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General description. The folic horizon (from L. folium, leaf) is a surface horizon, or a subsurface horizon occurring at shallow depth, which consists of well-aerated organic soil material.
Diagnostic criteria. A folic horizon must have:
1. more than 20 percent (by weight) organic carbon (35 percent organic matter); and
2. water saturation for less than one month in most years; and
3. thickness of more than 10 cm. If a folic horizon is less than 20 cm thick, the upper 20 cm of the soil after mixing must contain 20 percent or more organic carbon.
Relationships with some other diagnostic horizons. Histic horizons have similar characteristics to the folic horizon; however, these are saturated with water for one month or more in most years. Moreover, the composition of the histic horizon is generally different from that of the folic horizon as the vegetative cover is often different.
General description. The fragic horizon (from L. fragilis, frangere, to break) is a natural noncemented subsurface horizon with a pedality and a porosity pattern such that roots and percolating water penetrate the soil only along interped faces and streaks. The natural character excludes plough pans and surface traffic pans.
Diagnostic criteria. A fragic horizon must have:
1. higher bulk density relative to the horizons above; and
2. less than 0.5 percent organic carbon; and
3. penetration resistance at field capacity more than 50 kN m-1; and
4. slaking or fracturing of an air-dry clod within 10 minutes when placed in water; and
5. no cementation upon repeated wetting and drying; and
6. thickness of at least 25 cm.
Field identification. A fragic horizon has a prismatic and/or blocky structure. The inner parts of the peas can have a relative high total porosity, including pores larger than 200 mm, but as a result of a dense outer rim of the peas no continuity exists between the inped pores and the interped pores and fissures. The fragic horizon is devoid of active faunal burrowing activity, except occasionally along the interped streaks. As a result of this 'closed box system', more than 90 percent of the soil volume cannot be explored by the root systems and is isolated from percolating water. An estimate or measurement of this soil volume can only be made by combining both vertical and horizontal sections of the fragic horizon.
The ped interface or streak can have the colour, mineralogical and chemical characteristics of an eluvial or albic horizon, or meet the requirements of albeluvic tonguing. In the presence of a fluctuating water table this part of the soil is depleted of iron and manganese. As air remains trapped inside the peas, a concomitant iron accumulation is observed at the level of the ped surface and manganese accumulations will occur further inside the peas (stagnic colour pattern).
Fragic horizons are commonly loamy, but loamy sand and clay textures are not excluded. In the latter case the clay mineralogy is dominantly kaolinitic.
Dry clods are hard to extremely hard, moist clods are firm to extremely firm, and moist consistence can be brittle. A ped or clod from a fragic horizon tends to rupture suddenly rather than to undergo slow deformation when pressure is applied.
Relationships with some other diagnostic horizons. A fragic horizon may underlie, although not necessarily directly, an albic, cambic, spodic or argic horizon, unless the soil has been truncated. It can overlap partly or completely with an argic horizon. Laterally, fragic horizons may grade into (petro-)duric horizons in dry regions. Moreover, fragic horizons can have stagnic properties.
General description. The fulvic horizon (from L. fulvus, dark yellow) is a thick, black horizon at or near to the surface, which is usually associated with short-range-order minerals (usually allophane) or with organo-aluminium complexes. It has a low bulk density and contains a high amount of organic matter.
Diagnostic criteria. A fulvic horizon must have:
1. properties characteristic for andic horizons throughout its thickness; and
2. a Munsell colour value (moist) and chrome of 2 or less; and
3. a melanic index12 of more than 1.7 throughout; and
12 See Honna et al. (1988).
4. a weighted average of 6 percent or more organic carbon, and 4 percent or more organic carbon in all parts; and
5. cumulative thickness of at least 30 cm with less than 10 cm "non-fulvic" material in between.
Field identification. The intense dark colour, its thickness, as well as its usual association with pyroclastic deposits makes the fulvic horizon easy recognizable in the field. However, distinction between the fulvic and melanic horizon can only be made after laboratory analyses.
