Chapter 17: Family and Series Differentiae and Names

Families and series serve purposes that are largely pragmatic, the series name is abstract, and the technical family name is descriptive. In this chapter the descriptive terms used in the names of families are defined, the control sections to which the terms apply are given, and the criteria, including the taxa in which they are used, are indicated.

Family Differentiae for Mineral Soils and Mineral Layers of Some Organic Soils

The following differentiae are used to distinguish families of mineral soils and the mineral layers of some organic soils within a subgroup. The class names of these components are used to form the family name. The components are listed and defined in the same sequence in which the components appear in the family names.

Particle-size classes
Mineralogy classes
Cation-exchange activity classes
Calcareous and reaction classes
Soil temperature classes
Soil depth classes
Rupture-resistance classes
Classes of coatings
Classes of cracks

Particle-Size Classes and Their Substitutes

Definition of Particle-Size Classes and Their Substitutes for Mineral Soils

The first part of the family name is the name of either a particle-size class or a substitute for a particle-size class. The term particle-size class is used to characterize the grain-size composition of the whole soil, including both the fine earth and the rock and pararock fragments up to the size of a pedon, but it excludes organic matter and salts more soluble than gypsum. Substitutes for particle-size classes are used for soils that have andic soil properties or a high content of volcanic glass, pumice, or cinders.

The particle-size classes of this taxonomy represent a compromise between conventional divisions in pedologic and engineering classifications. Engineering classifications have set the limit between sand and silt at a diameter of 74 microns, while pedologic classifications have set it at either 50 or 20 microns. Engineering classifications have been based on grain-size percentages, by weight, in the soil fraction less than 74 mm in diameter, while textural classes in pedologic classifications have been based on percentages, by weight, in the fraction less than 2.0 mm in diameter. In engineering classifications, the separate very fine sand (diameter between 50 and 100 microns or 0.05 and 0.1 mm) has been subdivided at 74 microns. In defining the particle-size classes for this taxonomy, a similar division has been made, but in a different way. Soil materials that have a texture of fine sand or loamy fine sand normally have an appreciable amount of very fine sand, most of which is coarser than 74 microns. A silty sediment, such as loess, may also contain an appreciable amount of very fine sand, most of which is finer than 74 microns. Thus, in the design of particle-size classes for this taxonomy, the very fine sand has been allowed to "float" It is included with the sand if the texture (fine-earth fraction) of a soil is sand, loamy fine sand, or coarser. It is treated as silt, however, if the texture is very fine sand, loamy very fine sand, sandy loam, silt loam, or finer.

No single set of particle-size classes seems adequate to serve as family differentiae for all of the different kinds of soil. Thus, this taxonomy provides 2 generalized and 11 more narrowly defined classes, which permit relatively fine distinctions between families of soils for which particle size is important, while providing broader groupings for soils in which narrowly defined particle-size classes would produce undesirable separations. Thus, the term "clayey" is used for some soil families to indicate a clay content of 35 percent (30 percent in Vertisols) or more in specific horizons, while in other families the more narrowly defined terms "fine" and "very-fine" indicate that these horizons have a clay content either of 35 (30 percent in Vertisols) to 60 percent or of 60 percent or more in their fine-earth fraction. Fine earth refers to particles smaller than 2.0 mm in diameter. Rock fragments are particles 2.0 mm or more in diameter that are strongly cemented or more resistant to rupture and include all particles with horizontal dimensions smaller than the size of a pedon. Cemented fragments 2.0 mm or more in diameter that are in a rupture-resistance class that is less cemented than the strongly cemented class are referred to as pararock fragments. Pararock fragments, like rock fragments, include all particles between 2.0 mm and a horizontal dimension smaller than the size of a pedon. Most pararock fragments are broken into fragments 2.0 mm or less in diameter during the preparation of samples for particle-size analysis in the laboratory. Therefore, pararock fragments are generally included with the fine earth in the particle-size classes, although cinders, pumice, and pumicelike fragments are treated as fragments in the substitutes for classes, regardless of their rupture-resistance class.

Substitutes for particle-size classes are used for soils that have andic soil properties or a high content of volcanic glass, pumice, or cinders. These materials cannot be readily dispersed, and the results of dispersion vary. Consequently, normal particle-size classes do not adequately characterize these components. Substitutes for particle-size class names are used for those parts of soils that have andic soil properties or a high amount of volcanic glass, pumice, or cinders, as is the case with Andisols and many Andic and Vitrandic subgroups of other soil orders. Some Spodosols, whether identified in Andic subgroups or not, have andic soil properties in some horizons within the particle-size control section, and particle-size substitute class names are used for these horizons. Neither a particle-size class name nor a substitute for a particle-size class name is used for Psamments, Psammaquents, and Psammentic subgroups that are in a sandy particle-size class. These taxa have, by definition, either a sandy particle-size class or an ashy substitute class. The sandy particle-size class is considered redundant in the family name. The ashy substitute class, however, is named, if appropriate in these taxa.

Particle-size class names are applied, although with reservations, to spodic horizons and other horizons that do not have andic soil properties but contain significant amounts of allophane, imogolite, ferrihydrite, or aluminum-humus complexes. The isotic mineralogy class (defined below) is helpful in identifying these particle-size classes. In general, the weighted average particle-size class of the whole particle-size control section (defined below) determines what particle-size class name is used as a component of the family name.

Strongly Contrasting Particle-Size Classes

If the particle-size control section consists of two parts with strongly contrasting particle-size or substitute classes (listed below), if both parts are 12.5 cm or more thick (including parts not in the control section), and if the transition zone between them is less than 12.5 cm thick, both class names are used. For example, the family particle-size class is sandy over clayey if all of the following criteria are met: the soil meets criterion D (listed below) under the control section for particle-size classes or their substitutes; any Ap horizon is less than 30 cm thick; the weighted average particle-size class of the upper 30 cm of the soil is sandy; the weighted average of the lower part is clayey; and the transition zone is less than 12.5 cm thick. If a substitute name applies to one or more parts of the particle-size control section and the parts are not strongly contrasting classes, the name of the thickest part (cumulative) is used as the soil family name.

