Soil Organic Matter (SOM)

	Definitions
	Factors Influencing SOM
	Properties
	Classification

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Definitions

Soils represent a major pool (172 x 10¹⁰ t) in the cycling of C from the atmosphere to the biosphere and are the habitat for terrestrial photosynthetic organisms, which fix 11 x 10 ¹⁰ t C per year, about half of which eventually finds its way into soils. Organic matter in soils is represented by plant debris or litter in various stages of decomposition through to humus and includes the living organisms in the soil. Above ground plants (phytomass) are generally excluded from discussions of soil organic matter, but living roots are generally included.

 

The following definitions will be followed:

	Soil Organic Matter: Natural C-containing organic materials living or dead, but excluding charcoal.
	Phytomass: It is the above ground portion of materials of plant origin usually living, but may also include standing dead trees.
	Microbial Biomass: It is the living population of soil microrganisms.
	Litter: It comprises the dead plant and animal debris on the soil surface.
	Macroorganic Matter: Organic fragments from any source which are > 250µm (generally less decomposed than humus).
	Organic Carbon: The carbon content is commonly used to characterize the amount of organic matter in soils. Organic matter = 1.724 * percent organic carbon.
	Humus: Material remaining in soils after removal of macroorganic matter (generally material that has been more extensively physically and/or biochemically transformed as a result of soil forming processes than macroorganic matter). There are two major classes: the nonhumic substances (e.g. amino acids, lipids, carbohydrates) and humic substances (a series of high-moelcular-weight amorphous compounds).
	Humic Acids:  Dark-colored amorphous materials that can be extracted from the soil by a variety of reagents, such as strong bases or neutral salts and that are insoluble in dilute acid. This implies that humic acids contain primarily acidic functional groups, such as phenolic or carboxylic groups. Humic acids are composed of molecules with molecular weights in the range 20,000 to 1,360,000. They are considered to be polymerization products of fulvic acids and other decay products.
	Fulvic Acids: The organic materials that are extracted with humid acid but remain in solution upon acidification with dilute acid. This implies that fulvic acids contain acidic functional groups since it is soluble in strong bases and extracted with humic acids that fulvic acids also contain basic groups since it remains in solution upon acidification. Fulvic acids are composed of molecules with molecular weights in the range 275 to 2,100. They are considered to be decay products of higher plants and microbial residue.
	Humin: The strong base insoluble fraction.

 

The carbon cycle describes how carbon is circulated through the atmosphere, biosphere, pedosphere, and hydrosphere. The dead organic matter of the soil is colonized by (micro)organisms, which derive energy for growth from the oxidative decomposition of complex organic molecules. Decomposition is the biochemical breakdown of mineral and organic materials. During decomposition, anorganic elements are converted from organic compounds, a process called mineralization. For example, organic-N and -P is mineralized to NH ₄⁺ and H ₂PO₄ ^-, and C is converted to CO₂. The remainder of the substrate C used by the microorganisms is incorporated into their cell substance (biomass), which is called immobilization. The incroporated minerals are immobilized and released after the organisms die or decay. Humification is the formation of humus (complex organic polymers) from raw organic materials, such as fulvic acids, humic acids, or humin.

 

 

Figure 1. Transformation of soil organic matter within soil.

 

Figure 2. The carbon cycle: quantities and reservoirs. (units: 10 ⁹ metric tonnes)

 

Global Climate Change

Soil organic matter represents a major pool of carbon within the biosphere, estimated at about 1400 x 1015 g globally, roughly twice that in atmospheric CO₂. SOM may act as both a source and sink of carbon during global environmental change. Changes in climate are likely to influence the rate of accumulation and decomposition of carbon in SOM, both directly through changes in temperature and soil moisture, and indirectly through changes in plant growth and rhizodeposition. Other factors, especially changes in land use and management, may have even greater effects. Changes in land use or management may occur as a direct result of climate change or other environmental factors, or may be influenced by agricultural, economic or social policies.

 

SOM models (e.g. DAISY, RothC, CANDY, DNDC, CENTURY, and NCSOIL) embody our best understanding of soil carbon dynamics and may be used to predict how global environmental change will influence soil carbon, and to evaluate the likely effectiveness of different mitigation options.

