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How to Classify

What do we classify?

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 2m. 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 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.

In classifying the soil profile, it is necessary to identify various diagnostic horizons and materials. All terms used in the classification are consistent with those defined in the second edition of the Australian Soil and Land Survey Field Handbook (McDonald et al. 1990), or else are defined in the Glossary (indicated by italics).

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.

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, 1994). 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 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 0.3m 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 0.3m 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 Yellow-Orthic Tenosol/Black Vertosol.

If a buried soil cannot be classified, the sequence may be recorded as in the following example: Grey Kandosol/sulphidic 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. Note that Figure 1 is not to be used as a substitute for the key to soil orders.

figure 1

Figure 1.   Schematic summary of the orders (Note that this figure is not to be used as a 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, 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 eight 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 5.4 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.

A number of 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. A number of concepts have been borrowed from Soil Taxonomy, and some have originated in the South African classification (Soil Classification Working Group 1991), for example base status classes. A number of concepts from the Factual Key havealso been used e.g. 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'. Appendix 5 shows approximate correlations between the orders of the new scheme and classes of three other classifications formerly used in Australia.

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 horizon 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. 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 it is highly unlikely that any new orders will be introduced. 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 codes.

This two letter code in brackets is unique for that class name. The order code is given after the order heading. Similarly, a one letter code is given for the family criteria. This code system will allow 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]. Provision is also made for indicating confidence levels of the classification where class definitions involve the need for analytical 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.

Note that this 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
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; McDonald et al. 1990, pp.104-5) 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 percentages given in brackets are merely a guide and are based on those in McDonald et al. (1990). 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 Isbell (1988).

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 and those soils containing sulphuric and/or sulphidic materials, where at present data are very limited but extensive studies are in progress. Amendments to the classification are the responsibility of the Working Group on Land Resource Assessment which has representatives from relevant Territory, State and Commonwealth agencies.