Soil micromorphology is the study of size, shape, aggregation, etching, coating, accumulation, and depletion of minerals associated with various soil processes. Soil formation is a dynamic process with material continually being added, removed, and transformed. For example, water moving through a soil profile will pick up fine-textured material and deposit it as a coating along channels formed by the faces of adjacent peds.

Human activity can interrupt these processes and result in subtle differences in morphology. Water infiltration in the soil around a structure would be less than that in adjacent soil, because the structure would divert water away from it and compact the soil beneath it. Consequently, coatings on soil peds beneath the structure would be thinner, less well oriented, and have a different ratio of fine to coarse material than in the adjacent undisturbed soil.
 
 

Soil minerals and organic matter have weak electrostatic charge sites associated with their surface. While these sites may be positively or negatively charged, the predominant charge is negative. The magnitude of the charge associated with clay minerals may be very large. Thus, soils with a high clay content tend to have a larger negative charge. This charge attracts cations dissolved in soil solution. A soil's capability to replace cations on the surface with those in a soil solution at a given pH is called cation exchange capacity (CEC). We report CEC in centimoles of charge per kilogram of soil (cmol_c kg^-1 soil). Exchange occurs on a charge-for-charge basis.

While the total amount of exchange depends on charge, a cation's affinity for the surface is a function of its charge and hydrated radius. Cations with a high charge and small hydrated radius have a greater affinity for the surface. The attraction to the surface of cations commonly found in soil solution is in the order of Al³⁺>Ca²⁺>Mg²⁺>K⁺~NH₄⁺>Na⁺. Because human habitation often selectively enriches or depletes ions, a comparison of ion ratios may help explain land-use patterns.

In soil classification it is helpful to know the proportion of a soil's CEC occupied by basic and acidic cations. Basic cations are Ca²⁺, Mg²⁺, K⁺, and Na⁺. The sum of these four basic cations divided by the CEC is called the percent base saturation. The ratio of Ca²⁺ to Mg²⁺ is an indication of the degree of weathering. The relative depletion of Ca²⁺ versus Mg²⁺ is an indication of advanced weathering. Exchange acidity is a term given to the sum of Al³⁺ and H⁺ extracted in solution buffered at pH 8.2. Exchangeable acidity increases with leaching of basic cations and weathering.

Soil pH is a measure of the ability of soil minerals and organic matter to act as dilute acids and donate hydrogen ions into solution. A soil pH of <3.5 is normally associated with sulfur oxidation. This is common in coastal marshes and mine spoils, where buried sulfide minerals are exposed to oxygen. In forest soils, the acid-forming litter tends to keep soil pH below 5.5. Soil pH between 3.5 and 6.5 indicates free iron at the lower pH levels, and then aluminum hydrolysis at higher values controls pH. At pH values between 6.5 and 8.5, free CaCO₃ controls pH. Soil pH >8.5 indicates a high sodium content. These soils tend to be hard and very impermeable.

Soil organic matter is the partially decomposed residue of plants and animals. As it breaks down, it coats soil particles giving them a dark brown to black color. Organic matter can be categorized into three fractions: 

1. Plant litter and animal remains

These tend to be new additions to the soil. Depending on the climate, they decay relatively quickly, with a turnover time of some five years or less.
 
2. Microbial metabolites and stable cellular debris

This fraction includes humus, which gives soil many beneficial characteristics. It has a turnover time of fifty years or more. Table 3 details some of the effects humus has on soil.
 
3. Highly resistant fraction

These compounds may last in soil for 2,500 years or longer.

Organic matter accumulates near the surface where there is a high root density (A horizon). Various vegetation types have different effects on soil organic matter. Grasslands have a dense rooting pattern resulting in a thick dark colored A horizon. Forest soils have a thinner layer of organic staining. In areas of high rainfall, soluble organic matter can be washed out of a subsurface horizon (E horizon). It accumulates in a lower horizon and appears as a dark staining on particles.
 
 

Horizontal layers of soil called horizons can be described by their different morphological characteristics. Capital letters designate master horizons, which are further subdivided by Arabic numerals. Master horizons are used to describe similar appearing soil layers and should not be confused with diagnostic horizons used to classify soils.

The O horizon is a surface layer dominated by organic material. An O horizon may be found below the surface if it has been buried. Predominantly found in forested regions, the O horizon is composed of leaf litter in various stages of decay.

The A horizon is the uppermost mineral layer. It may lie below the O horizon. An A horizon has a high concentration of humus and is not dominated by the migration of clay, humus, aluminum, or iron into or out of the horizon. The humus content gives it a darker color than the horizon below.

The E horizon is a layer of eluviation where clay organic matter and iron and aluminum oxides have been leached out. Remaining material tends to be light colored and coarse textured. The E horizon is normally found below an O or an A horizon and above a B horizon. However, it may separate sections of a B horizon.

