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 phenolic functional groups (Broadbent and Bradford 1952). The number of these functional groups increases as decomposition progresses, and CEC values of 200 cmol kg1 and higher have been reported for some organic matter (Broadbent 1953). Note that the effect of decomposition on CEC is even greater when CEC is reported on a soil volume basis, rather than on a mass basis, due to the change in bulk density. The CEC of many organic soils is pH-dependent and may change from about 10 cmol kg1 at pH 3.7 (in 1:1 water) to over 100 cmol kg1 at pH 7 (Dolman and Buol 1967).
Most Histosols have bulk densities much less than 1 g cm3. Bulk density tends to increase with decomposition (Boelter 1965) from values as low as 0.02 g/cm3 in fibric materials to 0.3 g/cm3 or more in sapric materials as plant structural parts are broken down and voids collapse. The amount of mineral material and the type of vegetation contributing to the Histosol causes a wide variation in the bulk density. Histosols are commonly saturated and have an extremely high water-holding capacity, both on a weight
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and a volume basis. Much of the water is either in the larger pores (gravitational water) or in such small pores that it is unavailable for plant growth (Boelter and Blake 1964). In general, although the least decomposed organic soil materials have the greatest water content at saturation, they tend to have the lowest plant available water due to the large proportion of very large pores that empty at low suction. Because Histosols shrink considerably on drying, their moisture characteristics are best expressed on a wet bulk volume basis or measured in the field (Boelter 1964).
Hydraulic conductivity decreases significantly as decomposition proceeds from fibric through hemic to sapric materials. In many Histosols, the degree of decomposi-tion increases with increasing depth, thus hydraulic conductivity decreases with depth, in many cases by two or three orders of magnitude (Hartshorn et al. 2003). Also, horizontal hydraulic conductivity may be two or three times greater than vertical hydraulic conductivity. These conditions favor slow vertical movement of water and significant lateral flow of water through and out of Histosol landscapes.
Uses of HistosolsHistosols are used for production of crops, trees, wildlife, and recreation. Organic material is harvested for (1) horticultural potting materials, (2) chemicals (extractable), (3) fuel for heating and for generation of electricity, and (4) treatment of wastewater and sewage.
Management of Histosols for growing crops (including trees; Berguson et al. 1983) involves drainage (Stephens 1955; Roe 1936), irrigation when appropriate, fertilization, cultivation (Farnham 1983), and several special harvesting practices.
Table 13.1. Selected representative or mean values for properties of organic soil materials in relation to degree of decomposition
Organic carbon (%) 37 35 31
Total nitrogen (%) 1.4 1.6 1.8C:N 28 25 21Cellulose (% dry weight) 23 22 16pH 4.5 4.9 5.1CEC
c/kg) 83 88 101
c/L) 21 44 76
Bulk density, field moist (g/cm3) 0.020.08 0.070.18 0.100.30Water content (% by volume, 10 kPa) 20 50 75Water content (% by volume, 1500 kPa) 10 15 25Vertical hydraulic conductivity (cm/s) 0.045 0.00056 0.00027Horizontal hydraulic conductivity (cm/s) 0.089 No data 0.00027
Source: Rabenhorst and Swanson 2000; Everett 1983.
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The Houghton soils of Figure 13.2 are suitable for production of carrots, onions, potatoes, mint, and salad crops. The low-bulk density of Histosols permits root crops to attain perfect shapes. Forest production improves with drainage of Histosols (Maki 1974). Drainage firms the soil enough to support field equipment. However, dewatering leads to oxidation of the soil by volatilization, chiefly as water and carbon dioxide, and to further subsidence due to consolidation. The oxidation process does release plant nutrients from the organic materials, but far too slowly to satisfy needs of growing plants. Hence, careful fertilization of crops on Histosols must include not only macronutrients, but also micronutrients, such as copper, according to soil tests. Bodies of Histosols are typically in depressions, which, in temperate zones, may have cold microclimates because of cold air drainage to the lowlands. In cranberry bogs, when night frosts are predicted, spray irrigation and even flooding are used to save the crop in these landscape positions with short growing seasons.
