Glossary of WRB diagnostic horizons for 
agricultural problem soils

• Albic horizon

• Argic horizon

• Calcic horizon

• Ferralic horizon

• Histic horizon

• Natric horizon

• Salic horizon

• Spodic horizon

• Sulfuric horizon

• Vertic horizon

Albic horizon

General description. The albic horizon (from L. albus, white) is a light coloured subsurface horizon from which clay and free iron oxides have been removed, or in which the oxides have been segregated to the extent that the colour of the horizon is determined by the colour of the sand and silt particles rather than by coatings on these particles. It generally has a weakly expressed soil structure or lacks structural development altogether. The upper and lower boundaries are normally abrupt or clear. The morphology of the boundaries is variable and sometimes associated with albeluvic tonguing. Albic horizons usually have coarser textures than the overlying or underlying horizons, although this difference with respect to an underlying spodic horizon may only be slight. Many albic horizons are associated with wetness and contain evidence of gleyic or stagnic properties.
Diagnostic criteria. An albic horizon must have:
1. Munsell colour, dry: a. value of either 7 or 8 and a chrome of 3 or less; or
b.  value of 5 or 6 and a chrome of 2 or less; and
2. Munsell colour, moist:  a.  a value 6, 7 or 8 with a chrome of 4 or less; or
b.  a value of 5 and a chrome of 3 or less; or
c.  a value of 4 and a chrome of 2 or less 5 . A chrome of 3 is permitted if the parent materials have a hue of 5YR or redder, and the chrome is due to the colour of uncoated silt or sand grains; and
3. thickness: at least 1 cm.
Field identification. Identification of albic horizons in the field is based on Munsell soil colours. In addition to the colour determination, checks can be made using a x 10 hand-lens to verify if coatings on sand and silt-sized particles are absent.
Additional characteristics. The presence of coatings around sand and silt grains can be determined using an optical microscope for analysing thin sections. Uncoated grains usually show a very thin rim at their surface. Coatings may be of an organic nature, consist of iron oxides, or both, and are dark coloured under translucent light. Iron coatings become reddish in colour under reflected light, while organic coatings remain brownish-black.
Relationships with some other diagnostic horizons. Albic horizons are normally overlain by humus-enriched surface horizons (mollic, umbric or ochric horizons) but may be at the surface due to erosion or artificial removal of the surface layer. They can be considered as an extreme type of eluvial horizon, and usually occur in association with illuvial horizons such as an argic, natric or spodic horizon, which they overlie. In sandy materials albic horizons can reach considerable thickness, up to several metres, especially in humid tropical regions, and associated diagnostic horizons may be hard to establish.

