Diagnostic Surface Horizons: The Epipedon

The epipedon (Gr. epi, over, upon, and pedon, soil) is ahorizon that forms at or near the surface and in which most ofthe rock structure has been destroyed. It is darkened by organic matter or shows evidence of eluviation, or both. Rock structure as used here and in other places in this taxonomy includes fine stratification (less than 5 mm) in unconsolidated sediments(eolian, alluvial, lacustrine, or marine) and saprolite derived from consolidated rocks in which the unweathered minerals and pseudomorphs of weathered minerals retain their relative positions to each other.

Any horizon may be at the surface of a truncated soil. The following section, however, is concerned with eight diagnostic horizons that have formed at or near the soil surface. These horizons can be covered by a surface mantle of new soil material. If the surface mantle has rock structure, the top of the epipedon is considered the soil surface unless the mantle meets the definition of buried soils in chapter 1. If the soil includes a buried soil, the epipedon, if any, is at the soil surface and the epipedon of the buried soil is considered a buried epipedon and is not considered in selecting taxa unless the keys specifically indicate buried horizons, such as those in Thapto-Histic subgroups. A soil with a mantle thick enough to have a buried soil has no epipedon if the soil has rock structure to the surface or has an Ap horizon less than 25 cm thick that is underlain by soil material with rock structure. The melanic epipedon (defined below) is unique among epipedons. It forms commonly in volcanic deposits and can receive fresh deposits of ash. Therefore, this horizon is permitted to have layers within and above the epipedon that are not part of the melanic epipedon.

CHAPTER 4

Horizons and Characteristics Diagnostic for the Higher Categories22 Soil Taxonomy

A recent alluvial or eolian deposit that retains stratifications (5 mm or less thick) or an Ap horizon directly underlain by such stratified material is not included in the concept of the epipedon because time has not been sufficient for soil-forming processes to erase these transient marks of deposition and for diagnostic and accessory properties to develop. An epipedon is not the same as an A horizon. It may include part or all of an illuvial B horizon if the darkening by organic matter extends from the soil surface into or through the B horizon.

• Anthropic Epipedon

The anthropic epipedon has the same limits as the mollic epipedon in color, structure, and organic-carbon content. It formed during long-continued use of the soil by humans, either as a place of residence or as a site for growing irrigated crops. In the former case, disposal of bones and shells has supplied calcium and phosphorus and the level of phosphorus in the epipedon is too high for a mollic epipedon. Such epipedons occur in the humid parts of Europe, the United States, and South America and probably in other parts of the world, mostly in kitchen middens. The high level of phosphorus in the anthropic epipedons is not everywhere accompanied by a base saturation of 50 percent or more, but it is accompanied by a relatively high base saturation if compared with the adjacent soils. In arid regions some long-irrigated soils have an epipedon that is like the mollic epipedon in most chemical and physical properties. The properties of the epipedon in these areas are clearly the consequence of irrigation by humans. Such an epipedon is grouped with the anthropic epipedons, which developed under human habitation. If not irrigated, such an epipedon is dry in all its parts for more than 9 months in normal years. Additional data about anthropic epipedons from several parts of the world may permit future improvements in this definition.

Required Characteristics

In summary, the anthropic epipedon shows some evidence of disturbance by human activity and meets all of the requirements for a mollic epipedon, except for one or both of the following:

1. 1,500 milligrams per kilogram or more P2 O5 soluble in 1 percent citric acid and a regular decrease in P2O5 to a depth of 125 cm; or

2. If the soil is not irrigated, all parts of the epipedon are dry for 9 months or more in normal years.

• Folistic Epipedon

The folistic epipedon consists of organic material (defined in chapter 3), unless the soil has been plowed. This epipedon normally is at the soil surface, although it can be buried. If the soil has been plowed, the organic-carbon requirements are lower than the requirements for organic soil materal because of the need to accommodate the oxidation that occurs when the soil is plowed. Folistic epipedons occur rimarily in cool, humid regions of the world. They differ from histic epipedons because they are saturated with water for less than 30 days (cumulative) in normal years (and are not artificially drained). Taxa for soils with folistic epipedons above the series level are not currently recognized in this taxonomy. The folistic epipedon is used only with mineral soils.

