This glossary does not attempt to define all morphological terms used in the classification. It mainly deals with those that are not defined in the Field Handbook (McDonald et al. 1990). Where applicable all definitions are consistent with usage in the Field Handbook.
These occur in soils which contain significant amounts of volcanic glass and short-range-order minerals such as allophane. Chemical tests and Soil Taxonomy requirements are given in Soil Survey Staff (1994).
An argic horizon is a subsoil horizon(s) consisting of distinct lamellae, usually 5 to 10 mm thick but occasionally up to 0.1 m or greater. They occur as sharply defined, horizontal to subhorizontal layers which are appreciably more clayey than the adjacent sandy or sandy loam soil material. Consistence strength is stronger, and colour is usually darker and redder or browner than the adjacent soil.
The most common known occurrences are in the mallee dune landscapes of Victoria-South Australia.
In the Field Handbook (p.105) B horizons are defined, in part, as having a concentration of silicate clay, iron, aluminium, organic material, or several of these. There is no mention of carbonate in the definition, although elsewhere (p.108) the subscript k is used to denote an accumulation of carbonate, as in B2k. In contrast, Soil Survey Division Staff (1993) now has the following criteria as a requisite for a B horizon: "illuvial concentration of silicate clay, iron, aluminium, humus, carbonates, gypsum, or silica, alone or in combination". This definition is used in this classification.
In some shallow, stony soils B horizon material may only be present in fissures within the parent rock or saprolite. In such cases there should be 50% or more (visual abundance estimate) of B horizon material for it to qualify as a B horizon for the purposes of this classification (See also "What do we classify" and transitional horizons).
This refers to the sum of exchangeable basic cations (Ca, Mg, K and Na) expressed in cmol (+) kg-1 clay. This sum is obtained by multiplying the sum of the reported basic cations (which are determined on a soil fine earth basis) by 100 and dividing by the clay percentage of the sample. Where this is not available it may be approximated from the field texture using the figures given on pp. 118-120 of the Field Handbook. Three classes are defined: Dystrophic - the sum is less than 5; Mesotrophic - the sum is between 5 and 15 inclusive; Eutrophic - the sum is greater than 15. An estimate of the sum of basic cations for the B horizon of an individual soil may be obtained from its classification if the B horizon maximum texture is recorded in the family criteria.
One which contains more than 20% (visual abundance estimate) of bauxite nodules or concretions which are mostly uncemented, and has a minimum thickness of 0.1 m.
Presence of carbonate segregations or fine earth (soil matrix) effervescence with 1M HCl.
An horizon that is usually identified as a Bk, BCk, 2Bk or 2BCk horizon, or one containing fragments of a cemented (suffix 'm') equivalent of these horizons. As noted in the Field Handbook (p. 108), the suffix k is usually recorded only if there are more than 10% of the calcareous segregations. However in soil with no carbonate except for one horizon with few (2-10%) segregations, this could be designated with a suffix k. See also calcrete, calcrete pan and cemented pans.
In the Field Handbook calcrete is described as both a pan (ie. a soil horizon, such as Bkm) and as a substrate material. However, the definition is the same in both cases, viz 'any cemented terrestrial carbonate accumulation that may vary significantly in morphology and degree of cementation'. The latter may be regarded as indicating the material must be hard. According to this broad definition, calcrete can obviously encompass a wide range of calcareous material although not the common soft segregations of finely divided carbonate, nor accumulations of pedogenic carbonate in the form of discrete nodules or concretions. Unfortunately, the term has been widely used in southern Australia for an almost infinite variety of forms of calcium carbonate. For the purposes of this classification, the term is used strictly as defined in the Field Handbook. See also calcrete, calcrete pan and cemented pans.
A moderately, strongly or very strongly cemented layer of calcrete which is either continuous, or if discontinuous or broken, consists of at least 90% of hard calcrete fragments, most of which are more than 0.2 m in smallest dimension.
