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General characteristics. The umbric horizon (from L. umbra, shade) is a thick, dark coloured, base-desaturated surface horizon rich in organic matter.
Diagnostic criteria. An umbric horizon must have:
1. soil structure sufficiently strong that the horizon is not both massive and hard or very hard when dry. Very coarse prisms larger than 30 cm in diameter are included in the meaning of massive if there is no secondary structure within the prisms; and
2. Munsell colours with a chrome of less than 3.5 when moist, a value darker than 3.5 when moist and 5.5 when dry, both on broken and crushed samples. The colour value is at least one unit darker than that of the C horizon (both moist and dry) unless the C horizon has a colour value darker than 4.0, moist, 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; and
3. base saturation (by 1 M NH4OAc) of less than 50 percent on a weighted average throughout the depth of the horizon; and
4. organic carbon content of 0.6 percent (1 percent organic matter) or more throughout the thickness of mixed horizon (usually it is more than 2 to 5 percent, depending on the clay content). The organic carbon content is at least 0.6 percent more than the C horizon if the colour requirements are waived because of dark coloured parent materials; and
5. the following thickness requirements:
a. 10 cm or more if resting directly on hard rock, a petroplinthic or petroduric horizon, or overlying a cryic horizon; or
b. at least 20 cm and more than one-third of the thickness of the solum where the solum is less than 75 cm thick; or
c. more than 25 cm where the solum is more than 75 cm thick.
The measurement of the thickness includes transitional AB, AE and AC horizons.
The requirements for an umbric horizon must be met after the first 20 cm are mixed, as in ploughing.
Field identification. The main field characteristics used to identify the presence of an umbric horizon are its dark colour and its structure. In general, umbric horizons tend to have a lesser grade of soil structure than mollic horizons.
As a guide, most umbric horizons have an acid soil reaction (pH (H2O, 1:2.5) of less than about 5.5) which represents a base saturation of less than 50 percent. An additional indication for the acidity is a rooting pattern in which most of the roots tend to be horizontal, in the absence of a physical root restricting barrier.
Relationships with some other diagnostic horizons. The base saturation requirement sets the umbric horizon apart from the mollic horizon, which otherwise is very similar. The upper limit of organic carbon content varies from 12 percent (20 percent organic matter) to 18 percent (30 percent organic matter) which is the lower limit for the histic horizon, or 20 percent, the lower limit of a folic horizon.
Limits with base-desaturated fulvic and melanic horizons are set by the combination of the intense dark colour, the high organic carbon content, the thickness and the characteristics associated with andic horizons in these two horizons. Otherwise, umbric horizons frequently occur in association with andic horizons.
Some thick, dark coloured, organic-rich, base-desaturated surface horizons occur which are formed as a result of human activities such as deep cultivation and manuring, the addition of organic manures, the presence of ancient settlements, kitchen middens, etc. (cf. anthropedogenic horizons). These horizons can usually be recognized in the field by the presence of artifacts, spade marks, contrasting mineral inclusions or stratification indicating the intermittent addition of manurial material, a relative higher position in the landscape, or by checking the agricultural history of the area. If hortic or plaggic horizons are present, either the 0.5 M NaHCO3 P2O5 analysis (Gong et al., 1997) or the 1 percent citric acid soluble P2O5 analysis may give an indication.
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 slickensides19; and
19 Slickensides are polished and grooved ped surfaces which are produced by one soil mass sliding past another.
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.
General description. The vitric horizon (from L. vitrum, glass) is a surface or subsurface horizon dominated by volcanic glass and other primary minerals derived from volcanic ejecta.
Diagnostic criteria. A vitric horizon must have:
1. 10 percent or more volcanic glass and other primary minerals in the fine earth fraction; and either:
2. less than 10 percent clay in the fine earth fraction; or
3. a bulk density > 0.9 kg dm3; or
4. Alox + 1/2Feox20 >0.4 percent; or
20 Alox and Feox are acid oxalate (pH 3) extractable aluminium and iron, respectively (method of Blakemore et al., 1987).
