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.
Histosols: Organic Soils
Histosols (from the Greek, histos, tissue, and the Latin, solum, soil) is the name given in Soil Taxonomy to soils composed mainly of organic soil materials, do not have permafrost, and do not have andic properties dominant in the upper 60 cm of the soil. Organic soil materials are saturated with water for at least 30 days or are artificially drained and contain 12 to 18% organic carbon by weight, excluding live roots, depending on clay content. (See Figure 2.3.) If saturated for fewer than 30 days, organic soil materials must contain 20% or more organic carbon (Soil Survey Staff 1999). Histosols occupy approximately 1% of the land area (Wilding 2000). Many organic soils with permafrost that were previously classified as Histosols are now Histels, a suborder of the Gelisols occupying about 0.8% of the earths land area.
SettingHistosols occur at all latitudes, but about 90% of the Histosols occur in the boreal zone of North America, northern Europe (especially Finland and Sweden), Canada, and Russia (Everett 1983; Rabenhorst and Swanson 2000; Lindbo and Kozlowski 2006). Less extensive areas of Histosols are present in some lowlands throughout the tropics especially in Asia (Andriesse 1974). In this regard, the global distribution of Histosols is similar to that of Spodosols (Chapter 17). In the U.S. outside of Alaska, Histosols are locally extensive on the coastal plains of the Southeast, in the upper Great Lakes region, in southern Florida, and in central California near the confluence of the Sacramento and San Joaquin rivers. Elsewhere, Histosols occur only locally, where landscapes are very poorly drained or in alpine settings where cold tempera-tures retard organic matter decomposition. (See Figure 13.1.)
A general condition that must be met in the Histosols is that the rate of organic matter production must exceed the rate of organic matter decomposition. This condition may be met when water tables are maintained very near the soil surface formost of the year. As a result, less oxygen is available for aerobic microbial decom-position of organic residues. The anaerobic respiration process is less efficient than aerobic decomposition, organic substrates are less completely metabolized, and microbially synthesized biomolecules may be resistant to further oxidation (Anderson 1995). Thus, Histosols are most common in climates where precipitation exceeds evapotranspiration. Cool climates promote this process because of reduced microbial
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decomposition, hence, the increased presence of organic matter in freezing conditions and low evaporation. The accumulation of peats in a wetland area generally depends more strongly on the decomposition rate than on the biomass production rate; decomposition rates are more a function of temperature than of precipitation (Mausbach and Richardson 1994).
A variety of terms are used to describe organic soils or landscapes dominated by them (Stanek and Worley 1983) including mire, moor, bog, peatland, muskeg (Canadian Algonquin term), pocosin (Carolinian term), fen, marsh, and swamp. These names apply to particular ecosystems or landscapes characterized by specific biotic communities, hydrologic regimes, reaction (pH), nutrient status, and pattern of microrelief and ponds. For example, bogs are nutrient poor, have acidic organic soils, and are dominantly rain fed. Fens or swamps (if forested) have less acidic organic soils, are more nutrient rich, and are fed dominantly by groundwater or runoff. Histosols also occur in subaqueous settings, for example, in intertidal areas, where the soils are submerged most of the time (Soil Survey Staff 2010).
Hydrology clearly is a very important factor in the formation and maintenance of Histosols and can be considered as a component of both climate and topography (Mausbach and Richardson 1994). Histosols may occupy a variety of landscape positions (Figure 13.2), and their properties depend largely on the hydrologic regime. Histosols often occur on the lowest, wettest parts of the landscape. Those Histosols that occupy landscape depressions are typically independent of climate (aclimatic) and are the result of a high water table. Some Histosols are formed on hillslope seep
Figure 13.1. An alpine Cryohemist in Switzerland, formed mostly from sedges, with gray strata of mineral soil material. The black bar represents 50 cm. The water table is at a depth of about 80 cm. For color detail, please see color plate section.
