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Chapter 3: Diagnostic horizons, properties and materials
Soil horizons, properties and materials are intended to reflect features which are widely recognized as occurring in soils and which can be used to describe and define soil classes. They are considered to be "diagnostic" when they reach a minimum degree of expression, which is determined by appearance, measurability, importance, relevance and quantitative criteria. To be considered diagnostic, soil horizons also require a minimum thickness, which must be appraised in relation to bioclimatic factors (e.g. an albic horizon in boreal regions is not expected to be as thick as one in the tropics).
The diagnostic horizons, properties and materials are described, where possible, giving a general description, the diagnostic criteria, possibilities for field identification and additional characteristics. Some relationships with other important diagnostic horizons are also given.
The cation exchange capacity (CEC), used as a criterion in the definition of diagnostic horizons or properties as well as in the key to the reference soil groups, is essentially meant to reflect the nature of the mineral component of the exchange complex. However, the CEC determined on the total earth fraction is also influenced by the amount and kind of organic matter present. Where low clay activity is a diagnostic property, it may be desirable to deduct CEC linked to the organic matter, using a graphical method4 for individual profiles (Bennema and Camargo, 1979; Brinkman, 1979; Klamt and Sombroek, 1988).
4 The method involves regressing the amount of organic C (expressed in g) against the measured CEC (pH 7) expressed in cmolc kg-1 clay. With the resultant equation tile contribution of the organic C to tile CEC can be calculated, and the corrected CEC of the clay be determined. Uniform clay mineralogy throughout tile profile should be assumed.
The terminology used to describe soil morphology is that adopted in the Guidelines for Soil Profile Description (FAO, 1990). Colour notations are according to the Munsell Soil Color Charts (KIC, 1990). Chemical and physical characteristics are expressed on the basis of the methods given in the Procedures for Soil Analysis (Van Reeuwijk, 1995).
For WRB purposes the diagnostic horizons, defined in Revised Legend (FAO, 1988), have been used as a basis, with the exception of the fimic horizon which has not been retained. New ones are introduced, such as andic, anthropedogenic (anthraquic, hydragric, hortic, irragric, plaggic and ferric horizons), chernic, cryic, duric, ferric, folic, fragic, fulvic, melanic, nitic, petroduric, petroplinthic, plinthic, salic, takyric, vertic, vitric and yermic horizons. Some of these horizons replace FAO's diagnostic properties and phases.
General description. The albic horizon (from L. albus, white) is a light coloured subsurface horizon from which clay and free iron oxides have been removed, or in which the oxides have been segregated to the extent that the colour of the horizon is determined by the colour of the sand and silt particles rather than by coatings on these particles. It generally has a weakly expressed soil structure or lacks structural development altogether. The upper and lower boundaries are normally abrupt or clear. The morphology of the boundaries is variable and sometimes associated with albeluvic tonguing. Albic horizons usually have coarser textures than the overlying or underlying horizons, although this difference with respect to an underlying spodic horizon may only be slight. Many albic horizons are associated with wetness and contain evidence of gleyic or stagnic properties.
Diagnostic criteria. An albic horizon must have:
1. Munsell colour, dry:
a. value of either 7 or 8 and a
chrome of 3 or less; or
b. value of 5 or 6 and a chrome of
2 or less; and
2. Munsell colour, moist:
a. a value 6, 7 or 8 with a chrome
of 4 or less; or
b. a value of 5 and a chrome of 3
or less; or
c. a value of 4 and a chrome of 2
or less5. A chrome of 3 is permitted if the
parent materials have a hue of 5YR or redder, and the
chrome is due to the colour of uncoated silt or sand
3. thickness: at least 1 cm.
5 Colour requirements have been slightly changed with respect to those defined in FAO (1988) and Soil Survey Staff (1996) to accommodate albic horizons, which show a considerable shift in chrome upon moistening. Such albic horizons occur frequently in, for example, the southern African region.
Field identification. Identification of albic horizons in the field is based on Munsell soil colours. In addition to the colour determination, checks can be made using a x 10 hand-lens to verify if coatings on sand and silt-sized particles are absent.