General description. The gypsic horizon (from L. gypsum) is a non-cemented horizon containing secondary accumulations of gypsum (CaSO4.2H2O) in various forms.
Diagnostic criteria. A gypsic horizon must have:
1. 15 percent or more gypsum; if the horizon contains 60 percent or more gypsum, it becomes a hypergypsic horizon (from Gr. hyper, superseding, and L. gypsum). The percentage gypsum is calculated as the product of gypsum content, expressed as cmolc kg-1 soil, and the equivalent weight of gypsum (86) expressed as a percentage; and
2. thickness of at least 15 cm, also for hypergypsic horizons.
Field identification. Gypsum may be found in the form of pseudomycelia, as coarse-sized crystals (individualized, as nests, beards or coatings, or as elongated groupings of fibrous crystals) or as compact powdery accumulations. The latter form gives the gypsic horizon a massive structure and a sandy texture. The distinction between compact powdery accumulations and the others is important in terms of soil potentiality.
Gypsic horizons can be associated with calcic horizons but occur always in separate positions within the soil profile, because of the higher solubility of gypsum with respect to lime.
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 to establish the presence of a gypsic horizon and the distribution of the gypsum in the soil mass.
Relationships with some other diagnostic horizons. When hypergypsic horizons become indurated, transition takes place to the petrogypsic horizon, the expression of which may be as massive or platy structures.
In dry regions gypsic horizons are associated with calcic or salic horizons. Calcic and gypsic horizons usually occupy distinct positions in the soil profile as the solubility of calcium carbonate is different from that of gypsum. They normally can be clearly distinguished from each other by the morphology (see calcic horizon). Salic and gypsic horizons also occupy different positions for the same reasons.
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:
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.
(see Anthropedogenic horizons)
(see Anthropedogenic horizons)
(see Anthropedogenic horizons)
General description. The melanic horizon (from Gr. melanos, black) is a thick, black horizon at or near to the surface, which is usually associated with short-range-order minerals (usually allophane) or with organo-aluminium complexes. It has a low bulk density and contains a high amount of organic matter of a type which is thought to result from large amounts of root residues supplied by a graminaceous vegetation.
Diagnostic criteria. A melanic horizon must have:
1. properties and characteristic for andic horizons throughout its thickness; and
2. a Munsell colour value (moist) and chrome of 2 or less, and
3. a melanic index13 of 1.70 or less throughout; and
13 See Honna et al. (1988).
4. a weighted average of 6 percent or more organic carbon, and 4 percent or more organic carbon in all parts; and
5. cumulative thickness of at least 30 cm with less than 10 cm "non-melanic" material in between.
Field identification. The intense dark colour, its thickness, as well as its usual association with pyroclastic deposits makes the melanic horizon easy to recognize in the field. The relationship with grassland vegetation can only be established under natural conditions, otherwise it may be inferred from historical records. However, laboratory analyses to determine the type of organic matter may be necessary to identify unambiguously the melanic horizon.
General description. The mollic horizon (from L. mollis, soft) is a well structured, dark coloured surface horizon with a high base saturation and a moderate to high content in organic matter.
Diagnostic criteria. A mollic 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 (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. both broken and crushed samples have a Munsell chrome of less than 3.5 when moist, a value darker than 3.5 when moist and 5.5 when dry. If there is more than 40 percent finely divided lime, the limits of colour value dry are waived; the colour value, moist, should be 5 or less. The colour value must be at least one unit darker than that of the C horizon (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; and
3. an organic carbon content of 0.6 percent (1 percent organic matter) or more throughout the thickness of mixed horizon. The organic carbon content is at least 2.5 percent if the colour requirements are waived because of finely divided lime, or 0.6 percent more than the C horizon if the colour requirements are waived because of dark coloured parent materials; and
4. a base saturation (by 1 M NH4OAc) of 50 percent or more on a weighted average throughout the depth of the horizon; and
5. the following thickness:
a. 10 cm or more if resting directly on hard rock, a petrocalcic, petroduric or petrogypsic 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 of a mollic horizon includes transitional horizons in which the characteristics of the surface horizon are dominant - for example, AB, AE or AC.
