Annex 2
Diagnostic horizons, properties and materials

Diagnostic Horizons

Albic horizon

An albic horizon must:

1. have a Munsell colour, dry, with a value of 7 or 8 and a chroma of 3 or less; or a value of 5 or 6 and chroma of 2 or less; and
2. have a Munsell colour, moist, with a value of 6, 7 or 8 and a chroma of 4 or less; or a value of 5 and a chroma of 3 or less; or a value of 4 and chroma of 2 or less¹ (a chroma of 3 is permitted if parent materials have a hue of 5 YR or redder and the chroma is due to the colour of uncoated silt or sand grains); and
3. have a thickness of 1 cm or more.

Andic horizon

An andic horizon must have all of the following:

1. a bulk density at field capacity (no prior drying) of less than 0.9 kg dm^-3; and
2. 10 percent or more clay and an (Al_ox + ½Fe_ox) value² in the fine earth fraction of 2 percent or more; and
3. 70 percent or more phosphate retention; and
4. less than 10 percent volcanic glass in the fine earth fraction; and
5. a thickness of 30 cm or more.

Anthraquic horizon (see Anthropedogenic horizons)

Anthropedogenic horizons

Note that the precise diagnostic criteria for anthropedogenic horizons are still under review. According to present knowledge they are described as follows.

A terric horizon (from L. terra, earth) results from addition of earthy manure, compost or mud over a long period of time. The terric horizon has a non-uniform textural differentiation with depth. The source material and/or underlying substrates influence the colour of the terric horizon. Base saturation (in 1 M NH₄OAc at pH 7.0) is more than 50 percent.

An irragric horizon (from L. irrigare, to irrigate, and agricolare, to cultivate) is a light coloured (Munsell colour value and chroma, moist, both greater than 3), uniformly structured surface layer, developed through long-continued irrigation with sediment-rich water. Clay and carbonates are evenly distributed and the irragric horizon has more clay, particularly fine clay, than the underlying soil material. The weighted average organic carbon content exceeds 0.5 percent, decreasing with depth but remaining at least 0.3 percent at the lower limit of the irragric horizon.

A plaggic horizon (from Dutch plag, sod) has a uniform texture, usually sand or loamy sand. The weighted average organic carbon content exceeds 0.6 percent. The base saturation (in 1 M NH₄OAc at pH 7.0) is less than 50 percent. The content of P₂O₅ extractable in 1 percent citric acid is more than 0.25 percent within 20 cm of the surface (frequently more than 1 percent).

A hortic horizon (from L. hortus, garden) results from deep cultivation, intensive fertilisation and/or long-continued application of organic wastes. It is a dark coloured horizon with Munsell colour value and chroma (moist) of 3 or less. The hortic horizon has a weighted average organic carbon content of 1 percent or more, and more than 100 mg kg^-1 (0.5 M NaHCO³ extractable) P₂O₅ in the fine earth fraction of the upper 25 cm layer. Base saturation (in 1 M NH₄OAc at pH 7.0) is 50 percent or more.

An anthraquic horizon (from Gr. anthropos, human, and L. aqua, water) represents a puddled layer or a plough pan. Characteristically, plough pans have a platy structure; they are compacted and slowly permeable to water. Yellowish-brown, brown or reddish-brown rust mottles occur along cracks and root holes. The bulk density of the plough pan is at least 20 percent greater than that of the puddled layer, whereas its porosity is 10 to 30 percent less than that of the puddled layer. Non-capillary porosity is 2 to 5 percent.

A hydragric horizon (from Gr. hydros, water, and L. agricolare, to cultivate) is a subsurface horizon with characteristics associated with wet cultivation:

• Iron-manganese accumulation or coatings of illuvial Fe and Mn; or Twice as much dithionite-citrate extractable iron than in the surface horizon(s), or more, or 4 times as much dithionite-citrate extractable manganese or more; or
• Redoximorphic features associated with wet cultivation; and
• Thickness of more than 10 cm.