Subsidence in Florida was found by Stephens (1956) to be about 3 cm per year under cultivation. The rate of subsidence was directly related to depth of artificial drainage and could be predicted by the following formula: x = (y 2.45)/14.77, where x is the annual subsidence in inches and y is the average depth of drainage in inches. The subsidence rate in Florida was about twice as fast as subsidence of drained Histosols in Indiana where the formula x = (y 9.6)/23 was applicable. Undoubtedly, the difference is related to higher temperatures and lack of freezing in Florida. Another study in Florida (McDowell et al. 1969) indicated that 1.8 meters of a Histosol were lost after 50 years of drainage, or according to the researchers calculations, about as much as had been created in 1,200 years. Therefore, a descrip-tion of Histosols is incomplete without reference to their susceptibility to subsid-ence. As a result of subsidence, soil maps of Histosols in Floridas Everglades (Collins et al. 1986) must be updated regularly. Ground-penetrating radar (Baraniak 1983) is useful in making continuous recordings of thickness of Histosols along transects. In the agricultural part of the Everglades, 66% of the acreage of Lauderhill muck (euic, hyperthermic Lithic Haplosaprists) had thinned during the period 19741984 to Dania muck (euic, hyperthermic, shallow Lithic Haplosaprists). It is estimated that between the years 1914 and 2000, soils of 90,000 ha of the original 250,000 ha ofHistosols used for agricultural production in this region decreased in average thickness from 3.6 m to 30 cm.
The hydrologic characteristics of Histosols are closely related to the degree of decomposition (Table 13.1). In particular, highly decomposed materials (sapric) may have hydraulic conductivities lower than those for clay-textured mineral sediment (Boelter 1965), so low that open ditches cannot effectively control the water table unless very close spacing is used (Boelter 1974). An effective system for drainage of Haplosaprists with extremely low hydraulic conductivity is to space the ditches about 100 m apart and shape the fields between ditches to a relief of about 25 cm, providing for surface runoff and lateral flow to the ditches.
Fire hazard increases after drainage of Histosols. Peat and muck fires are difficult to control and may burn for several months, polluting the air, as well as destroying the
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organic material. Once the organic soil has been destroyed by fire, farming may continue on the remaining mineral substratum, which is usually poorly drained (Davis and Engberg 1955; Jongedyk et al. 1950, 1954; Mirza and Irwin 1964; Neller 1944). In some areas, resistant, hard logs and stumps concentrate on the surface as the other organic materials oxidize. These create difficulties for the operation of equipment (Dolman and Buol 1967). Decomposition and drying of surface organic material make the soil very susceptible to wind erosion due to low bulk density and low cohesion.
Building structures and roads on Histosols is usually difficult. The organic material has very low bearing capacity, and most structures must be placed on foundations that extend to underlying mineral material. If an area is drained just prior to time of construction, subsidence of the soils becomes apparent only gradually. Ultimately the structures are left standing far above the ground level. In the case of house construction on piles driven into the mineral soil, the building becomes a house on posts perched above the lawn. The garage stands above the driveway and is inaccessible to the car (Slusher et al. 1974). Peatland (Histosol) waters are known to corrode both concrete and metal (MacFarlane and Williams 1974).
Horticultural value of Histosol materials lies in their use in potting soil, as conditioners for mineral soil, as plant stimulants, and as a component of some fertilizers. A vast array of chemicals (including dopplerite, a calcium salt of a humic acid of commercial and medicinal value) has formed in Histosols (Gary et al. 1983; Pihlaja et al. 1983). Useful waxes have been separated from peat in Finland (Peltola et al. 1983).
Histosols and their parent materials are used for fuel, either in burning directly or after conversion to methane by gasification (LeMasters et al. 1983). Technology is well advanced for mining, dewatering, and burning organic materials to heat buildings and to generate electricity (Farnham 1978; Chornet et al. 1982; Dubbe et al. 1982). Peat is used extensively as a fuel in northern Europe especially Belarus, Russia, Finland, and Ireland (Rabenhorst and Swanson 2000). Comparative energy values of fuels, in BTU lb1, dry, are as follows: wood, 6,000; grasses and sedges, 7,500; fibric soil, 7,300; sapric soil, 8,300; lignite, 9,500; anthracite, 14,000.