Argic horizon

General description. The argic horizon (from L. argilla, white clay) is a subsurface horizon which has a distinctly higher clay content than the overlying horizon. The textural differentiation may be caused by an illuvial accumulation of clay, by predominant pedogenetic formation of clay in the subsoil or destruction of clay in the surface horizon, by selective surface erosion of clay, by biological activity, or by a combination of two or more of these different processes. Sedimentation of surface materials which are coarser than the subsurface horizon may enhance a pedogenetic textural differentiation. However, a mere lithological discontinuity, such as may occur in alluvial deposits, does not qualify as an argic horizon.
Soils with argic horizons often have a specific set of morphological, physico-chemical and mineralogical properties other than a mere clay increase. These properties allow various types of 'argic' horizons to be distinguished and to trace their pathways of development. Main subtypes are lixi-, luvi-, abrupti- and plan-argic horizons, and natric and nitic horizons.
The argic B horizon as defined in the Revised Legend of the Soil Map of the World (FAO, 1988) is taken as a reference, with one modification. The requirement to observe in the field '... at least 1 percent clay skins on ped surfaces and in pores...' is changed into 5 percent. This change is based on the notion that there is no 1:1 correspondence between the amount of clay skins on ped surfaces and in pores, and the percentage of the thin section occupied by oriented clay. Even if 100 percent of the ped surfaces are covered by clay skins, the thin section will in its major part be occupied by the matrix of the soil and voids.
Diagnostic criteria. An argic horizon must have:
1. texture of sandy loam or finer and at least 8 percent clay in the fine earth fraction; and
2.  more total clay than an overlying coarser textured horizon (exclusive of differences which result from a lithological discontinuity only) such that:
a. if the overlying horizon has less than 15 percent total clay in the fine earth fraction, the argic horizon must contain at least 3 percent more clay; or
b. if the overlying horizon has 15 percent or more and less than 40 percent total clay in the fine earth fraction, the ratio of clay in the argic horizon to that of the overlying horizon must be 1.2 or more; or
c.  if the overlying horizon has 40 percent or more total clay in the fine earth fraction, the argic horizon must contain at least 8 percent more clay; and
3. an increase in clay content within a vertical distance of 30 cm if an argic horizon is formed by clay illuviation. In any other case the increase in clay content between the overlying and the argic horizon must be reached within a vertical distance of 15 cm; and
4. autochthonous rock structure is absent in at least half the volume of the horizon; and 5. thickness of at least one tenth of the sum of the thickness of all overlying horizons and at least 7.5 cm thick. If the argic horizon is entirely composed of lamellae, the lamellae must have a combined thickness of at least 15 cm. The coarser textured horizon overlying the argic horizon must be at least 18 cm thick or 5 cm if the textural transition to the argic horizon is abrupt (see abrupt textural change).
Field identification. Textural differentiation is the main feature for recognition of argic horizons in the field. The illuvial nature may be established in the field using a x10 hand-lens if clear clay skins occur on ped surfaces, in fissures, in pores and in channels. An 'illuvial' argic horizon should at least in some part show clay skins on at least 5 percent of both horizontal and vertical ped faces and in the pores.
Clay skins are often difficult to detect in soils with a smectitic mineralogy as these are destroyed regularly by shrink-swell movements. The presence of clay skins in 'protected' positions, e.g. in pores, should be sufficient to meet the requirements for an 'illuvial' argic horizon.
Additional characteristics. The illuvial character of an argic horizon can best be established using thin sections. Diagnostic 'illuvial' argic horizons must show areas with oriented clays that constitute on average at least 1 percent of the entire cross-section. Other tests involved are particle size distribution analysis, to determine the increase in clay content over a specified depth, and the fine clay 9 /total clay analysis. In 'illuvial' argic horizons the fine clay/total clay ratio is larger than in the overlying horizons, caused by preferential eluviation of fine clay particles.
If the soil shows a lithological discontinuity over or within the argic horizon, or if the surface horizon has been removed by erosion, or if only a plough layer overlies the argic horizon, the illuvial nature must be clearly established.
A lithological discontinuity, if not clear from the field (data), can be identified by the percentage of coarse sand, fine sand and silt, calculated on a clay-free basis (international particle size distribution or using the additional groupings of the USDA system or other), or by changes in the content of gravel and coarser fractions. A change of at least 20 percent (relative) of any of the major particle size fractions can be regarded as diagnostic for a lithological discontinuity. However, it should only be taken into account if it is located in the section of the profile where the clay increase occurs and if there is evidence that the overlying layer was coarser textured.
Although this is a simplified way of treating lithological discontinuities, not much more can be done with the data commonly available. On the other hand, particle size discontinuities are of main interest for the argic horizon and will show if the overlying material was very much different and coarser, even without considering clay loss due to eluviation or other processes.
Relationships with some other diagnostic horizons. Argic horizons are normally associated with and situated below eluvial horizons, i.e. horizons from which clay and iron have been removed. Although initially formed as a subsurface horizon, argic horizons may occur at the surface as a result of erosion or removal of the overlying horizons. Some clay-increase horizons may have the set of properties which characterize the ferralic horizon, i.e. a low CEC and ECEC (effective CEC), a low content of water-dispersible clay and a low content of weatherable minerals, all over a depth of 50 cm. In such cases a ferralic horizon has preference over an argic horizon for classification purposes. However, an argic horizon prevails if it overlies a ferralic horizon and it has, in its upper part over a depth of 30 cm, 10 percent or more water-dispersible clay, unless the soil material has geric properties or more than 1.4 percent organic carbon.
Argic horizons also lack the structure and sodium saturation characteristics of the natric horizon.