Aniso Class

If the particle-size control section includes more than one pair of the strongly contrasting classes, listed below, then the soil is assigned to an aniso class named for the pair of adjacent classes that contrast most strongly. The aniso class is considered part of the particle-size class name and is set off by commas after the particle-size name. An example is a sandy over clayey, aniso, mixed, active, mesic Aridic Haplustoll.

Generalized Particle-Size Classes

Two generalized particle-size classes, loamy and clayey, are used for shallow classes (defined below) and for soils in Arenic, Grossarenic, and Lithic subgroups. The clayey class is used for all strongly contrasting particle-size classes with more than 35 percent clay (30 percent in Vertisols). The loamy particle-size class is used for contrasting classes, where appropriate, to characterize the lower part of the particle-size control section. The generalized classes, where appropriate, are also used for all strongly contrasting particle-size classes that include a substitute class. For example, loamy over pumiceous or cindery (not fine-loamy over pumiceous or cindery) is used.

Six generalized classes, defined later in this chapter, are used for Terric subgroups of Histosols and Histels.

Control Section for Particle-Size Classes or Their Substitutes in Mineral Soils

The particle-size and substitute class names listed below are applied to certain horizons, or to the soil materials within specific depth limits, that have been designated as the particle-size control section. The lower boundary of the control section may be at a specified depth (in centimeters) below the mineral soil surface or below the upper boundary of an organic layer with andic soil properties, or it may be at the upper boundary of a root-limiting layer. Unless otherwise indicated, the following are considered root-limiting layers in this chapter: a duripan; a fragipan; petrocalcic, petrogypsic, and placic horizons; continuous ortstein; and densic, lithic, paralithic, and petroferric contacts. The following list of particle-size control sections for particular kinds of mineral soils is arranged as a key. This key, like other keys in this taxonomy, is designed in such a way that the reader makes the correct classification by going through the key systematically, starting at the beginning and eliminating one by one all classes that include criteria that do not fit the soil in question. The soil belongs to the first class for which it meets all of the criteria listed. The upper boundary of an argillic, natric, or kandic horizon is used in the following key. This boundary is not always obvious. If one of these horizons is present but the upper boundary is irregular or broken, as in an A/B or B/A horizon, the depth at which half or more of the volume has the fabric of an argillic, natric, or kandic horizon should be considered the upper boundary.

Key to the Control Section for Particle-Size Classes or Their Substitutes in Mineral Soils

For mineral soils that have a root-limiting layer (listed above) within 36 cm of the mineral soil surface or below the upper boundary of organic soil materials with andic soil properties, whichever is shallower: From the mineral soil surface or the upper boundary of the organic soil materials with andic soil properties, whichever is shallower, to the root-limiting layer; or
For Andisols: Between either the mineral soil surface or the upper boundary of an organic layer with andic soil properties, whichever is shallower, and the shallower of the following: (a) a depth 100 cm below the starting point or (b) a root-limiting layer; or
For those Alfisols, Ultisols, and great groups of Aridisols and Mollisols, excluding soils in Lamellic subgroups, that have an argillic, kandic, or natric horizon that has its upper boundary within 100 cm of the mineral soil surface and its lower boundary at a depth of 25 cm or more below the mineral soil surface or that are in a Grossarenic or Arenic subgroup, use 1 through 4 below. For other soils, go to D below.
1. Strongly contrasting particle-size classes (defined and listed later) within or below the argillic, kandic, or natric horizon and within 100 cm of the mineral soil surface: The upper 50 cm of the argillic, kandic, or natric horizon or to a depth of 100 cm, whichever is deeper, but not below the upper boundary of a root-limiting layer; or
2. All parts of the argillic, kandic, or natric horizon in or below a fragipan: Between a depth of 25 cm from the mineral soil surface and the top of the fragipan; or
3. A fragipan at a depth of less than 50 cm below the top of the argillic, kandic, or natric horizon: Between the upper boundary of the argillic, kandic, or natric horizon and the top the fragipan; or
4. Other soils that meet C above: Either the whole argillic, kandic, or natric horizon if 50 cm or less thick or the upper 50 cm of the horizon if more than 50 cm thick.
For those Alfisols, Ultisols, and great groups of Aridisols and Mollisols that are in a Lamellic subgroup or have an argillic, kandic, or natric horizon that has its upper boundary at a depth of 100 cm or more from the mineral surface and that are not in a Grossarenic or Arenic subgroup: Between the lower boundary of an Ap horizon or a depth of 25 cm from the mineral soil surface, whichever is deeper, and 100 cm below the mineral soil surface or a root-limiting layer, whichever is shallower; or
For other soils that have an argillic or natric horizon that has its lower boundary at a depth of less than 25 cm from the mineral surface: Between the upper boundary of the argillic or natric horizon and a depth of 100 cm below the mineral soil surface or a root-limiting layer, whichever is shallower; or
All other mineral soils: Between the lower boundary of an Ap horizon or a depth of 25 cm below the mineral soil surface, whichever is deeper, and the shallower of the following: (a) a depth of 100 cm below the mineral soil surface or (b) a root-limiting layer.

Key to the Particle-Size and Substitute Classes of Mineral Soils

This key, like other keys in this taxonomy, is designed in such a way that the reader makes the correct classification by going through the key systematically, starting at the beginning and eliminating one by one all classes that include criteria that do not fit the soil or layer in question. The class or substitute name for each layer within the control section must be determined from the key. If any two layers meet the criteria for strongly contrasting particle-size classes (listed below), the soil is named for that strongly contrasting class. If more than one pair meets the criteria for strongly contrasting classes, the soil is also in an aniso class named for the pair of adjacent classes that contrast most strongly. If the soil has none of the strongly contrasting classes, the weighted average soil materials within the particle-size control section generally determine the class. Exceptions are soils that are not strongly contrasting and that have a substitute class name for one or more parts of the control section. In these soils the class or substitute name of the thickest (cumulative) part within the control section is used to determine the family name.

