ABSTRACT
The discussion on the categories in soil classification has a long history (Dmitriev, 1991). Initially, pedologists considered each soil profile to be both representative and typical of its own class. However, the extension of soil survey resulted in recognizing the great heterogeneity of soil morphology and properties. Thus, a number of concepts were proposed to define the objects of soil classification (Arnold, 1983). The first version of the US Soil Taxonomy defined the soil individual as a small landscape called a polypedon and its sampling unit was a pedon, which was considered to be the object of classification (Buol et al, 1977). Many discussions dealt with the definition of the uniformity and size of the pedon (Dmitriev, 1991); however, Sokolov (1978) noted that most specialists ignored these features and considered the object of classification to be the profile itself. In more recent publications we can find the opinion, based on cognitive psychology, that it is not the natural object which is classified, but our idea of this object (Haskett, 1995). A similar thought is outlined in the modern French soil classification (AFES, 1998). French scientists proposed to separate the soil mantle (a real three-dimensional natural body), soil profile (vertical change of a certain property in a soil pit; e.g. a morphological profile, a humidity profile, etc.) and solum, which is the concept of a soil as a sequence of pedogenically developed horizons. The object of classification is solum, a sequence of horizons (or ideas of horizons). This approach seems logical, because in a strict sense, except in our mind, there are neither soil classes nor soil profiles. In reality there is a substrate, with some properties changing in vertical and lateral directions that we divide into horizons by overlaying the structure of our mind on the real object. The fact that the projection is subjective does not make it ‘less scientific’, if we use a unified technique for distinguishing horizons and soil classification.