Organic debris that has accumulated by colluvial and alluvial processes when torrential rain occurs following extensive bushfires. The material has a low bulk density (<1 t m-3) and consists of variably carbonised plant remains, ranging from little-altered vegetative material to charcoal and humified plant debris. Small amounts of mineral soil are also usually present. The main difference from organic materials is the much lower degree of plant decomposition, ie. an absence of material that could be classed as peat.
The following table is a summary of the classes used in the classification for various kinds and amount of calcium carbonate.
|soft carbonate||hard carbonate|
|Hypocalcic||>0 & <2%||<20%|
In the Field Handbook a pan is defined as an indurated and/or cemented soil horizon and thus horizons such as Bcm, Bkm and Bqm could be interpreted to represent strongly developed B horizons, with consequent effects on the classification of some soils, eg. Kandosols and Tenosols. The Field Handbook also recognised that it can be difficult to determine if materials such as calcrete, ferricrete, silcrete etc. are indeed soil horizons or are better identified as substrate materials, ie. do not show pedological development or are paleo-features.
To avoid the above problem, cemented pans such as calcrete, silcrete, red-brown hardpan, ferricrete, petroferric horizon and petroreticulite are recognised as diagnostic substrate features and hence excluded as criteria of B horizon development. Note that the Podosol diagnostic horizons are not regarded as substrate materials.
Clear or abrupt textural B horizon
The boundary between the horizon (normally a B2t) and the overlying horizon (which must be thicker than 0.03 m and is normally an A but occasionally a B1 horizon) is clear, abrupt or sharp and is followed by a clay increase giving a strong texture contrast:
- If the clay content of the material above the clear, abrupt or sharp boundary is less than 20%, (and/or has a field texture of sandy loam or less) then the clay content immediately below must be at least twice as high. However, there must be a minimum of 20% clay (and/or a minimum field texture of sandy clay loam) at the top of the B horizon.
- If the material above the transition has 20% clay or more but less than 35% clay (and/or has a field texture of sandy clay loam or greater but less than light clay), then the material below must show an absolute increase of at least 20% clay, eg. 25% increasing clearly, sharply or abruptly to at least 45%, (and/or a field texture of light medium clay or greater). Note that a clear or abrupt textural change is not allowed within the clay range.
Note: The field textures listed in (a) and (b) above must be regarded only as guidelines. Some discrepancies may arise in soils with high organic matter, silt, fine sand or soft carbonate contents, and in soils with strongly subplastic B horizons. If there are apparent discrepancies between field texture and laboratory data, the first step is to repeat the assessments if possible. If these remain unchanged the classifiers should use their own judgement based on how they think the soil behaves. In some such cases field textures may be a better guide to soil behaviour than particle size data.
Note also that the above definition is not directly equivalent to that of the duplex primary profile form of the Factual Key (Northcote, 1979).
An earthy pan which is very fine sandy (0.02-0.05mm). Fragments, both wet and dry, slake in water. Densipans are less stable on exposure than underlying or overlying horizons.
Base status is less than 5 cmol (+) kg-1 clay.
ESP (Exchangeable sodium percentage)
Since the review by Northcote and Skene(1972), an ESP of 6 has been widely used in Australia as a critical limit for the adverse effects of sodicity. ESP is conventionally defined as exchangeable sodium expressed as a percentage of the cation exchange capacity (CEC) - both usually determined in Australia at pH 7 or 8.5. In acid soils, particularly those with variable charge colloids, CEC at pH 7 or 8.5 will normally be higher than that determined at the soil pH. Hence it is more realistic to determine the effective cation exchange capacity (ECEC) (method 15J1 of Rayment and Higginson 1992), or to use an unbuffered method to determine CEC, and to use these values to calculate ESP in soils with pH around 5.5 or less. See also Comment after definition of Kurosols (p. 64).