5. phosphate retention > 25 percent; and
6. thickness of at least 30 cm.
Field identification. The vitric horizon can be identified in the field with relative ease. It can occur as a surface horizon, however, it may also occur buried under some tens of centimetres of recent pyroclastic deposits. It has a fair amount of organic matter and a low clay content. The sand and silt fractions are still dominated by unaltered volcanic glass and other primary minerals (may be checked by x 10 hand-lens).
Relationships with some other diagnostic horizons. Vitric horizons are closely linked with andic horizons, into which they may eventually develop. The amount of volcanic glass and other primary minerals, together with the amount of non-crystalline or paracrystalline pedogenetic minerals mainly separates the two horizons.
Vitric horizons may overlap with several diagnostic surface horizons, viz. the fulvic, melanic, mollic, umbric and ochric horizons.
General description. The yermic horizon (from Sp. yermo, desert) is a surface horizon which usually, but not always, consists of surface accumulations of rock fragments ("desert pavement") embedded in a loamy vesicular crust and covered by a thin aeolian sand or loess layer.
Diagnostic criteria. A yermic horizon must have:
1. aridic properties; and
a. a pavement which is varnished or includes wind-shaped gravel or stones ("ventifacts"); or
b. a pavement and a vesicular crust; or
c. a vesicular crust above a platy A horizon, without a pavement.
Field identification. A yermic horizon comprises a vesicular crust at the surface and underlying A horizon(s). The crust, which has a loamy texture, shows a polygonal network of desiccation cracks, often filled with inblown material, which extend into the underlying horizons. Crust and the A horizon(s) below have a weak to moderate platy structure.
Relationships with some other diagnostic horizons. Yermic horizons often occur in association with other diagnostic horizons characteristic for desert environments (salic, gypsic, duric, calcic and cambic horizons). In very cold deserts (e.g. Antarctica) they may occur associated with cryic horizons. Under these conditions coarse cryoclastic material dominates and there is little dust to be deflated and deposited by wind. Here a dense pavement with varnish, ventifacts, aeolian sand layers and soluble mineral accumulations may occur directly on loose C horizons, without a vesicular crust and underlying A horizons.
Abrupt textural change
Continuos hard rock
Strongly humic properties
Ferralic, geric, gleyic and stagnic and strongly humic properties as well as abrupt textural change, continuous hard rock and permafrost are retained from the Revised Legend of the Soil Map of the World (FAO, 1988) as they reflect specific soil conditions rather than horizons. The term secondary carbonates is preferred to soft powdery lime as used in the Revised Legend. Newly defined diagnostic properties are albeluvic tonguing, alic and aridic properties.
Abrupt textural change
General description. An abrupt textural change is a very sharp increase in clay content within a limited depth range.
Diagnostic criteria. An abrupt textural change requires either:
1. doubling of the clay content within 7.5 cm if the overlying horizon has less than 20 percent clay; or
2. 20 percent (absolute) clay increase within 7.5 cm if the overlying horizon has 20 percent or more clay. In this case some part of the lower horizon should have at least twice the clay content of the upper horizon.
General description. The term albeluvic tonguing (from L. albus, white, and eluere, to wash out) is connotative of penetrations of clay and iron-depleted material into an argic horizon. When peas are present, albeluvic tongues occur along ped surfaces. Redoximorphic characteristics and stagnic properties are not necessarily present.
Diagnostic criteria. Albeluvic tongues must:
1. have the colour of an albic horizon; and
2. have greater depth than width, with the following horizontal dimensions:
a. 5 mm or more in clayey argic horizons; or
b. 10 mm or more in clay loamy and silty argic horizons; or
c. 15 mm or more in coarser (silt loam, loam or sandy loam) argic horizons; and
3. occupy more than 10 percent of the volume in the first 10 cm of the argic horizon, estimated from or measured on both vertical and horizontal sections; and
4. have a particle size distribution matching that of the eluvial horizon overlying the argic horizon.