Mineral soils that have, in the thickest part of the control section (if the control section is not in one of the strongly contrasting particle-size classes listed below), or in a part of the control section that qualifies as an element in one of the strongly contrasting particle-size classes listed below, or throughout the control section, a fine-earth component (including associated medium and finer pores) of less than 10 percent of the total volume and that meet one of the following sets of substitute class criteria:
1. Have, in the whole soil, more than 60 percent (by weight) volcanic ash, cinders, lapilli, pumice, and pumicelike1 fragments and, in the fraction coarser than 2.0 mm, two-thirds or more (by volume) pumice and/or pumicelike fragments.
  Pumiceous
  or
2. Have, in the whole soil, more than 60 percent (by weight) volcanic ash, cinders, lapilli, pumice, and pumicelike fragments and, in the fraction coarser than 2.0 mm, less than two-thirds (by volume) pumice and pumicelike fragments.
  Cindery
  or
3. Other mineral soils that have a fine-earth component of less than 10 percent (including associated medium and finer pores) of the total volume.
  Fragmental
  or
Other mineral soils that have a fine-earth component of 10 percent or more (including associated medium and finer pores) of the total volume and meet, in the thickest part of the control section (if the control section is not in one of the strongly contrasting particle-size classes listed below), or in a part of the control section that qualifies as an element in one of the strongly contrasting particle-size classes listed below, or throughout the control section, one of the following sets of substitute class criteria:
1. They:
  1. Have andic soil properties and have a water content at 1500 kPa tension of less than 30 percent on undried samples and less than 12 percent on dried samples; or
  2. Do not have andic soil properties, have a total of 30 percent or more of the fine-earth fraction in the 0.02 to 2.0 mm fraction, and have (by grain count) 30 percent or more of that fraction consisting of volcanic glass, glass aggregates, glass-coated grains, and other vitric volcaniclastics; and
  3. Have one of the following;
    1. A total of 35 percent or more (by volume) rock and pararock fragments, of which two-thirds or more (by volume) is pumice or pumicelike fragments.
      Ashy-pumiceous
      or
    2. 35 percent or more (by volume) rock fragments.
      Ashy-skeletal
      or
    3. Less than 35 percent (by volume) rock fragments.
      Ashy
      or
2. Have a fine-earth fraction that has andic soil properties and that has a water content at 1500 kPa tension of 12 percent or more on air-dried samples or of 30 to 100 percent on undried samples; and
  1. Have a total of 35 percent or more (by volume) rock and pararock fragments, of which two-thirds or more (by volume) is pumice or pumicelike fragments.
    Medial-pumiceous
    or
  2. Have 35 percent or more (by volume) rock fragments.
    Medial-skeletal
    or
  3. Have less than 35 percent (by volume) rock fragments.
    Medial
    or
3. Have a fine-earth fraction that has andic soil properties and that has a water content at 1500 kPa tension of 100 percent or more on undried samples; and
  1. Have a total of 35 percent or more (by volume) rock and pararock fragments, of which two-thirds or more (by volume) is pumice or pumicelike fragments.
    Hydrous-pumiceous
    or
  2. Have 35 percent or more (by volume) rock fragments.
    Hydrous-skeletal
    or
  3. Have less than 35 percent (by volume) rock fragments.
    Hydrous
    or
Note: In the following classes, "clay" excludes clay-size carbonates. Carbonates of clay size are treated as silt. If the ratio of percent water retained at 1500 kPa tension to the percentage of measured clay is 0.25 or less or 0.6 or more in half or more of the particle-size control section or part of the particle-size control section in strongly contrasting classes, then the percentage of clay is estimated by the following formula: Clay % = 2.5(% water retained at 1500 kPa tension - % organic carbon).
Other mineral soils that, in the thickest part of the control section (if part of the control section has a substitute for particle-size class and is not in one of the strongly contrasting particle-size classes listed below), or in a part of the control section that qualifies as an element in one of the strongly contrasting particle-size classes listed below, or throughout the control section, meet one of the following sets of particle-size class criteria:
1. Have 35 percent or more (by volume) rock fragments and a fine-earth fraction with a texture of sand or loamy sand, including less than 50 percent (by weight) very fine sand.
  Sandy-skeletal
  or
2. Have 35 percent or more (by volume) rock fragments and less than 35 percent (by weight) clay.
  Loamy-skeletal
  or
3. Have 35 percent or more (by volume) rock fragments.
  Clayey-skeletal
  or
4. Have a texture of sand or loamy sand, including less than 50 percent (by weight) very fine sand in the fine-earth fraction.
  Sandy
  or
5. Have a texture of loamy very fine sand, very fine sand, or finer, including less than 35 percent (by weight) clay in the fine-earth fraction (excluding Vertisols), and are in a shallow family (defined below) or in a Lithic, Arenic, or Grossarenic subgroup, or the layer is an element in a strongly contrasting particle-size class (listed below) and the layer is the lower element or the other element is a substitute for particle-size class.
  Loamy
  or
6. Have, in the fraction less than 75 mm in diameter, 15 percent or more (by weight) particles with diameters of 0.1 to 75 mm (fine sand or coarser, including rock fragments up to 7.5 cm in diameter) and, in the fine-earth fraction, less than 18 percent (by weight) clay.
  Coarse-loamy
  or
7. Have, in the fraction less than 75 mm in diameter, 15 percent or more (by weight) particles with diameters of 0.1 to 75 mm (fine sand or coarser, including rock fragments up to 7.5 cm in diameter) and 18 to 35 percent (by weight) clay (Vertisols are excluded).
  Fine-loamy
  or
8. Have, in the fraction less than 75 mm in diameter, less than 15 percent (by weight) particles with diameters of 0.1 to 75 mm (fine sand or coarser, including rock fragments up to 7.5 cm in diameter) and, in the fine-earth fraction, less than 18 percent (by weight) clay.
  Coarse-silty
  or
9. Have, in the fraction less than 75 mm in diameter, less than 15 percent (by weight) particles with diameters of 0.1 to 75 mm (fine sand or coarser, including rock fragments up to 7.5 cm in diameter) and, in the fine-earth fraction, 18 to 35 percent (by weight) clay (Vertisols are excluded).
  Fine-silty
  or
10. Have 35 percent or more (by weight) clay (more than 30 percent in Vertisols) and are in a shallow family (defined below) or in a Lithic, Arenic, or Grossarenic subgroup, or the layer is an element in a strongly contrasting particle-size class (listed below).
  Clayey
  or
11. Have (by weighted average) less than 60 percent (by weight) clay in the fine-earth fraction.
  Fine
  or
12. Have 60 percent or more clay.
  Very-fine

Strongly Contrasting Particle-Size Classes

The purpose of strongly contrasting particle-size classes is to identify changes in pore-size distribution or composition that are not identified in higher soil categories and that seriously affect the movement and retention of water and/or nutrients.