 

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Factors Influencing SOM

The content of organic matter content is a function of the soil forming factors. Jenny (1930) found that for loamy soils in the United States the effect of soil forming factor to OM were in the order:

 

climate      vegetation       topography, parent material      time

 

Climate

Climate, i.e., precipitation and temperature, influence the amount and type of vegetation as well as the rate of decomposition. The organic matter content of a soil increases with increasing decomposition up to the limit set by temperature. In Figure 3. it is shown, that SOM increases from the east side of the Rocky Mountains to the east with increasing precipitation, and from south to north with decreasing temperature. In soils, every 10 ^oC increase in mean annual temperature results in the organic matter content being reduced by about 1/3 to 1/2, if all other factors are constant. For example, the carbon content to a depth of 1 m is 2 kg/m ² in the Badlands of North Dakota, 21 kg/m ² in some poorly-drained, fine-textured soils of eastern North Dakota. Organic soils (Histosols) contain about 75 kg/m ² (Franzmeier et al., 1985).

 

 

Figure 3. Distribution of soil organic matter in the United States, as related to climate and vegetation (adapted from Schreiner et al., 1938).

 

Generally, cold and arid climate tends to slow down the microbial processes within soil, in particular decomposition and mineralization. Therefore, those soils contain large portions of organic matter as plant debris (macroorganic matter) than as humus. The same effect is observed in acid to very acid soils. The warmer the climate the higher the rates of microbial processes, i.e., the lower the organic matter content in those soils.

 

The soil moisture content also has a remarkable effect of soil organic matter decomposition and accumulation. Waterlogged soils tend to accumulate organic matter because the microbial processes, in particular decomposition and mineralization, are slowed down. In aquic moisture regimes the drainage and soil aeration is poor (anaerobic conditions). Anaerobic oxidation of organic residues is less efficient than aerobic oxidation. If organic matter is accumulated the soil development is towards organic soils (Histosols). Histosols generally form in wet, poorly aerated sites, such as shallow lakes and ponds, depression areas, swamps, and bogs and are the end product of natural eutrophication.

 

Vegetation/Soil Organisms

Vegetation affects soil organic matter by the type, amount, and placement of the organic residues. The composition of organic matter in soil can be related to the nature of the soil floral and faunal community. When biomass is added to the soil, three general reactions take place:

 

	The bulk of the material undergoes enzymatic oxidation with carbon dioxide, water and heat as major products,
	N, P and S are released and/or immobilized by a series of reactions unique to each specific element,
	Compounds resistant to further immediate microbial reaction are formed either from compounds in the initial material or by microbial synthesis.

 

The rates of decomposition, even for simple substrates such as glucose, vary widely due to differnces in water content, temperature, pH and the availability of nutrients such as P and N to support microbial activity. However, the simpler monomers from carbohydrates, proteins, fats, and many polyphenolic materials are decomposed within weeks in soil environments. Polymers (complex compounds) such as hemi-cellulose or cellulose are decomposed more slowly and their resistance to decomposition increase with complexity. It is essential to emphasize that many of the organic compounds found in soils result from in-situ synthesis mediated by microbial processes. Some natural polymers may persist in soils for years:

 

	Cellulose because it is crystalline and often encrusted with lignin and thus not readily accessible to microorganisms,
	Polyphenols in polymers such as humic materials, and waxes, which are both characteristically recalcitrant (i.e., resistant to rapid microbial breakdown).

 

Decomposition reactions are catalyzed by enzymes. Generally, when the C : N ratio is > 25, net immobilization occurs, whereas at ratios < 25 net mineralization is likely. Classically, organic matter has been characterized via various extraction/fractionation procedures into non-humic (lipids, carbohydrates and other 'simple' organic compounds) and the more complex humic susbstances (humic acids, fulvic acids and humin). These divisions do not align well with current understanding of the biological and biochemical processes operating during decomposition and stabilization of organic material in soil.

 

Generally, litter from coniferous trees, such as pine, are undergoing a slow decomposition, whereas the litter from deciduous trees, such as elm, ash, oak, and birch, are easy to decompose. Lignin (complex phenolic polymer) is a significant proportion of straw and coniferous litter, which takes a long time to decompose. Coniferous litter tend to be acidic and low in bases, which promotes greater amounts of soil weathering. Annual species, such as grasses, tend to add organic residues not only to the surface, but due to death and decay of the roots. Also, the residue from annual species tends to have higher base contents than are found with perennials. Therefore, a thicker, darker A horizon is formed under grass than under deciduous or coniferous forest.