General description. The term alic properties (from L. alumen, alum) is connotative of very acid mineral soil material with a high amount of exchangeable aluminium.
Diagnostic criteria. Alic properties apply to mineral soil material which has all of the following physical and chemical characteristics:
1. a cation exchange capacity (by 1 M NH4OAc) equal to or more than 24 cmolc kg-1 clay; and
a. a total reserve in bases (TRB = exchangeable plus mineral Ca, Mg, K and Na) of the clay which is 80 percent or more of the TRB of the soil; or
b. a silt/clay ratio of 0.60 or less; and
3. a pH (KCl) of 4.0 or less; and
4. a KCl extractable Al content of 12 cmolc kg-1 clay or more, and an KCl extractable Al/CECclay21 ratio of 0.35 or more; and
21 CECclay: cation exchange capacity (by 1 M NH4OAc) of tile clay fraction, corrected for organic matter.
5. an aluminium saturation (exch. Al/ECEC x 100) of 60 percent or more.
General description. The term aridic properties combines a number of properties which are common in surface horizons of soils occurring under arid conditions and where pedogenesis exceeds new accumulation at the soil surface by aeolian or alluvial activity.
Diagnostic criteria. Aridic properties are characterized by all of the following:
1. organic carbon content of less than 0.6 percent22 if texture is sandy loam or finer, or less than 0.2 percent if texture is coarser than sandy loam, as a weighted average in the upper 20 cm of the soil or down to the top of a B horizon, a cemented horizon, or to rock, whichever is shallower; and
22 The organic carbon content may be higher if the soil is periodically flooded, or if it has an electrical conductivity of the saturated paste extract of 4 dS m-1 or more somewhere within 100 cm of the soil surface.
2. evidence of aeolian activity in one or more of the following forms:
a. the sand fraction in some subhorizon or in inblown material filling cracks contains a noticeable proportion of rounded or subangular sand particles showing a matt surface (use a x 10 hand-lens). These particles make up 10 percent or more of the medium and coarser quartz sand fraction; or
b. wind-shaped rock fragments ("ventifacts") at the surface; or
c. aeroturbation (e.g. crossbedding); or
d. evidence of wind erosion or deposition, or both; and
3. both broken and crushed samples have a Munsell colour value of 3 or more when moist and 4.5 or more when dry, and a chrome of 2 or more when moist; and
4. base saturation (by 1 M NH4OAc) of more than 75 percent, but normally 100 percent.
Additional remarks. The presence of acicular ("needle-shaped") clay minerals (e.g. palygorskite and sepiolite) in soils is considered connotative of a desert environment, but it has not been reported in all desert soils. This may be due to the fact that under arid conditions acicular clays are not produced but only preserved, provided they exist in the parent material or in the dust that falls on the soil.
Continuos hard rock
Definition. Continuous hard rock is material underlying the soil, exclusive of cemented pedogenetic horizons such as a petrocalcic, petroduric, petrogypsic and petroplinthic horizons, which is sufficiently coherent and hard when moist to make hand digging with a spade impractible. The material is considered continuous if only a few cracks 10 cm or more apart are present and no significant displacement of the rock has taken place.
General description. Ferralic properties (from L. ferrum, iron, and alumen, alum) refer to mineral soil material which has a relative low cation exchange capacity. It also includes soil materials which would qualify for a ferralic horizon apart for their coarse texture.
Diagnostic characteristics. Ferralic properties apply to mineral soil materials which have either:
1. a cation exchange capacity (by 1 M NH4OAc) of less than 24 cmolc kg-1 clay; or
2. a cation exchange capacity (by 1 M NH4OAc) of less than 4 cmolc kg-1 soil, both in at least some subhorizon of the B horizon or the horizon immediately underlying the A horizon.