Argic horizon

An argic horizon must:

1. have a texture of sandy loam or finer and at least 8 percent clay in the fine earth fraction; and
2. have more `total' clay than an overlying coarser textured horizon (exclusive of differences, which result from a lithological discontinuity), such that:
• 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
• if the overlying horizon has between15 and 40 percent `total' clay in the fine earth fraction, the ratio of `total' clay in the argic horizon to that in the overlying horizon must be 1.2 or more; or
• 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. have a markedly increased clay content relative to the overlying horizon, within a vertical distance of 30 cm if the argic horizon is formed by clay illuviation or within a vertical distance of 15 cm in any other case; and
4. have no autochthonous rock structure in at least half the volume of the horizon; and
5. have a thickness of at least one tenth of the accumulated thickness of all overlying horizons with a minimum of 7.5 cm. If the argic horizon is entirely composed of lamellae, these 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 under abrupt textural change).

Calcic horizon

A calcic horizon must:

1. show evidence of secondary carbonates accumulation, and
2. have an equivalent calcium carbonate content of 15 percent or more in the fine earth fraction (hypercalcic horizons contain more than 50 percent calcium carbonate equivalent in the fine earth fraction); and
3. have a thickness of 15 cm or more.

Cambic horizon

A cambic horizon must:

1. have a texture of sandy loam or finer; and
2. have soil structure, which is at least moderately developed whereas autochthonous rock structure is absent from at least half the volume of the horizon; and
3. show evidence of alteration in one or more of the following forms:
• stronger chroma, redder hue, or more clay than the underlying horizon;
• evidence of removal of carbonates.
4. carbonates are absent in the parent material and in the dust that falls on the soil, presence of soil structure and absence of rock structure are enough evidence of alteration; and
5. lack the brittle consistence (moist) that is typical of a fragic horizon; and
6. have
• a cation exchange capacity (in 1 M NH₄OAc at pH 7.0) greater than 16 cmol(+) kg^-1 clay; or
• an effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) greater than 12 cmol(+) kg^-1 clay; or
• 10 percent or more weatherable minerals in the 50-200 mm fraction⁴; or
• 10 percent or more water dispersible clay; and
7. have a thickness of 15 cm or more, with the base of the horizon at least 25 cm below the soil surface.

Chernic horizon

A chernic horizon must:

1. have granular or fine subangular blocky soil structure; and
2. have, in the upper 15 cm of the horizon, or immediately below any plough layer, in both broken and crushed samples with a Munsell chroma of less than 2.0 when moist, a value darker than 2.0 when moist and 3.0 when dry. The colour value, moist, must be 3 or less if there is more than 40 percent finely divided lime, or if the texture of the horizon is loamy sand or coarser. The colour value must be at least one unit darker than that of the C-horizon⁵ (both moist and dry), unless the soil is derived from dark coloured parent material. If a C-horizon is not present, comparison should be made with the horizon immediately underlying the surface horizon; and
3. have 50 percent or more (by volume) of the horizon consisting of wormholes, worm casts, and/or filled animal burrows; and
4. have at least 1.5 percent organic carbon (2.5 percent organic matter) throughout after mixing. The organic carbon content must be at least 6 percent if the colour requirements are waived because of presence of finely divided lime, or at least 1.5 percent more than that of the C-horizon if colour requirements are waived because of dark coloured parent materials; and
5. have 80 percent base saturation or more (in 1 M NH₄OAc at pH 7.0); and
6. have a thickness of at least 35 cm. Note that the thickness measurement of a chernic horizon includes transitional horizons in which the characteristics of the surface horizon are dominant - for example, AB, AE or AC.

Cryic horizon

A cryic horizon must:

1. have a (soil) temperature at or below 0^oC for two or more years in succession; and
2. have - in the presence of interstitial soil water, evidence of cryoturbation, frost heave, cryogenic sorting, thermal cracking, or ice segregation;
3. or have - in the absence of interstitial soil water, evidence of thermal contraction of frozen soil material; and
4. have platy or blocky macro-structure resulting from vein ice development, and orbicular, conglomeratic and banded micro-structure resulting from sorting of coarse soil material.