 

A sequence for decomposition of litter would look like this, whereas it starts with low decomposition and ends with high rates of decomposition:

coniferous trees      straw       deciduous trees      grass

 

Figure 4. Organic matter content in a grassland and a forest soil profile (modified after Foth et al., 1984).

 

In Figure 4. the differences in organic matter content in a grassland and a forest soil profile are shown. Grassland soils contain more SOM than forest soils under similar environmental conditions. The distribution of SOM is more uniformly distributed through the grassland profile than in a forest soil.

 

On agricultural land the application of mineral fertilizers, manure or the practice of green manuring influence the organic matter content in soils. The application of manure tends to increase soil organic matter because of the supply of nutrients and organic material to the soil .

 

Most of the soil organisms are concentrated in the top 15 - 25 cm of soil because C substrates are more plentiful there. Estimates of microbial biomass C range from 500 to 2,000 kg/ha to 15-cm depth. The macro- and mesofaunal biomass ranges from 2 to 5 t/ha, with earthworms making the largest single contribution. Microorganisms use litter and other organic compounds for respiration, where organic material is mineralized and CO₂ and inorganic elements are released. The prokaryotes include the bacteria and actinomycetes, the eukaryotes include the fungi, algae and protozoa. They can be classified in heterotrophs, which require C in the form of organic molecules for growth, and the autotrophs, which can synthesize their cell substance from the C of CO₂, harnessing the energy of sunlight (in the case of photosynthetic bacteria and algae) or chemical energy from the oxidation of inorganic compounds (the chemoautotrophs). Another way of subdividing the microorganisms is on their requirement of O ₂: (i) the aerobes, those requiring O ₂ as the terminal acceptor of electrons in respiration (ii) the facultative anaerobes, those normally requiring O ₂ but able in anaerobic conditions to use NO ₃ ^- and other inorganic compounds as electron acceptor in respiration (iii) the obligate anaerobes, those which grow only in the absence of O ₂.

 

Table 1. Annual rate of litter return to the soil (White, 1987).

Land use / Vegetation type	Organic C [t/ha]
Alpine and arctic forest	0.1 - 0.4
Arable land	1.0 - 2.0
Temperate grassland	2.0 - 4.0
Coniferous forest	1.5 - 3.0
Deciduous forest	1.5 - 4.0
Tropical rainforest	5.0 - 10.0

 

Topography

Topography affects the amount of surface runoff, erosion and deposition. If erosion removes soil from the shoulder or backslope areas of a hillslope, thinner and light-colored soils remain where the organic matter content is low. Soils found on footslope or toeslope areas generally show a higher organic matter content and thicker A horizon. Because soil moisture often differs across a hillslope microbial activity is affected as well. For example, north-facing slopes are generally wetter and soil temperature is lower compared to south-facing slopes, therefore the humus content is higher in north and lower in the south facing slopes.

 

Parent Material

On sandy soils less organic matter is found than on silty or clayey soils. This can be explained by the characteristics of different sized particles. Sandy soils are well aerated and tend to have a low soil moisture content, which are environmental conditions favor for low organic matter content. Vice versa, clayey soils are less aerated with a high amount of fine micropores and tend to have a higher soil moisture content than medium and fine textured soils, hence, they tend to have a high organic matter content. Furthermore, calcareous or Al/Fe rich soils tend to have higher organic matter contents.

 

Time

"Turnover times" for organic C in soils can be derived by dividing the organic matter content of the soil by the annual biomass input and expressing the answer in years. The turnover time for global C is 30 to 40 years, but varies by orders of magnitude for different ecosystems (the estimates are gross averages and subject to error). Organic soils (Histosols) whose formation is favored by water logging may have turnover times exceeding 2000 y and soils of tundra regions where low temperatures retard oxidation may have turnover times exceeding 100 y. In contrast, the shortest turnover times of about 4 y apply to equatorial forests. Although net primary production is at a maximum in these ecosystems, rapid decomposition precludes appreciable accumulation of soil organic matter.