The following particle-size or substitute classes are considered strongly contrasting if both parts are 12.5 cm or more thick (including parts not in the particle-size control section; however, substitute class names are used only if the soil materials to which they apply extend 10 cm or more into the upper part of the particle-size control section) and if the transition zone between the two parts of the particle-size control section is less than 12.5 cm thick.

Some classes, such as sandy and sandy-skeletal, have been combined in the following list. In those cases the combined name is used as the family class if part of the control section meets the criteria for either class.

Ashy over clayey
Ashy over clayey-skeletal
Ashy over loamy-skeletal
Ashy over loamy
Ashy over medial-skeletal
Ashy over medial (if the water content at 1500 kPa tension in dried samples of the fine-earth fraction is 10 percent or less for the ashy materials and 15 percent or more for the medial materials)
Ashy over pumiceous or cindery
Ashy over sandy or sandy-skeletal
Ashy-skeletal over fragmental or cindery (if the volume of the fine-earth fraction is 35 percent or more [absolute] greater in the ashy-skeletal part than in the fragmental or cindery part)
Cindery over loamy
Cindery over medial-skeletal
Cindery over medial
Clayey over fine-silty (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Clayey over fragmental
Clayey over loamy (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Clayey over loamy-skeletal (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Clayey over sandy or sandy-skeletal
Clayey-skeletal over sandy or sandy-skeletal
Coarse-loamy over clayey
Coarse-loamy over fragmental
Coarse-loamy over sandy or sandy-skeletal (if the coarse-loamy material contains less than 50 percent fine sand or coarser sand)
Coarse-silty over clayey
Coarse-silty over sandy or sandy-skeletal
Fine-loamy over clayey (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Fine-loamy over fragmental
Fine-loamy over sandy or sandy-skeletal
Fine-silty over clayey (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Fine-silty over fragmental
Fine-silty over sandy or sandy-skeletal
Hydrous over clayey-skeletal
Hydrous over clayey
Hydrous over fragmental
Hydrous over loamy-skeletal
Hydrous over loamy
Hydrous over sandy or sandy-skeletal
Loamy over sandy or sandy-skeletal (if the loamy material contains less than 50 percent fine sand or coarser sand)
Loamy over pumiceous or cindery
Loamy-skeletal over cindery (if the volume of the fine-earth fraction is 35 percent or more [absolute] greater in the loamy-skeletal part than in the cindery part)
Loamy-skeletal over clayey (if there is an absolute difference of 25 percent or more between clay percentages of the fine-earth fraction in the two parts of the control section)
Loamy-skeletal over fragmental (if the volume of the fine-earth fraction is 35 percent or more [absolute] greater in the loamy-skeletal part than in the fragmental part)
Loamy-skeletal over sandy or sandy-skeletal (if the loamy material has less than 50 percent fine sand or coarser sand)
Medial over ashy (if the water content at 1500 kPa tension in dried samples of the fine-earth fraction is 15 percent or more for the medial materials and 10 percent or less for the ashy materials)
Medial over ashy-pumiceous or ashy-skeletal (if the water content at 1500 kPa tension in dried samples of the fine-earth fraction is 15 percent or more for the medial materials and 10 percent or less for the ashy part)
Medial over clayey-skeletal
Medial over clayey
Medial over fragmental
Medial over hydrous (if the water content at 1500 kPa tension in undried samples of the fine-earth fraction is 75 percent or less for the medial materials)
Medial over loamy-skeletal
Medial over loamy
Medial over pumiceous or cindery
Medial over sandy or sandy-skeletal
Medial-skeletal over fragmental or cindery (if the volume of the fine-earth fraction is 35 percent or more [absolute] greater in the medial-skeletal part than the fragmental or cindery part)
Pumiceous or ashy-pumiceous over loamy
Pumiceous or ashy-pumiceous over medial-skeletal
Pumiceous or ashy-pumiceous over medial
Pumiceous or ashy-pumiceous over sandy or sandy-skeletal
Sandy over clayey
Sandy over loamy (if the loamy material contains less than 50 percent fine sand or coarser sand)
Sandy-skeletal over loamy (if the loamy material contains less than 50 percent fine sand or coarser sand)

Mineralogy Classes

The mineralogy of soils is known to be useful in making predictions about soil behavior and responses to management. Some mineralogy classes occur or are important only in certain taxa or particle-size classes, and others are important in all particle-size classes. The following key to mineralogy classes is designed to make those distinctions.

Control Section for Mineralogy Classes

The control section for mineralogy classes is the same as that defined for the particle-size classes and their substitutes.

Key to Mineralogy Classes

For soils with strongly contrasting particle-size classes, the mineralogy for both named particle-size classes or substitutes are given, unless they are the same. Examples are an ashy over clayey, mixed (if both the ashy and clayey parts are mixed), superactive, mesic Typic Vitraquand and a clayey over sandy or sandy-skeletal, smectitic over mixed, thermic Vertic Haplustept.