In some dystrophic soils, problems can arise when low levels of exchangeable sodium give rise to relatively high ESP values. In such cases there is insufficient evidence that ESP values greater than 6 have a deleterious effect on soil physical properties equivalent to that in less acid soils with higher base status. Further experience may indicate a need for a minimum level of exchangeable sodium to be introduced.
A related problem is the sensitivity of the analytical procedures when values for exchangeable cations and CEC and ECEC are very low. It is probably not advisable to calculate ESP when the CEC or ECEC is 3 cmol (+) kg-1 or less and exchangeable sodium is 0.3 cmol (+) kg-1 or less. As an indicator of sodicity, such calculations are likely to be quite misleading.
Finally, it must be remembered that the effect of ESP on behaviour such as dispersion is also influenced by other soil properties such as organic matter content, clay mineralogy, cation composition, sesquioxide content, and particularly electrolyte concentration of the soil and of any applied irrigation water.
Base status is greater than 15 cmol (+) kg-1 clay.
One which contains more than 20% (visual abundance estimate) of ferruginous nodules or concretions which are mostly uncemented, and has a minimum thickness of 0.1 m. Most of the nodules contain at least some manganese, and in some situations the majority (if not all) of the nodules may be transported from elsewhere.
This refers to the amount (visual abundance estimate) of gravel-sized (>2 mm) materials that occur on the surface and in the A1 horizon and include hard (when moist) coarse fragments and segregations of pedogenic origin. The most common examples of the latter are carbonate and ferruginous nodules and/or concretions.
One which contains more than 20% of visible gypsum that is apparently of pedogenic origin, and has a minimum thickness of 0.1 m. Where the upper boundary of the gypsic horizon first occurs below 1 m depth it is disregarded in the classification.
In the classification hard is used as a general term to indicate strength. Hard nodules or segregations means their strength is such that they cannot be broken between the thumb and forefinger; ie. strong in the Field Handbook (p. 147). When referring to pans hard means moderately cemented or stronger (Field Handbook p. 143). When referring to substrate material hard means moderately strong or stronger (Field Handbook p. 156).
This is a humus-rich surface or near surface horizon that is 0.2 m or more thick and has insufficient organic carbon to qualify as organic material. The average value for the humose horizon is more than 4% organic carbon [Walkley-Black x 1.3 or a total combustion method (Rayment and Higginson 1992, Methods 6A1 or 6B2).] (but less than 12%) if the mineral fraction contains no clay, or 6% or more organic carbon (but less than 18%) if the mineral fraction contains 60% or more clay; with proportional contents of organic carbon between these limits (see Fig. 2). Approximate loss-on-ignition values are given under organic materials below.
This definition is based on that used in England and Wales (Avery 1990).
If the humose surface layer is less than 0.2 m it will not be specifically recognised as a separate texture at the family level but will be assigned to the relevant mineral soil texture class eg. sandy, loamy, etc. The one exception occurs in the Leptic Tenosols where a subhumose subgroup is provided.
Figure 2. Limits of organic and humose soil materials (after
The dashed line is used for materials seldom saturated with water.
One which contains more than 20% (visual abundance estimate) of black manganiferous nodules or concretions which are mostly uncemented, and has a minimum thickness of 0.1 m. Most nodules also contain some iron.
A loose, earthy material consisting chiefly of an intimate mixture of clay and calcium carbonate, commonly formed in freshwater lakes. The carbonate content may range from about 30 to 90% (Bates and Jackson 1987).
This is a dark surface or near surface horizon that has insufficient organic carbon to qualify as a humose horizon, and has little if any evidence of stratification. It has all of the following properties:
- moist colour is black throughout (ie. value 3 or less and chroma 2 or less - see Colour classes) and dry colour value is 5 or less.
- a minimum thickness of 0.2 m (in soils with a clear or abrupt textural B horizon the minimum thickness must be present within the A horizon)
- the major part of the horizon has more than a weak grade of structure in which the most common ped size is 10-20 mm or less. This condition may be waived for an Ap horizon or when dry consistence strength is weak or less.
