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Soil Survey Technical Note 10

Buried Soils and Their Effect on Taxonomic Classification
 

Purpose

The purpose of this technical note is to clarify the meaning of “buried soils” and how they affect the taxonomic classification of soils when “Keys to Soil Taxonomy” is used. Accurate classification requires that users understand the definitions of (1) a buried soil, (2) a buried genetic horizon, and (3) a surface mantle of new soil material. Incorrectly identifying these features could result in the selection of diagnostic features that should not be used in classification and thus could result in inaccurately classified soils. This document was revised in July 2014 to conform to changes in the 12th edition of the “Keys to Soil Taxonomy.”
 

Discussion

The “buried soil” concept was originally developed in soil taxonomy for use on such landscapes as flood plains where a dike had burst or a natural levee was breached by floodwaters; near volcanoes where very recent ash or pumice deposits occur; or in areas where dunes are moving across the landscape, creating mantles of new surficial material (Smith, 1986). All of these mantles consist of alluvial or eolian deposits on preexisting soils. Recent interest in human-transported materials and in parent materials for subaqueous soils has expanded the application of the buried soils concept.

The definition of “buried soil” involves evaluating the thickness of the surface mantle of new soil material and determining the proper subsequent role of the mantle in taxonomic classification. The thickness of the surface mantle determines how properties of the mantle and underlying diagnostic horizons are considered when “Keys to Soil Taxonomy” is used to classify soils into the higher categories of the system (Soil Survey Staff, 2014). The definitions of “buried soil,” “buried genetic horizon,” and “surface mantle of new soil material” are presented in the following paragraphs.

A buried soil is a sequence of one or more genetic horizons covered with a surface mantle of new soil material that is 50 cm or more thick. Any horizons or layers underlying a plaggen epipedon are also considered to be a buried soil.

A buried genetic horizon is an identifiable buried horizon with major genetic features that were developed before burial. Buried genetic horizons are connoted by the use of suffix symbol “b” in horizon designations. It is important to note that buried genetic horizons are not always part of a horizon sequence that meets the definition of a buried soil.

A surface mantle of new soil material is a layer of natural or human-deposited mineral material that is largely unaltered, at least in the lower part, and is underlain by one or more buried genetic horizons. The mantle may have a diagnostic surface horizon (epipedon),  a cambic horizon, or both, but it has no other diagnostic subsurface horizons. A surface mantle of new soil material can be of any thickness. However, if one or more genetic horizons have formed within the material making up the surface mantle, a layer that is at least 7.5 cm thick and is not part of any diagnostic horizon must remain at the base of the material. Human-transported material is considered in the same context as naturally transported alluvial or eolian deposits and can form a surface mantle of new soil material, providing it meets the conditions described above.

The required unaltered lower part of a surface mantle of new soil material is often designated as a C horizon or, rarely, as a transitional horizon, such as AC or BC. Such horizons commonly have rock structure as evidence of both their young age and lack of pedogenic alteration. Rock structure (Soil Survey Staff, 2014) includes fine stratifications (5 mm or less thick) in unconsolidated sediments (eolian, alluvial, lacustrine, or marine). A surface mantle of new soil material is often easily identified in a soil profile by having an abrupt lower horizon boundary and a lithologic discontinuity with the underlying horizons.

An underlying sequence of genetic horizons is not considered a “buried soil” if the surface mantle of new soil material is less than 50 cm thick. In this case, the surface mantle of new soil material can be used to establish a phase of the mantled soil or, if the mantle affects the use of the soil, another soil series.
 

General Rules

Soil profiles that have a surface mantle of new soil material underlain by older soil material present unique challenges to proper classification. Because there are effectively two soils present (a “new” soil underlain by an older soil), a determination is required as to which soil takes precedence for classification purposes. Also, some criteria for various taxonomic classes specify a condition to be met within a specific depth from “the soil surface.” Among these criteria are those for determining the soil moisture and temperature control sections; depth to aquic conditions; presence of andic and vitrandic properties; the value of certain properties, such as base saturation or organic carbon content at a given depth; and some soil family criteria. Such determinations require the correct starting point for measurements. These issues are discussed below.
 

Which Soil is Classified?

