Anthropic Epipedon

Required Characteristics 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 P₂O₅ 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

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:
   1. 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

   2. 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:
   1. 16 percent or more if the mineral fraction contains 60 percent or more clay; or

   2. 8 percent or more if the mineral fraction contains no clay; or

   3. 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay.

Histic Epipedon

Required Characteristics

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

1. Consists of organic soil material that:
   1. 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

   2. 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:
   1. 16 percent or more if the mineral fraction contains 60 percent or more clay; or

   2. 8 percent or more if the mineral fraction contains no clay; or

   3. 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay.

Most histic epipedons consist of organic soil material (defined in chapter 2). Item 2 provides for a histic epipedon that is an Ap horizon consisting of mineral soil material. A histic epipedon consisting of mineral soil material can also be part of a mollic or umbric epipedon.

Melanic Epipedon

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:
   1. Andic soil properties throughout; and

   2. A color value, moist, and chroma (Munsell designations) of 2 or less throughout and a melanic index of 1.70 or less throughout; and

   3. 6 percent or more organic carbon as a weighted average and 4 percent or more organic carbon in all layers.

Mollic Epipedon

Required Characteristics

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

1. When dry, either or both:
   1. Structural units with a diameter of 30 cm or less
      or secondary structure with a diameter of 30 cm or less; or

   2. 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:
   1. 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

   2. 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

   3. 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 NH₄OAc) of 50 percent or more; and

5. An organic-carbon content of:
   1. 2.5 percent or more if the epipedon has a color value, moist, of 4 or 5; or

   2. 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

   3. 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:
   1. 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

   2. 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

   3. 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

   4. 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

   5. 18 cm or more if none of the above conditions apply; and

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

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

   3. 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 °C or higher, if the soil is not irrigated; and

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

Umbric Epipedon

Required Characteristics

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

1. When dry, either or both:
   1. Structural units with a diameter of 30 cm or less or secondary structure with a diameter of 30 cm or less; or

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

2. 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 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:
   1. 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

   2. 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:
   1. 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

   2. 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

   3. 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

   4. 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

   5. 18 cm or more if none of the above conditions apply; and

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

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

   3. 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 °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.

1. All argillic horizons must meet both of the following requirements:
   1. One of the following:
      1. If the argillic horizon is coarse-loamy, fine-loamy, coarse-silty, fine-silty, fine, or very-fine or is loamy or clayey, including skeletal counterparts, it must be at least 7.5 cm thick or at least one-tenth as thick as the sum of the thickness of all overlying horizons, whichever is greater; or

      2. If the argillic horizon is sandy or sandy-skeletal, it must be at least 15 cm thick; or

      3. If the argillic horizon is composed entirely of lamellae, the combined thickness of the lamellae that are 0.5 cm or more thick must be 15 cm or more; and

   2. Evidence of clay illuviation in at least one of the following forms:
      1. Oriented clay bridging the sand grains; or

      2. Clay films lining pores; or

      3. Clay films on both vertical and horizontal surfaces of peds; or

      4. Thin sections with oriented clay bodies that are more than 1 percent of the section; or

      5. If the coefficient of linear extensibility is 0.04 or higher and the soil has distinct wet and dry seasons, then the ratio of fine clay to total clay in the illuvial horizon is greater by 1.2 times or more than the ratio in the eluvial horizon; and

2. If an eluvial horizon remains and there is no lithologic discontinuity between it and the illuvial horizon and no plow layer directly above the illuvial layer, then the illuvial horizon must contain more total clay than the eluvial horizon within a vertical distance of 30 cm or less, as follows:
   1. If any part of the eluvial horizon has less than 15 percent total clay in the fine-earth fraction, the argillic horizon must contain at least 3 percent (absolute) more clay (10 percent versus 13 percent, for example); or

   2. If the eluvial horizon has 15 to 40 percent total clay in the fine-earth fraction, the argillic horizon must have at least 1.2 times more clay than the eluvial horizon; or

   3. If the eluvial horizon has 40 percent or more total clay in the fine-earth fraction, the argillic horizon must contain at least 8 percent (absolute) more clay (42 percent versus 50 percent, for example).

Albic (L. albus, white) materials are soil materials with a color that is largely determined by the color of primary sand and silt particles rather than by the color of their coatings. This definition implies that clay and/or free iron oxides have been removed from the materials or that the oxides have been segregated to such an extent that the color of the materials is largely determined by the color of the primary particles.

Required Characteristics

Albic materials have one of the following colors:

1. Chroma of 2 or less; and either
   1. A color value, moist, of 3 and a color value, dry, of 6 or more; or

   2. A color value, moist, of 4 or more and a color value, dry, of 5 or more; or

2. Chroma of 3 or less; and either
   1. A color value, moist, of 6 or more; or

   2. A color value, dry, of 7 or more; or

3. Chroma that is controlled by the color of uncoated grains of silt or sand, hue of 5YR or redder, and the color values listed in item 1-a or 1-b above.