- pH (1:5 H2O) is 5.5 or greater throughout the major part of the horizon.
Base status is between 5 and 15 cmol (+) kg-1 clay inclusive.
An horizon in which mottle abundance is greater than 10% (visual abundance estimate) and contrast between colours is distinct to prominent. Colour patterns due to biological or mechanical mixing, and inclusions of weathered substrate materials, are not included. As pointed out (see Comment - Hydrosols), mottling does not necessarily imply that oxidising and reducing conditions are currently occurring in the soil in most years.
These are plant-derived organic accumulations that are either:
- saturated with water for long periods or are artificially drained and, excluding live plant tissue, (i) have 18% or more organic carbon [Walkley-Black x 1.3 or a total combustion method. (Rayment and Higginson 1992, Methods 6A1 or 6B2).] if the mineral fraction is 60% or more clay, (ii) have 12% or more organic carbon if the mineral fraction has no clay, or (iii) have a proportional content of organic carbon between 12 and 18% if the clay content of the mineral fraction is between zero and 60% (see Fig. 2); or
- saturated with water for no more than a few days and have 20% or more organic carbon.
This definition is the same as that used in Soil Taxonomy and is very similar to that used in England and Wales (Avery 1990).
Loss-on-ignition (LOI) may be used as an estimate of organic carbon. For non-calcareous soils, the relationship between organic carbon and LOI was found by Spain et al. (1982) to be influenced by clay content. For the range of organic carbon contents of interest, approximate conversions are:
|when clay is||< 20%||LOI = 2.0 x organic carbon|
|20-60%||LOI = 2.3 x organic carbon|
|> 60%||LOI = 2.7 x organic carbon.|
As noted in the Field Handbook, peats may be assessed by examining the degree of decomposition and distinctness of plant remains. This may be assisted by using a modification of the von Post field test (see Avery, 1990 p.90), in which a sample of the wet peat is squeezed in the closed hand and the colour of the liquid expressed, the proportion extruded between the fingers, and the nature of the plant residues are observed.
- Fibric Peat. Undecomposed or weakly decomposed organic material; plant remains are distinct and identifiable; yields clear to weakly turbid water; no peat escapes between fingers.
- Hemic Peat. Moderately to well-decomposed organic material; plant remains recognisable but may be rather indistinct and difficult to identify; yields strongly turbid to muddy water; amount of peat escaping between fingers ranges from none up to one-third; residue is pasty.
- Sapric Peat. Strongly to completely decomposed organic material; plant remains indistinct to unrecognisable; amounts ranging from about half to all escape between fingers; any residue is almost entirely resistant remains such as root fibres and wood.
Peaty horizon (P and O2 horizons in Field Handbook)
This is a surface or near surface layer of organic materials at least 0.2 m thick overlying mineral soil and which does not qualify as an Organosol. Such soils are designated as a peaty subgroup. In cases where the soil has a surface layer of organic materials less than 0.2 m thick but does not qualify for an Organosol (eg. as in Definition (ii) of Organosols), it will be recognised at the family level as having a peaty 'texture'. The one exception occurs in the Leptic Tenosols where a subpeaty subgroup is provided. In the peaty and subpeaty subgroups there will be a repetition of texture at the family level.
Ferruginous, ferromanganiferous or aluminous nodules or concretions cemented in place into indurated blocks or large irregular fragments.
Unless otherwise specified, pH refers to 1:5 H20 (pHw). Approximate equivalents for pHw and pHCa (1:5 soil: 0.01M CaC12) for the critical pH values used in the classification are as follows: (based on regressions given by Ahern et al. (1995) for large numbers of Queensland surface and subsoil samples)
- pHw of 5.5 is approximately equivalent to pHCa of 4.6
- pHw of 4.0 is approximately equivalent to pHCa of 3.5
A reticulite horizon (see below) that is always indurated in the greater part both before and after exposure.