Peatlands are being recognized more and more as important sanctuaries for unique plant communities (insectivorous species, orchids, tundra species) and associated animals. For many years, peatlands were avoided and ignored, or were viewed by many people as wastelands, that were of value only if drained and cultivated. We now recognize the value of these lands as components of wetlands that provide habitat for plants and animals, act as filters of N, P, sediment, and other contaminants (Farnham 1974; Nichols 1981; Loxham and Burghardt 1983), and, while forming, act as sinks of atmospheric carbon (Hartshorn et al. 2003). Undisturbed bodies of Histosols hold scientific interest and aesthetic value (including the heaths fragrance, mentioned by McDonald ) that attract visitors in their leisure time. Scientific areas have been set aside in several states of the United States as gene pools, research areas, and nature appreciation landscapes.
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Describing HistosolsConventions useful in describing mineral soils, as shown in Chapter 2, are not wholly applicable to Histosols. Each layer of a Histosol is described in the following terms: color, fiber content before and after rubbing, structure, consistence, roots, reaction, boundary, and color of the sodium pyrophosphate extract (Soil Survey Staff 1999). Moist and dry colors are determined, when possible, and in addition, the rubbed color of the material is determined after it has been pressed or rubbed in the hands. Unrubbed fiber content is an estimate of the volume of plant fibers observable in a freshly broken surface. Rubbed fiber content is estimated after the material has been rubbed between the fingers to break down extremely rotten fibers. The estimate of fiber content excludes live roots. Structure, consistence, content of live roots, reaction, and boundaries are described as in mineral soils. Sodium pyrophosphate extract color (Munsell) is measured after absorbing solution from a mixture of soil material and sodium pyrophosphate on white filter paper (Kaila 1956; Soil Survey Staff 1996).
Additional information of importance may include the botanical origin of the fibers, notes on included thin strata of organic or mineral soil material, presence of logs and stumps, mineral content, and presence of sulfate minerals (e.g., jarosite) or sulfidic material (sulfide-containing material that becomes very acidic upon oxidation due to sulfuric acid production). All depths are recorded from the soilair interface.
Kinds of Organic Soil Materials. Three kinds of organic soil materialsfibric, hemic, and sapricare recognized on the basis of the extent of decomposition of the plant material from which they are formed (Soil Survey Staff 1999).
Before defining and describing these three soil materials, it is necessary first to describe fibers as they occur in organic soils. Fibers are short pieces of plant tissue in organic soil materials (except for live roots) that (1) are large enough to be retained on a 100-mesh sieve when the materials are sieved after sodium hexametaphosphate dispersion; (2) show cellular structure of the source plants from which they are derived; and (3) must be 2 cm or less in their smallest dimension, or are decomposed enough so they can be crushed with the fingers.
Fibric soil materials are defined as organic soil materials that either (1) contain three-fourths or more by volume of fibers after rubbing, or (2) contain two-fifths or more by volume of fibers after rubbing and yield a sodium pyrophosphate extract with color value and chroma of 7/1, 7/2, 8/1, 8/2, or 8/3. Fibric materials are generally lighter in color, and browner or redder than more decomposed organic soil materials.
Hemic soil materials are between the less decomposed fibric and the more decomposed sapric materials.
Sapric soil materials are the most highly decomposed of the three kinds of organic soil materials. They have a rubbed fiber content of less than one-sixth by volume and yield sodium pyrophosphate extracts with Munsell color value less than 8, and with value and chroma that are below or to the right of a line drawn to exclude value/chroma of 5/1, 6/2, and 7/3. They are mostly dark gray to black, have the smallest amount of plant fiber, highest bulk density, and lowest water content (on dry weight basis) at saturation.
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In addition to fibric, hemic, and sapric materials, humilluvic material is also recognized.
Humilluvic material (illuvial humus) accumulates in lower layers of some Histosols (usually old, drained, and cultivated). The humilluvic material is highly soluble in sodium pyrophosphate and commonly accumulates at a contact with sandy mineral soil material. To be recognized in classifying Histosols, humilluvic material must make up one-half or more (volume) of a soil layer 2-cm or more thick. Hydraulic conductivity of humilluvic material is very slow.