Relatively unaltered layers of light colored sand, volcanic ash, or other materials deposited by wind or water are not considered albic materials, although they may have the same color and apparent morphology. These deposits are parent materials that are not characterized by the removal of clay
and/or free iron and do not overlie an illuvial horizon or other soil horizon, except for a buried soil. Light colored krotovinas or filled root channels should be considered albic materials only if they have no fine stratifications or lamellae, if any sealing along the krotovina walls has been destroyed, and if these intrusions have been leached of free iron oxides and/or clay after deposition.

Andic Soil Properties

Andic soil properties result mainly from the presence of significant amounts of allophane, imogolite, ferrihydrite, or aluminum-humus complexes in soils. These materials, originally termed "amorphous" (but understood to contain allophane) in the 1975 edition of Soil Taxonomy (USDA, SCS, 1975), are commonly formed during the weathering of tephra and other parent materials with a significant content of volcanic glass. Although volcanic glass is or was a common component in many Andisols, it is not a requirement of the Andisol order.

Required Characteristics

To be recognized as having andic soil properties, soil materials must contain less than 25 percent (by weight) organic carbon and meet one or both of the following requirements:

1. In the fine-earth fraction, all of the following:
   1. Aluminum plus ¹/2 iron percentages (by ammonium oxalate) totaling 2.0 percent or more; and

   2. A bulk density, measured at 33 kPa water retention, of 0.90 g/cm³ or less; and

   3. A phosphate retention of 85 percent or more; or

2. In the fine-earth fraction, a phosphate retention of 25 percent or more, 30 percent or more particles 0.02 to 2.0 mm in size, and one of the following:
   1. Aluminum plus ¹/2 iron percentages (by ammonium oxalate) totaling 0.40 or more and, in the 0.02 to 2.0 mm fraction, 30 percent or more volcanic glass; or

   2. Aluminum plus ¹/2 iron percentages (by ammonium oxalate) totaling 2.0 or more and, in the 0.02 to 2.0 mm fraction, 5 percent or more volcanic glass; or

   3. Aluminum plus ¹/2 iron percentages (by ammonium oxalate) totaling between 0.40 and 2.0 and, in the 0.02 to 2.0 mm fraction, enough volcanic glass so that the glass percentage, when plotted against the value obtained by adding aluminum plus ¹/2 iron percentages in the fine-earth fraction, falls within the shaded area of figure 1.

Figure 1.—Soils that are plotted in the shaded area have andic soil properties. A soil has these properties if the fraction less than 2.0 mm in size has phosphate retention of more than 25 percent and the 0.02 to 2.0 mm fraction is at least 30 percent of the fraction less than 2.0 mm in size.

1. Show evidence of pedogenesis within the aggregates or, at a minimum, on the faces of the aggregates; and

2. Slake when air-dry fragments of the natural fabric, 5 to 10 cm in diameter, are submerged in water; and

3. Have a firm or firmer rupture-resistance class and a brittle manner of failure when soil water is at or near field capacity; and

4. Restrict the entry of roots into the matrix when soil water is at or near field capacity.

1. 3 percent or more (absolute) higher than in the overlying eluvial horizon (e.g., 13 percent versus 10 percent) if any part of the eluvial horizon has less than 15 percent clay in the fine-earth fraction; or

2. 20 percent or more (relative) higher than in the overlying eluvial horizon (e.g., 24 percent versus 20 percent) if all parts of the eluvial horizon have more than 15 percent clay in the fine-earth fraction.

1. Abrupt textural contacts.—An abrupt change in particle-size distribution, which is not solely a change in clay content resulting from pedogenesis, can often be observed.

2. Contrasting sand sizes.—Significant changes in sand size can be detected. For example, if material containing mostly medium sand or finer sand abruptly overlies material containing mostly coarse sand and very coarse sand, one can assume that there are two different materials. Although the materials may be of the same mineralogy, the contrasting sand sizes result from differences in energy at the time of deposition by water and/or wind.

3. Bedrock lithology vs. rock fragment lithology in the soil.—If a soil with rock fragments overlies a lithic contact, one would expect the rock fragments to have a lithology similar to that of the material below the lithic contact. If many of the rock fragments do not have the same lithology as the underlying bedrock, the soil is not derived completely from the underlying bedrock.

4. Stone lines.—The occurrence of a horizontal line of rock fragments in the vertical sequence of a soil indicates that the soil may have developed in more than one kind of parent material. The material above the stone line is most likely transported, and the material below may be of different origin.

5. Inverse distribution of rock fragments.—A lithologic discontinuity is often indicated by an erratic distribution of rock fragments. The percentage of rock fragments decreases with increasing depth. This line of evidence is useful in areas of soils that have relatively unweathered rock fragments.

6. Rock fragment weathering rinds.—Horizons containing rock fragments with no rinds that overlie horizons containing rocks with rinds suggest that the upper material is in part depositional and not related to the lower part in time and perhaps in lithology.