 

Reference

Foth H.D., 1984. Fundamentals of Soil Science. John Wiley & Sons, New York.

 

Franzmeier D.P., Lemme G.D., and Miles R.J., 1985. Organic Carbon in Soils of North Central United States. Soil Sci. Soc. Am. J., 49: 702 - 708.

 

Jenny H., 1930. A Study on the Influence of Climate upon Nitrogen and Organic Matter Content of Soil, Missouri Agr. Exp. Sta. Res. Bull. No 52, University of Missouri, Columbia, Mo. 

 

Schreiner O., and Brown B.E., 1938. Soil Nitrogen. In: Soils and Men. USDA Yearbook, Washington D.C.: 361 - 376.

 

White R.E., 1987. Introduction to the Principles and Practice of Soil Science. Blackwell Sci. Publ., Oxford, London, Boston.

 

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Properties

Cation Exchange Capacity

Organic matter makes a substantial contribution to the cation exchange capacity (CEC) of the whole soil, and hence to the retention of exchangeable cations. This is because humification produces organic colloids of high specific surface area. It should be stressed, that the CEC of soil organic matter is completely pH-dependent and buffered over a wide range of H ⁺ ion concentrations. The functional groups, such as the -COOH (carboxylic) and the -OH (phenolic groups), dissociate H ⁺ and thus can accept cations such as K ⁺, Na⁺, Ca ²⁺ ,or Mg²⁺. These cations are generally considered to be part of a reservoir of exchangeable cations in the soil. The approximate CEC of organic matter varies between 1500 - 5000 cmol/kg. From 7 - 20 % of the CEC of many soils is caused by organic matter.

 

Interaction of SOM with Clay-Size Material

The relationship between clay type and content and organic matter accumulation and stabilization is complex. This is because clay content is usually correlated with other factors that result in organic matter production. In particular, clay content is often correlated with greater plant growth for chemical (plant nutrients) and physical (water regime) reasons and results in greater annual input of C. There is also evidence that clay type and associated cations influence organic matter stabilization. Vice versa, the presence of organic matter is of great importance in the formation and stabilization of soil structure. The fulvic and humic acids and their polymers are adsorbed on to mineral surfaces by the functional groups, of which the most important ones are carboxyl (-COOH), carbonyl (-C=O), hydroxyl (-OH), amino (=NH), and amine (-NH ₂). Large uncharged polymers (e.g. polysaccharides) can be adsorbed by hydrogen-bonding and by van der Waals' forces, and also function as bonding agent between mineral particles.

 

Field and laboratory experiments using additions of 14C-labelled organic compounds have been conducted to evaluate the fate of organic additions to soils of contrasting textures. The finer textured soils typically show a larger initial flush of microbial activity that is followed by greater incorporation and stabilization of organic matter in the soil than found in coarser textured soils.

 

Porosity exerts a strong influence on the fate of residues added to the soil because it define the domains in which microorganisms can function and those smaller domains into which organic molecules can migrate and become physically isolated from microbial attack. According to Kilbertus (1980), bacteria function only in pores that are at least 3 times their own diameter. Thus, bacteria are excluded from much of the pore space in soils, an effect that becomes more pronounced with increasing clay content. Thus, in clay-rich soils the physical separation between microorganisms and organic molecules can be extensive and account in part for their tendency to have larger accumulations than coarser-textured soils formed under otherwise comparable conditions.

 

It has been suggested that stabilization of organic molecules may occur between quasi-crystals (a packet of several layers) and within interlayers of 2:1 swelling clays such as montmorillonite. This mechanism has been inferred from examination of high resolution transmission electron micrographs that show presence of organic molecules within ~1.0 µm diameter pores between clay crystals. It is assumed that these domains provide considerable protection against microbial attack. Humic substances coat, partially or totally, mineral particles such as clay, often protecting the coated particles from weathering.

 

Cation Bridges and Retention of Organic Matter

Polyvalent cations (e.g., Ca²⁺, Mg ²⁺, Fe³⁺, Al ³⁺) play a major role in the stabilization of organic and inorganic colloids - when in abundance limiting their ability to shrink and swell - favoring a flocculated (stable) condition. Polyvalent cations serve as bridges between negatively charged clays (inorganic colloids) and negatively charged organic colloids, which enhances structural stability.