The B horizon is a subsurface layer showing evidence of one or more of the following processes:

1. illuvial accumulation of alumino-silicate clay, iron, aluminum, gypsum, or silica;

 
2. carbonate removal;

 
3. residual concentration of sesquioxides;

 
4. coating of sesquioxides, which makes the horizon conspicuously lower in color value, higher in chroma, or redder in hue without apparent illuviation of iron than that found in the overlying and underlying horizons;

 
5. alteration that forms silicate clay or liberates oxides, or both, and that forms a granular, blocky, or prismatic structure if volume changes accompany changes in moisture context; or 

 
6. brittleness.

The C horizon is a layer of minimal alteration. Material may be similar to or unlike that from which the other horizons formed. C horizons lack the properties of O, A, E or B horizons, and can include coprogenous earth (sedimentary peat), diatomaceous earth, saprolite, unconsolidated bedrock, and other uncemented geologic materials or materials soft enough for excavation with moderate difficulty. 

An R layer refers to hard bedrock. Material is cemented and manual excavation is impossible. Intrusive soils can be found in rare cracks in the bedrock. Examples of R layer material include: granite, basalt, quartzite, indurated limestone, or sandstone.

Transitional horizons are dominated by properties of one master horizon but have the subordinate properties of another. These are designated by two capital letters, for example, AB, EB, BE, or BC. The first letter represents the dominant horizon characteristics, the second indicates the weaker expressed characteristics.

A second type of transitional horizon has two distinct parts with recognizable properties of the two master horizons indicated by the capital letters. Parts of one surround the other. This type of transitional horizon is designated by a capital letter for the part with the greatest volume, followed by a slash and another capital letter for the secondary part (for example, E/B, B/E, or B/C).

Master horizons are further divided by subordinate characteristics, which usually do not apply to transitional horizons. Subordinate distinctions are identified by lower-case letters, called suffix symbols. In some cases, they describe an accumulation of material. This means that the so-designated horizons contain more of the material in question than is presumed to have been present in the parent material. For example, Bt refers to a B horizon with more clay than normal. The symbols and their meanings follow.

• a - highly decomposed organic material

Used with O to indicate the most highly decomposed organic materials, which have rubbed fiber content of less than 17 percent of the volume.
 
• b - buried genetic horizon

Used in mineral soils to indicate identifiable buried horizons with major genetic features that were developed before burial. Genetic horizons may or may not have formed in the overlying material, which may be either like or unlike the assumed parent material of the buried soil. This symbol is not used in organic soils or to separate an organic from a mineral layer.
 
• c - concretions or nodules

Indicates a significant accumulation of concretions or nodules. Cementation is required, but the cementing agent is not specific, except that it cannot be silica. The symbol is not used if the concretions or nodules consist of dolomite or calcite, or more soluble salts. It is used if the nodules or concretions are enriched with minerals that contain iron aluminum, manganese, or titanium.
 
• d - physical root restriction

Indicates root-restricting layers in naturally occurring or man-made unconsolidated sediments or materials, such as dense basal till, plow pans, and other mechanically compacted zones.
 
• e - organic material of intermediate composition

Used with O to indicate organic materials of intermediate composition with rubbed fiber content between 17 and 40 percent (by volume).
 
• f - frozen soil

Indicates permanent ice content in a horizon or layer. The symbol is not used for seasonally frozen layers or for so-called dry permafrost (material that is colder than 0º but does not contain ice).
 
• g - strong gleying

Indicates either that iron has been reduced and removed during soil formation, or that saturation with stagnant water has preserved it in a reduced state. Most of the affected layers have a chroma of 2 or less, and many have redox concentrations. The low chroma can represent either the color of reduced iron or the color of uncoated sand and silt particles from which the iron has been removed. The symbol g is not used for materials of low chroma that have no history of wetness, such as some shales or E horizons. If g is used with B, pedogenic change in addition to gleying is implied. The horizon is designated Cg if no other pedogenic change besides gleying has occurred.
 
• h - illuvial accumulation of organic matter

Used with B to indicate the accumulation of illuvial, amorphous, dispersible organic-matter-sesquioxide complexes if the sesquioxide component is dominated by aluminum but is present only in small quantities. The organo-sesquioxide material coats sand and silt particles. In some horizons, these coatings have coalesced, filled pores, and cemented the horizon. The symbol h is also used in combination with s, as in Bhs, if the amount of sesquioxide component is significant but the color value and chroma of the horizon when moist is 3 or less.
 
• i - slightly decomposed organic matter

Used with O to indicate the least decomposed of the organic materials. Its rubbed fiber content is 40 percent or more (by volume).
 
• k - accumulation of carbonates

Indicates an accumulation of alkaline-earth carbonates, commonly calcium carbonate.
 