Duric horizon

A duric horizon must:

1. have 10 percent or more (by volume) of durinodes with the following properties:
• durinodes do not break down in concentrated hydrochloric acid (HCl), but break down in hot concentrated potassium hydroxide (KOH) after treatment with HCl; and
• durinodes are firm or very firm, and brittle when wet, both before and after treatment with acid; and
• durinodes have a diameter of 1 cm or more; and
2. have a thickness of 10 cm or more.

Ferralic horizon

A ferralic horizon must:

1. have sandy loam or finer particle size and have less than 90 percent (by weight) gravel, stones or petroplinthic (iron-manganese) concretions; and
2. have a cation exchange capacity (in 1 M NH₄OAc at pH 7.0) of 16 cmol(+) kg^-1 clay or less and have an effective cation exchange capacity (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of less than 12 cmol(+) kg^-1 clay; and
3. have less than 10 percent water-dispersible clay, unless the soil material has geric properties or contains more than 1.4 percent organic carbon; and
4. have less than 10 percent weatherable minerals in the 50-200 mm fraction; and
5. have no characteristics diagnostic for the andic horizon; and
6. have a thickness of at least 30 cm.

Ferric horizon

A ferric horizon must:

1. have coarse mottles with hues redder than 7.5YR and/or chromas in excess of 5 that cover more than 15 percent of the exposed surface area; or
2. have discrete nodules, up to 2 cm in diameter, whose exteriors are enriched and weakly cemented or indurated with iron and have redder hue or stronger chroma than the interior; and
3. have a thickness of at least 15 cm.

Folic horizon

A folic horizon must:

1. have more than 20 percent (by weight) organic carbon (35 percent organic matter); and
2. not be saturated with water for more than one month in most years; and
3. have a thickness greater 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.

Fragic horizon

A fragic horizon must:

1. have greater (bulk) density than overlying horizons; and
2. have less than 0.5 percent organic carbon; and
3. have a penetration resistance at field capacity in excess of 50 kN m^-2; and
4. display slaking or fracturing of an air-dry clod within 10 minutes after being placed in water; and
5. have no cementation brought about by repeated wetting and drying; and
6. have a thickness of at least 25 cm.

Fulvic horizon

A fulvic horizon must:

1. have properties characteristic for andic horizons throughout the fulvic horizon; and
2. have a Munsell colour value (moist) and chroma of 2 or less; and
3. have a melanic index⁶ of more than 1.7 throughout; and
4. have a weighted average organic carbon content of 6 percent or more and 4 percent organic carbon or more in all parts of the fulvic horizon; and
5. have a cumulative thickness of at least 30 cm with less than 10 cm "non-fulvic" material in between.

Gypsic horizon

A gypsic horizon must:

1. contain 15 percent or more gypsum. Note that the horizon qualifies as a hypergypsic horizon (from Gr. hyper, superseding, and L. gypsum) if it contains 60 percent or more gypsum. The percentage gypsum is calculated as the product of gypsum content, expressed in cmol(+) kg^-1 soil, and the equivalent weight of gypsum expressed as a percentage (i.e. 86); and
2. have a thickness of at least 15 cm (also for hypergypsic horizons).

Histic horizon

A histic horizon must:

1. have 18 percent (by weight) organic carbon (30 percent organic matter) or more if the mineral fraction comprises 60 percent or more clay; or
2. have 12 percent (by weight) organic carbon (20 percent organic matter) or more if the mineral fraction has no clay; or
3. have 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. In materials characteristic for andic horizons, the organic carbon content must be more than 20 percent (35 percent organic matter); and
4. be saturated with water for at least one month in most years (unless artificially drained); and
5. have a thickness of 10 cm or more. A histic horizon less than 20 cm thick must have 12 percent or more organic carbon after mixing down to a depth of 20 cm.

Hydragric horizon (see anthropedogenic horizons)

Hortic horizon (see anthropedogenic horizons)

Irragric horizon (see anthropedogenic horizons)

Melanic horizon

A melanic horizon must:

1. have the properties and characteristic of andic horizons; and
2. have Munsell colour value (moist) and chroma of 2 or less; and
3. have a melanic index⁷ of 1.70 or less throughout; and
4. have a weighted average organic carbon content of 6 percent or more and 4 percent or more organic carbon in all parts; and
5. have a cumulative thickness of at least 30 cm with less than 10 cm "non-melanic" material in between.