7. Shape of rock fragments.—A soil with horizons containing angular rock fragments overlying horizons containing well rounded rock fragments may indicate a discontinuity. This line of evidence represents different mechanisms of transport (colluvial vs. alluvial) or even different transport distances.

8. Soil color.—Abrupt changes in color that are not the result of pedogenic processes can be used as indicators of discontinuity.

9. Micromorphological features.—Marked differences in the size and shape of resistant minerals in one horizon and not in another are indicators of differences in materials.

1. Laboratory data—visual scan.—The array of laboratory data is assessed in an attempt to determine if a field-designated discontinuity is corroborated and if any data show evidence of a discontinuity not observed in the field. One must sort changes in lithology from changes caused by pedogenic processes. In most cases the quantities of sand and coarser fractions are not altered significantly by soil-forming processes. Therefore, an abrupt change in sand size or sand mineralogy is a clue to lithologic change. Gross soil mineralogy and the resistant mineral suite are other clues.

2. Data on a clay-free basis.—A common manipulation in assessing lithologic change is computation of sand and silt separates on a carbonate-free, clay-free basis (percent fraction, e.g., fine sand and very fine sand, divided by percent sand plus silt, times 100). Clay distribution is subject to pedogenic change and may either mask inherited lithologic differences or produce differences that are not inherited from lithology. The numerical array computed on a clay-free basis can be inspected visually or plotted as a function of depth.

   Another aid used to assess lithologic changes is computation of the ratios of one sand separate to another. The ratios can be computed and examined as a numerical array, or they can be plotted. The ratios work well if sufficient quantities of the two fractions are available. Low quantities magnify changes in ratios, especially if the denominator is low.

1. A pH value in water (1:1) of 5.9 or less and an organic-carbon content of 0.6 percent or more; and

2. One or both of the following:
   1. An overlying albic horizon that extends horizontally through 50 percent or more of each pedon and, directly under the albic horizon, colors, moist (crushed and smoothed sample), as follows:
      1. Hue of 5YR or redder; or

      2. Hue of 7.5YR, color value of 5 or less, and chroma
         of 4 or less; or

      3. Hue of 10YR or neutral and a color value and chroma of 2 or less; or

      4. A color of 10YR 3/1; or

   2. With or without an albic horizon and one of the colors listed above or hue of 7.5YR, color value, moist, of 5 or less, chroma of 5 or 6 (crushed and smoothed sample), and one or more of the following morphological or chemical properties:
      1. Cementation by organic matter and aluminum, with or without iron, in 50 percent or more of each pedon and a very firm or firmer rupture-resistance class in the cemented part; or

      2. 10 percent or more cracked coatings on sand grains; or

      3. Aluminum plus ¹/2 iron percentages (by ammonium oxalate) totaling 0.50 or more, and half that amount or less in an overlying umbric (or subhorizon of an umbric) epipedon, ochric epipedon, or albic horizon; or

      4. An optical-density-of-oxalate-extract (ODOE) value of 0.25 or more, and a value half as high or lower in an overlying umbric (or subhorizon of an umbric) epipedon, ochric epipedon, or albic horizon.

1. Clay minerals: All 2:1 lattice clays, except for one that is currently considered to be an aluminum-interlayered chlorite. Sepiolite, talc, and glauconite are also included in this group of weatherable clay minerals, although they are not everywhere of clay size.

2. Silt- and sand-sized minerals (0.02 to 0.2 mm in diameter): Feldspars, feldspathoids, ferromagnesian minerals, glass, micas, zeolites, and apatite.

Coprogenous Earth

A layer of coprogenous earth (sedimentary peat) is a limnic layer that:

1. Contains many fecal pellets with diameters between a few hundredths and a few tenths of a millimeter; and

2. Has a color value, moist, of 4 or less; and

3. Either forms a slightly viscous water suspension and is nonplastic or slightly plastic but not sticky, or shrinks upon drying, forming clods that are difficult to rewet and often tend to crack along horizontal planes; and

4. Either yields a saturated sodium-pyrophosphate extract on white chromatographic or filter paper that has a color value of 7 or more and chroma of 2 or less (fig. 2) or has a cation-exchange capacity of less than 240 cmol(+) per kg organic matter (measured by loss on ignition), or both.

Diatomaceous Earth

A layer of diatomaceous earth is a limnic layer that:

1. If not previously dried, has a matrix color value of 3, 4,
   or 5, which changes irreversibly on drying as a result of the irreversible shrinkage of organic-matter coatings on diatoms (identifiable by microscopic, 440 X, examination of dry samples); and

2. Either yields a saturated sodium-pyrophosphate extract on white chromatographic or filter paper that has a color value of 8 or more and chroma of 2 or less or has a cation-exchange capacity of less than 240 cmol(+) per kg organic matter (by loss on ignition), or both.

Marl

A layer of marl is a limnic layer that:

1. Has a color value, moist, of 5 or more; and

2. Reacts with dilute HCl to evolve CO₂.

The color of marl usually does not change irreversibly on drying because a layer of marl contains too little organic matter, even before it has been shrunk by drying, to coat the carbonate particles.