• m - cementation or induration

Indicates continuous or nearly continuous cementation. The symbol m is used for horizons that are more than 90 percent cemented, although they may be fractured. The cemented layer is physically root-restrictive. The predominant cementing agent (or the two dominant cementing agents) may be indicated by using defined letter suffixes, singly or in pairs. Following are some suffix combinations and what they indicate:
 
•  km - cementation by carbonates;
•  qm - cementation by silica;
•  sm - cementation by iron;
•  ym - cementation by gypsum;
•  kqm - cementation by lime and silica; and
•  zm - cementation by salts more soluble than gypsum.

 
• n - accumulation of sodium

Indicates an accumulation of exchangeable sodium.
 
• o - residual accumulation of sesquioxides

 
• p - tillage or other disturbance

Indicates a disturbance of the surface layer by mechanical means, pasturing, or similar uses. A disturbed organic horizon is designated Op. A disturbed mineral horizon is designated Ap, even though it is clearly a former E, B, or C horizon.
 
• q - accumulation of silica

Indicates an accumulation of secondary silica.
 
• r - weathered or soft bedrock

Used with C to indicate root-restrictive layers of saprolite, such as weathered igneous rock, or of soft bedrock, such as partly consolidated sandstone, siltstone, and shale. Excavation difficulty is low to high.
 
• s - illuvial accumulation of sesquioxides and organic matter

Used with B to indicate an accumulation of illuvial, amorphous, dispersible, organic-matter-sesquioxide complexes if both organic-matter and sesquioxide components are significant, and if color value and chroma of the horizon when moist is 4 or more. The symbol is also used in combination with the symbol h, as in Bhs, if both the organic-matter and sesquioxide components are significant, and if the color value and chroma, moist, is 3 or less.
 
• ss - presence of slickensides

Indicates the presence of slickensides. Slickensides result directly from the swelling of clay minerals and shear failure, commonly at angles of 20 to 60 degrees above horizontal. They are indicators that other vertic characteristics, such as wedge-shaped peds and surface cracks, may be present.
 
• t - accumulation of silicate clay

Indicates an accumulation of silicate clay that has either formed and subsequently been translocated within the horizon or has been moved into the horizon by illuviation, or both. At least some part of the horizon should show evidence of clay accumulation either as coatings on surfaces of peds or in pores, or as lamellae or bridges between mineral grains.
 
• v - plinthite

Indicates the presence of iron-rich humus-poor reddish material that is firm or very firm when moist and hardens irreversibly when exposed to the atmosphere and to repeated wetting and drying.
 
• w - development of color or structure

Used with B to indicate the development of color and structure, or both, with little or no apparent illuvial accumulation of material. It should not be used to indicate a transitional horizon.
 
• x - fragipan character

Indicates a genetically developed layer with a combination of firmness, brittleness, and commonly a higher bulk density than adjacent layers. Some part of the layer is physically root-restrictive.
 
• y - accumulation of gypsum

 
• z - accumulation of salts more soluble than gypsum

Master horizons describe a soil profile, while diagnostic horizons are used to classify soils. Whereas master horizons are based on appearance, diagnostic horizons are based on soil formation processes. These two classification schemes are not complementary. Diagnostic horizons can contain all or part of more than one master horizon.

An epipedon is the surface, or uppermost soil horizon. It may be thinner than the soil profile A horizon, or include the E or part or all of the B horizon. Epipedons derived from bedrock lack rock structure and are normally darkened by organic matter.

Anthropic epipedon
While similar to the mollic epipedon, the anthropic epipedon contains greater than 250 ppm citric acid soluble P₂O₅ with or without a 50 percent base saturation and requires that the soil is moist three months or more over 8 to 10 years. It is commonly found in fields cultivated over long periods of time.

Histic epipedon
This organic horizon is water saturated long enough for reduced conditions to occur unless artificially drained. It is 20 to 60 cm thick and has a low bulk density often less than 1 g cm-3. The actual organic matter content is dependent on the percent clay. If the soil has not been plowed, it must contain between 12 percent or more organic carbon with no clay and 18 percent or more organic carbon with 60 percent or more clay. When the soil has been plowed, the organic carbon content is from 8 percent with no clay to 16 percent with 60 percent or more clay.

Melanic epipedon
This thick, black surface horizon with a high organic matter content formed in volcanic ejecta. It has a minimum thickness of 30 cm, contains 6 percent or more organic carbon, and has volcanic mineral-like allophane throughout.

Mollic epipedon
This epipedon is a soft dark grassland soil. Its organic carbon content is 0.6 percent or more resulting in a color value of 3 or less moist, 5 or less dry. Its base saturation is 50 percent or more. It measures a minimum of 18 cm thick if not directly above a petrocalcic horizon, duripan, or a lithic or paralithic contact, and contains less than 250 ppm P₂O₅. Moist three months or more each year, it cannot have both hard consistence and massive structure.