Case 1.—The surface mantle is 50 cm or more thick, and it is underlain by a buried soil. In this case, the classification is based primarily on the soil formed in the surface mantle of new soil material. The soil order is determined based on the diagnostic horizons (if any) in the mantle. The surface of the new mantle is the starting point for depth measurements to determine soil moisture and temperature classes, depth to aquic conditions, depth to and thickness of diagnostic horizons or other diagnostic characteristics listed in the “Keys to Soil Taxonomy,” and control sections used for soil family classes. Additional properties, such as organic carbon distribution and base saturation, may also be required at depths measured from the surface of the mantle and that fall within the buried soil profile. However, diagnostic horizons present within the buried soil are not considered unless the criteria in the “Keys to Soil Taxonomy” specifically indicate the presence of buried horizons (such as in thapto-histic subgroups). The presence of buried horizons, as well as their key properties, may be included in the range in characteristics for the series or as phase criteria. See examples 2 and 3 below.

Case 2.—The surface mantle is less than 50 cm thick, and it is underlain by a sequence of one or more buried genetic horizons. In this case, the classification is based primarily on the soil below the surface mantle of new soil material. The top of the underlying original soil is considered as the soil surface for determining the soil order and the depth to and thickness of any diagnostic horizons or other characteristics listed in the “Keys to Soil Taxonomy.” However, the surface of the new mantle is used as the starting point for depth measurements to determine soil moisture and temperature classes, depth to aquic conditions, andic or vitrandic properties, such properties as organic carbon distribution and base saturation, and control sections used for soil family classes that are measured from the soil surface. See example 1 below.

If the newer deposit of soil material has undergone pedogenic development such that any part of the lowest 7.5 cm of the newer deposit meets the criteria for any diagnostic horizon, then the newer deposit is not considered a surface mantle of new soil material and the underlying soil is not a buried soil. In this case, the entire profile is considered as one soil, mostly likely having one or more lithologic discontinuities. See examples 4, 5, and 6 below.

The concepts of buried soils and buried genetic horizons are easily conflated, resulting in classification errors. It is important to differentiate the concepts. Every buried soil has a buried genetic horizon or a sequence of buried genetic horizons, but not every soil profile with a buried genetic horizon (designated with suffix symbol “b”) has a buried soil. A profile may not qualify as having a buried soil because the mantle is too thin or because pedogenesis welded soil horizons that formed in parent materials of differing ages (Ruhe and Olson, 1980). The process of welding gradually changes soil properties (e.g., clay content) and thus obscures the appearance and recognition of lithologic discontinuities. The result of welding is that some soil profiles can display genetic horizons, such as a Bt horizon over a 2Bt horizon, that are currently part of one pedon and are both currently receiving additions of suspended or dissolved materials. See examples 4, 5, and 6 below.
 

Summary

Properly applying the definition of a “buried soil” helps to ensure correct taxonomic classification when “Keys to Soil Taxonomy” is used. The concept of buried soils was originally developed for use on active subaerial landscapes, such as flood plains. The concept is also applicable to areas subjected to deposition of human-transported materials and to the submerged landscapes and parent materials of subaqueous soils. Buried soils require evidence of a diagnostic horizon underlying either a plaggen epipedon or a surface mantle of new soil material (including human-transported material) that is 50 cm or more thick.

The classification of a soil profile that has a buried soil is based mostly on the properties within the surface mantle and not on the presence of the diagnostic horizons within the buried soil. The diagnostic horizons in buried soils are not used for selecting taxa (except for thapto-histic subgroups) in the higher categorical levels of soil taxonomy. Specific properties of the buried horizons, however, may be used when measurements that are required from the current soil surface fall within a buried genetic horizon. Examples of such properties include organic carbon distribution, base saturation, and properties used in some family criteria. The properties of buried diagnostic horizons and features can also be used at the series level or for phase distinctions of series, if needed.

A surface mantle of new soil material is largely unaltered, at least in the lower part. These materials do not need to be naturally formed through typical pedogenic processes. Human-transported materials may also be considered surface mantles of new soil material. A thin surface mantle does not impact selection of taxa in the higher categorical levels of soil taxonomy (except as noted in the discussion above). If the mantle affects the use of the soil, however, it may be used to establish a phase of the mantled soil, to establish a new soil series, or to differentiate existing soil series (Soil Survey Staff, 2014).

The identification of a buried genetic horizon does not always indicate the presence of a buried soil. Buried genetic horizons may be used to classify soils that do not also meet the requirements for a buried soil. Identification of buried genetic horizons is critical for identifying past soil disturbance and placement of human-transported materials, such as fill.

For additional information and examples regarding the classification of soils that have a surface mantle of new material and buried soils, see the job aid: “Identifying/Classifying Buried Soil Horizons” (Galbraith, 2011).
 

