SOIL TAXONOMY

A comprehensive soil classification system designed by the United States Department of Agriculture (USDA), principally for use in the United States, but which seeks to classify all world soils and has been extensively used for international comparisons of soils and the transfer of technical information about soils and soil management. It was first published in 1975 and has undergone eight revisions; the most recent version was published in 1999, but there is a constant process of update and revision as more data are gathered. It is a hierarchical classification from Series at a local scale through increasingly general categories: Family, Subgroup, Great Group, Suborder and Order. In 1975, there were 10 Orders: Alfisols, Aridisol, Entisol, Histosol, Inceptisol, Mollisol, Oxisol, Spodosol, Ultisol and Vertisol. Two further Orders were added in 1999: Andisol and Gelisol. The names for the Orders are derived from Greek or Latin roots that convey the concept of the nature of the soil or its genesis. The nomenclature at different levels of the classification gives an indication of the key characteristics used in the differentiation.

The following example is of the Order Mollisol (a very dark-coloured, base-rich soil of the steppes, with a surface layer rich in organic matter and of high base status). The formative element, ‘oll’, is carried through lower levels of the classification as the order name, the prefix ‘aqua’ indicates the presence of poor drainage and ‘argi’ indicates the presence of a clay-enriched subsoil. ‘Typic’ indicates that the soil is typical of the central concept of the Great Group.

Currently, there are 64 Suborders, >300 Great Groups and >2,400 Subgroups. Below the subgroup level, the subdivisions are Families, which might, for example, distinguish soils of similar physical and chemical properties and Series, indicating the geographical locality of the soil and where it was first described. Key features in the classification of soils are the diagnostic soil horizons, many of which are broadly similar to those described for the world reference base for soil resources (WRB). Direct correspondence between classes in different soilclassification systems is infrequent; the Table seeks to draw broad comparisons between the Reference Soil Groups in WRB and Soil Taxonomy.