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 (Kchler 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 ). 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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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. Thepedon is about 1 m deep. For color detail, please seecolor plate section.
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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), atoposequence 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 ahigh 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
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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 (Lundstrm 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).
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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 (0.51.5 m in diameter) in the solum and of mineral particles (222 m 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 200years, indicating rapid turnover. In profile 2b, under coniferous forest, the rate ofeluviation 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
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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 amechanism 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 ahydroxyaluminum-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
Typic Haplorthod Oxyaquic Haplorthod
3% element or ODOE
0 1 2 3
Figure 17.4. Distribution of oxalate extractable aluminum (Alo), iron (Feo), sodium pyr...