Calcic horizon

General description. The calcic horizon (from L. calx, lime) is a horizon in which secondary calcium carbonate (CaCO3) has accumulated either in a diffuse form (calcium carbonate present only in the form of fine particles of 1 mm or less, dispersed in the matrix) or as discontinuous concentrations (pseudomycelia, cutans, soft and hard nodules, or veins). The accumulation may be in the parent material, or in subsurface horizons, but it can also occur in surface horizons as a result of erosion. If the accumulation of soft carbonates becomes such that all or most of the pedological and/or lithological structures disappear and continuous concentrations of calcium carbonate prevail, the horizon is named a hypercalcic horizon (from Gr. hyper, superseding, and L. calxis, lime).
Diagnostic criteria. A calcic horizon must have:
1. calcium carbonate equivalent content in the fine earth fraction of 15 percent or more (for hypercalcic horizons more than 50 percent calcium carbonate equivalent in the fine earth fraction); and
2. thickness at least 15 cm, also for the hypercalcic horizon.
Field identification. The presence of calcium carbonate can be identified in the field using a 10% HCl solution. The degree of effervescence (audible only, visible as individual bubbles, or foam-like) is an indication of the amount of lime present. This test is important if only diffuse distributions are present.
Other indications for the presence of a calcic or hypercalcic horizon are:
1. soil colours which are more or less white, pinkish to reddish, or grey; and
2. a low porosity (inter-aggregate porosity in the (hyper-)calcic horizon is usually less than that in the horizon immediately above and possibly also less than in the horizon directly underneath).
Calcium carbonate content may decrease with depth, but this is often difficult to establish, particularly if the calcic horizon occurs in the deeper subsoil. Accumulation of secondary lime is therefore sufficient to diagnose a (hyper-)calcic horizon.
Additional characteristics. Determination of the amount of calcium carbonate (by weight) and the changes within the soil profile of the calcium carbonate content are the main analytical criteria for establishing the presence of a calcic horizon. Determination of the pH (H2O) enables distinction between accumulations with a basic ('calcic') character (pH 8.0 - 8.7) due to the dominance of CaCO3, and those with an ultrabasic ('non-calcic') character (pH > 8.7) because of the presence of MgCO3 or Na2CO3.
In addition, microscopical analysis of thin sections may reveal the presence of dissolution forms in horizons above or below a calcic horizon, evidence of silicate epigenesis (isomorphous substitution of quartz by calcite), or the presence of other calcium carbonate accumulation structures, while clay mineralogical analyses of calcic horizons often show clays characteristic of confined environments, such as montmorillonites, attapulgites and sepiolites.
Relationships with some other diagnostic horizons. When hypercalcic horizons become indurated, transition takes place to the petrocalcic horizon, the expression of which may be massive or as platy structures.
In dry regions and in the presence of sulphate-bearing soil- or groundwater solutions, calcic horizons occur associated with gypsic horizons. Calcic and gypsic horizons usually occupy different positions in the soil profile because of the difference in solubility of calcium carbonate and gypsum, and normally they can be clearly distinguished from each other by the difference in morphology. Gypsum crystals tend to be needle-shaped, often visible with the naked eye, whereas pedogenetic calcium carbonate crystals are much finer in size.