 

In neutral and alkaline soils, Ca²⁺ and Mg²⁺ are the major cations responsible for bridging and the hydroxypolyvalent cations, Fe³⁺ and Al³⁺, serve a similar role in acid soils and those with a large amount of hydrous oxides. There are empirical observations that calcareous soils tend to have larger accumulations of organic matter than their non-calcareous neighbors. Liming experiments provide some insight into the role of Ca²⁺ in conversion of plant residue into stable organic matter. Addition of CaSO ₄ or CaCO₃ to soil containing 14C-labelled wheat straw produces an initial 'priming effect' on microbial biomass activity resulting in accelerated release of CO ₂ that is followed by a greater retention and stabilization of organic matter than found in control treatments (i.e., no Ca ²⁺ addition). Thus, the 'priming effect' of Ca ²⁺ addition to the soil appears to be transient and the long term effect is one of stabilization of organic matter. The proposed mechanism of stabilization is the formation of Ca²⁺ cation bridges.

 

The mechanisms that control Fe³⁺ and Al³⁺ linkages with organic molecules are poorly understood. Fe³⁺ is only sparingly soluble in most soils and occurs mainly in hydrous oxide forms, some of which may be positively charged at low pH because of protonation or addition of hydrogen ions to surface exposed hydroxyl groups. Such positive charged surfaces may attract negatively charged organic molecules. A similar generalized mechanism probably operates with hydrous oxides of Al ³⁺ . However, at low pH soils may exhibit Al ³⁺ toxicity to vegetation, which would tend to limit C inputs into the soil.

 

The chelation process results in the formation of chelates, which are stable complexes containing organic compounds and metallic cations, which are trapped within the ring structures. The complexes can hardly be dissolved. Chelates formed with certain di- and polyvalent cations are the most stable, the stability falling in the order Cu > Fe = Al > Mn = Co > Zn.

 

Soil Moisture

Humic and fulvic acids are considered to be hydrophilic colloids. As such, they have a high affinity for water and are solvated in aquaeous solution.

 

Organic compounds (organic colloids < 2 micrometer) have the characteristic to increase field capacity because they tend to hydrolize. Generally, organic matter can hold up to 20 times its weight in water. This is important particularly for sandy soils to improve soil moisture conditions during summer seasons, when precipitation is limited and evapotranspiration rates are high. If organic matter becomes dry it is prone to wind erosion and can be transported over wide distances.

 

Soil Temperature

Because of the dark black color of organic compounds the adsorption of solar radiation is high and reflection low, therefore soils high in SOM tend to warm up faster than soils low in SOM.

 

Buffering

Organic matter exhibits buffering in slightly acid, neutral, and alkaline range. This buffering helps to maintain an uniform reaction in the soil.  

 

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Classification

Litter Classification

Litter accumulation and its extent of decomposition on the soil surface (O horizons) differs widely among ecosystems and locally within ecosystems. Climatic factors exert a strong influence on the rate of biomass turnover, and the composition of plant debris and mode(s) of its incorporation into the soil influence activity of the fauna and flora involved in the various transformation processes. The following classification of litter layers or O horizons is based on C:N ratios of the plant debris. In general, debris with a large amount of N is associated with large amount of water soluble organic compounds (e.g., amino acids, sugars) and elements such as S and P that stimulate microbial activity and thus initial degradation of the debris.

 

	Mull: low C:N <25, species - alder, false acacia, ash, grasses, legumes (N-fixers), ameliorators.
	Moder: intermediate C:N 30-45, species - oak, beach.
	Mor: high C:N >60, species - conifers, ericaceous plants, acidifiers.

 

A hypothetical soil profile under deciduous species could be described as follows: There is a loose litter layer 2 - 5 cm deep under which the soil is well aggregated, porous, dark-brown in color, and has a granular structure. Below there is a deep A (approximately 30 - 50 cm) of a C : N ratio 10 - 15. The litter accumulation would be classified as mull. In contrast, a hypothetical soil profile under coniferous species could be described as follows: The surface litter is thick (5 - 20 cm) and ramified by plant roots and a fungi mycelium. There is a sharp transition between the organic and underlying mineral soil layers. The litter would be classified as mor.