Required Characteristics

The folistic epipedon is defined as a layer (one or more horizons) that is saturated for less than 30 days (cumulative) in normal years (and is not artificially drained) and either:

1. Consists of organic soil material that:

a. Is 20 cm or more thick and either contains 75 percent or more (by volume) Sphagnum fibers or has a bulk density, moist, of less than 0.1; or b. Is 15 cm or more thick; or 2. Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic-carbon content (by weight) of:

a. 16 percent or more if the mineral fraction contains 60 percent or more clay; or 8 percent or more if the mineral fraction contains no clay; or 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay. Most folistic epipedons consist of organic soil material (defined in chapter 3). Item 2 provides for a folistic epipedon that is an Ap horizon consisting of mineral soil material.

• Histic Epipedon

The histic epipedon consists of organic soil material (peat or muck) if the soil has not been plowed. If the soil has been plowed, the epipedon normally has a high content of organic matter that results from mixing organic soil material with some mineral material. The histic epipedon either is characterized by saturation and reduction for some time in normal years or has been artificially drained. It is normally at the soil surface, although it can be buried.

Required Characteristics

The histic epipedon is a layer (one or more horizons) that is characterized by saturation (for 30 days or more, cumulative)Horizons and Characteristics Diagnostic for the Higher Categories 23 and reduction for some time during normal years (or is artificially drained) and either:

1. Consists of organic soil material that:

a. Is 20 to 60 cm thick and either contains 75 percent or more (by volume) Sphagnum fibers or has a bulk density, moist, of less than 0.1; or b. Is 20 to 40 cm thick; or 2. Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic-carbon content (by weight) of: a. 16 percent or more if the mineral fraction contains 60 percent or more clay; or b. 8 percent or more if the mineral fraction contains no clay; or 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay.

• Melanic Epipedon

The melanic epipedon is a thick, dark colored (commonly black) horizon at or near the soil surface (photo 5). It has high concentrations of organic carbon, generally associated with short-range-order minerals or aluminum-humus complexes. The intense dark colors are attributed to the accumulation of organic matter from which "Type A" humic acids areextracted. This organic matter is thought to result from large amounts of root residues supplied by a gramineous vegetation and can be distinguished from organic matter formed under forest vegetation by the melanic index. The suite of secondary minerals generally is dominated by allophane, and the soil material has a low bulk density and a high anion adsorption capacity.

Required Characteristics

The melanic epipedon has both of the following:

1. An upper boundary at, or within 30 cm of, either the mineral soil surface or the upper boundary of an organic layer with andic soil properties (defined below), whichever is shallower; and

2. In layers with a cumulative thickness of 30 cm or more within a total thickness of 40 cm, all of the following:

a. Andic soil properties throughout; and A color value, moist, and chroma (Munsell designations) of 2 or less throughout and a melanic index of 1.70 or less throughout; and 6 percent or more organic carbon as a weighted average and 4 percent or more organic carbon in all layers.

• Mollic Epipedon

The mollic epipedon is a relatively thick, dark colored, humus-rich surface horizon (or horizons) in which bivalent cations are dominant on the exchange complex and the grade of structure is weak to strong (photos 6 and 7). These properties are common in the soils of the steppes in the Americas, Europe, and Asia.