General description. Geric properties (from Gr. geraios, old) refers to mineral soil material which has a very low effective cation exchange capacity or even acts as an anion exchanger.
Diagnostic criteria. Mineral soil material has geric properties if it has either:
1. 1.5 cmolc or less of exchangeable bases (Ca, Mg, K, Na) plus unbuffered 1 M KCl exchangeable acidity per kg clay; or
2. a delta pH (pHKCl minus pHwater) of +0.1 or more.
General description. Soil materials develop gleyic properties (from the Russian local name gley, mucky soil mass) if they are completely saturated with groundwater, unless drained, for a period that allows reducing conditions to occur (this may range from a few days in the tropics to a few weeks in other areas), and show a gleyic colour pattern.
Diagnostic criteria. Reducing conditions23 are evident by:
1. a value of rH in the soil solution of 19 or less; or
2. the presence of free Fe2+ as shown by the appearance of either:
a. a solid dark blue colour on a freshly broken surface of a field-wet soil sample, after spraying it with a potassium ferric cyanide (K3Fe(III)(CN)6) solution; or
b. a strong red colour on a freshly broken surface of a field-wet soil sample after spraying it with a a,a, dipyridyl solution in 10% acetic acid; and
3. a gleyic colour pattern24 reflecting oxirnorphic25 and/or reductomorphic26 properties either:
a. in more than 50 percent of the soil mass; or
b. in 100 percent of the soil mass below any surface horizon.
23 The basic measure for reduction in soil materials is the rH. This measure is related to the redox potential (Eh) and corrected for the pH as shown in the following formula:
24 A gleyic colour pattern results from a redox gradient between the groundwater and capillary fringe causing an uneven distribution of iron and manganese (hydr)oxides. In the lower part of the soil and/or inside the peas the oxides are either transformed into insoluble Fe/Mn(II) compounds or they are translocated both processes leading to the absence of colours with a Munsell hue redder than 2.5Y. Translocated iron and manganese compounds can be concentrated in oxidized form (Fe(III) Mn(lV)) recognizable by a 10% H2O2 test in the field on ped surfaces or in (bio)pores ("rusty root channels"), and towards the surface even in the matrix.
25 Oximorphic properties reflect alternating reducing and oxidizing conditions as is the case in the capillary fringe and in the surface horizon(s) of soils with fluctuating groundwater levels. Oximorphic properties are expressed by reddish brown (ferrihydrite) or bright yellowish brown (goethite) mottles or as bright yellow (jarosite) mottles in acid sulphate soils. In loamy and clayey soils the iron (hydr)oxides are concentrated on aggregate surfaces and the walls of larger pores (e.g. old root channels).
26 Reductomorphic properties reflect permanently wet conditions and are expressed by neutral (white to black: N1/ to N8/) or bluish to greenish (2.5Y, 5Y, 5G, 5B) colours in more than 95 percent of the soil matrix. In loamy and clayey material blue-green colours dominate due to Fe (II,III) hydroxy salts (green rust). If the material is rich in sulphur blackish colours prevail due to iron sulphides. In calcareous material whitish colours are dominant due to calcite and/or siderite. Sands are usually light grey to white in colour and often also impoverished in iron and manganese.
The upper part of a reductomorphic horizon may show up to 5 percent rusty colours mainly around channels of burrowing animals or plant roots.
Field identification. Iron and manganese (hydr)oxides in soils with gleyic properties are redistributed to the outside of the peas and towards the soil surface from where oxygen is derived. The resulting colour pattern (reddish, brownish or yellowish colours near the ped surface or in the upper part of the profile, together with grayish/bluish colours in the inside of the peas or deeper in the soil) indicates if gleyic conditions occur. Also, the dipyridyl test often gives a good indication if ferric iron is present in the soil solution.
Definition. Permafrost is a layer in which the temperature is perennially at or below 0°C for at least two consecutive years.
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