Ochric epipedon
This epipedon does not meet the definitions of any other surface horizon. It does not have the thickness, percent organic carbon, or color to be a mollic or umbric epipedon. The ochric epipedon extends to the first illuvial (B) horizon.

Plaggen epipedon
This man-made horizon is 50 cm or more thick and has resulted from centuries of accumulation of sod, straw, and manure, for example. It commonly contains artifacts such as pottery and bricks.

Umbric epipedon
Mollic-like in thickness, organic carbon content, color, P₂O₅ content, consistence, and structure, this epipedon has less than 50 percent base saturation.

Diagnostic subsurface horizons can be categorized as weakly developed horizons, as horizons featuring an accumulation of clay, organic matter, or inorganic salts, as cemented horizons, or as strongly acidic horizons.

Agric
This horizon forms under a plow layer. It normally has lamellae (finger-shaped concentrations of material) of illuvial humus, silt, and clay.

Albic
Clay, humus, and other coatings have been leached from this eluvial horizon, leaving light-colored sand and silt particles.

Argillic
This illuvial horizon of mostly high-charged layer silicate clay has clay films on the faces of peds or some indication of clay movement. It is at least one-tenth the thickness of all overlying horizons. If the overlying horizon has less than 15 percent clay, the argillic has 3 percent more clay than the eluvial horizon above. If the overlying horizon has 15 to 40 percent clay, the argillic has 1.2 times that amount. If the overlying horizon has over 40 percent clay, the argillic has 8 percent more clay.

Calcic
Measuring 15 cm or more thick, this horizon is not indurated or cemented, and has evidence of calcium carbonate movement. It has a 15 percent or more CaCO₃ equivalent unless there is below 18 percent clay, then the requirement is a 5 percent or more CaCO₃ equivalent. If the horizon is cemented, it is classified as petrocalcic.

Cambic
This horizon shows some evidence of alterations but is very weakly developed between A and C horizons. The cambic horizon has less illuviation evidence than found in the argillic and spodic horizons. 

Duripan
This subsurface horizon is cemented by silica in more than 50 percent of its volume. It dissolves in concentrated basic solution or alterating acid and then basic solutions, but does not slake in HCl.

Fragipan
A fragipan is a brittle horizon situated at some depth below an eluvial horizon. It has a low organic matter content, lower bulk density than overlying horizons, and hard or very hard consistence when dry.

Glossic
This transitional horizon has parts of an eluvial horizon and the remnants of a degrading argillic, kandic, or natric horizon.

Gypsic
An illuvial horizon, the gypsic is 15 cm or more thick with 5 percent or more gypsum and at least 1 percent by volume visible gypsum. If the horizon is cemented, it is classified as petrogypsic.

Kandic
The kandic is a horizon with an illuvial accumulation of 1:1 (kaolinite-like) clay that has a CEC of less than 16 cmolc kg-1 clay. A clay increase within 15 cm of the overlying horizon is 4 percent or more if the surface has less than 20 percent clay; 20 percent or more if the surface has 20 to 40 percent clay; or 8 percent or more if the surface has greater than 40 percent clay. The horizon is 30 cm thick unless there is a lithic, paralithic, or petroferric contact, in which case minimum thickness is 15 cm. Organic carbon constantly decreases with increasing depth.

Natric
The natric horizon is similar to the argillic horizon with the additional characteristics of columnar structure. It has an exchangeable sodium percentage of 15 percent or more.

Oxic
The oxic horizon contains highly weathered clays. It is 30 cm or more thick and has a CEC of less than 16 cmolc kg^-1 clay. Less than 10 percent of the minerals are weatherable. Within a distance of 15 cm, there is an increase in clay of 4 percent or less if the surface horizon contains less than 20 percent clay; less than 20 percent if the surface contains 20 to 40 percent clay; or 8 percent or less if the surface contains 40 percent or more clay.

Placic
This subsurface horizon is cemented by iron, iron and manganese, or iron and organic matter.

Salic
Measuring 15 cm or more thick, the salic horizon contains at least 2 percent soluble salt. A 1:1 soil to water extract has an electrical conductivity of 30 dSm^-1 (decisiemens per meter) or more.

Sombric
The sombric horizon has an illuvial accumulation of humus that is not associated with aluminum (spodic) or sodium (natric).

Spodic
This illuvial horizon contains high pH dependent charge material. A sandy-textured horizon, it has an accumulation of humus with aluminum and/or iron.

Sulfuric
The sulfuric horizon forms as a result of draining soil with a high sulfide content that is oxidized to sulfates, drastically reducing the pH. It is at least 15 cm thick and has a pH of 3.5 or less.