina et al. 2005), primarily due to the spodic material color requirements.

Spodosols are identified on the basis of the occurrence of spodic materials and spodic horizons. Spodic materials have the following characteristics: a pH in water (1:1) of 5.9 or less and at least 0.6% organic carbon; and either (1) underlie an albic horizon and have either a reddish hue or are very dark gray or black (specific Munsell colors are required); or (2) with or without an albic horizon, have specific color requirements (either reddish or very dark) and are (a) cemented by organic matter and aluminum, or (b) have cracked coatings of organic matter and sesquioxides on sand grains, or (c) have at least 0.50% oxalate extractable aluminum plus one-half iron and less than half that amount in an overlying horizon, or (d) have an optical density of the oxalate extract (ODOE) of 0.25 and less than half that value in an overlying horizon. A spodic horizon is defined as an illuvial layer at least 2.5-cm thick that is not part of an Ap horizon and contains at least 85% spodic material. At the order level, Spodosols are generally defined as mineral soils that have a spodic horizon and that do not have permafrost or gelic materials.

Spodosols are subdivided into five suborders (Table 17.1, Figure 17.6), which are briefly defined as follows.

Aquods (for example, Figure 17.1) are Spodosols that have aquic conditions within 50 cm of the mineral soil surface in most years and have a histic epipedon or redoximorphic features in the upper 50 cm. These soils occupy local depressions or large areas of low relief and a high water table.


371

Table 17.1. Suborders and great groups in the Spodosols order

Suborders Great Groups

Aquods Cryaquods have a cryic soil temperature regime.Alaquods have less than 0.10% oxalate extractable iron in 75% or more of the

spodic horizon.Fragiaquods have a fragipan within 100 cm.Placaquods have a placic horizon within 100 cm in 50% or more of each pedon.Duraquods have a cemented horizon within 100 cm in 90% or more of each

pedon.Epiaquods have episaturation.Endoaquods—other Aquods.

Gelods Humigelods have 6.0% or more organic carbon in a layer 10 cm or more thick in the spodic horizon.

Haplogelods—other Gelods.Cryods Placocryods have a placic horizon within 100 cm in 50% or more of each pedon.

Duricryods have a cemented horizon within 100 cm in 90% or more of each pedon.

Humicryods have 6.0% or more organic carbon in a layer 10 cm or more thick in the spodic horizon.

Haplocryods—other Cryods.Humods Placohumods have a placic horizon within 100 cm in 50% or more of each pedon.

Durihumods have a cemented horizon within 100 cm in 90% or more of each pedon.

Fragihumods have a fragipan within 100 cm.Haplohumods—other Humods.

Orthods Placorthods have a placic horizon within 100 cm in 50% or more of each pedon.Durorthods have a cemented horizon within 100 cm in 90% or more of each

pedon.Fragiorthods have a fragipan within 100 cm.Alorthods have less than 0.10% oxalate extractable iron in 75% or more of the

spodic horizon.Haplorthods—other Orthods.

Aquods

HumodsGelods

CryodsOrthods

Cold

Wet

Much OC in spodic

Figure 17.6. Diagram showing some relationships among suborders of Spodosols.



Gelods are better-drained Spodosols of high latitudes and altitudes that have gelic soil temperature regimes. These Spodosols occur in association with Gelisols but do not have permafrost. Soils with spodic-like horizons and permafrost are classified as Spodic subgroups of Gelisols.

Cryods have cryic soil temperature regimes (thus, are warmer than the Gelods) and generally occur at lower latitudes and altitudes than the Gelods. These are the most extensive of the Spodosols.

Humods are Spodosols that are not as poorly drained as Aquods and have spodic horizons containing 6.0% or more organic carbon.

Orthods (for example, Figure 17.2; Figure 17.3, profile 2a; Figure 17.4) are the better-drained, warmer Spodosols with spodic horizons containing less than 6.0% organic carbon.

Criteria for great groups of Spodosols are quite uniform and include fragipans, cemented horizons, placic horizons, aluminum or organic carbon content of the spodic horizon, and for the Aquods only, a cryic soil temperature regime and nature of saturation.

PerspectiveThe coarse texture and corresponding relatively limited total particle surface area of materials in which many Spodosols have formed make possible differentiation of contrasting E and B horizons. Many profiles observed in glaciated terrains are of young and incipient Spodosols. Some well-developed Spodosols in nonglaciated landscapes are probably steady-state systems that have maintained themselves for many thousands of years. Quantitative criteria for Spodosols select those with B horizons in which short-range-order organometallic and aluminosilicate compounds have accumulated (chiefly by illuviation) in significant amounts. Supplies of organic acids, metal ions, and silica that form the spodic horizon (generally Bs, Bh, and Bhs morphologic horizons) come from (1) decomposition of roots, surface litter, and organisms, and weathering of mineral particles, (2) mobilization of these compounds mainly in O, A, and E horizons, and (3) immobilization of the compounds in the B horizon by a number of possible mechanisms, including precipitation, chemisorption, flocculation, and decomposition or polymerization of the organic constituents. Collectively these processes are referred to as podzolization. The emphasis, in soil classification, on the spodic horizon should not divert attention from the biological, chemical, and physical dynamics of the surficial horizons (O, A, E) in natural ecosystems and the Ap horizon in cultivated areas. Low temperatures or a high water table in most Spodosols favor maintenance of relatively high organic matter contents in the spodic horizon. Cementation of portions of that horizon or an overlying albic horizon may further preserve the short-range-order materials and alter soil hydrology and the rhizosphere.

Spodosols with high water tables (or artificially drained) are placed in the suborder Aquods. Better drained Spodosols with gelic soil temperature regimes are



Gelods; those with cryic soil temperature regimes are Cryods; those organic carbon-rich spodic horizons are Humods; and those with aluminum- and iron-dominated spodic horizons are Orthods.

Spodosols are found in a wide range of ecosystems, dominantly in cold coniferous forests, but ranging into the tropical rain forests. Most are coarse, often sandy textured, and acidic to very acidic in reaction. Although often rejected for agricultural use because of their sandy, acidic condition, they are important agricultural soils in many areas.