1. Saturation is characterized by zero or positive pressure in the soil water and can generally be determined by observing free water in an unlined auger hole. Problems may arise, however, in clayey soils with peds, where an unlined auger hole may fill with water flowing along faces of peds while the soil matrix is and remains unsaturated (bypass flow). Such free water may incorrectly suggest the presence of a water table, while the actual water table occurs at greater depth. Use of well sealed piezometers or tensiometers is therefore recommended for measuring saturation. Problems may still occur, however, if water runs into piezometer slits near the bottom of the piezometer hole or if tensiometers with slowly reacting manometers are used. The first problem can be overcome by using piezometers with smaller slits and the second by using transducer tensiometry, which reacts faster than manometers. Soils are considered wet if they have pressure heads greater than -1 kPa. Only macropores, such as cracks between peds or channels, are then filled with air, while the soil matrix is usually still saturated. Obviously, exact measurements of the wet state can be obtained only with tensiometers. For operational purposes, the use of piezometers is recommended as a standard method.

   The duration of saturation required for creating aquic conditions varies, depending on the soil environment, and is not specified.

   Three types of saturation are defined:

   1. Endosaturation.—The soil is saturated with water in all layers from the upper boundary of saturation to a depth of 200 cm or more from the mineral soil surface.

   2. Episaturation.—The soil is saturated with water in one or more layers within 200 cm of the mineral soil surface and also has one or more unsaturated layers, with an upper boundary above a depth of 200 cm, below the saturated layer. The zone of saturation, i.e., the water table, is perched on top of a relatively impermeable layer.

   3. Anthric saturation.—This term refers to a special kind of aquic conditions that occur in soils that are cultivated and irrigated (flood irrigation). Soils with anthraquic conditions must meet the requirements for aquic conditions and in addition have both of the following:
      1. A tilled surface layer and a directly underlying slowly permeable layer that has, for 3 months or more in normal years, both:
         1. Saturation and reduction; and

         2. Chroma of 2 or less in the matrix; and

      2. A subsurface horizon with one or more of the following:
         1. Redox depletions with a color value, moist, of 4 or more and chroma of 2 or less in macropores; or

         2. Redox concentrations of iron; or

         3. 2 times or more the amount of iron (by dithionite citrate) contained in the tilled surface layer.

2. The degree of reduction in a soil can be characterized by the direct measurement of redox potentials. Direct measurements should take into account chemical equilibria as expressed by stability diagrams in standard soil textbooks. Reduction and oxidation processes are also a function of soil pH. Obtaining accurate measurements of the degree of reduction in a soil is difficult. In the context of this taxonomy, however, only a degree of reduction that results in reduced iron is considered, because it produces the visible redoximorphic features that are identified in the keys. A simple field test is available to determine if reduced iron ions are present. A freshly broken surface of a field-wet soil sample is treated with alpha,alpha-dipyridyl in neutral, 1-normal ammonium-acetate solution. The appearance of a strong red color on the freshly broken surface indicates the presence of reduced iron ions. A positive reaction to the alpha,alpha-dipyridyl field test for ferrous iron (Childs, 1981) may be used to confirm the existence of reducing conditions and is especially useful in situations where, despite saturation, normal morphological indicators of such conditions are either absent or obscured (as by the dark colors characteristic of melanic great groups). A negative reaction, however, does not imply that reducing conditions are always absent. It may only mean that the level of free iron in the soil is below the sensitivity limit of the test or that the soil is in an oxidized phase at the time of testing. Use of alpha,alpha-dipyridyl in a 10 percent acetic-acid solution is not recommended because the acid is likely to change soil conditions, for example, by dissolving CaCO₃.

   The duration of reduction required for creating aquic conditions is not specified.

3. Redoximorphic features associated with wetness result from alternating periods of reduction and oxidation of iron and manganese compounds in the soil. Reduction occurs during saturation with water, and oxidation occurs when the soil is not saturated. The reduced iron and manganese ions are mobile and may be transported by water as it moves through the soil. Certain redox patterns occur as a function of the patterns in which the ion-carrying water moves through the soil and as a function of the location of aerated zones in the soil. Redox patterns are also affected by the fact that manganese is reduced more rapidly than iron, while iron oxidizes more rapidly upon aeration. Characteristic color patterns are created by these processes. The reduced iron and manganese ions may be removed from a soil if vertical or lateral fluxes of water occur, in which case there is no iron or manganese precipitation in that soil. Wherever the iron and manganese are oxidized and precipitated, they form either soft masses or hard concretions or nodules. Movement of iron and manganese as a result of redox processes in a soil may result in redoximorphic features that are defined as follows:
   1. Redox concentrations.—These are zones of apparent accumulation of Fe-Mn oxides, including:
      1. Nodules and concretions, which are cemented bodies that can be removed from the soil intact. Concretions are distinguished from nodules on the basis of internal organization. A concretion typically has concentric layers that are visible to the naked eye. Nodules do not have visible organized internal structure. Boundaries commonly are diffuse if formed in situ and sharp after pedoturbation. Sharp boundaries may be relict features in some soils; and

      2. Masses, which are noncemented concentrations of substances within the soil matrix; and

      3. Pore linings, i.e., zones of accumulation along pores that may be either coatings on pore surfaces or impregnations from the matrix adjacent to the pores.