Mollic horizon

A mollic horizon must:

1. have
• structural units with a diameter of 30cm or less or secondary structure with a diameter of 30cm or less (when dry); and/or
• a moderately hard or softer rupture resistance class (when dry); and
2. have a Munsell chroma of less than 3.5 (moist), a value darker than 3.5 (moist) and 5.5 (dry) in both broken and crushed samples. If there is more than 40 percent finely divided lime, the colour value (moist) must 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. If a C-horizon is not present, comparison should be made with the horizon immediately underlying the surface horizon; and
3. have 0.6 percent organic carbon (1 percent organic matter) or more throughout the (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 that of the C-horizon if the colour requirements are waived because of dark coloured parent materials; and
4. have a weighted average base saturation (in 1 M NH₄OAc at pH 7.0) of 50 percent or more throughout the depth of the horizon; and
5. have thickness specifications as follows:
• 10 cm or more if directly on hard rock, a petrocalcic, petroduric or petrogypsic horizon, or overlying a cryic horizon or material containing more than 40 percent CaCO₃; or
• 20 cm or more and more than one-third of the thickness of the solum if the solum is less than 75 cm thick; or
• 25 cm or more if the solum is more than 75 cm thick.
6. that the thickness specifications of a mollic horizon include 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.

Natric horizon

A natric horizon must:

1. have sandy loam or finer texture and at least 8 percent clay in the fine earth fraction; and
2. have more total clay than an overlying coarser textured horizon (exclusive of differences which result from a lithological discontinuity only) such that:
• 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
• 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 greater; or
• 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. have a distinct increase in clay content within a vertical distance of 30 cm if the natric horizon is formed by clay illuviation. Else, the increase in clay content between the overlying and the natric horizon must be reached within a vertical distance of 15 cm; and
4. have no rock structure in at least half the volume of the horizon; and
5. have 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. have an exchangeable sodium percentage (ESP⁸) greater than 15 within the upper 40 cm of the horizon, or more exchangeable magnesium plus sodium than calcium plus exchange acidity (at pH 8.2) within the same depth if the ESP exceeds 15 percent in some sub horizon within 200 cm from the surface of the soil; and
7. have a thickness of at least one tenth of the sum of the thickness of all overlying horizons, with a minimum value of 7.5 cm.

Note that a coarse(r)-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 under abrupt textural change).

Nitic horizon

A nitic horizon must:

1. have diffuse or gradual transitions to horizons immediately above and below the nitic horizon; and
2. have
• more than 30 percent clay; and
• a water-dispersible clay/total clay ratio of less than 0.10 (unless there is more than 0.6 percent organic carbon); and
• a silt/clay ratio of less than 0.40; and
3. have moderate to strong, nutty or polyhedric structure, with many shiny ped faces; and
4. have no gleyic or stagnic properties; and
5. have
• 4.0 percent or more citrate-dithionite extractable iron ("free" iron) in the fine earth fraction; and
• more than 0.20 percent acid oxalate (pH 3) extractable ("active") iron in the fine earth fraction; and
• a ratio between "active" and "free" iron of 0.05 or more; and
6. have a thickness of 30 cm or more.

Ochric horizon

An ochric horizon lacks fine stratification and has one (or more) of the following characteristics or properties:

1. Consistence is 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 chroma of 3.5 or more when moist, and a value of 3.5 or more when moist and 5.5 or more when dry. If there is more than 40 percent finely divided lime, the colour value, moist, must be more than 5; or
3. The organic carbon content is less than 0.6 percent (1 percent organic matter) throughout the (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. The thickness of the horizon is:
• less than 10 cm if resting directly on hard rock, a petrocalcic, petroduric or petrogypsic horizon, or overlying a cryic horizon; or
• 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
• 25 cm or less where the solum is more than 75 cm thick.

Petrocalcic horizon

A petrocalcic horizon must:

1. have a calcium carbonate equivalent of 50 percent (by weight) or more; and
2. have cementation to the extent that dry fragments do not slake in water and roots cannot enter; and
3. have extremely hard consistence when dry (cannot be penetrated by spade or auger); and
4. have a thickness of at least 10 cm, or 2.5 cm if it is laminar and rests directly on bedrock.