Limnic materials, present in some Histosols, include both organic and inorganic materials that were either deposited in water by algae or diatoms or originated from underwater and floating aquatic plants and were modified by aquatic animals. Coprogenous earth (sedimentary peat layer) is a limnic layer that has many fecal pellets, Munsell color value of 4 or less, forms viscous water suspensions, and is nonplastic or slightly plastic or shrinks upon drying and is difficult to rewet, and either has a saturated sodium pyrophosphate extract Munsell value of 7 or more and chroma of 2 or less or has a CEC less than 240 cmol kg1 of organic matter. Diatomaceous earth is a limnic layer composed mostly of diatoms that has a Munsell color value of 3, 4, or 5, if not previously dried, and yields a sodium pyrophosphate extract with a color value of 8 or more and chroma of 2 or less or has a CEC less than 240 cmol kg1 of organic matter, or both. Marl is a limnic layer composed mostly of carbonates that has a moist color value of 5 or more, and effervesces with dilute hydrochloric acid.
Horizon nomenclature has been developed that relates directly to diagnostic layers (Chapter 2). Organic horizons are denoted by O; subhorizons are designated as i (Oi) for fibric, e (Oe) for hemic, and a (Oa) for sapric. Limnic layers are designated as L layers.
Von Post (1924) developed a 10-stage scale of degree of decomposition of organic soil materials, based on the proportion of material remaining in the hand after squeezing a wet sample. If colorless liquid is produced upon squeezing, it is little decomposed; if all of the organic material escapes between the fingers, the soil is classified at the highest stage of decomposition. With practice, one can get good results with this technique in identifying fibric, hemic, and sapric materials. Fibric material produces only slightly turbid water and no material protrudes between the fingers (Von Post stages 13). Hemic material produces turbid water, and less than two-thirds of the original handful escapes between the fingers upon squeezing; that is, more than one-third of the handful is retained (Von Post stages 47). Upon squeezing wet sapric material, more than two-thirds of the sample extrudes between the fingers (Von Post stages 810).
Classification of HistosolsThree important elements are of major concern in defining the order: (1) a standard minimum content of soil organic matter; (2) a required thickness of this organic soil material; and (3) avoidance of criteria that would necessitate reclassification of the soil as a result of common agricultural practices such as drainage.
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Organic soil material, if saturated with water for more than 30 days or if artifi-cially drained, has at least 18% organic carbon if the mineral fraction has 60% or more clay; at least 12% organic carbon if no clay is present; or proportionately more organic carbon than a line connecting these points for intermediate clay contents. (See Figure 2.3.) If the soil is saturated for fewer than 30 days, it must contain 20% or more organic carbon.
Generally, Histosols do not have permafrost and do not have andic properties in 60% or more of the upper 60 cm of the soil, and half or more of the upper 80 cm of soil is organic soil material. Histosols are also identified if the organic materials rest on rock or fill or partially fill voids in fragmental, cindery, or pumiceous materials. If the bulk density of the soil is less than 0.1 g cm3, then three-fourths or more of the upper 80 cm must be organic soil material.
An arbitrary control section of 130 cm or 160 cm is defined for use in classifying Histosols, providing no densic, lithic, or paralithic layer or water or permafrost occurs in that depth. The thicker limit is used only when the surface 60 cm is more than 75% moss fibers or has a bulk density less than 0.1 g cm3. This control section is then subdivided into three layers referred to as surface, subsurface, and bottom tiers. The surface tier is 30-cm thick unless 75% or more of it consists of moss fiber or has a bulk density less than 0.1 g cm3, in which case it is 60-cm thick. The subsurface tier is 60-cm thick and may include mineral material, provided it is not densic, lithic, or paralithic. The bottom tier is 40-cm thick or it extends to the top ofa densic, lithic, or paralithic layer, or a layer of water or permafrost, whichever isshallower.