Oxisols and "kandi" and "kanhap" great groups of Alfisols and Ultisols that in the mineralogy control section have:
1. More than 40 percent iron oxide (more than 28 percent Fe), by dithionite citrate, in the fine-earth fraction.
  Ferritic
  or
2. More than 40 percent gibbsite in the fine-earth fraction.
  Gibbsitic
  or
3. Both:
  1. 18 to 40 percent iron oxide (12.6 to 28 percent Fe), by dithionite citrate, in the fine-earth fraction; and
  2. 18 to 40 percent gibbsite in the fine-earth fraction.
    Sesquic
  or
4. 18 to 40 percent iron oxide (12.6 to 28 percent Fe), by dithionite citrate, in the fine-earth fraction.
  Ferruginous
  or
5. 18 to 40 percent gibbsite in the fine-earth fraction.
  Allitic
  or
6. More than 50 percent (by weight) kaolinite plus halloysite, dickite, nacrite, and other 1:1 and nonexpanding 2:1 layer minerals and gibbsite in the fraction less than 0.002 in size and more kaolinite than halloysite.
  Kaolinitic
  or
7. More than 50 percent (by weight) halloysite plus kaolinite and allophane in the fraction less than 0.002 in size.
  Halloysitic
  or
8. All other properties.
  Mixed
  or
Other soil layers or horizons, in the mineralogy control section, that have a substitute class that replaces the particle-size class, other than fragmental, and that:
1. Have a sum of 8 times the Si (percent by weight extracted by acid oxalate) plus 2 times the Fe (percent by weight extracted by acid oxalate) of 5 or more, and 8 times the Si is more than 2 times the Fe.
  Amorphic
  or
2. Other soils that have a sum of 8 times the Si (percent by weight extracted by acid oxalate) plus 2 times the Fe (percent by weight extracted by acid oxalate) of 5 or more.
  Ferrihydritic
  or
3. Other soils that have 30 percent or more (by grain count) volcanic glass in the 0.02 to 2.0 mm fraction.
  Glassy
  or
4. All other soils that have a substitute class.
  Mixed
  or
Other mineral soil layers or horizons, in the mineralogy control section, in all other mineral soil orders and in Terric subgroups of Histosols and Histels that have:
1. Any particle-size class and more than 40 percent (by weight) carbonates (expressed as CaCO3) plus gypsum, with gypsum constituting more than 35 percent of the total weight of carbonates plus gypsum, either in the fine-earth fraction or in the fraction less than 20 mm in size, whichever has a higher percentage of carbonates plus gypsum.
  Gypsic
  or
2. Any particle-size class and more than 40 percent (by weight) carbonates (expressed as CaCO3) plus gypsum, either in the fine-earth fraction or in the fraction less than 20 mm in size, whichever has a higher percentage of carbonates plus gypsum.
  Carbonatic
  or
3. Any particle-size class, except for fragmental, and more than 40 percent (by weight) iron oxide (extractable by dithionite citrate), reported as Fe2O3 (or 28 percent reported as Fe), in the fine-earth fraction.
  Ferritic
  or
4. Any particle-size class, except for fragmental, and more than 40 percent (by weight) hydrated aluminum oxides, reported as gibbsite and bohemite, in the fine-earth fraction.
  Gibbsitic
  or
5. Any particle-size class, except for fragmental, and more than 40 percent (by weight) magnesium-silicate minerals, such as the serpentine minerals (antigorite, chrysotile, and lizardite) plus talc, olivines, Mg-rich pyroxenes, and Mg-rich amphiboles, in the fine-earth fraction.
  Magnesic
  or
6. Any particle-size class, except for fragmental, and a total iron oxide, by weight (Fe extracted by citrate-dithionite times 1.43 is used to determine the iron oxide), plus percent (by weight) gibbsite of more than 10 percent in the fine-earth fraction.
  Parasesquic
  or
7. Any particle-size class, except for fragmental, and more than 20 percent (by weight) glauconitic pellets in the fine-earth fraction.
  Glauconitic
or
Other mineral soil layers or horizons, in the mineralogy control section, of soils in all other mineral orders and in Terric subgroups of Histosols and Histels, in a clayey, clayey-skeletal, fine or very-fine particle-size class, that in the fraction less than 0.002 mm in size:
1. Have more than one-half (by weight) halloysite plus kaolinite and allophane and more halloysite than any other single mineral.
  Halloysitic
  or
2. Have more than one-half (by weight) kaolinite plus halloysite, dickite, and nacrite, and other 1:1 or nonexpanding 2:1 layer minerals or gibbsite, and less than 10 percent (by weight) smectite.
  Kaolinitic
  or
3. Have more smectite (montmorillonite, beidellite, and nontronite), by weight, than any other single kind of clay mineral.
  Smectitic
  or
4. Have more than one-half (by weight) illite (hydrous mica) and commonly more than 4 percent K2O.
  Illitic
  or
5. Have more vermiculite than any other single kind of clay mineral.
  Vermiculitic
  or
6. In more than one-half of the thickness, meet all of the following:
  1. Have no free carbonates; and
  2. The pH of a suspension of 1 g soil in 50 ml 1 M NaF is more than 8.4 after 2 minutes; and
  3. The ratio of 1500 kPa water to measured clay is 0.6 or more.
    Isotic
  or
7. All other soils in this category.
  Mixed
or
All other mineral soil layers or horizons, in the mineralogy control section, that have:
1. More than 40 percent (by weight) (80 percent by grain count) mica and stable mica pseudomorphs in the 0.02 to 2.0 mm fraction.
  Micaceous
  or
2. More than 25 percent (by weight) (65 percent by grain count) mica and stable mica pseudomorphs in the 0.02 to 2.0 mm fraction.
  Paramicaceous
  or
3. In more than one-half of the thickness, all of the following:
  1. No free carbonates; and
  2. The pH of a suspension of 1 g soil in 50 ml 1 M NaF is more than 8.4 after 2 minutes; and
  3. A ratio of 1500 kPa water to measured clay of 0.6 or more.
    Isotic
  or
4. More than 90 percent (by weight) silica minerals (quartz, chalcedony, or opal) and other extremely durable minerals that are resistant to weathering, in the 0.02 to 2.0 mm fraction.
  Siliceous
  or
5. All other properties.
  Mixed

Cation-Exchange Activity Classes

The cation-exchange activity classes help in making interpretations of mineral assemblages and of the nutrient-holding capacity of soils in mixed and siliceous mineralogy classes of clayey, clayey-skeletal, coarse-loamy, coarse-silty, fine, fine-loamy, fine-silty, loamy, loamy-skeletal, and very-fine particle-size classes. Cation-exchange activity classes are not assigned to Histosols and Histels, and they are not assigned to Oxisols and "kandi" and "kanhap" great groups and subgroups of Alfisols and Ultisols because assigning such classes to them would be redundant. Cation-exchange activity classes are not assigned to the sandy, sandy-skeletal, or fragmental particle-size class because the low clay content causes cation-exchange activity classes to be less useful and less reliable.