Additional characteristics. The presence of coatings around sand and silt grains can be determined using an optical microscope for analysing thin sections. Uncoated grains usually show a very thin rim at their surface. Coatings may be of an organic nature, consist of iron oxides, or both, and are dark coloured under translucent light. Iron coatings become reddish in colour under reflected light, while organic coatings remain brownish-black.
Relationships with some other diagnostic horizons. Albic horizons are normally overlain by humus-enriched surface horizons (mollic, umbric or ochric horizons) but may be at the surface due to erosion or artificial removal of the surface layer. They can be considered as an extreme type of eluvial horizon, and usually occur in association with illuvial horizons such as an argic, natric or spodic horizon, which they overlie. In sandy materials albic horizons can reach considerable thickness, up to several metres, especially in humid tropical regions, and associated diagnostic horizons may be hard to establish.
General description. The andic horizon (from Japanese An, dark, and Do, soil) is a horizon resulting from moderate weathering of mainly pyroclastic deposits. However, they may also be found in association with non-volcanic materials (e.g. loess, argilites and ferralitic weathering products). Their mineralogy is dominated by short-range-order minerals, and they are part of the weathering sequence in pyroclastic deposits (tephric soil material (r) vitric horizon (r) andic horizon).
Andic horizons may be found both at the surface and in the subsurface. They also often occur as layers, separated by non-andic layers. As a surface horizon, andic horizons generally contain a high amount of organic matter (more than 5 percent), are very dark coloured (Munsell value and chrome, moist, is 3 or less), have a fluffy macrostructure and often a smeary consistence. They are light in weight (have a low bulk density), and have mostly silt loam or finer textures. Andic surface horizons rich in organic matter may be very deep, reaching often a thickness of 50 cm or more (pachic characteristic). Andic subsurface horizons are generally somewhat lighter coloured.
Andic horizons may have different properties, depending on the type of dominant weathering process acting upon the soil material. They may exhibit thixotropy, i.e. the soil material changes, under pressure or by rubbing, from a plastic solid into a liquified stage and back into the solid condition. In perhumid climates, humus-rich andic horizons may contain more than 100 percent water (by volume) compared to their oven-dry volume (hydric characteristic).
Two major types of andic horizons are recognized, one in which allophane and similar minerals are predominant (the sil-andic type), and one in which aluminium complexed by organic acids prevails (the alu-andic type). The sil-andic horizon has an acid to neutral soil reaction, while the alu-andic horizon varies from extremely acid to acid.
Diagnostic criteria. An andic horizon must have the following physical, chemical and mineralogical properties (Shoji et al, 1996; Berding, 1997):
1. bulk density of the soil at field capacity (no prior drying) of less than 0.9 kg dm-3; and
2. 10 percent or more clay and an Alox + 1/2Feox6 value in the fine earth fraction of 2 percent or more; and
6 Alox and Feox are acid oxalate extractable aluminium and iron, respectively (method of Blakemore et al., 1987).
3. phosphate retention of 70 percent or more; and
4. volcanic glass content in the fine earth fraction of less than 10 percent; and
5. thickness of at least 30 cm.
Sil-andic horizons have an acid oxalate (pH 3) extractable silica (Siox) of 0.6 percent or more while alu-andic horizons have a Siox of less than 0.6 percent (or, alternatively, an Alpy7/Alox ratio of less than 0.5 and 0.5 or more, respectively).
7 Alpy: pyrophosphate extractable aluminium.
Field identification. Andic horizons may be identified using the pH NaF field test developed by Fieldes and Perrott (1966). A pH NaF of more than 9.5 indicates an abundant presence of allophanic products and/or organo-aluminium complexes. The test is indicative for most andic horizons, except for those very rich in organic matter. However, the same reaction occurs in spodic horizons and in certain acid clayey soils, which are rich in aluminium interlayered clay minerals.