Properties

The mollic epipedon is defined in terms of its morphology rather than its genesis. It consists of mineral soil material and is at the soil surface, unless it underlies a histic epipedon or thin surface mantle, as explained earlier in this chapter. If the surface layer of organic material is so thick that the soil is recognized as a Histosol (defined below), the horizon that at one time was a mollic epipedon is considered to be buried and no longer meets the definition of an epipedon. The mollic epipedon has soil structure strong enough that less than one-half of the volume of all parts has rock structure and one-half or more of the horizon is not both hard, very hard, or harder and massive when dry. In this definition very coarse prisms, with a diameter of 30 cm or more, are treated as if they were the same as massive unless there is secondary structure within the prisms. The restriction against hardness and structure applies only to those epipedons that become dry. A mollic epipedon can directly overlie deposits with rock structure, including fine stratifications, if the epipedon is 25 cm or more thick. The epipedon does not include any layer in which one-half or more of the volume has rock structure, including fine stratifications. The mollic epipedon has dark color and low chroma in 50 percent or more of its matrix. It typically has a Munsell color value of 3 or less when moist and of 5 or less when dry and chroma of 3 or less when moist. If its structure is fine granular or fine blocky, the sample, when broken, may show only the color of the coatings of peds. The color of the matrix in such situations can be determined only by crushing or briefly rubbing the sample. Prolonged rubbing should be avoided because it may cause darkening of a sample if soft iron- manganese concretions are present. Crushing should be just sufficient to mix the coatings with the matrix. The dry color value should be determined after the crushed sample is dry enough for continued drying to produce no further change and the sample has been smoothed to eliminate shadows. Normally, the color value is at least 1 Munsell unit lower or the chroma at least 2 units lower (both moist and dry) than that of the 1C horizon (if one occurs). Some parent materials, such24 Soil Taxonomy as loess, cinders, basalt, or carbonaceous shale, can also have dark color and low chroma. Soils that formed in such materials can accumulate appreciable amounts of organic matter but commonly have no visible darkening in the epipedon. In these dark colored materials, the requirement that the mollic epipedon have a lower color value or chroma than the C horizon is waived if the surface horizon(s) meets all of the other requirements for a mollic epipedon and, in addition, has at least 0.6 percent more organic carbon than the C horizon. Finely divided CaCO3 acts as a white pigment and causes soils to have a high color value, especially when dry. To compensate for the color of the carbonates, the mollic epipedon is allowed to have lighter color than normal if the epipedon averages more than 15 percent carbonates. If the fine-earth fraction has a calcium carbonate equivalent of 15 to 40 percent, the limit for the dry color value is waived. If it has a calcium carbonate equivalent of 40 percent or more, the limit for the dry color value is waived and the moist color value is 5 or less.

The mollic epipedon forms in the presence of bivalent cations, particularly calcium. The base saturation by the NH4 OAc method is required to be 50 percent or more throughout the epipedon.

The mollic epipedon is thought to be formed mainly through the underground decomposition of organic residues in the presence of these cations. The residues that are decomposed are partly roots and partly organic residues from the surface that have been taken underground by animals. Accumulation and turnover of the organic matter in the mollic epipedon probably are rapid. The radiocarbon age (mean residence time) of the organic carbon is mostly 100 to 1,000 years. A high percentage of the organic matter is so-called "humic acid." The minimum organic-carbon content throughout the thickness of the mollic epipedon is 0.6 percent in most mollic epipedons. Exceptions are (1) a minimum of 2.5 percent organic carbon in epipedons that have a color value, moist, of 4 or 5 and a fine-earth fraction with a calcium carbonate equivalent of 40 percent or more and (2) a minimum of 0.6 percent more organic carbon than in the C horizon in epipedons in which the C horizon has a color as dark as or darker than the color of the epipedon. The maximum organic-carbon content of a mollic epipedon is the same as for mineral soil material. Some Ap horizons that approach the lower limit of a histic epipedon can be part of the mollic epipedon.

The minimum thickness of the mollic epipedon depends on the depth and texture of the soil. The minimum thickness is for soils with an epipedon that is loamy very fine sand or finer and that is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan. These soils have a minimum thickness of 10 cm. Soils that are 10 to 18 cm deep have a mollic epipedon if the whole soil meets all of the criteria for a mollic epipedon when mixed. The minimum thickness is 25 cm for: (1) all soils with a texture throughout the epipedon of loamy fine sand or coarser; (2) all soils that have no diagnostic horizons or features below the epipedon; and (3) soils that are 75 cm or more deep to a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan, are more than 75 cm deep to the upper boundary of any identifiable secondary carbonates, and are more than 75 cm deep to the lower boundary of any argillic, cambic, kandic, natric, oxic, or spodic horizon (all defined below).