Petroduric horizon

A petroduric horizon must:

1. have cementation or induration in more than 50 percent of the horizon; and
2. show 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. have less than 50 percent (by volume) of its mass 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. be laterally continuous to the extent that roots cannot penetrate except along vertical fractures. The latter must have a horizontal spacing of 10 cm or more; and
5. have a thickness of 10 cm or more.

Petrogypsic horizon

A petrogypsic horizon must:

1. contain 60 percent or more gypsum. The percentage gypsum is calculated as the product of gypsum content, expressed as cmol(+) kg^-1 soil, and the equivalent weight of gypsum (86) expressed as a percentage; and
2. be cemented to the extent that dry fragments do not slake in water and the horizon cannot be penetrated by roots; and
3. have a thickness of 10 cm or more.

Petroplinthic horizon

A petroplinthic horizon must:

1. have 10 percent (by weight) or more citrate-dithionite extractable iron, at least in the upper part of the horizon; and
• have a ratio of acid oxalate (pH 3) extractable iron over citrate-dithionite extractable iron of less than 0.10⁹; and
2. contain less than 0.6 percent (by weight) organic carbon; and
3. be cemented to the extent that dry fragments do not slake in water and the horizon cannot be penetrated by roots; and
4. have a thickness of 10 cm or more.

Plaggic horizon (see Anthropedogenic horizons)

Plinthic horizon

A plinthic horizon must:

1. have 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, hard aggregates on exposure to repeated wetting and drying with free access of oxygen; and
2. have
• 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
• a ratio of acid oxalate (pH 3) extractable iron to citrate-dithionite extractable iron of less than 0.10¹⁰; and
3. contain less than 0.6 percent (by weight) organic carbon; and
4. have a thickness of 15 cm or more.

Salic horizon

A salic horizon must, throughout its depth:

1. have
• an Electrical Conductivity (ECe) of the saturation extract of more than 15 dS m^-1 at 25^oC at some time of the year; or
• an ECe of more than 8 dS m^-1 at 25^oC if the pH_(H2O) of the saturation extract exceeds 8.5 (for alkaline carbonate soils) or is less than 3.5 (for acid sulphate soils); and
2. have a product of thickness (in cm) times salt percentage of 60 or more; and
3. have a thickness of 15 cm or more.

Spodic horizon

A spodic horizon must:

1. have
• a Munsell hue of 7.5YR or redder with value of 5 or less and chroma of 4 or less when moist and crushed; or
• a hue of 10YR with value of 3 or less and chroma of 2 or less when moist and crushed; or
• a sub-horizon, 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
• distinct organic pellets between sand grains; and
2. contain 0.6 percent or more organic carbon; and
3. have a soil-pH (1:1 in water) of 5.9 or less; and
4. have
• at least 0.50 percent Al_ox + ½Fe_ox¹¹ and have two times or more Al_ox+ ½Fe_ox than an overlying umbric, ochric, albic or anthropedogenic horizon; or
• an ODOE-value (`Optical Density of the Oxalate Extract') of 0.25 or more, which also is two times or more the value of the overlying horizons; and
5. have a thickness of 2.5 cm or more and an upper limit below 10 cm from the mineral soil surface, unless permafrost is present within 200 cm depth.

Sulfuric horizon

A sulfuric horizon must:

1. have a soil-pH < 3.5 (in 1:1 water suspension); and
2. have
• yellow/orange jarosite [KFe₃(SO₄)₂(OH)₆] or yellowish-brown schwertmannite [Fe₁₆O₁₆(SO₄)₃(OH)₁₀.10H₂O] mottles; or
• concretions and/or mottles with a Munsell hue of 2.5Y or more and a chroma of 6 or more; or
• underlying sulfidic soil materials; or
• 0.05 percent (by weight) or more of water-soluble sulphate; and
3. have a thickness of 15 cm or more.

