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General description. The ochric horizon (from Gr. ochros, pale) is a surface horizon lacking fine stratification and which is either light coloured15, or thin, or has an low organic carbon content, or is massive and (very) hard when dry.
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In arid and semi-arid environments ochric horizons occur which have a light or bleached colour (commonly grey) when dry which turns darker on moistening ("bleached surface horizons"). They do not qualify for an albic horizon because of the colour requirements in both dry and moist state. They are characterized by low (usually <0.4%; South African results) organic carbon and free iron oxide contents. They are coarse textured, show signs of the development of a platy structure and the presence of a thin surface crust. In Australia it is known as bleached A horizon (Northcote, 1979), while in South Africa (Soil Classification Working Croup, 1991) it is defined on the second (family) level of classification as a bleached (orthic) A horizon.
The bleached surface horizon has many negative influences on soil use. Its low physical stability causes the subsoil to remain relatively dry after most rain events. As a result emergence of plant seeds does not readily take place. This is further enhanced by the platy structure and crust formation. In arid regions this phenomenon can lead to large areas without plant coverage (barren land), which are highly susceptible to soil erosion.
Diagnostic criteria. An ochric horizon lacks fine stratification and has one (or more) of the following characteristics or properties:
1. both massive and hard or very hard when dry. Very coarse prisms (prisms larger than 30 cm in diameter) are included in the meaning of massive if there is no secondary structure within the prisms;
or
2. both broken and crushed samples have a Munsell chrome of 3.5 or more when moist, a value of 3.5 or more when moist and 5.5 when dry. If there is more than 40 percent finely divided lime, the colour value, moist, should be more than 5; or
3. an organic carbon content of less than 0.6 percent (1 percent organic matter) throughout the thickness of mixed horizon. The organic carbon content must be less than 2.5 percent if there is more than 40 percent finely divided lime; or
4. thickness of:
a. less than 10 cm if resting directly on hard rock, a
petrocalcic, petroduric or petrogypsic horizon, or overlying a cryic horizon; or
b. less than 20 cm or less than one-third of the thickness of the solum where the solum is less than 75 cm thick; or
c. 25 cm or less where the solum is more than 75 cm thick.
Relationships with some other diagnostic horizons. Ochric horizons have direct linkages with mollic or umbric horizons. The absence of fine stratification sets an ochric horizon apart from unaltered recent sediments.
General description. A petrocalcic horizon (from Gr. petros, rock, and L. calx, lime) is an indurated calcic horizon, which is cemented by calcium carbonate and, in places, by calcium and some magnesium carbonate. It is either massive or platy in nature, and extremely hard.
Diagnostic criteria. A petrocalcic horizon must have:
1. a calcium carbonate equivalent of 50 percent (by weight) or more;
and
2. cementation to the extent that dry fragments do not slake in water and roots cannot enter; and
3. extremely hard consistence when dry so that it cannot be penetrated by spade or auger; and
4. thickness of at least 10 cm, or 2.5 cm if it is laminar and rests directly on bedrock.
Field identification. Petrocalcic horizons occur as non-platy calcrete, either massive or nodular in nature, or as platy calcrete, of which the following types are the most frequent:
-
lamellar calcrete: superimposed separate petrified layers varying in thickness from a few millimetres to several centimetres. The colour is generally white or pink.
- petrified lamellar calcrete: one or several extremely hard layers, having grey or, more often, pink colours. They are generally more cemented than the lamellar calcrete and the internal organization is very massive (no fine lamellar structures, but coarse lamellar structures may be present).
Non-capillary pores in petrocalcic horizons are filled, and the hydraulic conductivity is moderately slow to very slow.
Relationships with some other diagnostic horizons. In arid regions petrocalcic horizons may occur in association with (petro-)duric horizons in which it may laterally grade. Petrocalcic and duric horizons are differentiated by the cementing agent. In petrocalcic horizons calcium and some magnesium carbonate constitutes the main cementing agent while some accessory silica may be present. In duric horizons silica is the main cementing agent, with or without calcium carbonate. Petrocalcic horizons also occur in association with gypsic, hypergypsic, or petrogypsic horizons. Associated surface horizons are usually ochric horizons.