   2. Redox depletions.—These are zones of low chroma (chromas less than those in the matrix) where either Fe-Mn oxides alone or both Fe-Mn oxides and clay have been stripped out, including:
      1. Iron depletions, i.e., zones that contain low amounts of Fe and Mn oxides but have a clay content similar to that of the adjacent matrix (often referred to as albans or neoalbans); and

      2. Clay depletions, i.e., zones that contain low amounts of Fe, Mn, and clay (often referred to as silt coatings or skeletans).

   3. Reduced matrix.—This is a soil matrix that has low chroma in situ but undergoes a change in hue or chroma within 30 minutes after the soil material has been exposed to air.

   4. In soils that have no visible redoximorphic features, a reaction to an alpha,alpha-dipyridyl solution satisfies the requirement for redoximorphic features.

Field experience indicates that it is not possible to define a specific set of redoximorphic features that is uniquely characteristic of all of the taxa in one particular category. Therefore, color patterns that are unique to specific taxa are referenced in the keys.

Anthraquic conditions are a variant of episaturation and are associated with controlled flooding (for such crops as wetland rice and cranberries), which causes reduction processes in the saturated, puddled surface soil and oxidation of reduced and mobilized iron and manganese in the unsaturated subsoil.

Cryoturbation

Cryoturbation (frost churning) is the mixing of the soil matrix within the pedon that results in irregular or broken horizons, involutions, accumulation of organic matter on the permafrost table, oriented rock fragments, and silt caps on rock fragments.

Densic Contact

A densic contact (L. densus, thick) is a contact between soil and densic materials (defined below). It has no cracks, or the spacing of cracks that roots can enter is 10 cm or more.

Densic Materials

Densic materials are relatively unaltered materials (do not meet the requirements for any other named diagnostic horizons or any other diagnostic soil characteristic) that have a noncemented rupture-resistance class. The bulk density or the organization is such that roots cannot enter, except in cracks. These are mostly earthy materials, such as till, volcanic mudflows, and some mechanically compacted materials, for example, mine spoils. Some noncemented rocks can be densic materials if they are dense or resistant enough to keep roots from entering, except in cracks.

Densic materials are noncemented and thus differ from paralithic materials and the material below a lithic contact, both of which are cemented.

Densic materials have, at their upper boundary, a densic contact if they have no cracks or if the spacing of cracks that roots can enter is 10 cm or more. These materials can be used to differentiate soil series if the materials are within the series control section.

Gelic Materials

Gelic materials are mineral or organic soil materials that show evidence of cryoturbation (frost churning) and/or ice segregation in the active layer (seasonal thaw layer) and/or the upper part of the permafrost. Cryoturbation is manifested by irregular and broken horizons, involutions, accumulation of organic matter on top of and within the permafrost, oriented rock fragments, and silt-enriched layers. The characteristic structures associated with gelic materials include platy, blocky, or granular macrostructures; the structural results of sorting; and orbiculic, conglomeric, banded, or vesicular microfabrics. Ice segregation is manifested by ice lenses, vein ice, segregated ice crystals, and ice wedges. Cryopedogenic processes that lead to gelic materials are driven by the physical volume change of water to ice, moisture migration along a thermal gradient in the frozen system, or thermal contraction of the frozen material by continued rapid cooling.

Glacic Layer

A glacic layer is massive ice or ground ice in the form of ice lenses or wedges. The layer is 30 cm or more thick and contains 75 percent or more visible ice.

Lithic Contact

A lithic contact is the boundary between soil and a coherent underlying material. Except in Ruptic-Lithic subgroups, the underlying material must be virtually continuous within the limits of a pedon. Cracks that can be penetrated by roots are few, and their horizontal spacing is 10 cm or more. The underlying material must be sufficiently coherent when moist to make hand-digging with a spade impractical, although the material may be chipped or scraped with a spade. The material below a lithic contact must be in a strongly cemented or more cemented rupture-resistance class. Commonly, the material is indurated. The underlying material considered here does not include diagnostic soil horizons, such as a duripan or a petrocalcic horizon.

A lithic contact is diagnostic at the subgroup level if it is within 125 cm of the mineral soil surface in Oxisols and within 50 cm of the mineral soil surface in all other mineral soils. In organic soils the lithic contact must be within the control section to be recognized at the subgroup level.

Paralithic Contact

A paralithic (lithiclike) contact is a contact between soil and paralithic materials (defined below) where the paralithic materials have no cracks or the spacing of cracks that roots can enter is 10 cm or more.