The cation-exchange capacity (CEC) is determined by NH4OAc at pH 7 on the fine-earth fraction. The CEC of the organic matter, sand, silt, and clay is included in the determination. The criteria for the classes use ratios of CEC to the percent, by weight, of silicate clay, both by weighted average in the control section. In the following classes "clay" excludes clay-size carbonates. If the ratio of percent water retained at 1500 kPa tension to the percentage of measured clay is 0.25 or less or 0.6 or more in half or more of the particle-size control section (or part in contrasting families), then the percentage of clay is estimated by the following formula: Clay % = 2.5(% water retained at 1500 kPa tension - % organic carbon).

Control Section for Cation-Exchange Activity Classes

The control section for cation-exchange activity classes is the same as that used to determine the particle-size and mineralogy classes. For soils with strongly contrasting particle-size classes, where both named parts of the control section use a cation-exchange activity class, the class associated with the particle-size class that has the most clay is named. For example, in a pedon with a classification of loamy over clayey, mixed, active, calcareous, thermic Typic Udorthent, the cation-exchange activity class "active" is associated with the clayey part of the control section.

Key to Cation-Exchange Activity Classes

Soils that are not Histosols, Histels, or Oxisols, that are not in "kandi" or "kanhap" great groups or subgroups of Alfisols and Ultisols, that are in either a mixed or siliceous mineralogy class, that are not in a fragmental, sandy, or sandy-skeletal particle-size class or any substitute for a particle-size class, and that have a ratio of cation-exchange capacity (by NH4OAc at pH 7) to clay (percent by weight) of:
1. 0.60 or more.
  Superactive
2. 0.40 to 0.60.
  Active
3. 0.24 to 0.40.
  Semiactive
4. Less than 0.24.
  Subactive
  or
All other soils: No cation-exchange activity classes used.

Calcareous and Reaction Classes of Mineral Soils

The presence or absence of carbonates, soil reaction, and the presence of high concentrations of aluminum in mineral soils are treated together because they are so intimately related. There are four classes-calcareous, acid, nonacid, and allic. These are defined later in the key to calcareous and reaction classes. The classes are not used in all taxa, nor is more than one used in the same taxa.

Use of the Calcareous and Reaction Classes

The calcareous, acid, and nonacid classes are used in the names of the families of Entisols, Aquands, and Aquepts, except they are not used in any of the following:

Duraquands and Placaquands
Sulfaquepts, Fragiaquepts, and Petraquepts
Sandy, sandy-skeletal, cindery, pumiceous, or fragmental families
Families with carbonatic or gypsic mineralogy

The calcareous class, in addition to those listed above, is used in the names of the families of Aquolls, except it is not used with any of the following:

Calciaquolls, Natraquolls, and Argiaquolls
Cryaquolls and Duraquolls that have an argillic horizon
Families with carbonatic or gypsic mineralogy

The allic class is used only in families of Oxisols.

Control Section for Calcareous and Reaction Classes

The control section for the calcareous class is one of the following:

Soils with a root-limiting layer that is 25 cm or less below the mineral soil surface: A 2.5-cm-thick layer directly above the root-limiting layer.
Soils with a root-limiting layer that is 26 to 50 cm below the mineral soil surface: The layer between a depth of 25 cm below the mineral soil surface and the root-limiting layer.
All other listed soils: Between a depth of 25 and 50 cm below the mineral soil surface.

The control section for the acid, nonacid, and allic classes is the same as that for particle-size classes.

Key to Calcareous and Reaction Classes

Oxisols that have a layer, 30 cm or more thick within the control section, that contains more than 2 cmol(+) of KCl-extractable Al per kg soil in the fine-earth fraction.
Allic
Other listed soils that, in the fine-earth fraction, effervesce (in cold dilute HCl) in all parts of the control section.
Calcareous
Other listed soils with a pH of less than 5.0 in 0.01 M CaCl2 (1:2) (about pH 5.5 in H2O, 1:1) throughout the control section.
Acid
Other listed soils with a pH of 5.0 or more in 0.01 M CaCl2 (1:2) in some or all layers in the control section.
Nonacid

It should be noted that a soil containing dolomite is calcareous and that effervescence of dolomite, when treated with cold dilute HCl, is slow.

The calcareous, acid, nonacid, and allic classes are listed in the family name, when appropriate, following the mineralogy and cation-exchange activity classes.

Soil Temperature Classes

Soil temperature classes, as named and defined here, are used as part of the family name in both mineral and organic soils. Temperature class names are used as part of the family name unless the criteria for a higher taxon carry the same limitation. Thus, frigid is implied in all cryic suborders, great groups, and subgroups and would be redundant if used in the names of families within these classes.

The Celsius (centigrade) scale is the standard. It is assumed that the temperature is that of a soil that is not being irrigated.

Control Section for Soil Temperature

The control section for soil temperature either is at a depth of 50 cm from the soil surface or is at the upper boundary of a root-limiting layer, whichever is shallower. The soil temperature classes, defined in terms of the mean annual soil temperature and the difference between mean summer and mean winter temperatures, are determined by the following key.

Key to Soil Temperature Classes

Gelisols that have a mean annual soil temperature as follows:
1. -10 °C or lower.
  Hypergelic
  or
2. -4 °C to -10 °C.
  Pergelic
  or
3. +1 °C to -4 °C.
  Subgelic
  or
Other soils that have a difference in soil temperature of 6 °C or more between mean summer (June, July, and August in the Northern Hemisphere) and mean winter (December, January, and February in the Northern Hemisphere) and a mean annual soil temperature of:
1. Lower than 8 °C (47 °F).
  Frigid
  or
2. 8 °C (47 °F) to 15 °C (59 °F).
  Mesic
  or
3. 15 °C (59 °F) to 22 °C (72 °F).
  Thermic
  or
4. 22 °C (72 °F) or higher.
  Hyperthermic
  or
All other soils that have a mean annual soil temperature as follows:
1. Lower than 8 °C (47 °F).
  Isofrigid
  or
2. 8 °C (47 °F) to 15 °C (59 °F).
  Isomesic
  or
3. 15 °C (59 °F) to 22 °C (72 °F).
  Isothermic
  or
4. 22 °C (72 °F) or higher.
  Isohyperthermic

Soil Depth Classes

Soil depth classes are used in all families that have a root-limiting layer at a specified depth from the mineral soil surface, except for those families in Lithic subgroups and those with a fragipan. The root-limiting layers included in soil depth classes are duripans; petrocalcic, petrogypsic, and placic horizons; continuous ortstein (90 percent or more); and densic, lithic, paralithic, and petroferric contacts. Soil depth classes for Histosols and Histels are given later in this chapter. One soil depth class name, "shallow," is used to characterize certain mineral soil families that have one of the depths indicated in the following key.