General description. The petroduric horizon (from Gr. petros, rock, and L. durum, hard), also known as duripan, is a subsurface horizon, usually reddish or reddish brown in colour, which is cemented mainly by secondary silica (SiO2, presumably opal and microcrystalline forms of silica). Air-dry fragments of petroduric horizons do not slake in water, even after prolonged wetting. Calcium carbonate may be present as accessory cementing agent. It is either massive, or has a platy or laminar structure.
Diagnostic criteria. A petroduric horizon must have:
1. cementation or induration in more than 50 percent of some subhorizon;
and
2. evidence of silica accumulation (opal or other forms of silica) e.g. as coatings in some pores, on some structural faces or as bridges between sand grains; and
3. less than 50 percent of the volume slaking in 1 M HCl even after prolonged soaking, but more than 50 percent slaking in concentrated KOH or in alternating acid and alkali; and
4. a lateral continuity such that roots cannot penetrate except along vertical fractures, which have a horizontal spacing of 10 cm or more; and
5. thickness of 10 cm or more.
Field identification. A petroduric horizon has a very to extremely firm consistence when moist, and is very or extremely hard when dry. Effervescence after applying 10% HCl may take place, but is probably not as vigorous as in petrocalcic horizons which look very similar. However, they may occur in conjunction with a petrocalcic horizon.
Relationships with other diagnostic horizons. In dry and arid climates petroduric horizons may grade laterally into petrocalcic horizons, and/or occur in conjunction with calcic or gypsic horizons which it normally overlies. In more humid climates petroduric horizons may grade laterally into fragic horizons.
General description. The petrogypsic horizon (from Gr. petros, rock, and L. gypsum) is a cemented horizon containing secondary accumulations of gypsum (CaSO4.2H2O).
Diagnostic criteria. A petrogypsic horizon must have:
1. 60 percent or more gypsum. The percentage gypsum is calculated as the product of gypsum content, expressed as cmol
c kg-1 soil, and the equivalent weight of gypsum (86) expressed as a percentage; and
2. cementation to the extent that dry fragments do not slake in water and it cannot be penetrated by roots; and
3. thickness of 10 cm or more.
Field identification. Petrogypsic horizons are whitish hard materials which contain dominantly gypsum. Sometimes, extremely hard and old petrogypsic horizons are capped by a thin laminar layer of about 1 cm thick.
Additional characteristics. Determination of the amount of gypsum in the soil to verify the required content and increase, as well as thin section analysis, are helpful techniques to establish the presence of a petrogypsic horizon and the distribution of the gypsum in the soil mass.
In thin sections the petrogypsic horizon shows a compacted microstructure with only a few cavities. The matrix is composed of densely packed lenticular gypsum crystals mixed with small amounts of detrital material. The matrix has a faint yellow colour in plain light. Irregular nodules formed by colourless transparent zones consist of coherent crystal aggregates with a hypidiotopic or xenotopic fabric and are mostly associated with (former) pores. Traces of biological activity (pedotubules) are sometimes visible.
Relationships with some other diagnostic horizons. As the petrogypsic horizon develops from a hypergypsic horizon, the two are closely linked. The degree of cementation distinguishes a petrogypsic from a hypergypsic horizon.
Petrogypsic horizons frequently occur associated with calcic horizons. Calcic and gypsic accumulations usually occupy different positions in the soil profile because the solubility of calcium carbonate is different fo that of gypsum. Normally they can be clearly distinguished from each other by their morphology (see calcic horizon).
General description. The petroplinthic horizon (from Gr. petros, rock, and plinthos, brick) is a continuous layer of indurated material, in which iron is an important cement and in which organic matter is absent, or present only in traces.
Diagnostic criteria. A petroplinthic horizon must have:
1.
a. 10 percent (by weight) or more citrate-dithionite extractable iron, at least in the upper part of the horizon;
and
b. ratio between acid oxalate (pH 3) extractable iron and citrate-dithionite extractable iron of less than 0.1016; and
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Estimated from data given by Varghese and Byju (1993).