The requirements for a mollic horizon must be met after the first 20 cm are mixed, as in ploughing.
Field identification. A mollic horizon can easily be identified by its dark colour, caused by the accumulation of organic matter, well developed structure (usually a granular or fine subangular blocky structure), an indication for high base saturation, and its thickness.
Relationships with some other diagnostic horizons. The base saturation of 50 percent separates the mollic horizon from the umbric horizon, which is otherwise similar. The upper limit of organic carbon content varies from 12 percent (20 percent organic matter) to 18 percent organic carbon (30 percent organic matter) which is the lower limit for the histic horizon or 20 percent, the lower limit for a folic horizon.
A special type of mollic horizon is the chernic horizon. It has a higher organic carbon content (1.5 percent or more), a specific structure (granular or fine subangular blocky), a very dark colour in its upper part, a high biological activity, and a minimum thickness of 35 cm.
Limits with high base-saturated 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, mollic horizons frequently occur in association with andic horizons.
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
14 ESP = exchangeable Na x 100/CEC.
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 cmolc 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.
General description. The nitic horizon (from L. nitidus, shiny) is a clay-rich subsurface horizon with as its main feature a moderately to strongly developed polyhedric or nutty structure with many shiny ped faces, which cannot or can only partially be attributed to clay illuviation.
Diagnostic criteria. A nitic horizon must have:
1. diffuse to gradual transitions to horizons immediately above and below (less than 20 percent change in clay content, over at least 12 cm; no abrupt colour change); and
a. more than 30 percent clay; and
b. water-dispersible clay/total clay ratio less than 0.10 (unless there is more than 0.6 percent organic carbon); and
c. silt/clay ratio is less than 0.40; and
3. moderate to strong, nutty or polyhedric structure, with many shiny pedfaces, which cannot or can only partially be associated with illuviation argillans in thin sections; and
4. Munsell colour value of 5 or less, and chrome of 4 or less, but no mottling of hydromorphic nature (gleyic or stagnic properties); and
a. 4.0 percent or more citrate-dithionite extractable iron ("free" iron) in the fine earth fraction; and
b. more than 0.20 percent acid oxalate (pH 3) extractable iron ("active" iron) in the fine earth fraction; and
c. ratio between "active" and "free" iron of 0.05 or more; and
6. minimum thickness of 30 cm, with gradual to diffuse transitions to horizons immediately above and below the nitic horizon.
Field identification. A nitic horizon has a clay loam or finer texture, although the material feels loamy. The change in clay content with the overlying and underlying horizons is gradual. The colours are of low value and chrome with hues often 2.5YR, but sometimes redder or yellower. There is no abrupt colour change with the horizons above and below. Mottling indicative of a hydromorphic nature is lacking. The structure is moderate to strong angular blocky which easily falls apart into flat edged or nut-shaped elements showing shiny ped faces which are either thin clay coatings or pressure faces.
Nitic horizons often contain magnetic minerals such as maghemite. Presence of such minerals can be tested using a magnet.
Additional characteristics. The cation exchange capacity (by 1 M NH4OAc), corrected for organic matter, is less than 36 cmolc kg-1 clay, and often below 24 cmolc kg-1 clay. The effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) is about half of the CEC. The moderate to low CEC and ECEC reflect the dominance of 1:1 lattice clays being both kaolinite and (meta-)halloysite.
Relationships with some other diagnostic horizons. The nitic horizon may be considered as a special type of argic horizon, or a strongly expressed cambic horizon, with specific properties such as a low amount of water dispersible clay and a high amount of active iron. As such the nitic horizon has preference over both for classification purposes. Its mineralogy (kaolinitic/(meta)halloysitic) sets it apart from most vertic horizons which have dominantly a smectitic mineralogy. However, laterally nitic horizons may grade into vertic horizons occurring in lower landscape positions. The well expressed soil structure, the high amount of active iron, and often medium cation exchange capacity in nitic horizons sets them apart from ferralic horizons.
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