Contact

The contact for this technical note is the National Leader for Soil Survey Standards, National Soil Survey Center, Lincoln, Nebraska.
 

Reference

Galbraith, J.M. 2011. Revised 2014. Identifying/classifying buried soil horizons. Virginia Tech University. http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050996.pdf (accessed 7 July 2014).

Ruhe, R.V., and C.G. Olson. 1980. Soil welding. Soil Sci. 130:132–139.

Smith, G.D. 1986. The Guy Smith interviews: Rationale for concepts in soil taxonomy. U.S. Department of Agriculture, Soil Conservation Service, and Cornell University, Department of Agronomy. Soil Management Support Serv. Tech. Monogr. 11:44–45.

soil survey staff. 2014. keys to soil taxonomy. 12th edition. u.s. department of agriculture, natural resources conservation service. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/home/?cid=NRCS142P2_053580 (accessed 7 July 2014).
 

Examples (Adapted from Galbraith, 2011)

Example 1: A soil with a mantle but no buried soil

Soil profile having A, C, Ab, Bwb, C'1, 2C2, and 3C3 horizons. Side bar text reads: Epipedon--Ochric. Textures ares ands throughout. There is a mantle because the lower 7.5 cm of the recent deposit is not part of a diagnostic horizon. There is an ochric epipedon (Ab) under the recent deposit, but there are no diagnostic subsurface horizons. The Bwb is too thin for a cambic horizon. Even if it were thick enough, it would be too sandy for a cambic horizon. There is no buried soil because the mantle on top is too thin (<50 cm thick). This soil is an Entisol, a Typic Xeropsamment.>




























Example 2: A soil with a mantle and a buried soil

Soil profile having Ap, C1, C2 and Ab horizons. Side bar text reads: Epipedon--none. What would have been an ochric epipedon is an Ap horizon that directly overlies freshly stratified sediments with an irregular decrease in carbon. Textures are loams and silt loams. There is no cambic horizon in the mantle. There is a buried mollic epipedon (Ab) and buried soil beginning at 182 cm. The mollic epipedon is not considered, except to confirm an irregular decrease in carbon. This soil is an Entisol, a Xerofluvent.





























Example 3: A soil with a mantle and a buried soil

Soil profile having A, C1, C2, Ab, C'1, 2C2, and 3C3 horizons. Side bar text reads: Epipedon--ochric. The A horizon is 10YR 6/2 dry and has 0.0 percent calcium carbonate. Textures are loams. There is a mantle because the lower 7.5 cm of the recent deposit is not part of a diagnostic horizon. There is no cambic horizon in the mantle. There is a buried ochric epipedon (Ab). There are no buried diagnostic subsurface horizons. The mantle on top is just thick enough to qualify the buried horizons as a buried soil (50 cm). This soil is an Entisol, a Typic Xerofluvent.



























Example 4: A soil with buried genetic horizons but no buried soil

A—0 to 4 cm; brown (7.5YR 4/4) loam; weak medium granular structure; friable; common very fine, fine, and medium roots; few fine flakes of mica; very strongly acid; clear smooth boundary.

Bw1—4 to 24 cm; dark yellowish brown (10YR 4/4) silty clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; common fine flakes of mica; few medium faint brown (10YR 5/3) iron depletions; very strongly acid; gradual wavy boundary.

Bw2—24 to 46 cm; dark yellowish brown (10YR 4/4) clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; many fine flakes of mica; common medium faint grayish brown (10YR 5/2) iron depletions and common medium distinct strong brown (7.5YR 4/6) masses of oxidized iron; very strongly acid; gradual wavy boundary.

Ab—46 to 60 cm; black (10YR 2/1) loam; weak fine granular structure; very friable; many fine roots; few fine pebbles; common fine flakes of mica; strongly acid; abrupt wavy boundary.

Btgb—60 to 80 cm; light brownish gray (10YR 6/2) clay loam; moderate medium subangular blocky structure; friable; moderately sticky and moderately plastic; many medium prominent yellowish brown (10YR 5/6) friable iron masses; common distinct clay films on faces of peds; few fine pebbles; common fine flakes of mica; strongly acid; gradual wavy boundary.

Cg—80 to 120 cm; gray (5Y 6/1) loam; massive; friable; many medium prominent dark yellowish brown (10YR 4/4) friable iron masses; about 5 percent, by volume, fine pebbles; few fine flakes of mica; strongly acid.