The minimum thickness is one-third of the thickness from the mineral soil surface to any of the features described in the paragraph above if (1) the texture throughout the epipedon is loamy very fine sand or finer and (2) depth to the feature described in the paragraph above is between 54 and 75 cm. The minimum thickness is 18 cm for all other soils. The mollic epipedon has less than 1,500 milligrams per kilogram of P2O5 soluble in 1 percent citric acid or has an irregular decrease in the amounts of P2O5 with increasing depth below the epipedon, or there are phosphate nodules within the epipedon. This restriction is intended to exclude plow layers of very old arable soils and kitchen middens that, under use, have acquired the properties of a mollic epipedon and to include the epipedon of a soil developed in highly phosphatic parent material.

Required Characteristics

The mollic epipedon consists of mineral soil materials and has the following properties:

1. When dry, either or both:

a. Structural units with a diameter of 30 cm or less or secondary structure with a diameter of 30 cm or less; or

b. A moderately hard or softer rupture-resistance class; and

2. Rock structure, including fine (less than 5 mm) stratifications, in less than one-half of the volume of all parts; and

3. One of the following:

a. All of the following:

(1) Colors with a value of 3 or less, moist, and of 5 or less, dry; and

(2) Colors with chroma of 3 or less, moist; and

(3) If the soil has a C horizon, the mollic epipedon has a color value at least 1 Munsell unit lower or chroma at least 2 units lower (both moist and dry) than that of the C horizon or the epipedon has at least 0.6 percent more organic carbon than the C horizon; or b. A fine-earth fraction that has a calcium carbonate equivalent of 15 to 40 percent and colors with a value and chroma of 3 or less, moist; or

c. A fine-earth fraction that has a calcium carbonate equivalent of 40 percent or more and a color value, moist, of 5 or less; and

4. A base saturation (by NH4 OAc) of 50 percent or more; and

5. An organic-carbon content of:

a. 2.5 percent or more if the epipedon has a color value, moist, of 4 or 5; or

b. 0.6 percent more than that of the C horizon (if one occurs) if the mollic epipedon has a color value less than 1 Munsell unit lower or chroma less than 2 units lower (both moist and dry) than the C horizon; or

c. 0.6 percent or more; and

6. After mixing of the upper 18 cm of the mineral soil or of the whole mineral soil if its depth to a densic, lithic, or paralithic contact, petrocalcic horizon, or duripan (all defined below) is less than 18 cm, the minimum thickness of the epipedon is as follows:

a. 10 cm or the depth of the noncemented soil if the epipedon is loamy very fine sand or finer and is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan that is within 18 cm of the mineral soil surface; or

b. 25 cm or more if the epipedon is loamy fine sand or coarser throughout or if there are no underlying diagnostic horizons (defined below) and the organic-carbon content of the underlying materials decreases irregularly with increasing depth; or

c. 25 cm or more if all of the following are 75 cm or more below the mineral soil surface:

(1) The upper boundary of any pedogenic lime that is present as filaments, soft coatings, or soft nodules; and (2) The lower boundary of any argillic, cambic, natric, oxic, or spodic horizon (defined below); and (3) The upper boundary of any petrocalcic horizon, duripan, or fragipan; or

d. 18 cm if the epipedon is loamy very fine sand or finer in some part and one-third or more of the total thickness between the top of the epipedon and the shallowest of any features listed in item 6-c is less than 75 cm below the mineral soil surface; or

e. 18 cm or more if none of the above conditions apply; and

7. Phosphate:

a. Content less than 1,500 milligrams per kilogram soluble in 1 percent citric acid; or

b. Content decreasing irregularly with increasing depth below the epipedon; or

c. Nodules are within the epipedon; and

8. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 or higher, if the soil is not irrigated; and

9. The n value (defined below) is less than 0.7.

• Ochric Epipedon

The ochric epipedon fails to meet the definitions for any of the other seven epipedons because it is too thin or too dry, has too high a color value or chroma, contains too little organic carbon, has too high an n value or melanic index, or is both massive and hard or harder when dry (photos 8 and 9). Many ochric epipedons have either a Munsell color value of 4 or more, moist, and 6 or more, dry, or chroma of 4 or more, or they include an A or Ap horizon that has both low color values and low chroma but is too thin to be recognized as a mollic or26 Soil Taxonomy umbric epipedon (and has less than 15 percent calcium carbonate equivalent in the fine-earth fraction). Ochric epipedons also include horizons of organic materials that are too thin to meet the requirements for a histic or folistic epipedon.

