Spodosols

U of Fl --- Sabine Grunwald Soil and Water

Spodosols

	Environmental Conditions
	Processes
	Properties
	Classification
	Distinguishing Characteristics

Spodosols

Summary:

	Vegetation: coniferous or mixed coniferous/deciduous forest, heath vegetation, ericaceous shrubs, alpine grasses, sedges
	Climate: Major settings in the humid boreal climatic zone
	Soil moisture regime: mostly udic (xeric, aquic)
	Major soil property: placic horizon, ortstein, coarse soil texture, low exchangeable bases, low pH
	Diagnostic horizons: spodic, (albic)
	Epipedon: histic, plaggen, umbric, or ochric
	Major processes: podzolization
	Characteristics: Soils with subsoil accumulation of humus and sesquioxides

Spodosols - Environmental Conditions

Climate: Spodosols may have any soil temperature regime. They are largely confined to humid areas that extend from high latitudes to the tropics. But, especially extensive in cool-humid, mid to high latitudes, e.g. Alaska, Northern Gt. Lakes, Canada, Northern New England, and comparable bioclimatic zones in Europe, Soviet Union, Japan, Argentina and New Zealand. Intertropical areas include coastal plains of Florida, Madagascar, Brazil, Australia, Brazil and Surinam. When they form in hot, humid tropical regions and other warm, humid areas, they occur mostly in quartz-rich sands that have fluctuating level of ground water. The moisture regime of Spodosols is mostly udic, but a few have a xeric moisture regime. Some have aquic conditions (suborder: Aquods).

Vegetation: The formation/occurrence of Spodosols is correlated with certain vegetation associations. Vegetation that can supply mobile and sesquioxide-mobilizing organic compounds favors the formation of Spodosols: (i) production of humic and fulvic acids (low molecular weight organics), (ii) vegetation with high cellulose contents / acidic, low-base litter, which decomposes slowly, and (iii) the release of phenolic and carboxylic compounds in throughfall and stemflow that are able to chelate or immobilize Fe an Al. Typical vegetation for Spodosol development are found in New Zealand - Kauri pine (Agathis australis); N-Hemisphere - Hemlock (Tsuga canadensis), Heath vegetation (Calluna vulgaris & Erica sp.), alpine grasses, sedges, and conifers (Pinus, Cupressus sp.); Spodosols are also found under Picea, Larix, Thuya, Populus, Quercus, Betula and understory plants such as Vaccinium.

Spodosols are used for forestry, pasture, hay, and for cultivated crops (e.g. blueberries). Management can decrease acidity by adding lime and raise nutrient level by fertilization. Ploughing mixes the O, A, or E horizon with the underlying horizon or break open cemented layers and degrades the spodic horizon to some extent due to aeration. In some European countries human activities such as burning, cutting, and grazing of heather by sheep preserve Spodosols. The rationale for these activities is to conserve those typical landscape settings with heather vegetation and Spodosols for future generations.

Treethrow is a common phenomenon in Spodosol dominated landscapes because of the shallow root system and rigid trunks, which is due to limited root penetration in cemented layers.

Relief: Spodosols form on slopes ranging from nearly level to very steeply sloping and on surfaces in which the water table ranges from very deep to fluctuating near the surface. They do not seem to form in a soil that is permanently saturated with water. The iron content of spodic horizons depends on the watertable levels - spodic horizons that are saturated with water for prolonged periods may be depleted of iron because it is reduced and mobilized.

Parent Material: Typically, Spodosols are formed in very coarse silty or coarser (i.e. increase in sand) textured material, e.g. sandy loam, loamy sand, sand. Siliceous or leached carbonaceous parent materials favor the development of Spodosols. In the United States most Spodosols occur in late-Pleistocene or Holocene deposits. Some materials were originally calcareous, but carbonates were leached before the spodic horizon developed. Spodosols form also in weathered material from rocks poor in Ca and Mg (e.g. sandstone, granite). The content of iron-bearing minerals in the parent material influences the kind of spodic horizon that will develop and the degree of development of an E horizon.

Time: In glaciated terrain Spodosols are relatively young - about 10,000 years. Some Spodosols are even younger when parent material, vegetation, and climate favors the formation of a spodic diagnostic horizon (<500 y). A cemented spodic horizon takes longer time periods to form (approximately 3,000 to 8,000 y).

Spodosols - Processes

Spodosols represent a more limited grouping of soils than that encompassed by Podzols (a grouping used in previous US classification systems and current systems in Europe, the Soviet Union and elsewhere). Podzol is a term from the Russian pod (beneath) and zol (ash), referring to the light-colored E horizon. In contrast to Podzols, Spodosols are not characterized by evidence of clay translocation, even though some may contain argillic horizons. Podzols differ from Spodosols in terms of the requirement of an eluvial and an illuvial horizon, whereas the requirement for Spodosols is only an illuvial spodic horizon.

