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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 (Sombroek, 1986). 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 clay9/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.

9 Fine clay: <0.2 µm.

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.

Cambic horizon

General description. The cambic horizon (from L. cambiare, to change) is a subsurface horizon showing evidence of alteration relative to the underlying horizons. It lacks the set of properties diagnostic for a ferralic, argic, natric or spodic horizon and the dark colours, organic matter content and structure of a histic, folic, mollic or umbric horizon.

Diagnostic criteria. A cambic horizon must have:

1. texture in the fine earth fraction of sandy loam or finer; and

2. soil structure which is at least moderately developed or autochthonous rock structure is absent in at least half the volume of the horizon; and

3. evidence of alteration in one or more of the following forms:

a. stronger chrome, redder hue, or higher clay content than the underlying horizon; or

b. evidence of removal of carbonates. A cambic horizon always has less carbonate than an underlying horizon with calcium carbonate accumulation. However, not all primary carbonates have to be leached from a horizon in order for it to qualify as a cambic horizon. If all coarse fragments in the underlying horizon are completely coated with lime, some of these fragments in the cambic horizon are partly free of coatings. If the coarse fragments in the horizon showing calcium carbonate accumulation are coated only on the underside, those in the cambic horizon should be free of coatings; or

c. if carbonates are absent in the parent material and in the dust that falls on the soil, the required evidence of alteration is satisfied by the presence of soil structure and absence of rock structure; and

4. not have the brittle consistence (moist) typical for the fragic horizon; and

5. either-cation exchange capacity (by 1 M NH4OAc) of more than 16 cmolc kg-1 clay;
or -an effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of less than 12 cmolc kg-1 clay;
or -a content of 10 percent or more weatherable minerals in the 50-200 mm fraction10.

10 Instead of analysing the weatherable mineral content, this requirement may be replaced by the analysis of the total reserve in bases (TRB = exchangeable plus mineral Ca, Mg, K and Na). A TRB of 25 cmolc kg-1 soil correlates well with an amount of 10 percent weatherable minerals in the 50-200 pm fraction.

6. thickness of at least 15 cm and a base at least 25 cm below the soil surface.

Relationships with some other diagnostic horizons. The cation exchange capacity/effective cation exchange capacity/weatherable mineral requirements set the cambic horizon apart from the ferralic horizon.

Chernic horizon

General description. The chernic horizon (from Russian chern, black) is a special type of mollic horizon. It is a deep, well structured, blackish surface horizon with a high base saturation, a high content in organic matter and a high biological activity.

Diagnostic criteria. A chernic horizon must have:

1. granular or fine subangular blocky soil structure; and

2. both broken and crushed samples with a Munsell chrome of less than 2.0 when moist, a value darker than 2.0 when moist and 3.0 when dry. If there is more than 40 percent finely divided lime, or if the texture of the horizon is loamy sand or coarser, the limits of colour value dry are waived; the colour value, moist, should be 3 or less. The colour value must be at least one unit darker than that of the C11 (both moist and dry), unless the soil is derived from dark coloured parent material, in which case the colour contrast requirement is waived. If a C horizon is not present, comparison should be made with the horizon immediately underlying the surface horizon. The above colour requirements apply to the upper 15 cm of the chernic horizon, or immediately below any plough layer; and

11 Reference is made here to the master horizon nomenclature as used in FAO's Guidelines for Soil Profile Description (1990); see Appendix 1).

3. 50 percent or more (by volume) of the horizon consisting of wormholes, wormcasts, and filled animal burrows; and

4. an organic carbon content of at least 1.5 percent (2.5 percent organic matter) throughout the thickness of mixed soil. The organic carbon content is at least 6 percent if the colour requirements are waived because of finely divided lime, or 1.5 percent more than the C horizon if the colour requirements are waived because of dark coloured parent materials; and

5. a base saturation (by 1 M NH4OAc) of 80 percent or more; and

6. thickness of at least 35 cm. The measurement of the thickness of a chernic horizon includes transitional horizons in which the characteristics of the surface horizon are dominant - for example, AB, AE or AC.