The Histosols order is divided into five suborders (Figure 13.4). Histosols that are saturated with water for fewer than 30 days per year are in the suborder Folists. Histosols that have positive water pressure at the surface for more than 21 hours each day are in the suborder Wassists. The Wassists were added as a Histosol suborder in the 11th edition of Keys to Soil Taxonomy (Soil Survey Staff 2010), to identify subaqueous soils dominated by organic soil materials. These subaqueous soils are common in the intertidal zone and adjacent areas submerged by water up to 2.5-m deep (Demas and Rabenhorst 1999; Bradley and Stolt 2003; Osher and Flannagan 2007). The other three suborders are differentiated on the basis of the degree of decomposition of the organic material, primarily in the subsurface tier. Fibrists have fibric material dominant in the subsurface tier and do not have a sulfuric horizon within 50 cm of the surface or sulfidic materials within 100 cm of the surface; Saprists have a subsurface tier dominated by sapric material; Hemists are other Histosols (that have a subsurface tier dominated by hemic organic material).
A listing of the great groups recognized in the Histosol order is given in Table 13.2. A variety of criteria are used to separate great groups. A cryic soil temperature regime is used in all of the suborders other than the Wassists. Soil moisture regime is used to separate the Folists because they are not saturated for long periods. Sulfidic materials and salinity are properties used to differentiate great groups
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13 / Histosols: Organic Soils 319
Saturated < 30 days
Submerged > 21 hours each day
Fibrists Hemists Saprists
Figure 13.4. Diagram showing some relationships among suborders of Histosols.
Table 13.2. Suborders and Great Groups in the Histosols Order
Suborder Great Group
Folists Cryofolists have a cryic soil temperature regime.Torrifolists have an aridic soil moisture regime.Ustifolists have an ustic or xeric soil moisture regime.Udifolistsother Folists.
Wassists Frasiwassists have electrical conductivity of less than 0.2 dS/m in all horizons within 100 cm.Sulfiwassists have sulfidic material within 50 cm.Haplowassistsother Wassists.
Fibrists Cryofibrists have a cryic soil temperature regimeSphagnofibrists have Sphagnum as 75% or more of the volume to a depth of 90 cm, or to
fragmental materials, a densic, lithic, or paralithic contact, or to a mineral soil contact if shallower.
Haplofibristsother Fibrists.Saprists Sulfosaprists have a sulfuric horizon within 50 cm.
Sulfisaprists have sulfidic materials within 100 cm.Cryosaprists have a cryic soil temperature regime.Haplosapristsother Saprists.
Hemists Sulfohemists have a sulfuric horizon within 50 cm.Sulfihemists have sulfidic materials within 100 cm.Luvihemists have a 2-cm or thicker horizon containing 50% or more by volume
humilluvic materials.Cryohemists have a cryic soil temperature regime.
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320 Soil Genesis and Classification
of the Wassists. Presence of moss fibers, humilluvic materials, sulfidic materials, and a sulfuric horizon are properties used to identify great groups in the other suborders. The nature of the bottom tier, including the presence of a mineral soil contact, is considered in the subgroup classification.
PerspectiveHistosols form in surficial bodies of organic material produced mostly by plants and generally accumulate under saturated, anaerobic conditions that slow decomposition (Folists are the exception) or in subaqueous settings where organic soil materials are dominant. Generally, to qualify as a Histosol, the soil must not have permafrost or andic properties, must contain at least 12 to 18% organic carbon by weight if satu-rated with water, or at least 20% organic carbon if not saturated, and be at least 40-cm thick. Other properties important for classification include degree of decomposition, positive water pressure due to submersion, pH, presence of sulfides or their acidic products of oxidation, temperature, salinity, and depth to mineral soil materials. Decomposition causes loss of carbon, an increase in bulk density, water-holding capacity and cation exchange capacity, a lower C:N ratio, and slower hydraulic con-ductivity. Histosols are mined for fuel and for horticultural purposes. Intensive pro-duction of root vegetables, salad crops, sugarcane, and other special crops involves artificial drainage, irrigation, and fertilization. Local conditions of soilscapes and of economic and cultural conditions determine the uses for these soils. Some areas are left undisturbed to serve as wildlife refuges and as natural bodies for storage and fil-tering of water and for flood control. If drained, these soils oxidize steadily, thereby contributing to atmospheric CO
2 levels. They subside due to this aeration and to con-
solidation by dewatering, and are susceptible to fires and wind erosion. Long-term land-use plans are needed for managed peatlands to maintain the soil-forming condi-tions that result in a net accumulation of organic soil materials.
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