Podzolization describes collectively the subprocesses of mobilization, eluviation of organic material, Al and Fe from the O, A, and E horizons, and the illuviation of these materials in the spodic horizon. The processes of Spodosol formation are complex and several hypotheses exist in literature.

Decomposition of surface litter, roots, and organisms produces organic acids (humic and fulvic acids). Leaching of any carbonates present, i.e., acidic condition in the upper horizons (O, A, and E horizons) is another prerequisite to the mobilization of organic matter and sesquioxides.

In general, a prerequisite for development of a spodic horizons is leaching of carbonates. In soils of high-base status microbial activity is generally vivid and organic materials tend to be oxidized or to form relatively insoluble Ca-humates. As depletion of bases occurs through leaching of soils in humid regions, bacterial decomposition of organic matter is retarded and minerals weather more rapidly releasing Al, Fe, and other elements. Soluble organic substances produced by fungi and other micro-floral and -faunal attack on plant litter persist and migrate downward with the soil solution. The development of Spodosols is forced under acidic, low-base litter by coniferous or mixed coniferous/deciduous forest, ericaceous shrubs, heath vegetation, alpine grasses, or sedges. Since the decomposing plant litter contains some Al and Fe taken up by plants, some soluble low-ratio (Al,Fe)-organic complexes may be formed directly in the O horizon that occur at the surface of most Spodosols.

Weathering of Spodosols is more intense towards the soil surface. Silt-sized silicates may weather to form mixed layer minerals (chlorite-vermiculite or chlorithe-mica) in the upper horizons. In the A and B horizons clay-sized quartz and feldspars is enriched. Inherited clay-sized silicates in the C horizon of Spodosols commonly include some combination of mica, chlorite and kaolinite. The marked difference in the extent of clay mineral weathering in the E and B horizon is though to be due to the fact that mineral surfaces in the B horizon are presumably coated with sesquioxides-organic surfaces and thus protected from weathering. Generally, ferromagnesian minerals near the surface are dissoluted and Fe, Al, Mg, Si, and associated elements are liberated.

There are several hypotheses describing the eluviation of material from the O, A, and E horizons to the illuvial (spodic) horizon:

I. Complexation of Fe and Al with organic acids: Humic substances can attack minerals and Al, Fe and other cations are liberated. Fe and Al build complexes with low molecular weight organics in surface horizons. The complexes are subsequently translocated in percolating water and precipitated in the B horizon. Continuing removal of weathering products from mineral surfaces by complexing action of organic solutions contributes to relatively rapid weathering in the E horizon, which is rich in quartz and other resistant minerals.

Soluble organic matter derived from decomposing plant litter has the capacity to remove Al and Fe from sesquioxide-organic complexes deposited in spodic horizons. If uncomplexed humic substances are added at a rate much faster than that of mineral weathering and release of Al and Fe in the solum, the sesquioxide-organic complex presumably attains a progressively lower metal to organic ratio and becomes increasingly soluble. Ultimately, the spodic horizon could be moved to a greater depth or destroyed and the low ratio complexes could move out with the percolating water.

Precipitation is assumed to occur because of changes in one or more of the following conditions in the subsoil: hydrology, chemistry and microbiology.

Precipitation of the (Al,Fe)-organic complexes might be related to the following mechanism (polymerization): Soluble humic substances form complexes with Al and Fe released from weathering surfaces of minerals in soil. Complexes having a low ratio of metal ion to carbon are soluble, but as loading increases, the complex becomes increasingly insoluble. The (Al, Fe)-organic complexes found in spodic horizons are commonly insoluble. Regardless of the process slow release of Al and Fe appears to be important, so that the metal-complex ratios sufficiently small to maintain the complex soluble in the upper portion of the soil.

Alternatively, the low-ratio complex may be adsorbed by insoluble high-ratio(Al,Fe)-organic complexes already deposited or by Al and Fe hydroxides (chemisorption ). The insoluble complex has the capacity to complex additional Al and Fe. Whatever the mechanism, the result is the same, the complex is deposited in soils with available sesquioxides. The site of deposition of sesquioxide-organic complexes depend upon the supply of weatherable minerals and disturbances such as faunal activity, blowdown of trees, etc.