Takyric horizon

A takyric horizon must:

1. have aridic properties; and
2. have platy or massive structure; and
3. have a surface crust which has all of the following properties:
• enough thickness (> 5 cm) so that it does not curl entirely upon drying; and
• polygonal desiccation cracks extending at least 2 cm deep when the soil is dry; and
• sandy clay loam, clay loam, silty clay loam or finer texture; and
• very hard consistence when dry and very plastic and sticky consistence when wet.

Terric horizon (see Anthropedogenic horizons)

Umbric horizon

An umbric horizon must, after the first 20 cm are mixed, as in ploughing:

1. have
• structural units with a diameter of 30cm or less or secondary structure with a diameter of 30cm or less (when dry); and/or
• a moderately hard or softer rupture resistance class (when dry); and
2. have a Munsell colour with a chroma of less than 3.5 when moist and a value darker than 3.5 when moist and 5.5 when dry, both on broken and crushed samples. The colour must be 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. have a base saturation (in 1 M NH₄OAc at pH 7.0) of less than 50 percent (weighted average throughout the depth of the horizon); and
4. contain, after mixing, 0.6 percent organic carbon (1 percent organic matter) or more throughout. The organic carbon content must be at least 0.6 percent more than that of the C-horizon if the colour requirements are waived because of dark coloured parent materials; and
5. have the following thickness specifications:
• 10 cm or thicker if resting directly on hard rock, a petroplinthic or petroduric horizon, or overlying a cryic horizon; or
• 20 cm or more and more than one-third of the thickness of the solum where the solum is less than 75 cm thick; or
• more than 25 cm where the solum is more than 75 cm thick.
6. that thickness specifications include transitional AB, AE and AC horizons.

Vertic horizon

A vertic horizon must:

1. contain 30 percent or more clay throughout; and
2. have wedge-shaped or parallelepiped structural aggregates with the longitudinal axis tilted between 10^o and 60^o from the horizontal; and
3. have intersecting slickensides¹²; and
4. have a thickness of 25 cm or more.

Vitric horizon

A vitric horizon must:

1. have 10 percent or more volcanic glass and other primary minerals in the fine earth fraction; and:
2. have
• a bulk density > 0.9 kg dm³; or
• Al_ox + ½Fe_ox¹³ >0.4 percent; or
• phosphate retention >25 percent; and
3. have a thickness of 30 cm or more.

Yermic horizon

A yermic horizon must:

1. have aridic properties; and
2. have
• a pavement that is varnished or includes wind-shaped gravel or stones ("ventifacts"); or
• a pavement and a vesicular crust; or
• a vesicular crust above a platy A-horizon, without a pavement, or
• a biological crust, 1 -2 mm thick.

Diagnostic properties

Abrupt textural change

An abrupt textural change is indicated by:

• doubling of the clay content within a vertical distance of 7.5 cm if the overlying horizon has less than 20 percent clay; or
• 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.

Albeluvic tonguing

Albeluvic tongues must:

1. have the colour of an albic horizon; and
2. have greater depth than width, with the following horizontal dimensions:
• 5 mm or more in clayey argic horizons; or
• 10 mm or more in clay loamy and silty argic horizons; or
• 15 mm or more in coarser (silt loam, loam or sandy loam) argic horizons; and
3. occupy more than 10 percent of the volume of the upper 10 cm part 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.

Alic properties

Alic properties apply to mineral soil material, which has all of the following physical and chemical characteristics:

1. have a cation exchange capacity (in 1 M NH₄OAc at pH 7.0) equal to or more than 24 cmol(+) kg^-1 clay; and
2. have
• a total reserve of 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
• a silt/clay ratio of 0.60 or less; and
3. have a pH (KCl) of 4.0 or less; and
4. have a KCl-extractable Al content of 12 cmol(+) kg^-1 clay or more, and a KCl-extractable Al/CEC_clay¹⁴ ratio of 0.35 or more; and
5. have 60 percent aluminium saturation (exch. Al/ECEC x 100) or more.