The ochric epipedon includes eluvial horizons that are at or near the soil surface, and it extends to the first underlying diagnostic illuvial horizon (defined below as an argillic, kandic, natric, or spodic horizon). If the underlying horizon is a B horizon of alteration (defined below as a cambic or oxic horizon) and there is no surface horizon that is appreciably darkened by humus, the lower limit of the ochric epipedon is he lower boundary of the plow layer or an equivalent depth (18 cm) in a soil that has not been plowed. Actually, the same horizon in an unplowed soil may be both part of the epipedon and part of the cambic horizon; the ochric epipedon and the subsurface diagnostic horizons are not all mutually exclusive. The ochric epipedon does not have rock structure and does not include finely stratified fresh sediments, nor can it be an Ap horizon directly overlying such deposits. The ochric epipedon by itself has few or no accessory characteristics, but an ochric epipedon in combination with other diagnostic horizons and features has many accessory characteristics. For example, if there is an underlying horizon in which clay has accumulated (defined later as an argillic horizon) and if the epipedon is seldom or never dry, carbonates are absent and base saturation is moderate or low in the major part of the epipedon unless the soil has been limed. If the texture is loamy, the structure breaks down easily when the soil is cultivated.

• Plaggen Epipedon

The plaggen epipedon is a human-made surface layer 50 cm or more thick that has been produced by long-continued manuring (photo 10). In medieval times, sod or other materials commonly were used for bedding livestock and the manure was spread on fields being cultivated. The mineral materials brought in by this kind of manuring eventually produced an appreciably thickened Ap horizon (as much as 1 m or more thick). In northwestern Europe this custom was associated with the poorly fertile, sandy Spodosols. The practice more or less ceased at the turn of the 19th century, when fertilizers became available.

The color of a plaggen epipedon and its organic-carbon content depend on the materials used for bedding. If the sod was cut from the heath, the plaggen epipedon tends to be black or very dark gray, to be rich in organic matter, and to have a wide carbon-nitrogen ratio. If the sod came from forested soils,

the plaggen epipedon tends to be brown, to have less organic matter, and to have a narrower carbon-nitrogen ratio. Commonly, the organic-carbon content ranges from 1.5 to 4 percent. Values commonly range from 1 to 4, moist, and chromas are 2 or less.

A plaggen epipedon can be identified by several means. Commonly, it contains artifacts, such as bits of brick and pottery, throughout its depth. There may be chunks of diverse materials, such as black sand and light gray sand, as large as the size held by a spade. The plaggen epipedon normally shows spade marks throughout its depth and also remnants of thin stratified beds of sand that were probably produced on the soil surface by beating rains and were later buried by spading. A map unit delineation of soils with plaggen epipedons would tend to have straight-sided rectangular bodies that are higher than the adjacent soils by as much as or more than the thickness of the plaggen epipedon.

• Umbric Epipedon

The umbric epipedon is a relatively thick, dark colored, humus-rich surface horizon or horizons (photo 11). It cannot be distinguished by the eye from a mollic epipedon, but laboratory studies show that the base saturation is less than 50 percent (by NH4OAc) in some or all parts.

The umbric epipedon is used for defining taxa at different levels. For those soils in which the content of organic matter is roughly proportional to the darkness of the color, the most satisfactory groupings appear to be those that assign soils with a thick, dark colored surface horizon and soils with a light colored or thin surface horizon to different suborders. Structure, bulk density, cation-exchange capacity, and other properties are related to the amount and type of organic matter in these soils. In those kinds of soil where dark color is not related to the content of organic matter, the soils that have light colored epipedons are separated from the soils that have dark colored epipedons only at lower categoric levels, if at all.