Ferralic horizon

General description. The ferralic horizon (from L. ferrum, iron, and alumen, alum) is a subsurface horizon resulting from long and intense weathering, in which the clay fraction is dominated by low activity clays, and the silt and sand fractions by highly resistant minerals, such as iron-, aluminium-, manganese- and titanium oxides.
Diagnostic criteria. A ferralic horizon must have:
1. a sandy loam or finer particle size and less than 90 percent (by weight) gravel, stones or petroplinthic (iron-manganese) concretions; and
2. a cation exchange capacity (by 1 M NH4OAc) of 16 cmolc kg^-1 clay or less and an effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of less than 12 cmolc kg^-1 clay; and
3. less than 10 percent water-dispersible clay, unless the soil material has geric properties or more than 1.4 percent organic carbon; and
4. less than 10 percent weatherable minerals in the 50-200 mm fraction; and
5. no characteristics diagnostic for the andic horizon; and
6. thickness of at least 30 cm.
Field identification. Ferralic horizons are associated with old and stable geomorphic surfaces. Generally, the macrostructure seems to be moderate to weak at first sight. However, typical ferralic horizons have a strong microaggregation ('pseudosand'). The consistence is usually friable, which gives the appearance as if 'the soil material flows like flour between the fingers'. Hand specimens of ferralic horizons are usually relatively light in weight because of the low bulk density. Indicative of the high porosity is the hollow sound many ferralic horizons produce when tapped.
Illuviation and stress features such as clay skins and pressure faces are generally lacking, although some illuviation cutans may occur in the lower part of the horizon. Boundaries of a ferralic horizon are normally diffuse and little differentiation in colour or particle size distribution within the horizon can be detected. It has a texture that is sandy loam or finer in the fine earth fraction and has less than 90 percent (by weight) gravel, stones or petroplinthic concretions.
Additional characteristics. As an alternative to the weatherable minerals requirement, a total reserve of bases (TRB = exchangeable plus mineral Ca, Mg, K and Na) of less than 25 cmolc kg^-1 soil may be indicative.
Relationships with some other diagnostic horizons. Ferralic horizons may meet the clay increase requirements which characterize the argic horizon. If the upper 30 cm of the clay-increase horizon contains 10% or more water-dispersible clay, an argic horizon has preference over a ferralic horizon for classification purposes, unless the soil material has geric properties or more than 1.4 percent organic carbon.
Acid ammonium oxalate (pH 3) extractable Fe, Al and Si (Alox, Feox , Siox) in ferralic horizons is very low, which sets it apart from the andic and nitic horizons. Andic horizons have at least Alox + ½ Feox >0.4 (in the presence of more than 10 percent volcanic glass particles in the fine earth fraction), and nitic horizons have a significant amount of active iron oxides: more than 0.2 percent acid oxalate (pH 3) extractable iron from the fine earth fraction which, in addition, is more than 5 percent of the citrate-dithionite extractable iron.
The limit with the cambic horizon is formed by the cation exchange capacity/effective cation exchange capacity/weatherable mineral requirements. Some cambic horizons have a low cation exchange capacity; however, the amount of weatherable minerals (or, alternatively, the total reserve in bases) is too high for a ferralic horizon. Such horizons represent an advanced stage of weathering and form the transition between the cambic and the ferralic horizon.

Histic horizon

General description. The histic horizon (from Gr. histos, tissue) is a surface horizon, or a subsurface horizon occurring at shallow depth, which consists of poorly aerated organic soil material.
Diagnostic criteria. A histic horizon must have:
1. either  - 18 percent (by weight) organic carbon (30 percent organic matter) or more if the mineral fraction comprises 60 percent or more clay;
 or -  12 percent (by weight) organic carbon (20 percent organic matter) or more if the mineral fraction has no clay;
 or -  a proportional lower limit of organic carbon content between 12 and 18 percent if the clay content of the mineral fraction is between 0 and 60 percent. If present in materials characteristic for andic horizons, the organic carbon content must be more than 20 percent (35 percent organic matter); and
2.  saturation with water for at least one month in most years (unless artificially drained); and
3.  thickness of 10 cm or more. A histic horizon less than 20 cm thick must have 12 percent or more organic carbon when mixed to a depth of 20 cm.