Another mechanism for the precipitation of material in the spodic horizon is due to microbial oxidation of organic matter, hence Al and Fe translocated to the B horizon as organic complexes are deposited as hydrous oxides due to microbial oxidation. The weakness in this hypothesis is generally the decrease of microbial activity with depth in soil. Furthermore, the resistance to microbial breakdown of humic substances in spodic horizons is high.

It has also been suggested the idea of mutual flocculation of negatively (humic) and positively (sesquioxides) charged sols.

II. Reduction of Fe³⁺ - translocation of Fe²⁺ - precipitation as Fe(OH)₂: The removal of Fe from the O, A, and E horizons is due to reduction of Fe³⁺ to Fe²⁺, the translocation of Fe²⁺ , which is more soluble, and the precipitation as Fe(OH)₂. Evidence against this mechanism of Fe eluviation is: (i) it does not account for the apparent overall similarity in the eluviation of Fe and Al in many Spodosols, and (ii) many coarse-textured Spodosols are saturated with water for only short time periods (reducing conditions). Oxidation of Fe²⁺ to Fe³⁺ and deposition as ferric hydroxide (Fe(OH)₂ ) occur under oxidizing conditions in the spodic horizon. Though such a mechanism may be operative in some soils, it is not likely to explain the development of a spodic horizon in all Spodosols.

III. Translocation and precipitation of Fe and Al due to low pH, i.e., acid weathering: Acid weathering of Al³⁺ (and iron) from the E horizon could account for eluviation of aluminum but low pH values (<= 4) are a prerequisite for this mechanism, which are commonly not found in many Spodosols.

IV. Translocation of sols: Movement of Al and Fe as hydrous oxide sols protected from flocculation by sols of opposite charge, such as humus or silica sols. However, organic matter is generally abundant in such soils and it is probable that the dissolved Al and Fe are complexed with humid substances. Flocculation of organic matter sols-colloidal humified organic matter might move as a negatively charged sol through the E horizon and be flocculated in the spodic horizon by cations. Evidence against this hypothesis is: (i) some spodic horizons are very poor in exchangeable cations, and (ii) most of the organic matter in many spodic horizons is fulvic acid, which forms true dilute aqueous solutions.

V. Formation of allophane / imogolite: Another hypothesis favors independent accumulation of Fe and Al in the subsoil either in situ formation of short range order aluminosilicate minerals (allophane, imogolite - amorphous character) or translocation of these pedogenically-formed minerals from surface horizons to the subsoil. Organic matter is assumed to accumulate independently.

Spodosols - Properties

Spodosols are characterized by the presence of a spodic diagnostic horizon , which represents a subsurface accumulation of soil organic matter (SOM) with aluminum and / or iron sesquioxides (the products of podzolisation). The general concept of a spodic horizon is: An illuvial horizon containing active amorphous material and organic matter and Al with or without Fe. The phrase 'amorphous material' refers to material that has high cation-exchange capacity, large surface area, and high water retention. There are four classes of spodic horizons: friable, cemented, nodular, and placic. The friable spodic horizon shows the highest porosity, low resistance to root penetration, and highest hydraulic conductivity compared to the other spodic horizons. The cemented, nodular, and placic spodic horizons are characterized by high resistance to root penetration and low hydraulic conductivity, which enforce (temporary) ponding in slight depressions, surface runoff, and a decrease in infiltration. A placic horizon is a thin (usually 2-10 mm), black to dark-reddish pan cemented by iron, by iron and manganese, or by iron-organic matter complexes, which is relatively impermeable to water and plant roots. They occur in layered material (e.g. placic horizons that transgresses subhorizons) or in non-layered material of relatively uniform particle-size distribution (e.g. placic horizons within the B horizon). Placic horizons are usually associated with perudic to aquic soil-moisture regimes in coastal regions. Ortstein is a cemented layer by amorphous (Al, Fe)-organic complex material. The cementation may be either continuously or discontinuously in nodules from a few cubic centimeters to a cubic meter in volume. In ortstein horizons, amorphous material coats and joins the skeletal grains and partially fills intergranular spaces. The development of ortstein is favored by low nutrient supply, high water table and associated shallow rooting of plants, low activity of microflora and fauna.

Spodic horizons have the following characteristics:

	>= 2.5 cm thick
	>= 85 % spodic materials
	not part of an Ap horizon
	pH <= 5.9
	organic carbon >= 0.6 %

One or more of the following:

	Strong color - hue 7.5 YR or redder; values and chroma (moist) <= 4
	A weaker color but accompanied by (i) ortstein in >= 50 % of the pedon, (ii) >= 10 % cracked coatings of organic matter and sesquioxides on sand grains, (iii) at least 0.50 % oxalate extractable aluminum plus one-half iron and less than half that amount in an overlying horizon, or (iv) an optical density of the oxalate extract (ODOE) of 0.25 and less than that value in an overlying horizon.

