Diagnostic Subsurface Horizons

clay accumulation with evidence of translocation (clay films, clay bridges on grains).
minimum thickness 7.5 cm if loamy or clayey, 15 cm if sandy.
Clay increase must occur within a vertical distance of 30 cm from the reference eluvial horizon.
% clay in eluvial; minimum clay increase in argillic.
  • < 15 % clay in eluvial then must have 3% absolute minimum clay increase in argillic
  • 15 - 40 clay in eluvial then must have 1.2 times eluvial (20% relative) clay increase in argillic
  • 40 - 60 clay in eluvial then must have 8% absolute clay increase in argillic
  • > 60 clay in eluvial then must have 8% absolute fine clay (< 0.2 micron) increase in argillic
properties of an argillic horizon, plus ESP > 15 or SAR > 13 within 40 cm of upper boundary of horizon, and structure of columnar, prismatic or blocky with tongues of eluvial material.
accumulation of low activity clay (CEC < 16 cmol (+)/kg clay and ECEC < 12 cmol(+)/kg clay), > 30 cm thick (usually), clay increase depends on clay content of overlying, coarser-textured horizon (but not the same as argillic).
accumulation of organo-metallic compounds, > 2.5 cm thick containing spodic material: specific color requirements, pH < 5.9, OC > 0.6%, oxalate extractable Al + 1/2 Fe > 0.50% and twice as much as in overlying horizon, or ODOE > 0.25 and twice as high as in overlying horizon. Orstein if cemented and > 25 mm thick.
horizon of alteration, redder colors than underlying horizon, or loss of rock structure, or development of soil structure, or loss of carbonates, does not meet requirements of spodic, oxic, argillic, or kandic, base is deeper than 25 cm, texture of loamy very fine sand, very fine sand or finer.
low activity clay (like kandic) but no or diffuse clay increase, > 30 cm thick, texture of sandy loam or finer, < 10 % weatherable minerals.
accumulation of CaCO3, generally > 15 cm thick, > 15% CaCO3 and > 5% more carbonate than an underlying horizon.
like calcic, but cemented, will not slake in water, > 10 cm thick, or > 1 cm thick if laminar cap rests directly on bedrock
accumulation of gypsum, > 15 cm thick, > 5% gypsum, > 1% visible secondary gypsum, product of % gypsum times thickness (cm) > 150.
like gypsic, > 10 cm thick, does not slake in water (cemented).
accumulation of salts more soluble than gypsum, > 15 cm thick, EC > 30 dS/m, product of EC times thickness (cm) > 900.
illuvial humus without sesquioxides.
> 15 cm thick, pH < 3.5, evidence of acidification by sulfuric acid.
silica-cemented, does not slake in water.
brittle, high density, but slakes in water, polygonal color pattern.
> 1 cm thick consisting of > 85% albic material: low chroma, high value soil material, color controlled by uncoated mineral grains.
illuvial accumulation of clay, silt and humus caused by long-term cultivation.
thin, Fe-Mn-organic carbon-cemented pan, < 25 mm thick.
> 5 cm thick, 15 to 85 % albic material, remnants of argillic, natric, or kandic horizon.
Densic material and contact
root restrictive, not cemented.
Paralithic material and contact
root restrictive, very weakly to moderately cemented.
Lithic material and contact
root restrictive, at least strongly cemented.
Buried Soils
See http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053568 [last accessed August 16, 2016]

A buried soil is covered with a surface mantle of new soil material that either is 50 cm or more thick or is 30 to 50 cm thick and has a thickness that equals at least half the total thickness of the named diagnostic horizons that are preserved in the buried soil. A surface mantle of new material that does not have the required thickness for buried soils can be used to establish a phase of the mantled soil or even another soil series if the mantle affects the use of the soil.

Any horizons or layers underlying a plaggen epipedon are considered to be buried. A surface mantle of new material, as defined here, is largely unaltered, at least in the lower part. It may have a diagnostic surface horizon (epipedon) and/or a cambic horizon, but it has no other diagnostic subsurface horizons, all defined later. However, there remains a layer 7.5 cm or more thick that fails the requirements for all diagnostic horizons, as defined later, overlying a horizon sequence that can be clearly identified as the solum of a buried soil in at least half of each pedon. The recognition of a surface mantle should not be based only on studies of associated soils.

Lithologic Discontinuity
Lithologic discontinuities are significant changes in particle size distribution or mineralogy that represent differences in lithology within a soil. A lithologic discontinuity can also denote an age difference. For information on using horizon designations for lithologic discontinuities, see the Soil Survey Manual (USDA, SCS, 1993). Not everyone agrees on the degree of change required for a lithologic discontinuity. No attempt is made to quantify lithologic discontinuities. The discussion below is meant to serve as a guideline. Several lines of field evidence can be used to evaluate lithologic discontinuities. In addition to mineralogical and textural differences that may require laboratory studies, certain observations can be made in the field. These include but are not limited to the following:
  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.
Slickensides are polished and grooved surfaces and generally have dimensions exceeding 5 cm. They are produced when one soil mass slides past another. Some slickensides occur at the lower boundary of a slip surface where a mass of soil moves downward on a relatively steep slope. Slickensides result directly from the swelling of clay minerals and shear failure. They are very common in swelling clays that undergo marked changes in moisture content.
Fe/Mn concretions
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