Natric horizon

General description. The natric horizon (from Dutch natrium, sodium) is a dense subsurface horizon with a higher clay content than the overlying horizon(s). The increase in clay content between the natric horizon and the overlying horizon must meet the same requirements as an argic horizon. Moreover, it has a high content in exchangeable sodium and/or magnesium.
Diagnostic criteria. A natric horizon must have:
1. texture of sandy loam or finer and at least 8 percent clay in the fine earth fraction; and
2. more total clay than an overlying coarser textured horizon (exclusive of differences which result from a lithological discontinuity only) such that:
a. if the overlying horizon has less than 15 percent total clay in the fine earth fraction, the natric horizon must contain at least 3 percent more clay; or
b. if the overlying horizon has 15 percent or more and less than 40 percent total clay in the fine earth fraction, the ratio of clay in the natric horizon to that of the overlying horizon must be 1.2 or more; or
c. if the overlying horizon has 40 percent or more total clay in the fine earth fraction, the natric horizon must contain at least 8 percent more clay; and
3.  an increase in clay content within a vertical distance of 30 cm if a natric horizon is formed by clay illuviation. In any other case the increase in clay content between the overlying and the natric horizon must be reached within a vertical distance of 15 cm; and
4.  rock structure is absent in at least half the volume of the horizon; and
5.  a columnar or prismatic structure in some part of the horizon, or a blocky structure with tongues of an eluvial horizon in which there are uncoated silt or sand grains, extending more than 2.5 cm into the horizon; and
6.  an exchangeable sodium percentage (ESP¹⁴) of more than 15 within the upper 40 cm, or more exchangeable magnesium plus sodium than calcium plus exchange acidity (at pH 8.2) within the same depth if the saturation with exchangeable sodium is more than 15 percent in some subhorizon within 200 cm of the surface; and
7.  thickness of at least one tenth of the sum of the thickness of all overlying horizons and at least 7.5 cm thick.
A coarser textured horizon overlying the natric horizon must be at least 18 cm thick or 5 cm if the textural transition to the natric horizon is abrupt (see abrupt textural change).
Field identification. The colour of the natric horizon ranges from brown to black, especially in the upper part. The structure is coarse columnar or prismatic, sometimes blocky, or may even be massive. Rounded and often whitish coloured tops of the structural elements are characteristic.
Both colour and structural characteristics depend on the composition of the exchangeable cations and the soluble salt content in the underlying layers. Often thick and dark coloured clay cutans or other plasma separations occur, especially in the upper part of the horizon. Natric horizons have a poor aggregate stability and very low permeability under wet conditions. When dry the natric horizon becomes hard to extremely hard. Soil reaction is strongly alkaline; pH (H2O) is more than 8.5.
Additional characteristics. Natric horizons are characterized by a high pH (H2O) which is frequently more than 9.0. Another measure to characterize the natric horizon is the sodium adsorption ratio (SAR) which has to be 13 cmol c 1 -1 or more. The SAR is calculated from soil solution data:
SAR = Na⁺ / [(Ca²⁺ + Mg²⁺) / 2]^0.5 cmolc/l
Micromorphologically, natric horizons show a specific fabric. The peptized plasma shows a strong orientation in a mosaic or parallel striated pattern. The plasma separations also show a high content in associated humus. Microcrusts, cutans, papules and infillings appear, when the natric horizon is impermeable.
Relationships with some other diagnostic horizons. A surface horizon usually rich in organic matter overlies the natric horizon. This horizon of humus accumulation varies in thickness from a few centimetres to more than 25 cm, and may be a mollic or ochric horizon. An albic horizon may be present between the surface and the natric horizon.
Frequently, a salt-affected layer occurs below the natric horizon. The salt influence may extend into the natric horizon which besides being sodic then also becomes saline. Salts present may be chlorides, sulphates or (bi-)carbonates.

Salic horizon

General description. The salic horizon (from L. sal, salt) is a surface or shallow subsurface horizon which contains a secondary enrichment of readily soluble salts, i.e. salts more soluble than gypsum (CaSO4.2H2O; log Ks = - 4.85 at 25°C).
Diagnostic criteria. A salic horizon must have, throughout its depth:
1. a. an electrical conductivity (EC) of the saturation extract of more than 15 dS m^-1 at 25°C at some time of the year; or
b. an EC of more than 8 dS m^-1 at 25°C if the pH (H2O) of the saturation extract exceeds 8.5 (for alkaline carbonate soils) or less than 3.5 (for acid sulphate soils); and
2. minimally 1 percent salt; and
3. product of thickness (in cm) times salt percentage of 60 or more; and
4. thickness of 15 cm or more.
Field identification. Circumstantial evidence usually points to the presence of a salic horizon. Halophyte vegetation like Tamarix and salt-tolerant crops are first indicators. Salt-affected layers often exhibit 'puffy' structures. Salts precipitate only after evaporation of the soil moisture. If the soil is moist or wet these precipitations need not to be present.
Salts may precipitate at the surface ('external Solonchaks') or at depth ('internal Solonchaks'). A salt crust at the surface is part of the salic horizon.