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areas due to local stratigraphy that causes lateral groundwater movement and local seepage on side slopes. Blanket peats and raised bogs depend on rainfall and often occupy the highest part of a poorly drained, low relief landscape. Fens often occur where groundwater discharge or stream inflow supplies water and nutrients. In perhumid climates, where precipitation exceeds evapotranspiration several months each year, Histosols form on flat uplands that occur on the centers of interstream divides and are known as pocosins (pocosinAmerican Indian word for swamp on a hill). Pocosins (bogs) are common in the lower coastal plain of North Carolina and actually are peat domes formed in drainage systems that were blocked and flooded between 10,000 and 15,000 years ago. Some Histosols of cool, humid moun-tainous regions (Mountain-top Histosols in Figure 13.2) are never saturated with water, except for a few days following heavy rain. They are either shallow or extremely rocky, and the plant roots grow only in the organic material.
Bodies of eutrophic Histosols (fens) are chemically influenced by input of nutri-ents (Verry 1981). Houghton muck (Figure 13.3) receives nutrients from stream and seepage water moving from adjacent slightly calcareous sandy glacial till. By contrast, the rain-fed, infertile raised bog, classified as Napoleon, receives nutrients only from precipitation and wind.
The parent materials for most Histosols are hydrophytic plants. Mosses of the genus Sphagnum dominate many Histosol landscapes. Other plant species commonly providing organic material to Histosols include sedges (Carex spp.), rushes (Juncus spp.), and cattails or tules (Typha spp. and Scirpus spp.). Ericaceous shrubs are common in some slightly better drained Histosol landscapes, as are stunted trees including spruce (Picea), pine (Pinus), hemlock (Tsuga), and willow (Salix). Just as rock composition affects soil properties of mineral soil, plant composition affects the
Histosols of pits(kettles)
Histosols ofseepage sites
Histosols of depressions
Figure 13.2. Idealized block diagram showing some relationships of Histosols to topographic position.
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properties of organic soils. Three broad classes of peat are identified on the basis of plant composition (Rabenhorst and Swanson 2000): moss peat (generally in bogs), sedge or herbaceous peat (generally in fens), and woody peat (generally in swamps).
The Histosols of the boreal zone occupy landscapes that were mostly covered by ice during the Pleistocene. These Histosol landscapes must be less than about 10,000 years old. At lower latitudes, most Histosols of coastal plains and estuaries probably have ages of 5,000 years or less. These Histosols probably began to form after sea level more or less stabilized at its current high stand following melting of ice during the late Pleistocene and early Holocene (Rabenhorst and Swanson 2000). Locally, Histosols may be considerably older than or younger than these general age ranges due to local hydrologic and topographic conditions.
Geological Processes of Organic Matter AccumulationThe geologic accumulation of organic materials and the extension of blanket peats over entire landscapes are termed paludization or paludification (Malmer 1975; Glaser 1987; Mausbach and Richardson 1994). The enlargement of such bodies is bysurface additions of organic materials. Anoxic conditions created by prolonged saturation, reducing the infusion of oxygen needed to oxidize the organic material, must predominate at the soil surface for paludization to occur. The slow hydraulic conductivity of many Histosols may play a positive feedback role in the paludization
Coloma loamy sand
Figure 13.3. Soilscape patterns of two Histosols, in association with Entisols in slightly calcareous tillterrain in Van Buren County, Michigan. Coloma soils are mixed, mesic Lamellic Udipsamments. Napoleon soils are dysic, mesic Typic Haplohemists. Houghton soils are euic, mesic Typic Haplosaprists. (After Bowman 1986, Sec. 3, T.1S., R.15W)
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process. Slow hydraulic conductivity leads to longer periods of saturation and anaerobic conditions, resulting in retarded organic matter decomposition, hence, more organic matter accumulates, leading to slow hydraulic conductivity (Hartshorn et al. 2003). Paludization can be a final stage in another process called terrestrialization (Malmer 1975), the in-filling of lakes and other depressions with sediment, and production of the classic peat land (Glaser 1987). These wetlands are well suited for Histosols formation because of the low organic matter decomposition rates under anaerobic conditions in the saturated soil. Paludization forms the blanket bogs on the mineral soil surface of a forested or tundra terrain and proceeds from a muskeg stage to a raised bog stage (Henselman 1970).