 

O horizons are described in the field in terms of their relative degree of decomposition using the following subscript designations:

 

	[O] a - Highly decomposed OM, rubbed fibre content < 1/6 of the volume.
	[O] e - OM of intermediate decomposition, rubbed fibre content 1/6 to 2/5 of volume.
	[O] i - Slightly decomposed OM, rubbed fibre content > 2/5 of volume.

 

The symbol 'h' is used for the illuvial accumulation of organic matter but only in combination with B horizons. The 'h' indicates an accumulation of illuvial, amorphous, dispersible organic matter with or without sesquioxides. The influence of tillage or other cultivation disturbance that mix the surface layer is denoted by a 'p'. The symbol 'p' is only used with the master horizon A or O, even if the material mixed by cultivation is from an E, B, or C horizon.

 

Diagnostic Surface Horizons: Epipedons

For the purposes of soil classification, diagnostic horizons have been developed to provide major distinctions among soils. Diagnostic horizons do not necessarily correspond with those described in the field, but are defined on the basis of specific depth limits and/or presence of specific properties. Diagnostic surface horizons are called epipedons, seven of which are recognized (mollic, ochric, mellanic, plaggen, histic, anthropic and umbric). Epipedons are horizons, which formed at the land surface in which rock/sedimentary structure has been replaced by soil structure, and has either been darkened by soil organic matter (SOM) and/or eluviated. Such a horizon may be covered by thin (< 50 cm) alluvial or eolian material without loosing its identity as an epipedon.

Note:

	If a fresh alluvial or eolian cover is > 50 cm thick, then the underlying horizon is considered to be part of a buried soil, which is indicated by the subscript 'b' following the master horizon designation, e.g., C, Ab, Eb...
	Any horizon may be at the surface following truncation (erosion) of the soil - in such cases, because the freshly exposed subsurface horizon had not formed at the surface, it would not qualify as an epipedon. Thus, some soils will not have a diagnostic epipedon (e.g. colluvial soils formed in closed depressions, reconstruction sites).

 

A simple key to understanding the distinctions among the seven diagnostic epipedons centers on (i) distinguishing between mineral and organic surfaces, (ii) the thorough understanding of the definition of the mollic epipedon, and (iii) the awareness of the field settings and conditions where the less common epipedons (e.g., umbric, melanic, anthropic, plaggen) are likely to occur.

 

Figure 5. illustrates the criterion used to distinguish between mineral and organic soil materials. Note that as clay content increases the amount of SOM required to meet the organic soil material designation increases. The rationale behind this reflects (i) the intrinsic influence that particle-size has on SOM stabilization in soil, and (ii) functional behavior of SOM in relation to particle-size (i.e., SOM has a comparatively stronger influence on soil behavior with decreasing particle-size).

 

Figure 5. Key to the epipedons in Soil Taxonomy.

 

The following list describes the epipedons and their major characteristics.

Histic Epipedon: The histic epipedon has an aquic condition for some time in most years or has been artificially drained, and either,

consists of organic soil material,which:

- is 20 to 60 cm thick and either contains 75% or more (by volume) sphagnum fibers or has a bulk density, moist, < 0.1 g/cm³ ; or

- is 20 to 40 cm thick and meets the organic carbon contents shown in Figure 6.

Is an Ap horizon which, when mixed to a depth of 25 cm, has an organic content (by weight) of:

- 16% or more if the mineral fraction contains 60% or more clay: or

- 8% or more if the mineral fraction contains no clay; or

- 8+ (clay % divided by 7.5) % or more if the mineral fraction contains < 60% clay.

Folistic Epipedon:  

Consists of organic soil material

Epipedon saturated for less than 30 days.

 

 

Figure 6. Organic matter (carbon) content required for soil horizons of different clay contents to all qualify as organic horizons. 

 

 

Mollic Epipedon: The mollic epipedon has the following properties:

Soil structure is strong enough so that 1/2 or more of the horizon is not massive when dry. Very coarse prisms, with a diameter of 30 cm or more, are included within the definition of massive if there is no secondary structure within the prisms.

Color crushed and smoothed has a Munsell value of 3 or less (moist) and 5 (dry), and a chroma of 3 or less (moist). Additional qualifications on these limits are outlined in Keys to Soil Taxonomy (KST).

Base saturation is 50% or more by the NH₄ OAc method.