Spodic horizon

General description. The spodic horizon (from Gr. spodos, wood ash) is a dark coloured subsurface horizon which contains illuvial amorphous substances composed of organic matter and aluminium, with or without iron. The illuvial materials are characterized by a high pH-dependent charge, a large surface area and high water retention.
Diagnostic criteria. A spodic horizon must have:
1. a. either-  a Munsell hue of 7.5YR or redder with value of 5 or less and chrome of 4 or less when moist and crushed;
or -  a hue of 10YR with value of 3 or less and chrome of 2 or less when moist and crushed; or
b.  a subhorizon which is 2.5 cm or more thick and which is continuously cemented by a combination of organic matter and aluminium, with or without iron ('thin iron pan'); or
c.  distinct organic pellets between sand grains; and
2.  0.6 percent or more organic carbon; and
3.  pH (1:1 in water) of 5.9 or less; and
4.  a. at least 0.50 percent Alox + ½Feox¹⁸ and have two times or more Alox + ½Feox than an overlying umbric, ochric, albic or anthropedogenic horizon; or 18 Alox and Feox : acid oxalate (pH 3) extractable aluminium and iron, respectively.
b.  an optical density of the oxalate extract (ODOE) value of 0.25 or more, which also is two times or more the value of the overlying horizons; and
5.  thickness of at least 2.5 cm and an upper limit below 10 cm of the mineral soil surface, unless permafrost is present within 200 cm depth.
Field identification. A spodic horizon normally underlies an albic horizon and meets the brownish black to reddish brown colours. Spodic horizons can also be characterized by the presence of a thin iron pan, or by the presence of organic pellets when weakly developed.
Relationships with some other diagnostic horizons. Spodic horizons can have similar characteristics as andic horizons rich in alumino-organic complexes. Sometimes only analytical tests can positively discriminate between the two. Spodic horizons have at least twice as much the Al ox + ½Fe ox percentages than an overlying umbric, ochric, albic or anthropedogenic horizon. This criterion normally does not apply to andic horizons in which the alumino-organic complexes are hardly mobile.

Sulfuric horizon

General description. The sulfuric horizon (from L. sulfur) is an extremely acid subsurface horizon in which sulphuric acid is formed through oxidation of sulphides.
Diagnostic criteria. A sulfuric horizon must have:
1.  pH < 3.5 in a 1:1 water suspension; and
2.  a.  either-  yellow/orange jarosite [KFe3(SO4)2(OH)6] or yellowish-brown schwertmannite [Fe16O16(SO4)3(OH)10.10H2O] mottles;
or -  concentrations with a Munsell hue of 2.5Y or more and a chrome of 6 or more; or
b.  superposition on sulfidic soil materials; or
c.  0.05 percent (by weight) or more water-soluble sulphate; and
3.  thickness of 15 cm or more.
Field identification. Sulfuric horizons generally contain yellow/orange jarosite or yellowish brown schwertmannite mottles. Moreover, soil reaction is extremely acid; pH (H2O) of less than 3.5 is not uncommon.
Relationships with some other diagnostic horizons. The sulfuric horizon often underlies a strongly mottled horizon with pronounced redoximorphic features (reddish to reddish brown iron hydroxide mottles and a light coloured, iron depleted matrix).

Vertic horizon

General description. The vertic horizon (from L. vertere, to turn) is a clayey subsurface horizon which as a result of shrinking and swelling has polished and grooved ped surfaces ('slickensides'), or wedge-shaped or parallelepiped structural aggregates.
Diagnostic criteria. A vertic horizon must have:
1.  30 percent or more clay throughout; and
2.  wedge-shaped or parallelepiped structural aggregates with a longitudinal axis tilted between 10° and 60° from the horizontal; and
3.  intersecting slickensides; and
4.  a thickness of 25 cm or more.
Field identification. Vertic horizons are clayey, and have a hard to very hard consistency. When dry, vertic horizons show cracks of 1 or more centimetre wide. In the field the presence of polished, shiny ped surfaces ("slickensides") which often show sharp angles with each other, is very obvious.
Additional characteristics. The coefficient of linear extensibility (COLE) is a measure for the shrink-swell potential and is defined as the ratio of the difference between the moist length and the dry length of a clod to its dry length: (Lm-Ld)/Ld, in which Lm is the length at 33 kPa tension and Ld the length when dry. In vertic horizons the COLE is more than 0.06.
Relationships with some other diagnostic horizons. Several other diagnostic horizons may also have high clay content, viz. the argic, natric and nitic horizons. These horizons lack the characteristic typical for the vertic horizon; however, they may be laterally linked in the landscape with the vertic horizon usually taking up the lowest position.