Aridic properties

Aridic properties are characterized by all of the following:

1. less than 0.6 percent¹⁵ organic carbon if the texture class 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 rock, whichever is shallower; and
2. evidence of aeolian activity in one or more of the following forms:
• 1 -100 cm long vertical cracks with inblown material. This material has a different colour or a different texture (often sandy) than the soil matrix or a noticeable proportion of rounded or subangular sand particles showing a matt surface; or
• wind-shaped rock fragments ("ventifacts") at the surface; or
• aeroturbation (e.g. crossbedding); or
• evidence of wind erosion or deposition, or both; or
• presence of 2% or more of palygorskkite or sepiolite in the clay fraction within a (sub) horizon within 50 cm from the surface, 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 chroma of 2 or more when moist; and
4. a base saturation (in 1 M NH₄OAc at pH 7.0) of more than 75 percent (normally 100 percent; this requirement is waived in lime-free Gypsisols).

Continuous hard rock

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 impracticable. The material is still considered continuous if only a few cracks 10 cm or more apart are present and no significant displacement of the rock has taken place.

Ferralic properties

Ferralic properties apply to mineral soil materials, which have:

• a cation exchange capacity (in 1 M NH₄OAc at pH 7.0) of less than 24 cmol(+) kg^-1 clay; or
• a cation exchange capacity (in 1 M NH₄OAc at pH 7.0) of less than 4 cmol(+) kg^-1 soil, in at least some subhorizon of the B-horizon or in the horizon immediately underlying the A-horizon.

Geric properties

Geric properties apply to mineral soil materials, which have:

• 1.5 cmol(+) or less of [exchangeable bases (Ca, Mg, K, Na) plus unbuffered 1 M KCl exchangeable acidity] per kg clay; or
• a delta pH (pH_KCl minus pH_water) of +0.1 or more.

Gleyic properties

Gleyic properties signify the occurrence of reducing conditions¹⁶ , evidenced by:

1. an rH-value in the soil solution of 19 or less; or
2. presence of free Fe²⁺ shown by a strong red colour after spraying a freshly broken surface of a field-wet soil sample it with a solution of 9.2% a,a dipyridyl in 10% acetic acid; and
3. a gleyic colour pattern¹⁷ reflecting oximorphic¹⁸ and/or reductomorphic¹⁹ properties,
• in more than 50 percent of the soil mass; or
• in 100 percent of the soil mass below any surface horizon.

Permafrost

Permafrost denotes a soil layer in which the temperature is continually at or below 0^oC for at least two consecutive years.

Secondary carbonates

Secondary carbonates are translocated lime, soft enough to be cut readily with a finger nail, precipitated the soil solution rather than inherited from a soil parent material. As a diagnostic property it should be present in significant quantities.

Field identification. Secondary carbonates must have some relation to the soil structure or fabric. Secondary carbonate accumulations may disrupt the fabric to form spheroidal aggregates or `white eyes', that are soft and powdery when dry, or secondary carbonates may be present as soft coatings in pores or on structural faces. If present as coatings, secondary carbonates cover 50 percent or more of the structural faces and are thick enough to be visible when moist. If present as soft nodules, they occupy 5 percent or more of the soil volume. Filaments (pseudomycelia), which come and go with changing moisture conditions, are not included in the definition of secondary carbonates.

Stagnic properties

Stagnic properties are indicative of reducing conditions, evident from:

1. a value of rH in the soil solution of 19 or less; or
2. presence of free Fe²⁺ as shown by the appearance of a strong red colour on a freshly broken surface of a field-wet soil sample after spraying it with a 9.2% a,a dipyridyl solution in 10% acetic acid; and
3. an albic horizon or a stagnic colour pattern
• in more than 50 percent of the soil volume if the soil is undisturbed; or
• in 100 percent of the soil volume if the surface horizon is disturbed by ploughing.

Strongly humic properties

To be strongly humic, soil material must have more than 1.4 percent organic carbon as weighted average over a depth of 100 cm from the soil surface (the same weighted average over 100 cm applies if the soil is 50-100 cm deep; soils less than 50 cm deep cannot be strongly humic). The calculation assumes a bulk density of 1.5 g cm^-3.

Vertic properties

To have vertic properties, a soil must:

1. have, after the upper 20 cm are mixed, 30 percent or more clay throughout upper 50 cm, and
2. have
• intersecting slickensides, and/or
• cracks, which open and close periodically, extend down to 50 cm from the soil surface or deeper and are 1cm or more wide at the surface.