Podosol diagnostic horizons
The various B horizons defined below consist of illuvial accumulations of amorphous organic matter-aluminium and aluminium-silica complexes, with or without iron in various combinations. Although some may qualify as cemented pans, they are not to be regarded as substrate materials.
Bs horizons. The usually bright colours indicate that iron compounds are strongly dominant or co-dominant and there is little evidence of organic compounds, apart from a few usually discontinuous patches in the upper B horizon or a thin band (< 0.05 m thick) at the A2/B junction. The upper boundary of the B horizon may be very uneven but otherwise the horizons are relatively uniform laterally. Iron concentrations may increase or decrease with depth. No strongly coherent Bs horizons have been recorded. Bs horizons may be non-reactive or give only a weak response to the reactive aluminium test. As a guide, Bs horizons usually have a hue of 5, 7.5 or 10YR, a value of 4 or 5, and a chroma of 4 - 8. The main feature distinguishing a Bs horizon from a tenic B horizon is some weak and irregular development of organic accumulations which extend laterally although discontinuously.
Note that the presence of a thin ironpan (placic horizon), which will be designated as Bsm, is not to be regarded as a Podosol diagnostic horizon because it may also occur in the B horizon of other soils, eg. Tenosols and Kandosols, and may also be present in C horizons or even parent rocks.
Bhs horizons. Iron and organic compounds are both prominent with the organic compounds distributed as streaks, patches or lumps so that concentrations of iron, aluminium and organic compounds have marked spatial variation. Such horizons may contain firm lumps of organic compounds but otherwise are weakly coherent and highly permeable, or they may be strongly coherent throughout, or contain strongly coherent subhorizons or pans. Bhs horizons always contain significant amounts of oxalate-extractable iron and aluminium and frequently silica, ie. imogolite-allophane complex is usually present in significant amounts and the horizons give a moderate to very strong response to the reactive aluminium test. As a guide, Bhs horizons usually have a hue of 2.5YR to 10YR, and value/chroma of 3/3, 3/4, 3/6, 4/3 or 4/4.
Bh horizons. Organic-aluminium compounds are strongly dominant with little or no evidence of iron compounds. Such horizons have a uniform appearance laterally and are relatively uniform vertically although concentrations of carbon and aluminium and the degree of coherence or cementation may change with depth. The horizons may be weakly or strongly coherent, or contain strongly coherent or cemented sub-horizons or pans, or overlie other kinds of pans or clay D horizons. Bh horizons are non-reactive or give only a weak response to the reactive aluminium test. Colours are usually dark with values <4 and chromas <3. In typical Bh horizons the sand grains are uncoated and the organic-aluminium complex is precipitated between the grains (Farmer et al. 1983).
Bh/Bhs horizons. These have a subhorizon, dominated by organic and aluminium compounds with relatively low iron (Bh), overlying the major horizon with prominent organic and iron compounds (Bhs). The dark horizon (Bh) may undulate but is usually discontinuous, and rests on or grades into a Bhs with a range in consistence as described above.
Basi horizons. These are brown, yellow-brown or pale brown cemented horizons that immediately underlie Bh horizons in some poorly drained Podosols. Despite their colour these horizons have low contents of acid oxalate-extractable iron but significant amounts of oxalate-extractable aluminium and silica. The cementing agency appears to be an imogolite-allophane complex with some organic-aluminium compounds. These horizons give a rapid strong or very strong response to the reactive aluminium test. Because of their bright colour and cementation many of these horizons have been included as ortstein in the past.
Bh/Basi horizons. Typical Bh horizons dominated by organic-aluminium compounds which may be weakly coherent or cemented and overlie a cemented Basi horizon.
