Classification is a basic requirement of all science and needs to be revised periodically as knowledge increases. It serves as a framework for organising our knowledge of Australian soils and provides a means of communication among scientists, and between scientists and those who use the land. The history of soil classification in Australia was reviewed by Isbell (1992), who noted that two classification schemes were widely used prior to 1996. The Handbook of Australian Soils (Stace et al. 1968) was largely a revision of the earlier great soil group scheme (Stephens 1953). The Factual Key (Northcote 1979) dates from 1960 and was essentially based on a set of about 500 profiles largely from south-eastern Australia. Moore et al. (1983) have discussed the advantages and disadvantages of these two schemes. Over the past three decades a vast amount of soils data has accumulated. This information needed to be incorporated into any new or revised national soil classification.
The selected option for a new Australian classification system was for a multi-categoric scheme with classes defined on the basis of diagnostic horizons or materials and their arrangement in vertical sequence as seen in an exposed soil profile, that is, soil rather than geographic attributes were to be used. In the new scheme, classes are based on real soil bodies, they are mutually exclusive, and the allocation of 'new' or 'unknown' individuals to the classes is by means of a key.
The guiding principles agreed to were:
- The classification should be a general purpose one as distinct from a technical or special purpose scheme.
- It should be based on Australian soil data and as far as possible the selected attributes should have significance to land use and soil management.
- It should be based on defined diagnostic attributes, horizons, or materials, the definitions of which, where appropriate, should be compatible with those of major international classification schemes.
- The entity to be classified is the soil profile, with no depth restrictions such as the arbitrary lower limit of 2 m used in Soil Taxonomy.
- Although the soil classification should be based as far as practicable on field morphological data, laboratory data must be used as appropriate. If possible, more use should be made of soil physical and engineering properties.;
- The scheme should be based on what is actually there rather than on what may have been present before disturbance by humans. Surface horizons should not be defined in terms of an 'after mixing' criterion as in Soil Taxonomy.
- The scheme should be a multi-categoric one, arranged in different levels of generalisation.
- The scheme should be flexible enough to accept new knowledge as it becomes available - it should be open-ended.
- The classification should give emphasis to relatively stable attributes as differentiae.
- The nomenclature must not be too complex, but be unambiguous.
The general guidelines above have mostly been followed in the new scheme. Unfortunately it has not been possible, because of lack of data, to make more use of soil physical and engineering properties.