2. less than 0.6 percent (by weight) organic carbon; and
3. cementation to the extent that dry fragments do not slake in water and it cannot be penetrated by roots; and
4. thickness of 10 cm or more.
Field identification. Petroplinthic horizons are extremely hard, usually rusty brown to yellowish brown coloured layers, which may be either massive, or show a reticulate or interconnected platy or columnar pattern, that encloses non-indurated material. They develop by irreversibly hardening of plinthite. The indurated layer may be fractured, but then the average lateral distances between the fractures must be 10 cm or more and the fractures themselves should not occupy more than 20 percent (by volume) of the layer.
Relationships with some other diagnostic horizons. Petroplinthic horizons are closely associated with plinthic horizons from which they develop. Often plinthic horizons can be traced by following petroplinthic layers which have formed, for example, in road cuts.
The low organic matter content separates the petroplinthic horizon from thin iron pans, bog iron and indurated spodic horizons as occurring in, for instance, Podzols, which do contain a fair amount of organic matter.
(see Anthropedogenic horizons)
General description. The plinthic horizon (from Gr. plinthos, brick) is a subsurface horizon which constitutes an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other constituents, and which changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen.
Diagnostic criteria. The plinthic horizon must have:
1. 25 percent (by volume) or more of an iron-rich, humus-poor mixture of kaolinitic clay with quartz and other diluents, which changes irreversibly to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen;
and
2.
a. 2.5 percent (by weight) or more citrate-dithionite extractable iron in the fine earth fraction, especially in the upper part of the horizon, or 10 percent in the mottles or concretions;
and
b. ratio between acid oxalate (pH 3) extractable iron and citrate-dithionite extractable iron of less than 0.1017; and
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Estimated from data given by Varghese and Byju (1993).
c. less than 0.6 percent (by weight) organic carbon; and
d. thickness of 15 cm or more.
Field identification. A plinthic horizon commonly shows red mottles, usually in platy, polygonal, vesicular or reticulate patterns. In a perennially moist soil, the plinthic material is usually not hard but firm or very firm and can be cut with a spade.
The plinthic material does not harden irreversibly as a result of a single cycle of drying and rewetting. Only repeated wetting and drying will change it irreversibly to an ironstone hardpan or to irregular aggregates, especially if it is also exposed to heat from the sun.
Additional criteria. Micromorphological studies may reveal the extent of impregnation of the soil mass by iron. In addition penetration resistance measurements and total amount of iron present may give an indication.
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.
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 Al
ox + ½Feox18 and have two times or more Alox + ½Feox than an overlying umbric, ochric, albic or anthropedogenic horizon; or
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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 Alox + ½Feox 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.
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).
General description. A takyric horizon (from Uzbek takyr, barren land) is a heavy textured surface horizon comprising a surface crust and a platy structured lower part. It occurs under arid conditions in periodically flooded soils.
Diagnostic criteria. A takyric horizon must have:
1.
aridic properties; and
2. a platy or massive structure; and
3. a surface crust which has all of the following properties:
a. enough thickness so that it does not curl entirely upon drying;
b. polygonal desiccation cracks extending at least 2 cm deep when the soil is dry;
c. sandy clay loam, clay loam, silty clay loam or finer texture;
d. very hard dry consistence and very plastic and sticky wet consistence; and
e. an electrical conductivity (EC) in the saturated paste of less than 4 dS m-1, or less than that of the horizon immediately below the takyric horizon.
Field identification. Takyric horizons are found in depressions in arid regions, where surface water, rich in clay and silt but relatively low in soluble salts, can accumulate and leach the upper soil horizons. Periodic salt leaching causes dispersion of clay and the formation of a thick, compact, fine-textured crust, which forms prominent polygonal cracks upon drying. Clay and silt often make up more than 80 percent of the crust material.
Relationships with some other diagnostic horizons. Takyric horizons occur in association with many diagnostic horizons, the most important ones being the salic, gypsic, calcic and cambic horizons. The low electrical conductivity and low soluble salt content of takyric horizons set them apart from the salic horizon.
(see Anthropedogenic horizons)