Sil-andic horizons generally have a field pH (H2O) of 5 or higher, while alu-andic horizons mainly have a field pH (H2O) of less than 4.5. If the pH (H2O) is between 4.5 and 5, additional tests may be necessary to establish the 'alu-' or 'sili-' characteristic of the andic horizon.
Relationships with some other diagnostic horizons. Vitric horizons are distinguished from andic horizons by their lesser rate of weathering. This is evidenced by a higher volcanic glass content in vitric horizons (> 10 percent of the fine earth fraction) and a lower amount of noncrystalline or paracrystalline pedogenetic minerals, as characterized by the moderate amount of acid oxalate (pH 3) extractable aluminium and iron in vitric horizons (Alox + 1/2Feox = 0.4-2.0 percent), by a higher bulk density (BD of vitric horizons is between 0.9 and 1.2 kg dm-3), and by a lower phosphate retention (25 -< 70 percent).
To separate andic horizons rich in organic matter from histic and folic horizons, andic horizons are not permitted to contain more than 20 percent organic carbon, while histic horizons with an organic carbon content between 12 and 20 percent are not permitted to have properties associated with andic horizons.
Spodic horizons, which also contain complexes of sesquioxides and organic substances, can have similar characteristics to andic horizons rich in alumino-organic complexes. Sometimes only analytical tests can discriminate between the two. Spodic horizons have at least twice as much Alox + 1/2Feox than an overlying umbric, ochric or albic horizon. This normally does not apply to andic horizons in which the alumino-organic complexes are virtually immobile.
(see Anthropedogenic horizons)
General description. Anthropedogenic horizons (from Gr. anthropos, human, and pedogenesis) comprise a variety of surface and subsurface horizons which result from long-continued cultivation. The characteristics and properties of these horizons depend much on the soil management practices used (see Table 1). Anthropedogenic horizons differ from anthropogenic soil materials, which are unconsolidated mineral or organic materials resulting largely from land fills, mine spoil, urban fill, garbage dumps, dredgings, etc., produced by human activities. These materials, however, have not been subject to a sufficiently long period of time to have received significant imprint of pedogenetic processes.
Continuous mechanical operations
extending below normal depth of field operations
Continuous applications of
organic/inorganic fertilizers without substantial
additions of mineral matter (e.g. manures, kitchen
refuse, compost, night soil, etc.)
Continuous applications of earthy
materials involving substantial additions of mineral
matter (e.g. sods, beach sand, earthy manures, etc.)
Continuous applications of
irrigation water with substantial amounts of
sediments (may also include fertilizers, soluble salts,
organic matter, etc.)
Processes associated with
submergence during cultivation; puddling of cultivation
layer; usually involving changes in aquic conditions.
Diagnostic subsoil features, such as illuvial
iron-manganese coatings, may develop under wet
cultivation, depending on depth of water table, texture,
presence of organic matter, etc.
The anthropedogenic horizons distinguished are the terric, irragric, plaggic, hortic, anthraquic and hydragric horizons. They occur over small areas in many parts of the world, notably on the old arable lands in western Europe, in the old irrigated plains in the Near East and China, in the old terraced landscapes in the Mediterranean region and the Arab peninsula, and in isolated spots in North and South America associated with long-continued Indian occupation, as well as in areas where paddy rice has been cultivated for a long time.
Diagnostic criteria. A terric horizon (from L. terra, earth) develops through addition of earthy manures, compost or mud over a long period of time. It has a non-uniform textural differentiation with depth. Its colour is related to the source material or the underlying substrate. Base saturation (by 1 M NH4OAc) 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 chrome, moist, is more than 3), uniformly structured surface layer, developed through long-continued irrigation with sediment-rich waters. Clay and carbonates are evenly distributed and it has a higher clay content, particularly fine clay, than the underlying original soil. Among the medium, fine and very fine sand fractions relative differences are not larger than 20 percent. It has a weighted average organic carbon content of more than 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 is more than 0.6 percent. The base saturation (by 1 M NH4OAc) is less than 50 percent while the P2O5 content extractable in 1 percent citric acid is high, at least more than 0.025 percent within 20 cm of the surface, but frequently more than 1 percent.
