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How to Classify

What do we classify?

Soils are natural bodies consisting of layers or horizons of mineral and/or organic constituents of variable thickness which differ from their parent material in the their morphological, physical, chemical and mineralogical properties and their biological characteristics (Birkeland 1984).

Because soils are three dimensional bodies their classification has always caused problems. In practice, in most countries, the entity classified is the soil profile, which is a vertical section through the soil from the surface through all of its horizons to the parent or substrate material. However, the lateral dimensions of the section may range from about 50mm to a metre or more depending on the method of examination.

It is sometimes difficult to distinguish soil from its parent material or underlying substrate, and to distinguish between soil and 'not soil'. Most concepts of soil involve the idea of an organised natural body at the surface of the earth that serves as a medium for plant growth. However, most engineers and geologists tend to regard soils mainly as weathered rock or regolith. The first edition of Soil Taxonomy (Soil Survey Staff 1975) noted that the lower limit of soil is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants. There are obvious problems with the latter part of this concept, and in Soil Taxonomy the lower limit of the soil that is classified is arbitrarily set at 2.0 m. This approach is rejected in the new classification, and the term pedologic organisation (McDonald et al. 1990) is used to distinguish soil materials. This is a broad concept used to include all changes in soil material resulting from the effect of the physical, chemical and biological processes that are involved in soil formation. Results of these processes include horizonation, colour differences, presence of pedality, texture and/or consistence changes. Obviously there are some difficulties in this approach, such as distinguishing between a juvenile soil and recently deposited sedimentary parent material.

Subjective judgement is often required, as in distinguishing between the Rudosols with only rudimentary pedologic organisation as opposed to slight development in the Tenosols. In Rudosols and some Arenosols there will be negligible, if any, horizon development.

In the special case of the Anthroposols - the 'human-made' soils - some departure from the above concept of soil is necessary. In this order human activities may have been mainly responsible for the creation of 'non-natural' parent materials as well as 'non-natural' alteration processes such as profound disturbance by mechanical or other means, or the addition of a wide range of anthropogenic materials to surface soils, including toxic chemical wastes.

Except for Anthroposols, the Australian Soil Classification is designed primarily for natural soils that have undergone at most, only minor change from human activities. However the classification of soils changed by the addition of liquids of variable attributes remains unresolved. The addition of liquid wastes, bore waters and other irrigation waters may alter the chemical and physical properties of soil substantially. For example, salts, acids, alkalis, suspended organic matter, nutrients and toxins in applied liquids may affect the pH, salinity, sodicity and fertility of the soil.

To be considered as a separate class of soil, the significant change from pre waste conditions should extend throughout the entire A horizon and into at least the upper part of the B horizon. These soils are not Anthroposols. However the soil attributes could be acknowledged in the soil classification by referring to the site as a phase of the pre-existing soil class (e.g. saline phase, sodic phase or alkaline phase of a Brown Chromosol)

In classifying the soil profile, it is necessary to identify various diagnostic features. All terms used in the classification are consistent with those defined in the third edition of the Field Handbook, or else are defined in the glossary. As the Field Handbook is currently being revised, some definitions in the glossary are aligned with the new Field Handbook, rather than the current (third) edition.

One of the most important features used in the classification is the B horizon. In some soils it may be present in variable amounts mainly in fissures in the parent rock or saprolite but even so it can still be of importance to use of the soil. The classification of such soils leads to a consideration of transitional horizons viz. BC, B/C and C/B. If the B horizon material occupies more than 50% (visual abundance estimate) of the horizon, ie. it is a B, BC or B/C horizon, the soil is deemed to possess a B horizon and is classified accordingly. If, however, the soil has a C/B horizon in which the B horizon component is between 10% and 50%, the soil will be classed as a Tenosol. If there is less than 10% of B horizon material and no pedological development other than a minimal A1 horizon, the soil would be classed as a Rudosol.

Although it is difficult to avoid genetic implications, it should be noted that a B horizon, for example, is identified by what it is, not by how it got there. Thus if there is a sequence where a sandy sedimentary layer overlies a clayey sedimentary layer and the system has been operating as a whole for sufficient time for soil forming factors to influence both, and also for the properties of one layer to influence the properties of the other, there is no reason why we cannot speak of these transformed layers as A and B horizons and classify the soil accordingly. Evidence of transformation commonly takes the form of the presence of an A2 horizon and an absence of a B horizon on the upper sedimentary layer. If there is no apparent transformation (commonly the presence of a B horizon in the upper sandy sedimentary layer above), and there is a sharp or clear horizon boundary, the clayey sedimentary layer below may be considered a lithologic discontinuity (McDonald and Isbell 2009) and identified as a D horizon.