Diagnostic materials

Anthropomorphic soil material

Anthropomorphic soil material (from Gr. anthropos, human) is unconsolidated mineral or organic material resulting (largely) from human activities. Anthropomorphic soil material has not, however, been subject to a sufficiently long period of soil formation to acquire distinct signs of pedogenetic alteration.

The following anthropomorphic soil materials are currently distinguished:

Aric soil material has 3 percent or more (by volume) fragments of diagnostic horizons, which are not arranged in any discernible order.

Garbic soil material is organic waste material as found in landfills containing dominantly organic waste products.

Reductic soil material refers to presence of waste products producing gaseous emissions (e.g. methane, carbon dioxide) resulting in anaerobic conditions in the material.

Spolic soil material refers to presence of material originating from industrial activities (mine spoil, river dredgings, highway constructions, etc.).

Urbic soil material refers to soil material containing more than 35 percent (by volume) of building rubble and artefacts.

Calcaric soil material

Calcaric soil material (from En. calcareous) shows strong effervescence in contact with 10 percent HCl. In practice, calcaric soil material contains more than 2 percent calcium carbonate equivalent.

Fluvic soil material

Fluvic soil material (from L. fluvius, river) is soil material, which shows stratification in at least 25 percent of the soil volume. Stratification is surmised if the organic carbon content decreases irregularly with depth but remains greater than 0.2 percent to a depth of 100 cm. Thin strata of sand may contain less organic carbon if underlying finer sediments, exclusive of buried A-horizons, contain more than 0.2 percent organic carbon. Fluvic soil material must be associated with structural water bodies (seas, lakes and rivers).

Gypsiric soil material

Gypsiric soil material (from L. gypsum) is mineral soil material, which contains 5 percent or more gypsum (by volume).

Organic soil material

Organic soil material must:

1. if saturated with water for long periods (unless artificially drained and excluding live roots), have:
• 18 percent organic carbon (30 percent organic matter) or more if the mineral fraction consists for 60 percent or more of clay; or
• 12 percent organic carbon (20 percent organic matter) or more if the mineral fraction has no clay; or
• an carbon content between 12 and 18 percent, proportional to the clay content, if the clay content of the mineral fraction is between 0 and 60 percent; or
2. if never saturated with water for more than a few days, have 20 percent or more organic carbon.

Sulfidic soil material

Sulfidic soil material must:

1. have 0.75 percent or more sulphur (dry weight) and less than three times more calcium carbonate equivalent than sulphur; and
2. have a soil-pH_(H2O) in excess of 3.5.

Tephric soil material

Tephric soil material must:

1. have 60 percent or more tephra; and
2. have less than 0.4 percent Al + ½Fe, extractable in acid oxalate (pH 3).

¹ Colour requirements differ slightly from those defined by FAO (1988) and Soil Survey Staff (1996). Modifications were made to accommodate albic horizons, which show a considerable shift in chroma upon moistening.

² Alox and Feox are acid oxalate extractable aluminium and iron.

³ Known as the Olsen routine method (Olsen et al., 1954).

⁴ 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 ?m fraction.

⁵ Reference is made here to the master horizon nomenclature as used in FAO's Guidelines for Soil Profile Description (1990); see Appendix 1).

⁶ See Honna et al. (1988).

⁷ See Honna et al. (1988).

⁸ ESP = exchangeable Na x 100 / CEC.

⁹ Estimated from data given by Varghese and Byju (1993).

¹⁰ Estimated from data given by Varghese and Byju (1993).

¹¹ Alox and Feox: acid oxalate (pH 3) extractable aluminium and iron, respectively.

¹² Slickensides are polished and grooved ped surfaces which are produced by one soil mass sliding past another.

¹³ Alox and Feox are acid oxalate (pH 3) extractable aluminium and iron, respectively (method of Blakemore et al., 1987).

¹⁴ CECclay: cation exchange capacity (by 1 M NH₄OAc) of the clay fraction, corrected for organic matter.

¹⁵ 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.

¹⁶ 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:

¹⁷ 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 peds 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(IV)), 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.

¹⁸ 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).

¹⁹ 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.