Paralithic Materials

Paralithic materials are relatively unaltered materials (do not meet the requirements for any other named diagnostic horizons or any other diagnostic soil characteristic) that have an extremely weakly cemented to moderately cemented rupture-resistance class. Cementation, bulk density, and the organization are such that roots cannot enter, except in cracks. Paralithic materials have, at their upper boundary, a paralithic contact if they have no cracks or if the spacing of cracks that roots can enter is 10 cm or more. Commonly, these materials are partially weathered bedrock or weakly consolidated bedrock, such as sandstone, siltstone, or shale. Paralithic materials can be used to differentiate soil series if the materials are within the series control section. Fragments of paralithic materials 2.0 mm or more in diameter are referred to as pararock fragments.

Permafrost

Permafrost is defined as a thermal condition in which a material (including soil material) remains below 0 °C for 2 or more years in succession. Those gelic materials having permafrost contain the unfrozen soil solution that drives cryopedogenic processes. Permafrost may be cemented by ice or, in the case of insufficient interstitial water, may be dry. The frozen layer has a variety of ice lenses, vein ice, segregated ice crystals, and ice wedges. The permafrost table is in dynamic equilibrium with the environment.

Soil Moisture Regimes

The term "soil moisture regime" refers to the presence or absence either of ground water or of water held at a tension of less than 1500 kPa in the soil or in specific horizons during periods of the year. Water held at a tension of 1500 kPa or more is not available to keep most mesophytic plants alive. The availability of water is also affected by dissolved salts. If a soil is saturated with water that is too salty to be available to most plants, it is considered salty rather than dry. Consequently, a horizon is considered dry when the moisture tension is 1500 kPa or more and is considered moist if water is held at a tension of less than 1500 kPa but more than zero. A soil may be continuously moist in some or all horizons either throughout the year or for some part of the year. It may be either moist in winter and dry in summer or the reverse. In the Northern Hemisphere, summer refers to June, July, and August and winter refers to December, January, and February.

Normal Years

In the discussions that follow and throughout the keys, the term "normal years" is used. A normal year is defined as a year that has plus or minus one standard deviation of the long-term mean annual precipitation. (Long-term refers to 30 years or more.) Also, the mean monthly precipitation during a normal year must be plus or minus one standard deviation of the long-term monthly precipitation for 8 of the 12 months. For the most part, normal years can be calculated from the mean annual precipitation. When catastrophic events occur during a year, however, the standard deviations of the monthly means should also be calculated. The term "normal years" replaces the terms "most years" and "6 out of 10 years," which were used in the 1975 edition of Soil Taxonomy (USDA, SCS, 1975).

Soil Moisture Control Section

The intent in defining the soil moisture control section is to facilitate estimation of soil moisture regimes from climatic data. The upper boundary of this control section is the depth to which a dry (tension of more than 1500 kPa, but not air-dry) soil will be moistened by 2.5 cm of water within 24 hours. The lower boundary is the depth to which a dry soil will be moistened by 7.5 cm of water within 48 hours. These depths do not include the depth of moistening along any cracks or animal burrows that are open to the surface.

If 7.5 cm of water moistens the soil to a densic, lithic, paralithic, or petroferric contact or to a petrocalcic or petrogypsic horizon or a duripan, the contact or the upper boundary of the cemented horizon constitutes the lower boundary of the soil moisture control section. If a soil is moistened to one of these contacts or horizons by 2.5 cm of water, the soil moisture control section is the boundary or the contact itself. The control section of such a soil is considered moist if the contact or upper boundary of the cemented horizon has a thin film of water. If that upper boundary is dry, the control section is considered dry.

The moisture control section of a soil extends approximately (1) from 10 to 30 cm below the soil surface if the particle-size class of the soil is fine-loamy, coarse-silty, fine-silty, or clayey; (2) from 20 to 60 cm if the particle-size class is coarse-loamy; and (3) from 30 to 90 cm if the particle-size class is sandy. If the soil contains rock and pararock fragments that do not absorb and release water, the limits of the moisture control section are deeper. The limits of the soil moisture control section are affected not only by the particle-size class but also by differences in soil structure or pore-size distribution or by other factors that influence the movement and retention of water in the soil.

Classes of Soil Moisture Regimes

The soil moisture regimes are defined in terms of the level of ground water and in terms of the seasonal presence or absence of water held at a tension of less than 1500 kPa in the moisture control section. It is assumed in the definitions that the soil supports whatever vegetation it is capable of supporting, i.e., crops, grass, or native vegetation, and that the amount of stored moisture is not being increased by irrigation or fallowing. These cultural practices affect the soil moisture conditions as long as they are continued.

Aquic moisture regime.—The aquic (L. aqua, water) moisture regime is a reducing regime in a soil that is virtually free of dissolved oxygen because it is saturated by water. Some soils are saturated with water at times while dissolved oxygen is present, either because the water is moving or because the environment is unfavorable for micro-organisms (e.g., if the temperature is less than 1 °C); such a regime is not considered aquic.