Key to Soil Depth Classes

Oxisols that are less than 100 cm deep (from the mineral soil surface) to a root-limiting layer and are not in a Lithic subgroup.
Shallow
or
Soils in all other mineral soil orders that are less than 50 cm deep (from the mineral soil surface) to a root-limiting layer and are not in a Lithic subgroup.
Shallow
or
All other mineral soils: No soil depth class used.

Rupture-Resistance Classes

In this taxonomy, some partially cemented soil materials, such as durinodes, serve as differentiae in categories above the family, while others, such as partially cemented spodic materials (ortstein), do not. No single family, however, should include soils both with and without partially cemented horizons. In Spodosols, a partially cemented spodic horizon is used as a family differentia. The following rupture-resistance class is defined for families of Spodosols:

Spodosols that have an ortstein horizon.
Ortstein
or
All other soils: No rupture-resistance class used.

Classes of Coatings (on Sands)

Despite the emphasis given to particle-size classes in this taxonomy, variability remains in the sandy particle-size class, which includes sands and loamy sands. Some sands are very clean, i.e., almost completely free of silt and clay, while others are mixed with appreciable amounts of finer grains. Clay is more efficient at coating sand than is silt. A weighted average silt (by weight) plus 2 times the weighted average clay (by weight) of more than 5 makes a reasonable division of the sands at the family level. Two classes of Quartzipsamments are defined in terms of their content of silt plus 2 times their content of clay.

Control Section for Classes of Coatings

The control section for classes of coatings is the same as that for particle-size classes or their substitutes and for mineralogy classes.

Key to Classes of Coatings

Quartzipsamments that have a sum of the weighted average silt (by weight) plus 2 times the weighted average clay of more than 5.
Coated
or
Other Quartzipsamments.
Uncoated

Classes of Permanent Cracks

Some Hydraquents consolidate or shrink after drainage and become Fluvaquents or Humaquepts. In the process they can form polyhedrons roughly 12 to 50 cm in diameter, depending on their n value and texture. These polyhedrons are separated by cracks that range in width from 2 mm to more than 1 cm. The polyhedrons may shrink and swell with changes in the moisture content of the soils, but the cracks are permanent and can persist for several hundreds of years, even if the soils are cultivated. The cracks permit rapid movement of water through the soils, either vertically or laterally. Such soils may have the same texture, mineralogy, and other family properties as soils that do not form cracks or that have cracks that open and close with the seasons. Soils with permanent cracks are very rare in the United States.

Control Section for Classes of Permanent Cracks

The control section for classes of permanent cracks is from the base of any plow layer or 25 cm from the soil surface, whichever is deeper, to 100 cm below the soil surface.

Key to Classes of Permanent Cracks

Fluvaquents or Humaquepts that have, throughout a layer 50 cm or more thick, continuous, permanent, lateral and vertical cracks 2 mm or more wide, spaced at average lateral intervals of less than 50 cm.
Cracked
or
All other Fluvaquents and Humaquepts: No class of permanent cracks used.

Family Differentiae for Histosols and Histels

Most of the differentiae that are used to distinguish families of Histosols and Histels have already been defined, either because they are used as differentiae in mineral soils as well as Histosols and Histels or because their definitions are used for the classification of some Histosols and Histels in categories higher than the family. In the following descriptions, differentiae not previously mentioned are defined and the classes in which they are used are enumerated.

The order in which family classes, if appropriate for a particular family, are placed in the technical family names of Histosols and Histels is as follows:

Particle-size classes
Mineralogy classes, including the nature of limnic deposits in Histosols
Reaction classes
Soil temperature classes
Soil depth classes (used only in Histosols)

Particle-Size Classes

Particle-size classes are used only for the family names of Terric subgroups of Histosols and Histels. The classes are determined from the properties of the mineral soil materials in the control section through use of the key to particle-size classes. The classes are more generalized than those for soils in other orders.

Control Section for Particle-Size Classes

The particle-size control section is the upper 30 cm of the mineral layer or of that part of the mineral layer that is within the control section for Histosols and Histels (given in chapter 3), whichever is thicker.

Key to Particle-Size Classes of Histosols and Histels

Terric subgroups of Histosols and Histels that have (by weighted average) in the particle-size control section:
1. A fine-earth component of less than 10 percent (including associated medium and finer pores) of the total volume.
  Fragmental
  or
2. A texture (of the fine earth) of sand or loamy sand, including less than 50 percent (by weight) very fine sand in the fine-earth fraction.
  Sandy or sandy-skeletal
  or
3. Less than 35 percent clay in the fine-earth fraction and a content of rock fragments of 35 percent or more of the total volume.
  Loamy-skeletal
  or
  A content of rock fragments of 35 percent or more of the total volume.
  Clayey-skeletal
  or
  A clay content of 35 percent or more in the fine-earth fraction.
  Clayey
  or
  All other Terric subgroups of Histosols and Histels.
  Loamy
  or
All other Histosols and Histels: No particle-size class used.

Mineralogy Classes

There are three different kinds of mineralogy classes recognized for families in certain great groups and subgroups of Histosols. The first kind is the ferrihumic soil material defined below. The second is three types of limnic materials-coprogenous earth, diatomaceous earth, and marl, defined in chapter 3. The third is mineral layers of Terric subgroups. The key to mineralogy classes for these mineral layers is the same as that for mineral soils. Terric subgroups of Histels also have the same mineralogy classes as those for mineral soils.

Ferrihumic Mineralogy Class

Ferrihumic soil material, i.e., bog iron, is an authigenic (formed in place) deposit consisting of hydrated iron oxide mixed with organic matter, either dispersed and soft or cemented into large aggregates, in a mineral or organic layer that has all of the following characteristics:

Saturation with water for more than 6 months per year (or artificial drainage);
2 percent or more (by weight) iron concretions having lateral dimensions ranging from less than 5 to more than 100 mm and containing 10 percent or more (by weight) free iron oxide (7 percent or more Fe) and 1 percent or more (by weight) organic matter; and
A dark reddish or brownish color that changes little on drying.