Pipey B horizons are characterised by pipes of bleached A2 horizon that penetrate both vertically and sometimes laterally > 50 cm into the B horizon, giving a tongued boundary on a profile face. The pipes are usually enclosed by dark organic compounds forming the pipe walls of Bh or Bh/Bhs materials which usually have a weak to firm consistence strength (ie. force 2-3) and are brittle when dry. The bleached A2 material consists of clean quartz grains that have lost any oxide coatings. In 'giant' Podosols the pipes may penetrate > 6 m into the B horizon.
Reactive aluminium test (Hewitt 1992).
This test indicates the presence of reactive hydroxy-aluminium groups, as occur for example in allophane and aluminium-humus complexes (Milne et al. 1991).
Using the procedure of Fieldes and Perrott (1966), 1 drop of saturated sodium fluoride solution is placed on a small test sample of soil, which has been smeared on to a filter paper treated with phenolphthalein indicator. The soil sample must be field moist. For classification, the reactivity of the soil sample is placed into one of the following classes.
|Reactivity Class||Class Definition|
|0 non-reactive||No colour within 2 minutes.|
|1 very weak||Pale red or light red (5R 6/1)
just discernible within 2 minutes.
|2 weak||Pale red or light red (5R 6/1)
within 1 minute.
|3 moderate||Red or weak red (5R 4 or5/-)
within 1 minute.
|4 strong||Dusky red or dark red (5R 3/-)
after 10 seconds.
|5 very strong||Dusky red or dark red (5R 3/-)
within 10 seconds.
An earthy pan which is normally reddish brown to red with a dense yet porous appearance. It is very hard, has an irregular laminar cleavage and some vertical cracks, and varies from less than 0.3m to over 30 m thick. Wavy black veinings, probably manganiferous, are a consistent feature while other more variable features include bedded and unsorted sand and gravel lenses and, less commonly, off-white veins of calcium carbonate. The red-brown hardpan appears to occur either as a cemented sediment or a cemented palaeosol (Wright 1983). It is one of a variable group of silica pans generally known as duripans (Soil Survey Staff 1994) that commonly occur in currently arid climates.
This is intended for strongly developed reddish, yellowish and greyish or white, more or less reticulately mottled horizons that can be hand-augered or cut with a spade. Ferruginous nodules or concretions may be present but are not diagnostic. When moist the material usually has at least a firm consistence strength, but following exposure the material may irreversibly harden. At depth it may grade into mottled saprolite.
The ESP of the fine earth soil material is 6 or greater.
One of the most important features of a soil is its depth or thickness, but it is frequently difficult to determine the lower limit of soil. For many purposes, depth of soil is considered to be synonymous with the rooting depth of plants, but because this may vary widely it is not always a suitable criterion. Thickness of solum (A + B horizon) is a measure that is useful in many soils, although it may be difficult in some soils to distinguish B from C horizon.
At the Family level, soil depth will be taken to mean either thickness of solum or depth to a cemented pan. In a particular soil it will be evident from the classification which criterion is used. However, depth to a thin ironpan will not be used because of the extremely irregular and convolute nature of most of such pans.
pH of the fine earth soil material is as follows:
- pHw (1:5 H20) is less than 5.5 or
- pHCa (1:5 soil : 0.01M CaCl2) is less than 4.6.
- pHw < 5.5 should be used as the critical limit when it is available.
Strongly coherent B horizon
These are Podosol B horizons in which the consistence strength ranges from very firm to strong throughout (ie. force 4-5), or they contain subhorizons with these properties. Included are pan-like materials that have been variously described as ortstein, coffee rock or sandrock. The consistence properties are usually independent of soil water status.
A subsoil, waterlogged, mineral or organic material that contains oxidisable sulfur compounds, usually iron disulfide (eg. pyrite, FeS2), that has a field pH of 4 or more but which will become extremely acid when drained. Sulfidic material is identified by a drop in pH by at least 0.5 unit to 4 or less (1:1 by weight in water, or in a minimum of water to permit measurement) when a 10mm thick layer is incubated at field capacity for 8 weeks. For a quick screening test that is not definitive, a 10 g sample treated with 50 ml of 30 % H202 will show a fall in pH to 2.5 or less. Caution: H202 is a strong oxidant and sulphides and organic materials will froth violently in a test tube which may become very hot.