It is noted that accurate classification may require assessment of the soil profile at depth to determine the presence or maximum development of a B horizon. Inspection to commonly standardised maximum depths of 1.0 m or 1.5 m for particular survey purposes may not be sufficient. In such circumstances, confidence levels with the classification may be affected.

The Australian Soil Classification uses a set of defined attributes, horizons and materials in the key to assign a soil profile to a class. Collectively these concepts are called diagnostic features. Diagnostic features used in the key are ranked mainly on the basis of their likely importance to use of the soil. Ranking is subjective and arbitrary to a varying extent (Isbell et al. 1997).

Some diagnostic features occur at or near the soil surface or must occur within a specified soil depth. However, others do not have a depth criterion. Although the Australian Soil Classification has no depth restriction, at the Great Group and Subgroup levels it is permissible to consider whether a diagnostic feature that only begins at considerable depth should apply. As a guide, if the feature begins below 1.5 m, judgement on the impact of this feature on soil performance may need to be made. The class relating to this diagnostic feature may not be the only option in the key. If it is considered that this feature has minimal influence on the performance of the soil and there are other options in the key, it is permissible to consider the next option.

Another well-known problem is how to deal with buried soils. No classification system has yet satisfactorily resolved this question. For the moment the approach adopted is a modified version of that used in Soil Taxonomy (Soil Survey Staff 1975). A buried soil may be overlain by another soil profile or by recently deposited material that has not had sufficient time to develop enough pedological features to meet any of the requirements for the defined Soil Orders. In such cases the overlying material shall be regarded as a phase 1 of the classified soil below. Typical examples would be very recent silty or sandy alluvium deposited on a flood plain, windblown sand, or a recent layer of volcanic ash.

If the soil material overlying the buried soil is less than 0.3 m thick and has pedological development sufficient only to qualify as a Rudosol, then it is also regarded as a depositional phase of the buried soil below. If the same overlying material is greater than 0.3 m thick it could be classified together with the buried soil as, for example, a Stratic Rudosol/Black Vertosol. If, however, the overlying material had sufficient pedological development for it to be classified other than as a Rudosol, it would be so classified irrespective of its thickness. An example would be Brown Tenosol/Black Vertosol.

If a buried soil cannot be classified, the sequence may be recorded as in the following example: Grey Kandosol/sulfidic clayey D horizon. In this example the buried soil has a clayey texture, using the same texture categories as in the family criteria.

Another situation which not uncommonly arises is the formation of a new soil in the A horizon of a pre-existing soil. This may also be covered as in the following example: Humosesquic, Semiaquic Podosol f Chromosolic, Redoxic Hydrosol. The symbol f indicates that the first named soil is forming in the A horizon of the second named soil.

Nature of the classification

The scheme is a general purpose, hierarchical one (Order, Suborder, Great Group, Subgroup, Family) and a diagrammatic view is shown in Figure 1.

figure 1

Figure 1.   Simplified key to the Orders

Note that this figure is a guide only and should not be used as a replacement for the complete key

All hierarchical schemes have both advantages and disadvantages. Among the former is the flexibility to classify a soil at whatever level of generalisation is desired. A perceived disadvantage is that as soils are grouped into higher categories, the assertions that can be made about any group become progressively fewer. This explains why some high-level groupings e.g. the Order Dermosols, can be criticised as containing a diverse range of soils. The goal of all successful hierarchical systems is to use criteria at the higher categories that carry the most accessory features along with those criteria.

Another related issue is some lack of consistency in the use of certain criteria in the hierarchy. The general philosophy, following Soil Taxonomy (Soil Survey Staff 1975), has been to select differentiae which seem to reflect the most important variables within the classes. It would be tidy, for instance, to have all Suborders based on colour. The fact is that while it is useful to use colour at the Suborder level for nine of the Orders, it does not give the 'best' class differentiation for other Orders where different criteria give a more effective subdivision e.g. in Podosols.