The ferrihumic mineralogy class is used for families of Fibrists, Hemists, and Saprists, but it is not used for Sphagnofibrists and Sphagnic subgroups of other great groups. If the ferrihumic class is used in the family name of a Histosol, no other mineralogy classes are used in that family because the presence of iron is considered to be by far the most important mineralogical characteristic.

Mineralogy Classes Applied Only to Limnic Subgroups

Limnic materials (defined in chapter 3) with a thickness of 5 cm or more are mineralogy class criteria if the soil does not also have ferrihumic mineralogy. The following family classes are used: coprogenous, diatomaceous, and marly.

Control Section for the Ferrihumic Mineralogy Class and Mineralogy Classes Applied to Limnic Subgroups

The control section for the ferrihumic mineralogy class and the classes applied to Limnic subgroups is the same as the control section for Histosols.

Mineralogy Classes Applied Only to Terric Subgroups

For Histosols and Histels in Terric subgroups, use the same key to mineralogy classes as that used for mineral soils unless a Histosol also has ferrihumic mineralogy.

Control Section for Mineralogy Classes Applied Only to Terric Subgroups

For Terric subgroups of Histosols and Histels, use the same control section for mineralogy classes as that used for the particle-size classes.

Key to Mineralogy Classes

Histosols (except for Folists), Sphagnofibrists, and Sphagnic subgroups of other great groups that have ferrihumic soil material within the control section for Histosols.
Ferrihumic
or
Other Histosols that have, within the control section for Histosols, limnic materials, 5 cm or more thick, that consist of:
1. Coprogenous earth.
  Coprogenous
  or
2. Diatomaceous earth.
  Diatomaceous
  or
  Marl.
  Marly
or
Histels and other Histosols in Terric subgroups: Use the key to mineralogy classes for mineral soils.
or
All other Histels and Histosols: No mineralogy class used.

Reaction Classes

Reaction classes are used in all families of Histosols and Histels. The two classes recognized are defined in the following key:

Histosols and Histels that have a pH value, on undried samples, of 4.5 or more (in 0.01 M CaCl2) in one or more layers of organic soil materials within the control section for Histosols.
Euic
or
All other Histosols and Histels.
Dysic

Soil Temperature Classes

The soil temperature classes of Histosols are determined through use of the same key and definitions as those used for mineral soils. Histels have the same temperature classes as other Gelisols.

Soil Depth Classes

Soil depth classes refer to the depth to a root-limiting layer, a fragmental particle-size class, or a cindery or pumiceous substitute class. The root-limiting layers included in soil depth classes of Histosols are duripans; petrocalcic, petrogypsic, and placic horizons; continuous ortstein; and densic, lithic, paralithic, and petroferric contacts. The following key is used for families in all subgroups of Histosols. The shallow class is not used in the suborder Folists.

Key to Soil Depth Classes

Histosols that are less than 18 cm deep to a root-limiting layer, to a fragmental particle-size class, or to a cindery or pumiceous substitute class.
Micro
or
Other Histosols, excluding Folists, that have a root-limiting layer, a fragmental particle-size class, or a cindery or pumiceous substitute class at a depth between 18 and 50 cm from the soil surface.
Shallow
or
All other Histosols: No soil depth class used.

Series Differentiae Within a Family

The function of the series is pragmatic, and differences within a family that affect the use of a soil should be considered in classifying soil series. The separation of soils at the series level of this taxonomy can be based on any property that is used as criteria at higher levels in the system. The criteria most commonly used include presence of, depth to, thickness of, and expression of horizons and properties diagnostic for the higher categories and differences in texture, mineralogy, soil moisture, soil temperature, and amounts of organic matter. The limits of the properties used as differentiae must be more narrowly defined than the limits for the family. The properties used, however, must be reliably observable or be inferable from other soil properties or from the setting or vegetation.

The differentiae used must be within the series control section. Differences in soil or regolith that are outside the series control section and that have not been recognized as series differentiae but are relevant to potential uses of certain soils are considered as a basis for phase distinctions.

Control Section for the Differentiation of Series

The control section for the soil series is similar to that for the family, but it differs in a few important respects. The particle-size and mineralogy control sections for families end at the upper boundary of a fragipan, duripan, or petrocalcic horizon because these horizons have few roots. In contrast to the control section for the series, the thickness of such horizons is not taken into account in the control sections for the family. The series control section includes materials starting at the soil surface and also the first 25 cm below a densic or paralithic contact if its upper boundary is less than 125 cm below the mineral soil surface. Properties of horizons and layers below the particle-size control section, a depth between 100 and 150 cm (or to 200 cm if in a diagnostic horizon) from the mineral soil surface, also are considered.

Key to the Control Section for the Differentiation of Series

The part of a soil to be considered in differentiating series within a family is as follows:

Mineral soils that have permafrost within 150 cm of the soil surface: From the soil surface to the shallowest of the following:
1. A lithic or petroferric contact; or
2. depth of 100 cm if the depth to permafrost is less than 75 cm; or
3. 5 cm below the upper boundary of permafrost if that boundary is 75 cm or more below the soil surface; or
4. 25 cm below a densic or paralithic contact; or
5. A depth of 150 cm; or
Other mineral soils: From the soil surface to the shallowest of the following:
1. A lithic or petroferric contact; or
2. A depth of either 25 cm below a densic or paralithic contact or 150 cm below the soil surface, whichever is shallower, if there is a densic or paralithic contact within 150 cm; or
3. A depth of 150 cm if the bottom of the deepest diagnostic horizon is less than 150 cm from the soil surface; or
4. The lower boundary of the deepest diagnostic horizon or a depth of 200 cm, whichever is shallower, if the lower boundary of the deepest diagnostic horizon is 150 cm or more below the soil surface; or
Organic soils (Histosols and Histels): From the soil surface to the shallowest of the following:
1. A lithic or petroferric contact; or
2. A depth of 25 cm below a densic or paralithic contact; or
3. A depth of 100 cm if the depth to permafrost is less than 75 cm; or
4. 25 cm below the upper boundary of permafrost if that boundary is between a depth of 75 and 125 cm below the soil surface; or
  The base of the bottom tier

Citation:
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].