Note: This definition is similar to that in Soil Taxonomy (Soil Survey Staff, 1994) but modified slightly by Dr David Dent, University of East Anglia and colleagues in CSIRO Division of Soils.
Soil material that has a pH less than 4 (1:1 by weight in water, or in a minimum of water to permit measurement) when measured in dry season conditions as a result of the oxidation of sulfidic materials (defined above). Evidence that low pH is caused by oxidation of sulfides is one of the following:
- yellow mottles and coatings of jarosite (hue of 2.5Y or yellower and chroma of about 6 or more).
- underlying sulfidic material.
Note: This definition is similar to that in Soil Taxonomy (Soil Survey Staff, 1994) but modified slightly by Dr David Dent, Bureau of Resource Sciences and colleagues in CSIRO.
Tenic B horizon
A usually weakly developed B2t, B2w or other B horizon (in terms of contrast between A horizons above and adjacent horizons below) of texture and/or colour and/or structure and/or presence of segregations of pedogenic origin (including carbonate). It usually is slightly different from the underlying horizon (excepting buried soils) in terms of a higher chroma, redder hue or higher clay content, but structure should be no more than weak grade and mottles or sesquioxidic segregations of pedogenic origin other than hard ferromanganiferous nodules or concretions should not exceed 10% in the major part of the horizon.
In many shallow stony soils, the tenic B horizon may be present only between rock fragments or in rock fissures (50% or more by visual abundance estimate). Where present in soils formed from sediments, weak evidence of stratification may be present. Weakly developed argic horizons may be present in some tenic B horizons. (See also B horizons and transitional horizons.).
In some soils underlain by a red-brown hardpan where there is no discernible A1 horizon and no underlying C horizon, it is difficult to identify a B horizon if there is little or no colour change or increase in texture or development of structure. Such layers of uniform soil materials without identifiable overlying or underlying horizons may be considered as a tenic B horizon if there is no evidence of alluvial stratification or aeolian cross-bedding within them.
These consist dominantly of tephra - unconsolidated, non-weathered or only slightly altered primary pyroclastic products of explosive volcanic eruptions. They include ash, cinders, lapilli, scoria, pumice and pumice-like vesicular pyroclastics. Volcanic bombs may occur, and some exotic ejecta such as limestone fragments.
Commonly a thin (2-10mm) black to dark reddish pan cemented by iron, iron and manganese, or iron-organic matter complexes. Rarely 40mm thick. It has a wavy or convolute form and usually occurs as a single pan. It is also known as a placic horizon (Soil Survey Staff 1994).
There are slight differences in the definitions of these horizons between the Soil Survey Manual (Soil Survey Division Staff 1993) and the Field Handbook. The definition used in this classification is that used in the Soil Survey Manual, viz:
- Horizons dominated by properties of one master horizon but having subordinate properties of another eg. BC. The first symbol indicates that the properties of the horizon so designated dominate the transitional horizon.
- Horizons with two distinct parts that have recognisable properties of the two kinds of master horizons indicated by the capital letters eg. C/B. The first symbol is that of the horizon with the greater volume. Most of the individual parts of one horizon component are surrounded by the other.
Unconsolidated mineral materials
This term is used to describe various unconsolidated materials below the solum, such as some C horizons, buried soils, sedimentary deposits of alluvial, colluvial or aeolian origin, and transported ferruginous nodules or concretions, such as occur in some ferric and bauxitic horizons.
Soil material with a clayey field texture (ie. light clay, medium clay, heavy clay) or 35% or more clay, which cracks strongly when dry and has slickensides and/or lenticular peds. See also Comment following the definition of Vertosols.