One or more of the following combinations of horizons may be recognized as spodic horizons in the field:

	Bh: OM accumulation
	Bs: Sesquioxides of Al and Fe
	Bhs: Mixture of the above

These horizons are typically associated with an bleached (gray to gray light color) E horizon (albic horizon), although this may be eroded or indistinguishable because of mixing with the A horizon by plowing. Below the spodic horizon there may be a fragipan or another sequum that has an argillic horizon.

The development of spodic amorphous pellets common in the upper part of the spodic horizon may be due to the removal of polygonally cracked coatings from the surface of sand grains, i.e., monomorphic organic matter results of dissolution and precipitation of humified organic matter. Destruction of the pellets as soil development continues would leave essentially uncoated sand grains, an E horizon. Another hypothesis for the development of amorphous pellets ist that they are fecal pellets of soil fauna, i.e., polymorphic organic matter is of biological origin.

Typically, the horizons (O, A, E, Bh, Bs, or Bsh, C) in Spodosols are very pronounced and the width of the boundaries are abrupt. There are Spodosols with extremely thick E and Bsh horizons (several meters thickness).

Organic matter dominates the microfabric of many spodic horizons. It includes fresh to partly decomposed plant root fragments and amorphous organic matter in various forms: (i) polymorphic, i.e., discontinuous mass with variable color and density: Pellets of polymorphic organic matter, root remains, and elements of fungi may occur in intergranular spaces as loose aggregates; (ii) monomorphic, i.e., continuous mass with uniform color and density: Smooth polygonally cracked, coatings on skeletal grains and bridges between them and discrete aggregates in intergranular spaces. The organic components vary widely in size, concentration of functional groups and solubility and they include low molecular weight organic acids as well as humic acids. The silt and clay in sandy spodic horizons may occur almost entirely as coatings on skeleton grains. These cutans may be impregnated by and coated with monomporphic (Al, Fe)-organic complex material. Some spodic horizons that are relatively rich in silt and clay have a spongy fabric consisting of poorly defined aggregates of organic matter, silt and clay.

Most Spodosols show a sandy to loamy soil texture with few clay-sized silicates. Their particle-size class is mostly sandy, sandy-skeletal, coarse-loamy, loamy-skeletal, or coarse-silty. Typically, bulk density is lower in the upper soil horizons compared to the C horizon (e.g. in a Spodosol of the Laurentides soil in Canada: E: 1.0, Bh: 0.8, Bs: 1.4, C: 1.9 g/cm³ - Wilding et al., 1983).

In most Spodosols the exchangeable bases are low and the pH-dependent exchange capacity of the B horizon is high. Spodosols with appreciable amounts of Al have large capacity to fix P, which is associated with the properties of the OM-sesquioxide complex in the spodic horizon.

Spodosols - Classification

Aquods: Aquods are the Spodosols of wet locations, characterized either by a shallow fluctuating water table or an extremely humid climate. They show a histic epipedon or redoximorphic features in the upper 50 cm. If the soil temperature regime is mesic or isomesic or warmer, most of them have a nearly white albic horizon thick enough to persist under cultivation or, in the wettest Aquods, a black surface horizon resting on a dark reddish brown spodic horizon that is virtually free of iron. Others have a placic horizon or a duripan, or are cemented by an amorphous mixture of sesquioxides and organic matter. The Aquods that do not have a placic horizon normally have a transitional horizon between the albic horizon and the spodic horizon, a feature virtually unique to these soils. Aquods have formed mainly in sandy materials of Pleistocene age. They may have any temperature regime. Water-loving plants of a wide variety, ranging from sphagnum in cold area to palms in the tropics, grow on these soils. These soils occupy local depressions or large areas of low relief and a high water table.

Cryods: Cryods are the Spodosols of high latitudes and/or high elevations that have a cryic soil temperature regime. Spodosol-like soils with pergelic soil temperature regimes are proposed to be classified as Spodi- great groups of Gelisols. Many Cryods have formed in volcanic ash or glacial drift, and some in residuum or colluvium on mountain slopes. They commonly have an O horizon over a very thin or intermittent albic horizon, which overlies a well-developed spodic horizon. Some have a placic horizon, ortstein, or other cemented soil layers within 100 cm of the soil surface. In many Cryods, the organic-carbon content in the upper part of the spodic horizon is relatively high. Vegetation is mostly coniferous forest or alpine tundra. In the United States they occur mostly in southeast Alaska and in the mountains of Washington and Oregon.