The fact that most classes are mutually exclusive inevitably means that soils on either side of a class boundary may appear to have more in common than they do with the 'central concept' of each adjoining class. An obvious example of this occurs in the Suborder classes defined by colour.

In general, intergrade soils are catered for at the subgroup level. As an example, there are sodic and vertic subgroups for Chromosols, which respectively indicate affinities with Sodosols and Vertosols. Another situation arises when similar soils are placed in different Orders because B horizon pH is say 5.3 in one soil and 5.6 in another; by definition the former soils are Kurosols and the latter Chromosols. However, the similarity between them is preserved by both Orders having essentially the same Suborders, Great Groups and Subgroups.

Several ideas have been taken from other classification schemes in Australia and overseas, e.g. the hierarchical framework of Order, Suborder, Great Group, Subgroup and Family is widely used elsewhere in the world. Several concepts have been borrowed from Soil Taxonomy (Soil Survey Staff 1975), and some have originated in the South African classification (Soil Classification Working Group 1991), for example base status classes. Several concepts from the Factual Key (Northcote 1979) have also been used, for example the use of strong texture contrast and colour at a high categorical level.

Throughout the text, where appropriate, brief reasons are given for particular decisions regarding the use of various differentiating criteria. These are found under the heading 'Comment'. Further explanation of many criteria are provided in Isbell et al (1997). Appendix 5 shows approximate correlations between the Orders of the new scheme and classes of two other classifications formerly used in Australia and two current international classifications.

A change from previous Australian classification schemes is the use of laboratory data (mainly chemical) at some levels in a number of Orders. Although some field soil surveyors have protested, no apology is made for this approach. Soil classification schemes being developed around the world are increasingly relying on laboratory data, particularly where soils with very similar morphology may have widely differing chemical properties. The same is true for most other sciences e.g. geology. In this scheme the need for laboratory data is minimised at the Order level, and where possible some guidelines are given to enable tentative field classification. A summary of the analytical requirements is given in appendix 4.

Operation and nomenclature

The classification is designed in the form of a number of keys. To classify a soil profile the following procedure should be adopted.
  1. Read the Key to the Soil Orders stepwise and select the first Order in the key that apparently includes the soil being studied, checking out diagnostic feature definitions in the glossary as needed.
  2. Turn to the page indicated, read the definition of the Order to ensure that it embraces the soil being studied.
  3. Then study the various keys to the Suborders, Great Groups and Subgroups, and select the first appropriate class where available. Note that the classes, particularly at the Subgroup level, must be examined sequentially, as they are often based on differentiating criteria which are thought to be of decreasing order of importance to the use of the soil. This of course is subjective, and the order in which the classes are arranged may be changed in the light of further knowledge.
  4. If a diagnostic feature the key at the Great Group and Subgroup level begins more than 1.5 m from the soil surface it may not have a significant impact on the performance of the soil. Refer to diagnostic features in the glossary for guidance on the use of such features in the classification.
  5. If a suitable class is not found at the Great Group or Subgroup level the classification at that level is not applied.
  6. To classify at the Family level, select the appropriate designations.

The scheme is open ended; new classes can be added if desired, although they will not necessarily follow on from the existing classes. However the introduction of any new Orders will require substantial justification and testing. Where possible, names are connotative, and often based on Latin or Greek roots, e.g. see Table 2. Suborder, Great Group and Subgroup class names are given in bold type after each class definition, together with their relevant code.

There is a two letter code in brackets that is unique and follows each class name. The Order code is given after the Order heading. Similarly, a one or two letter code is given for the Family criteria. This code system allows recording on field sheets, and also enable various database searches to be carried out. As an example, it will allow searches for particular criteria irrespective of the hierarchical level at which they are used in the classification. Provision is also made for instances where there is no appropriate class available [code ZZ], or when it is not possible to determine the class from the available information [code YY]. Appendix 2 lists the codes and equivalent class names for all codes used in the classification and appendix 3 lists the level at which class names and their codes occur in the Soil Orders.

Provision is also made for indicating confidence levels of the classification where class definitions involve the need for analytical data and/or more morphological data. In the appendices the full list of class names and codes is given, together with examples of their use.

The general form of the nomenclature is as follows:

Subgroup, Great Group, Suborder, Order; Family.