Field identification. The chernic horizon can be identified by its blackish colour, caused by the accumulation of organic matter, well developed structure (usually granular), high biological activity, mainly worms and other burrowing animals, and its thickness.

Relationships with some other diagnostic horizons. The special character of the chernic horizon with respect to the mollic horizon is expressed by its higher organic carbon content, the darker colours required, the high biological contribution to the soil structure, and its greater minimum depth. The upper limit of organic carbon content is 12 percent (20 percent organic matter) which is the lower limit for the histic horizon or 20 percent, the lower limit for a folic horizon.

Cryic horizon

General description. The cryic horizon (from Gr. kryos, cold, ice) is a perennially frozen soil horizon in mineral or organic soil materials.

Diagnostic criteria. A cryic horizon must have:

1. soil temperature at or below 0C for two or more years in succession; and

2.

a. in the presence of sufficient interstitial soil water, evidence of cryoturbation, frost heave, cryogenic sorting, thermal cracking, or ice segregation; or

b. in the absence of sufficient interstitial soil moisture, evidence of thermal contraction of the frozen soil material; and

3. platy or blocky macrostructures resulting from vein ice development, and orbicular, conglomeratic and banded microstructures resulting from sorting of coarse soil material.

Field identification. If soil moisture is present, cryic horizons show evidence of perennial ice segregation and/or cryogenic processes (mixed soil material, disrupted soil horizons, involutions (swirl-like patterns in soil horizons), organic intrusions, frost heave, separation of coarse from fine soil materials, cracks, patterned surface features such as earth hummocks, frost mounds, stone circles, nets and polygons).

If insufficient interstitial soil water is present, the cryic horizons are dry but thermal contraction features occur, although more weakly developed than those in cryic horizons with a higher moist content.

Relationships with some other diagnostic horizons. Cryic horizons may bear characteristics of histic, andic or spodic horizons, and may occur in association with salic, calcic, mollic, umbric or ochric horizons. In cold arid regions yermic horizons may be found in association with cryic horizons.

Duric horizon

General description. The duric horizon (from L. durum, hard) is a subsurface horizon showing weakly cemented to indurated nodules cemented by silica (SiO2), presumably in the form of opal and microcrystalline forms of silica ("durinodes").

Diagnostic criteria. A duric horizon must:

1. have 10 percent or more (by volume) of durinodes with the following properties:

a. do not break down in concentrated hydrochloric acid (HCl), but break down in hot concentrated potassium hydroxide (KOH) after treatment with HCl; and

b. are firm or very firm, and brittle when wet, both before and after treatment with acid; and

c. have a diameter of 1 cm or more; and

2. have a thickness of 10 cm or more.

Additional characteristics. Dry durinodes do not slake appreciably in water, but prolonged soaking can result in spelling of very thin platelets and in some slaking. In cross-section most durinodes are roughly concentric, and concentric stringers of opal may be visible under a hand lens.

Relationships with some other diagnostic horizons. In arid regions duric horizons occur associated with gypsic, petrogypsic, calcic and petrocalcic horizons. In more humid climates the duric horizon may grade info fragic horizons.

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 clayincrease 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 + 1/2Feox > 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.

Ferric horizon

General description. The ferric horizon (from L. ferrum, iron) is a horizon in which segregation of iron has taken place to such an extent that large mottles or concretions have formed and the inter-mottle/inter-concretionary matrix is largely depleted of iron. Generally, such segregation leads to poor aggregation of the soil particles in iron-depleted areas and compaction of the horizon.

Diagnostic criteria. A ferric horizon must have:

1. many (more than 15 percent of the exposed surface area) coarse mottles with hues redder than 7.5YR and chrome more than 5, or both; or

2. discrete nodules, up to 2 cm in diameter, the exteriors of the nodules being enriched and weakly cemented or indurated with iron and having redder hues or stronger chrome than the interiors; and

3. thickness of at least 15 cm.

Relationships with some other diagnostic horizons. If the amount of nodules reaches 10 percent or more (by volume) and the nodules harden irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen, the horizon is considered to be a plinthic horizon. Therefore, ferric horizons may, in tropical or subtropical regions, grade laterally into plinthic horizons. The transition between the two is often not very clear.


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