Humods: Humods are the relatively freely drained Spodosols that have a large accumulation of organic carbon in the spodic horizon (>= 6 % organic carbon). They have either a thin, intermittent or a distinct, continuous albic horizon over a spodic horizon, which in its upper part is nearly black and has a reddish hue. The hue normally becomes yellowier with depth. Humods are derived predominantly from Pleistocene or Holocene sediments. In the United States they may have developed mainly under coniferous forests. In western Europe they are common in areas of sandy materials, where heather (Calluna vulgaris) is, or used to be, a dominant plant. In most tropical regions most Humods have supported a rain forest. The soils of this suborder are not extensive in the United States. They are known to occur in the Pacific Northwest, mostly in small areas, and my exist in the Southeast.

Orthods: Orthods are the relatively freely drained Spodosols that have a horizon of accumulation containing aluminum, or aluminum and iron, and organic carbon. They have formed predominantly in coarse, acid Pleistocene or Holocene deposits under mostly coniferous forest vegetation. Orthods normally have an O horizon, an albic, and a spodic horizon, and they may have a fragipan. Some of these soils, however, have been mixed by roots of falling trees or by animals and have a very thin albic horizon or none. Under cultivation the albic horizon is very commonly mixed with part of the spodic horizon. In the United States, the moisture regime of Orthods is predominantly udic, but a few have a xeric regime. Their soil temperature regimes range from frigid to hyperthermic. The Orthods are extensive in the southeastern United States, the Northeast, the Great lakes states, and the mountains of the West. Orthods are the most common Spodosols in the northern parts of Europe.

Several characteristics are considered to define Spodosols on great group and subgroup level:

Spodosols with a placic horizon are defined on the great group and subgroup level (e.g. Placaquods, Placic Cryaquods). The same is true for soils with a duripan (e.g. Duraquods, Duric Cryaquods) or a fragipan (e.g. Fragiaquods, Fragic Haplorthents).

If andic soil properties such as low bulk densities but high water-retention capabilities are present the denotion 'Andic' is used (e.g. Andic Cryaquods, Andic Haplohumods).

The soil moisture regime is used to define 'Aquic' (redoximorphic features in one or more horizons within 75 cm of the mineral horizon, and also aquic conditions for some time in most years) (e.g. Aquic Duricryods) and 'Oxyaquic', which are soils saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years (e.g. Oxyaquic Duricryods). Spodosols which show episaturation are denoted by 'Epi' (e.g. Epiaquods).

Spodosols with a spodic horizon 10 cm or more thick in 50 percent or more of each pedon are differentiated as 'Entic' (e.g. Entic Cryaquods, Entic Haplocryods).

Spodosols with an argillic or a kandic horizon within 200 cm of the mineral soil surface are denoted as 'Alfic' (e.g. Alfic Alaquods) or 'Ultic' (e.g. Ultic Alaquods).

Shallow Spodosols are differentiated by using 'Lithic' (e.g. Lithic Cryaquods).

Epipedons are used for the differentiation of Spodosols on subgroup level: Histic epipedons (e.g. Histic Alaquods), ochric epipedons (e.g. Aeric Alaquods), plaggen epipedons (e.g. Plaggeptic Fragiaquods), or umbric epipedons (e.g. Umbric Epiaquods).

Soil texture is used to define 'Arenic' Spodosols, that have a sandy or sandy-skeletal particle-size class throughout a layer extending from the mineral soil surface to the top of the spodic horizon at a depth of 75 to 125 cm (e.g. Arenic Alorthods).

Spodosols - Distinguishing Characteristics

Within a given landscape, distribution of Spodosols may be fairly continuous, or it may be spotty.

The definition of Spodosols which require a spodic horizon but not necessarily an eluvial horizon (albic horizon) distinguish Spodosols from Podzols.

The separation between Spodosols and Andisols is difficult, because aluminosilicates and organo-metallic complexes occur in the B horizons of soils of both orders.

Low temperatures and high water table favor maintenance of a relatively high content of organic matter in the spodic horizon. If accumulation of organic matter becomes more pronounced Histosols form. For example, the expansion of bogs by lateral growth of sphagnum may bury Spodosols under Histosols in the taiga of Canada.

Soils with an very thick (>= 200 cm) albic horizon overlying a spodic horizon are excluded from the Spodosol order and grouped with the Entisols or Inceptisols. For example, where parent material is sandy and spodic horizons may lie at a depth of several meters Quartzipsamments occur. Some of the very deep spodic horizons may be buried, but it seems likely that others have formed at great depths because the overlying soil materials have very little iron and aluminum that could precipitate the organic carbon. In some the source of aluminum may be the ground water.

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