Organic carbon is either 0.6% or more through out the thickness of the mollic, or 2.5 % or more in layers that exhibit 'mollic' colors.

Thickness: After mixing the upper 18 cm of the mineral soil it meets the color and structure requirements outlined above. Additional qualifications on these limits are outlined in Keys to Soil Taxonomy (KST).

Phosphorous limits: The epipedon has < 250 ppm of P ₂O₅ soluble in 1% citric acid. This restriction distinguishes the mollic from cultural epipedons that have unusually large contents of P.

Soil moisture regime: If the soil is not irrigated, some part of the epipedon is moist 3 months or more (cumulative) per year in 8 out of 10 y, during times when the soil temperature is 5^oC or higher.

The n value is less than 0.7. Although many soils that have a mollic epipedon are poorly drained, a mollic does not have the same very high water content as sediments that have been continuously under water since deposition (i.e., they have acquired soil structure, which improves internal drainage).

Umbric Epipedon: The requirements for the umbric epipedon are the same for the mollic, except that base saturation is <50%.

Anthropic Epipedon: The requirements for the anthropic epipedon are the same for the mollic, except that P₂ O₅ soluble in 1% citric acid is > 250 ppm.

Plaggen Epipedon: The plaggen epipedon is a cultural surface horizon produced by long continued manuring. Its color depends on the nature of the manure. Commonly it contains artifacts, such as bits of bricks and pottery through out its depth.

Melanic Epipedon: The melanic epipedon is a thick black horizon which contains high concentrations of organic matter, usually associated with short-range-order minerals or aluminium-humus complexes. The intense black color is attributed to the accumulation of organic matter from which "Type A" humic acids are extracted. This organic matter is thought to result from large amounts of gramineous vegetation, and can be distinguished from organic matter formed under forest vegetation by the melanic index. Additional information about the melanic index is outlined in Keys to Soil Taxonomy (KST).

Ochric Epipedon: The ochric epipedon does not meet the requirements of any of the epipedons listed above, but does show signs of surface soil formation (i.e., soil structure, darkening by organic matter).

 

The umbric epipedon can not be simply be distinguished from the mollic epipedon in the field. A determination of base saturation is required to distinguish the >50% base saturated mollic from the <50% base saturated umbric. The plaggen epipedon and anthropic epipedon are not commonly found and both owe their origin to local human manipulation of the soil. The histic epipedon has large amounts of organic material overlying mineral subsoils. The histic epipedon is not used in reference to the soils that are classified as Histosols. The mellanic epipedon has restricted occurrence and is associated with soils formed in volcanic materials.

 

Table 2. Summary of epipedon names and important characteristics.

Epipedon Name	Derivation	Important characteristics
Histic	histos, tissue (Greek)	Thin, organic horizon saturated 30 consecutive days or more, unless drained. If mixed with mineral material, remains very high in organic matter
Plaggen	Plaggen, sod (German)	Overly thick mollic (> 50 cm) due to long continued manure application
Anthropic	anthropos, man (Greek)	Like mollic, but with a high phosphorus content due to long period of cultivation and fertilization
Mollic	mollis, soft (Latin)	Thick, well-structured, base saturation > 50 %, dark-colored mineral soil horizon
Umbric	umbra, shade (Latin)	Like mollic, but with base saturation < 50 %
Ochric	ochros, pale (Greek)	Surface mineral horizon that does not meet criteria for other epipedons

 

Diagnostic Organic Materials

Fibric Soil Material: In an unrubbed condition, fibers compose over 2/3 of the mass, and the material yields almost clear solutions when extracted with sodium pyrophosphate.

  Hemic Soil Material: In an unrubbed condition 1/3 to 2/3 of the total mass is composed of fibers (intermediate in decomposition between fibric and sapric).

Humilluvic Material: Illuvial humus that accumulates after prolonged cultivation of some acid organic soils.

Limnic Soil Material: These are organic or inorganic materials deposited in water by the action of aquatic organisms or derived from underwater and floating organisms. Marl, diatomaceous earth, and sedimentary peat (coprogeneous earth) are considered limnic materials.

Sapric Soil Material: In an unrubbed condition, less than 1/3 of the mass is composed of identifiable fibers and produced sodium pyrophosphate extracts with colors lower in value and higher in chroma than 10 YR 7/3.