An example is:

Bleached, Eutrophic, Red Chromosol; thin, gravelly, sandy/clayey, shallow, water repellent.

Note that the nomenclature can be shortened if desired, or if some levels of the hierarchy cannot be determined e.g. Red Chromosol; Bleached, Red Chromosol; Red Chromosol; thin, gravelly etc.

At the Subgroup level in particular, the differentiating criteria are frequently not mutually exclusive. This problem can be alleviated to some extent by combining attributes e.g. Bleached-Mottled, but usually judgement has been required in establishing the sequence of the subgroup classes. This was largely based on a subjective assessment of the subgroup properties in relation to use of the soil. In the six Orders where the Haplic subgroup is used it is placed last and defined as 'other soils with a whole coloured B horizon'. It should be noted that as well as having this particular property, it also does not have any of the properties of any class that precedes it in the list of Subgroups. This is the reason for the particular class name, derived from Gr. haplous, simple.

Table 2. Soil Order nomenclature

Name of Order Derivation Connotation
Anthroposols Gr. anthropos, man 'human-made' soils
Arenosols L. arena, sand sandy soils
Calcarosols L. calcis, lime calcareous throughout
Chromosols Gr. chroma, colour often bright coloured
Dermosols L. dermis, skin often with clay skins on ped faces
Ferrosols L. ferrum, iron high iron content
Hydrosols Gr. hydor, water wet soils
Kandosols Kandite (1:1) clay minerals
Kurosols pertaining to clay increase
Organosols dominantly organic materials
Podosols Rus. pod, under; zola, ash podzols
Rudosols L. rudimentum, a beginning rudimentary soil development
Sodosols influenced by sodium
Tenosols L. tenuis, weak, slight weak soil development
Vertosols L. vertere, to turn shrink-swell clays

There are apparent inconsistencies in the use of A and A1 horizons at the family level in various Orders. This is deliberate, for the following reasons. In the soils with strong texture contrast, it is thought that properties of the total A horizons (ie. A1, A2, A3) are important. In some other Orders where soil changes are very gradual with depth, and it is frequently difficult to distinguish between say A3 and B1 horizons, it is thought more appropriate to use A1 horizon at the family level. In some circumstances problems may arise with Ap horizons. In the strong texture contrast situation above, the Ap horizon will automatically be included, although in some soils with thin A horizons or where deep ploughing is practised, there is the probability that some of the B horizon will be incorporated in the Ap. In the case of A1 horizons, these will mostly equate with Ap horizons, although again there can be a problem with deep ploughing. In some A and A1 horizons, texture may not be uniform throughout. In these instances the texture of the major part of the horizon should be given.

Some soils may have surface horizons dominated by organic materials (O2 or P horizons; Field Handbook) which overlie an A1 horizon. In these cases the field texture at the family level will be given for the O2 or P horizon, i.e. peaty. In soils with peaty or subpeaty subgroups this will result in repetition at the family level. See also peaty horizon.

At the family level all textures are field textures. The clay percentages given in brackets are merely a guide and are based on those in the Field Handbook. In contrast, the clay content classes used in Vertosols are based on actual laboratory analyses.

Concluding Statement

Three points concerning use of the classification need to be emphasised. First, the best place to classify a soil is in the field, where the morphological requirements can readily be checked. Even if laboratory data are required for some classes, a tentative classification can usually be made and verified later. It is important therefore to always give the confidence level of the classification (see appendix 1). Second, to quote the South African Soil Classification Working Group (1992), "this soil classification has as its primary aim the identification and naming of soils according to an orderly system of defined classes, and so permit communication about soils in an accurate and consistent manner." Third, in the case of soil survey and mapping, the use of the scheme will not be any different to that of any existing classification; it must be coupled to soil mapping for it to yield information on the geographic distribution of soils. Recommendations for classification and mapping units in Australian soil surveys are provided by Powell (2008).

Finally, it should again be emphasised that no classification scheme is ever complete. As knowledge increases, so there must be future modifications to the scheme to incorporate this new knowledge. In this classification this axiom is particularly relevant in the case of the Anthroposols where at present data are very limited. Amendments to the classification are the responsibility of the National Committee on Soil and Terrain which has representatives from relevant Territory, State and Commonwealth agencies.

1A recommended use of soil phase has been given by Powell (2008).