It is not known how long a soil must be saturated before it is said to have an aquic moisture regime, but the duration must be at least a few days, because it is implicit in the concept that dissolved oxygen is virtually absent. Because dissolved oxygen is removed from ground water by respiration of micro-organisms, roots, and soil fauna, it is also implicit in the concept that the soil temperature is above biologic zero for some time while the soil is saturated. Biologic zero is defined as 5 °C in this taxonomy. In some of the very cold regions of the world, however, biological activity occurs at temperatures below 5 °C.

Very commonly, the level of ground water fluctuates with the seasons; it is highest in the rainy season or in fall, winter, or spring if cold weather virtually stops evapotranspiration. There are soils, however, in which the ground water is always at or very close to the surface. Examples are soils in tidal marshes or in closed, landlocked depressions fed by perennial streams. Such soils are considered to have a peraquic moisture regime.

Aridic and torric (L. aridus, dry, and L. torridus, hot and dry) moisture regimes.—These terms are used for the same moisture regime but in different categories of the taxonomy.

In the aridic (torric) moisture regime, the moisture control section is, in normal years:

Soils that have an aridic (torric) moisture regime normally occur in areas of arid climates. A few are in areas of semiarid climates and either have physical properties that keep them dry, such as a crusty surface that virtually precludes the infiltration of water, or are on steep slopes where runoff is high. There is little or no leaching in this moisture regime, and soluble salts accumulate in the soils if there is a source.

The limits set for soil temperature exclude from these moisture regimes soils in the very cold and dry polar regions and in areas at high elevations. Such soils are considered to have anhydrous conditions (defined earlier).

Udic moisture regime.—The udic (L. udus, humid) moisture regime is one in which the soil moisture control section is not dry in any part for as long as 90 cumulative days in normal years. If the mean annual soil temperature is lower than 22 °C and if the mean winter and mean summer soil temperatures at a depth of 50 cm from the soil surface differ by 6 °C or more, the soil moisture control section, in normal years, is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice. In addition, the udic moisture regime requires, except for short periods, a three-phase system, solid-liquid-gas, in part or all of the soil moisture control section when the soil temperature is above 5 °C.

The udic moisture regime is common to the soils of humid climates that have well distributed rainfall; have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration; or have adequate winter rains to recharge the soils and cool, foggy summers, as in coastal areas. Water moves downward through the soils at some time in normal years.

In climates where precipitation exceeds evapotranspiration in all months of normal years, the moisture tension rarely reaches 100 kPa in the soil moisture control section, although there are occasional brief periods when some stored moisture is used. The water moves through the soil in all months when it is not frozen. Such an extremely wet moisture regime is called perudic (L. per, throughout in time, and L. udus, humid). In the names of most taxa, the formative element "ud" is used to indicate either a udic or a perudic regime; the formative element "per" is used in selected taxa.

Ustic moisture regime.—The ustic (L. ustus, burnt; implying dryness) moisture regime is intermediate between the aridic regime and the udic regime. Its concept is one of moisture that is limited but is present at a time when conditions are suitable for plant growth. The concept of the ustic moisture regime is not applied to soils that have permafrost or a cryic soil temperature regime (defined below).

If the mean annual soil temperature is 22 °C or higher or if the mean summer and winter soil temperatures differ by less than 6 °C at a depth of 50 cm below the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years. It is moist, however, in some part either for more than 180 cumulative days per year or for 90 or more consecutive days.

If the mean annual soil temperature is lower than 22 °C and if the mean summer and winter soil temperatures differ by 6 °C or more at a depth of 50 cm from the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years, but it is not dry in all parts for more than half of the cumulative days when the soil temperature at a depth of 50 cm is higher than 5 °C. If in normal years the moisture control section is moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice, the moisture control section is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice.

In tropical and subtropical regions that have a monsoon climate with either one or two dry seasons, summer and winter seasons have little meaning. In those regions the moisture regime is ustic if there is at least one rainy season of 3 months or more. In temperate regions of subhumid or semiarid climates, the rainy seasons are usually spring and summer or spring and fall, but never winter. Native plants are mostly annuals or plants that have a dormant period while the soil is dry.

Xeric moisture regime.—The xeric (Gr. xeros, dry) moisture regime is the typical moisture regime in areas of Mediterranean climates, where winters are moist and cool and summers are warm and dry. The moisture, which falls during the winter, when potential evapotranspiration is at a minimum, is particularly effective for leaching. In areas of a xeric moisture regime, the soil moisture control section, in normal years, is dry in all parts for 45 or more consecutive days in the 4 months following the summer solstice and moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice. Also, in normal years, the moisture control section is moist in some part for more than half of the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is higher than 6 °C or for 90 or more consecutive days when the soil temperature at a depth of 50 cm is higher than 8 °C. The mean annual soil temperature is lower than 22 °C, and the mean summer and mean winter soil temperatures differ by 6 °C or more either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact if shallower.

Soil Temperature Regimes

Classes of Soil Temperature Regimes

Following is a description of the soil temperature regimes used in defining classes at various categoric levels in this taxonomy.