In Vermont, the biomass of Sphagnum moss has been observed (Osheyack and Worley 1981) to increase 16% annually for an average areal production of 370330 g m2 dry matter. This annual production rate is one-half the rate of boreal forests, one-third that of north temperate forests, one-quarter that of tropical forests, but 1.3 times that of alpine heath, 2.2 times that of alpine tundra meadow, 4 times thatof deserts, 1.5 times that of streams, and 3 times that of the open ocean. Mined bodies of Histosols in Scotland are reported (Robertson 1981) to regrow (heal) within 510 years, but no such healing has been noted in Minnesota where Typha spp. (cattail, with biomass production of 33 tonnes ha1 yr1) has been tried as a bioenergy crop following removal of peat (Garver et al. 1983). In some Histosols of the Florida Everglades (McDowell et al. 1969), the organic material a few centimeters above a limestone contact is about 4,300 years old, and the material 1.26meters above the limestone is about 1,250 years old.
Since genesis of Histosols depends on organic matter deposition, the deposition process is often considered to be geogenic rather than pedogenic. In this sense, one can consider the initial deposit of organic materials to be the parent material in which Histosols can form by alteration from recognizable organic forms of leaves, stems, and other plant parts, to unrecognizable organic material; and from a stratified or unstructured mass to granular, blocky, or prismatic structured horizons.
Pedogenic Biogeochemical ProcessesThe decomposition of organic matter is controlled by a number of interrelated factors of which moisture content, temperature, composition of the deposit, acidity, micro-bial activity, and time are the most important (Broadbent 1962). The alterations and reactions taking place during decomposition are complicated and only partially understood. The decomposition, transformation, and physical alteration of the initial organic material to produce Histosols are often referred to as ripening. In the Netherlands, Heuvelen et al. (1960), Jongerius and Pons (1962), and Pons (1960) have considered ripening processes of Histosols to begin as soon as microbial activity is promoted by entry of air into the organic deposit. Physical ripening primarily involves a decrease in volume due to consolidation and loss of material via decomposition. The amount of physical ripening depends upon the nature of plant
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remains, the content of mineral matter, and the depth of the water table. Chemical ripening is a combination of the chemical decomposition of the most labile components, the partial metabolism of more resistant components leaving still more resistant remnants, and the biosynthesis of new compounds as part of the microbial biomass (Kononova 1961; Anderson 1995). The chemical processes also include transformations of organic and inorganic sulfides, especially in coastal environments where brackish water provides dissolved sulfates (Rabenhorst and Swanson 2000). If the saturating conditions are terminated in sulfide-rich soils, often by engineered drainage systems, oxidation of sulfides produces sulfuric acid, which strongly acidifies the soils, and may lead to the formation of a sulfuric horizon (pH 3.5 or less and jarosite, underlying sulfidic materials, or at least 0.05% water-soluble sulfates). Biological ripening involves reduction in particle size, mixing of organic materials, and formation of peds and pedological features by organisms.
The first organic substrates to be metabolized by microorganisms, primarily fungi and bacteria, are the relatively simple biomolecules including amino acids, soluble proteins, and simple carbohydrates (Anderson 1995; Everett 1983). The products of this decomposition are CO
2 and water, plus energy, C, and N for the synthesis of
microbial biomass. More complex organic molecules including hemicellulose, cellulose, and lignin are metabolized less easily and yield a complex mixture of long aliphatic (polymethylene) chains, and aromatic compounds (containing phenol groups). This mixture is a combination of recalcitrant remnants of the initial organic substrates, plus microbially synthesized material (Anderson 1995). Anaerobic conditions prevent the complete oxidation of the reduced carbon in the organic molecules to CO
The decomposition process has a profound effect on the physical and chemical properties of organic soil materials (Table 13.1). As the organic molecules are decomposed and transformed, C is lost (e.g., cellulose), but N is conserved through microbial synthesis. The C:N ratio decreases as decomposition proceeds. The cation exchange capacity (CEC) of Histosols is derived primarily from carboxyl and phen...