Properties

The umbric epipedon consists of mineral soil material and is at the soil surface, unless it underlies either a recent deposit that is less than 50 cm thick and has fine stratification if not plowed or a thin layer of organic soil material. If the surface layer of organic material is so thick that the soil is recognized as a Histosol (defined below), the umbric epipedon is considered to be buried. The umbric epipedon has soil structure strong enough so that one-half or more of the horizon is not both hard, very hard, or harder and massive when dry. Very coarse prisms, with a diameter of 30 cm or more, are treated as if they were the same as massive if there is no secondary structure within the prisms. The restriction against massive and hardness applies only to those epipedons that become dry. The umbric epipedon has dark color and low chroma in 50 percent or more of its matrix. It has a Munsell color value of 3 or less, moist, and of 5 or less, dry, and chroma of 3 or less. If its structure is fine granular or fine blocky, the sample when broken may show only the color of the coatings of peds. The color of the matrix in such situations can be determined onlyHorizons and Characteristics Diagnostic for the Higher Categories 27 by crushing or briefly rubbing the sample. Prolonged rubbing should be avoided because it may cause darkening of a sample if soft iron-manganese concretions are present. Crushing should be just sufficient to mix the coatings with the matrix. The dry color value should be determined after the crushed sample is dry enough for continued drying to produce no further change and the sample has been smoothed to eliminate shadows.

Normally, the color value is at least 1 Munsell unit lower or the chroma at least 2 units lower (both moist and dry) than that of the C horizon (if present). Some parent materials, such as loess, cinders, alluvium, or shale, can also have dark color and low chroma. Soils that formed in such materials can accumulate appreciable amounts of organic matter but commonly show no visible darkening in the epipedon. In these dark colored materials, the requirement that the umbric epipedon have a lower color value or chroma than the C horizon is waived if the surface horizon(s) meets all of the other requirements for an umbric epipedon and, in addition, has at least 0.6 percent more organic carbon than the C horizon.

Base saturation by the NH4OAc method is required to be less than 50 percent in some or all parts of the epipedon. The umbric epipedon is thought to be formed mainly by the decomposition of organic residues. The residues that are decomposed are partly roots and partly organic residues from the surface that have been taken underground by animals. Accumulation and turnover of the organic matter in the umbric epipedon probably are slower than in the mollic epipedon. The aluminum ions may be somewhat toxic to some kinds of soil micro-organisms. The minimum organic-carbon content throughout the thickness of the umbric epipedon is 0.6 percent. The minimum thickness of the umbric epipedon is dependent on the depth and texture of the soil. The minimum thickness is for soils with an epipedon that is loamy very fine sand or finer (when mixed) and that is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan. These soils have a minimum thickness of 10 cm. Soils that are 10 to 18 cm deep have an umbric epipedon if the whole soil meets all of the criteria for an umbric epipedon when mixed. The minimum thickness is 25 cm for (1) all soils with a texture throughout the epipedon of loamy fine sand or coarser; (2) all soils that have no diagnostic horizons or features below the epipedon; and (3) soils that are 75 cm or more deep to a densic, lithic, or paralithic contact or a duripan and are more than 75 cm deep to the lower boundary of any argillic, cambic, kandic, natric, oxic, or spodic horizon (all defined below). The minimum thickness is one-third of the thickness from the mineral soil surface to any of the features in the paragraph above if (1) the texture in some or all parts of the epipedon is loamy very fine sand or finer and (2) depth to the feature listed in the paragraph above is between 54 and 75 cm below the mineral soil surface. The minimum thickness is 18 cm for all other soils. The umbric epipedon has less than 1,500 milligrams per kilogram of P2O5 soluble in 1 percent citric acid or has an irregular decrease in the amounts of P2O5 with increasing depth below the epipedon, or there are phosphate nodules within the epipedon. This restriction is intended to exclude plow layers of very old arable soils and kitchen middens that, under use, have acquired the properties of an umbric epipedon and to include the epipedon of a soil developed in highly phosphatic parent material. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 o C or higher and the soil isnot irrigated.