Cryic (Gr. kryos, coldness; meaning very cold soils).—Soils in this temperature regime have a mean annual temperature lower than 8 °C but do not have permafrost.

Frigid.—A soil with a frigid temperature regime is warmer in summer than a soil with a cryic regime, but its mean annual temperature is lower than 8 °C and the difference between mean summer (June, July, and August) and mean winter (December, January, and February) soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Mesic.—The mean annual soil temperature is 8 °C or higher but lower than 15 °C, and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Thermic.—The mean annual soil temperature is 15 °C or higher but lower than 22 °C, and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Hyperthermic.—The mean annual soil temperature is 22 °C or higher, and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

If the name of a soil temperature regime has the prefix iso, the mean summer and mean winter soil temperatures differ by less than 6 °C at a depth of 50 cm or at a densic, lithic, or paralithic contact, whichever is shallower.

Isofrigid.—The mean annual soil temperature is lower than 8 °C.

Isomesic.—The mean annual soil temperature is 8 °C or higher but lower than 15 °C.

Isothermic.—The mean annual soil temperature is 15 °C or higher but lower than 22 °C.

Isohyperthermic.—The mean annual soil temperature is 22 °C or higher.

Sulfidic Materials

Sulfidic materials contain oxidizable sulfur compounds. They are mineral or organic soil materials that have a pH value of more than 3.5 and that, if incubated as a layer 1 cm thick under moist aerobic conditions (field capacity) at room temperature, show a drop in pH of 0.5 or more units to a pH value of 4.0 or less (1:1 by weight in water or in a minimum of water to permit measurement) within 8 weeks.

Sulfidic materials accumulate as a soil or sediment that is permanently saturated, generally with brackish water. The sulfates in the water are biologically reduced to sulfides as the materials accumulate. Sulfidic materials most commonly accumulate in coastal marshes near the mouth of rivers that carry noncalcareous sediments, but they may occur in freshwater marshes if there is sulfur in the water. Upland sulfidic materials may have accumulated in a similar manner in the geologic past.

If a soil containing sulfidic materials is drained or if sulfidic materials are otherwise exposed to aerobic conditions, the sulfides oxidize and form sulfuric acid. The pH value, which normally is near neutrality before drainage or exposure, may drop below 3. The acid may induce the formation of iron and aluminum sulfates. The iron sulfate, jarosite, may segregate, forming the yellow redoximorphic concentrations that commonly characterize a sulfuric horizon. The transition from sulfidic materials to a sulfuric horizon normally requires very few years and may occur within a few weeks. A sample of sulfidic materials, if air-dried slowly in shade for about 2 months with occasional remoistening, becomes extremely acid.

Sulfuric Horizon

Required Characteristics

The sulfuric (L. sulfur) horizon is 15 cm or more thick and is composed of either mineral or organic soil material that has a pH value of 3.5 or less (1:1 by weight in water or in a minimum of water to permit measurement) and shows evidence that the low pH value is caused by sulfuric acid. The evidence is one or more of the following:

1. Jarosite concentrations; or

2. Directly underlying sulfidic materials (defined above); or

3. 0.05 percent or more water-soluble sulfate.

Literature Cited

Brewer, R. 1976. Fabric and Mineral Analysis of Soils. Second edition. John Wiley and Sons, Inc. New York, New York.

Childs, C.W. 1981. Field Test for Ferrous Iron and Ferric-Organic Complexes (on Exchange Sites or in Water-Soluble Forms) in Soils. Austr. J. of Soil Res. 19: 175-180.

Pons, L.J., and I.S. Zonneveld. 1965. Soil Ripening and Soil Classification. Initial Soil Formation in Alluvial Deposits and a Classification of the Resulting Soils. Int. Inst. Land Reclam. and Impr. Pub. 13. Wageningen, The Netherlands.

United States Department of Agriculture, Soil Conservation Service. 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Soil Surv. Staff. U.S. Dep. Agric. Handb. 436.

United States Department of Agriculture, Soil Conservation Service. 1993. Soil Survey Manual. Soil Surv. Div. Staff. U.S. Dep. Agric. Handb. 18.

Foot Notes

¹The concept of the kandic horizon and the "kandi" and "kanhapli" great groups of soils represent the work of the International Committee on the Classification of Low Activity Clays (ICOMLAC) chaired by Dr. Frank R. Moorman.

²In 1992, the term "aquic conditions" was introduced and other changes were made throughout this taxonomy as a result of recommendations submitted to NRCS by the International Committee on Aquic Moisture Regime (ICOMAQ), which was established in 1982 and was chaired initially by Dr. Frank Moormann, then by Dr. Johan Bouma.

Citations:
Primary Source: United States Department of Agriculture, Natural Resources Conservation Service. 1998. Keys to Soil Taxonomy, Eighth Edition. Soil Survey Staff.

Online Source: Pedosphere.com. 2001. Searchable Keys to Soil Taxonomy, Eighth Edition [Online WWW]. Available URL: http://www.pedosphere.com/resources/sg_usa/ [cite access date].

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