The Ab and Btgb horizons are buried genetic horizons. The material from 0 to 46 cm does not constitute a surface mantle of new soil material because the lower part (Bw2) does not contain unaltered material. The gradual boundary at 46 cm does not represent unaltered material. The profile does not represent a buried soil, and classification is based on the entire profile. Note that the original soil (46–120 cm) was a Typic Endoaqualf. After the addition of the newer material on top, the soil classifies as an Aeric Endoaqualf.
 

Example 5: A soil with a buried genetic horizon but no buried soil

^A—0 to 8 cm; brown (7.5YR 4/4) loam; weak medium granular structure; friable; common very fine, fine, and medium roots; 15 to 25 percent of the area has a 1/4 inch thick layer of asphalt coating that has broken up into fragments 3 inches in diameter; few fine flakes of mica; very strongly acid; clear smooth boundary.

^Bw1—8 to 24 cm; dark yellowish brown (10YR 4/4) silty clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; common fine flakes of mica; few medium faint brown (10YR 5/3) iron depletions; 5 percent gravel (asphalt); very strongly acid; gradual wavy boundary.

^Bw2—24 to 40 cm; dark yellowish brown (10YR 4/4) clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; many fine flakes of mica; common medium faint grayish brown (10YR 5/2) iron depletions and common medium distinct strong brown (7.5YR 4/6) masses of oxidized iron; 5 percent gravel (asphalt); very strongly acid; abrupt wavy boundary.

Btgb—40 to 80 cm; light brownish gray (10YR 6/2) clay loam; moderate medium subangular blocky structure; friable; moderately sticky and moderately plastic; many medium prominent yellowish brown (10YR 5/6) friable iron masses; common distinct clay films on faces of peds; few fine pebbles; common fine flakes of mica; strongly acid; gradual wavy boundary.

Cg—80 to 120 cm; gray (5Y 6/1) loam; massive; friable; many medium prominent dark yellowish brown (10YR 4/4) friable iron masses; about 5 percent, by volume, fine pebbles; few fine flakes of mica; strongly acid.

The Btgb horizon is a buried genetic horizon under human-transported material. The material from 0 to 40 cm does not constitute a surface mantle of new soil material because the lower part (^Bw2) does not contain unaltered material. The profile does not represent a buried soil, and classification (Aeric Endoaqulaf) is based on the entire profile.
 

Example 6: A soil with a buried genetic horizon deeper than 50 cm but no buried soil

Oa—0 to 10 cm; muck (sapric material), black (10YR 2/1) broken face, black (N 2.5/) rubbed; about 12 percent fiber, less than 5 percent rubbed; moderate medium granular structure; primarily herbaceous fibers; neutral (pH 7.0 in water); abrupt wavy boundary.

A10 to 14 cm; brown (7.5YR 4/4) clay loam; weak medium granular structure; friable; common very fine, fine, and medium roots; few fine flakes of mica; very strongly acid; clear smooth boundary.

Bw114 to 24 cm; dark yellowish brown (10YR 4/4) clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; common fine flakes of mica; few medium faint brown (10YR 5/3) iron depletions; very strongly acid; gradual wavy boundary.

Bw224 to 56 cm; dark yellowish brown (10YR 4/4) clay loam; weak medium subangular blocky structure; friable; common fine and medium roots; many fine flakes of mica; common medium faint grayish brown (10YR 5/2) iron depletions and common medium distinct strong brown (7.5YR 4/6) masses of oxidized iron; very strongly acid; gradual wavy boundary.

Ab56 to 60 cm; black (10YR 2/1) loam; weak fine granular structure; very friable; many fine roots; few fine pebbles; common fine flakes of mica; strongly acid; abrupt wavy boundary.

Cg160 to 80 cm; light brownish gray (10YR 6/2) loam; massive; friable; moderately sticky and moderately plastic; many medium prominent yellowish brown (10YR 5/6) friable iron masses; few fine pebbles; common fine flakes of mica; strongly acid; gradual wavy boundary.

Cg2—80 to 120 cm; gray (5Y 6/1) loam; massive; friable; many medium prominent dark yellowish brown (10YR 4/4) friable iron masses; about 5 percent, by volume, fine pebbles; few fine flakes of mica; strongly acid.

The Ab horizon is a buried genetic horizon. The material from 0 to 56 cm meets the thickness requirement for a buried soil, but the lower part does not contain unaltered material. The gradual boundary at 56 cm does not represent unaltered material. The profile does not represent a buried soil, and classification (Fluvaquentic Dystrudept) is based on the entire profile. Note that the original soil (56 to 120 cm) was likely a Typic Endoaquent.