Sediments that have been continuously under water since deposition have a very high water content and are unable to support livestock. Although some soils that have an umbric epipedon are very poorly drained, the umbric epipedon is required to have an n value (defined below) of less than 0.7. Several accessory properties are common in soils that have an umbric epipedon. These soils have the potential for toxicity from aluminum, and they are commonly low in calcium, magnesium, and potassium if lime and fertilizer have not been applied. These are accessory properties important to plant growth. The structure of the umbric epipedon facilitates the movement of moisture and air whenever the soil is not saturated with water. The content of organic matter indicates that the soil has received enough moisture to support fair to luxuriant plant growth in normal years. The umbric epipedon must be moist in at least some part for 3 months or more (cumulative) in normal years at times when the soil temperature is 5 C or higher at a depth of 50 cm and when the soil is not irrigated. Although the umbric epipedon is a surface horizon that can be truncated by erosion, its many important accessory properties suggest its use as a diagnostic horizon at a high categoric level. Some plaggen epipedons meet all of the requirements for an umbric epipedon but also show evidence of a gradual addition of materials during cultivation, whereas the umbric epipedon does not have the artifacts, spade marks, and raised surfaces that are characteristic of the plaggen epipedon.

Required Characteristics

The umbric epipedon consists of mineral soil materials and has the following properties:

1. When dry, either or both:

a. Structural units with a diameter of 30 cm or less or secondary structure with a diameter of 30 cm or less; or

b. A moderately hard or softer rupture-resistance class; and28 Soil Taxonomy

2. All of the following:

a. Colors with a value of 3 or less, moist, and of 5 or less, dry; and

b. Colors with chroma of 3 or less; and

c. If the soil has a C horizon, the umbric epipedon has a color value at least 1 Munsell unit lower or chroma at least 2 units lower (both moist and dry) than that of the C horizon or the epipedon has at least 0.6 percent more organic carbon than that of the C horizon; and

3. A base saturation (by NH4OAc) of less than 50 percent in some or all parts; and

4. An organic-carbon content of:

a. 0.6 percent more than that of the C horizon (if one occurs) if the umbric epipedon has a color value less than 1 Munsell unit lower or chroma less than 2 units lower (both moist and dry) than the C horizon; or

b. 0.6 percent or more; and

5. After mixing of the upper 18 cm of the mineral soil or of the whole mineral soil if its depth to a densic, lithic, or paralithic contact or a duripan (all defined below) is less than 18 cm, the minimum thickness of the epipedon is as follows:

a. 10 cm or the depth of the noncemented soil if the epipedon is loamy very fine sand or finer and is directly above a densic, lithic, or paralithic contact or a duripan that is within 18 cm of the mineral soil surface; or

b. 25 cm or more if the epipedon is loamy fine sand or coarser throughout or if there are no underlying diagnostic horizons (defined below) and the organic-carbon content of the underlying materials decreases irregularly with increasing depth; or

c. 25 cm or more if the lower boundary of any argillic, cambic, natric, oxic, or spodic horizon (defined below) is 75 cm or more below the mineral soil surface; or

d. 18 cm if the epipedon is loamy very fine sand or finer in some part and one-third or more of the total thickness between the top of the epipedon and the shallowest of any features listed in item 5-c is less than 75 cm below the mineral soil surface; or

e. 18 cm or more if none of the above conditions apply; and

6. Phosphate:

a. Content less than 1,500 milligrams per kilogram soluble in 1 percent citric acid; or

b. Content decreasing irregularly with increasing depth below the epipedon; or

c. Nodules are within the epipedon; and

7. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 o C or higher, if the soil is not irrigated; and

8. The n value (defined below) is less than 0.7; and

9. The umbric epipedon does not have the artifacts, spade marks, and raised surfaces that are characteristic of the plaggen epipedon.