A hortic horizon (from L. hortus, garden) results from deep cultivation, intensive fertilization and/or long-continued application of human and animal wastes and other organic residues. It is a dark coloured horizon with Munsell colour value and chrome (moist) of 3 or less. It has a weighted average organic carbon content of 1 percent or more, and 0.5 M NaHCO38 extractable P2O5 content is more than 100 mg kg-1 fine earth in the upper 25 cm (Gong et al., 1997). Base saturation (by 1 M NH4OAc) is 50 percent or more.
8 Known as the Olsen routine method (Olsen et al., 1954).
An anthraquic horizon (from Gr. anthropos, human, and L. aqua, water) comprises a puddled layer and a plough pan. Characteristically, the plough pan has a platy structure. It is compacted and has a very low infiltration rate. It shows yellowish-brown, brown or reddish-brown rust mottles along cracks and root holes. The bulk density of the plough pan is at least 20 percent (relative) higher than that of the puddled layer, whereas its porosity is 10 to 30 percent (relative) lower than the porosity in the puddled layer. The non-capillary porosity is 2 to 5 percent (about 60 percent (relative) of the non-capillary porosity of the associated puddled layer).
A hydragric horizon (from Gr. hydros, water, and L. agricolare, to cultivate) is a subsurface horizon associated with wet cultivation with one or more of the following characteristics:
layers of iron-manganese accumulation or illuvial Fe and Mn coatings; or
dithionite-citrate extractable iron is 2 times or more, or dithionite-citrate extractable manganese is 4 times or more that of the surface horizon(s); or
redox concentrations; or
redox depletions with a colour value ³ 4 and chrome £ 2 in macropores associated with wet cultivation; and
thickness of more than 10 cm.
Field identification. The terric, irragric and plaggic horizons all show evidence of surface raising, which may be inferred either from field observation or from historical records. The horizons are thoroughly mixed and usually contain artifacts such as pottery fragments, cultural debris or refuse, which are often very small (less than 1 cm in diameter) and much abraded. The ferric and plaggic horizons are built up gradually from earthy additions (compost, sods or soddy materials mixed with farmyard manure, litter, mud, beach sands, etc.) and may contain stones, randomly sorted and distributed, while the irragric horizon is built up gradually from irrigation deposits.
Few soil characteristics differentiate the ferric and plaggic horizons from each other. Terric horizons usually show a high biological activity, have a neutral to slightly alkaline soil reaction (pH (H2O) is normally more than 7.0), and may contain free lime. The colour is strongly related to the source material or the underlying substrate. Buried soils may be observed at the base of the horizon although the contact can be obscured by mixing.
The plaggic horizon has brownish or blackish colours, related to the origin of source materials and its soil reaction is slightly to strongly acid. It shows evidence of agricultural operations such as spade marks as well as old cultivation layers. Plaggic horizons often overlie buried soils although the original surface layers may be mixed. The lower boundary is usually clear.
The irragric horizon shows evidence of considerable biological activity and has more than 25 percent earthworm casts by volume. The lower boundary is clear and irrigation deposits may be present below.
The hortic horizon is also thoroughly mixed and stratification, if present originally, is not preserved. Artifacts and cultural debris are common, but often much abraded. Earthworm casts take up more than 25 percent of the volume. Tillage marks or evidence of mixing of the soil can be present. Buried soils may be preserved but they are usually incorporated in the horizon.
The anthraquic horizon comprises the puddled layer and the plough pan of a soil under long continued paddy cultivation. The puddled layer has colours associated with reduction, accompanied by low hue mottles and Fe-Mn cutans on ped faces and pore walls. It is very dispersible, shows sorting of soil aggregates and has vesicular pores.
The hydragric horizon has either reduction features in pores such as coatings or halos with a colour hue of 2.5Y or yellower and a chrome (moist) of 2 or less, or segregations of iron and/or manganese in the matrix as a result of the oxidative environment. It usually shows grey clay-fine silt and clay-silt-humus cutans on ped faces.
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