In several countries, physical measurements are being used in soil classification to help define classes of shrink-swell clay soils. In South Africa (Soil Classification Working Group 1991), the definition of a vertic A horizon (which is the definitive feature of soils equivalent to Vertosols) includes either slickensides or a plasticity index greater than 32 (using the SA Standard Casagrande cup to determine liquid limit) or greater than 36 (using the British Standard cone to determine liquid limit). Cracking is regarded as an accessory property, as is linear shrinkage which is stated to be usually greater than 12%.
Soil Taxonomy relies solely on morphology for the definition of Vertisols (as does FAO-Unesco 1990), but in the definition of vertic subgroups in Soil Taxonomy (Soil Survey Staff 1999) a linear extensibility [The linear extensibility (LE) of a soil layer is the product of the thickness, in centimetres, multiplied by the COLE of the layer in question. The LE of a soil is the sum of these products for all soil horizons (Soil Survey Staff 1999).] of 6 cm or more is offered as an alternative to the usual morphological requirements of cracks, and slickensides or wedge-shaped aggregates. However, the 6 cm minimum applies to the soil in the upper 100 cm of the profile, or the depth to a lithic or paralithic contract, whichever is shallower. This hardly seems appropriate to a common Australian situation where thick sandy A horizons overlie shrink-swell B horizons, particularly as in most engineering situations topsoils tend to be removed.
In Australia, COLE is seldom determined other than for research purposes and hence there is no appropriate data base of representative Australian clay soils. In contrast, standard engineering tests (Atterberg limits and linear shrinkage) are widely used by engineers and some soil conservation departments. Unfortunately, it is often not possible to relate the test values to specific kinds of soil, let alone the presence or absence of morphological features such as slickensides and lenticular peds. One relevant paper is that of Mills et al. (1980) who found in a study of 14 clay subsoils (three of which were Vertosols) in New South Wales that linear shrinkage was an appropriate method to predict shrink-swell activity but this was not related to morphology. Critical linear shrinkage limits of Mills et al.(1980) and for several other engineering authorities are given by Hicks (1991). Linear shrinkage values of 12-17% are rated as being marginal or moderate, with greater than 17% rated as a critical or high shrink-swell potential. However, Holland and Richards (1982) suggest that in arid and semi-arid climates, where pronounced short wet and long dry periods lead to major moisture changes, the linear shrinkage lower limits for moderate and high shrink-swell potential be 5% and 12% respectively.
McKenzie et al. (1994) have suggested that because the natural soil fabric is destroyed in the standard linear shrinkage test, the results can be difficult to relate to field behaviour. They have developed a rapid modified linear shrinkage test in which disruption to the natural soil fabric is reduced. This method was found to be a good predictor of COLE (r2 = 0.88) with the slope of the regression line close to unity. The standard linear shrinkage was found to be a weaker predictor of COLE (r2 = 0.79). In the 26 samples used, (that included two Vertosol profiles), the value for the standard linear shrinkage was always equal to or greater than the modified method.
There is obviously a need for further testing of all shrinkage methods on a wide range of Australian soils, and in particular to relate values to field morphology, as the latter may not always be a reliable guide to shrink-swell behaviour, particularly if salt and carbonate contents are high. McGarry (1995) has reviewed the various methods currently used to measure soil shrinkage.
For present classification purposes it is difficult to give firm guidelines. In the interim, a linear shrinkage of about 8% or greater by the modified version or about 12% or greater by the standard linear shrinkage (and/or a plasticity index > 32-36) will help differentiate soils with vertic properties from others.
Weakly coherent B horizon.
These are Podosol B horizons in which the consistence strength ranges from loose to firm (ie. force 0-3), although they may contain firm to very firm lumps (ie. force 3-4) associated with accumulations of organic compounds, and occasionally there may be some hard sesquioxide nodules present. They do not contain pans of any kind.