The Categories of Soil Taxonomy
- The soil orders are the highest category, and they differentiate one soil from another by the presence or absence of certain properties that are influenced by some soil-forming processes. These processes are, for example, addition and loss of nutrients and transformation and translocation of clay minerals. Three soil orders are described as examples.
- The order Andisols are volcanic ash soils dominated by non crystalline or poorly crystalline materials. They have high phosphorus retention, high water-holding capacity, and high cation exchange capacity. The resulting properties are due to weathering and mineral transformation.
- Oxisols, on the other hand, are one of the weathered soils of the tropics with low cation exchange capacity, dominated by kaolinite clay mineral and oxides of iron, aluminum, titanium, etc. The resulting properties are due not only to weathering but also to losses of the bases (calcium, magnesium, potassium) and transformation.
- Soils such as Ultisols, however, show clay films in the subsoil, indicating translocation of clay materials from the upper portion of the soil profile. Ultisols have low base content, are dominated by oxides and other secondary minerals, and give rise to properties which are indicative of infertile soils. In addition to translocation, the dominant processes are losses of bases and transformation of minerals.
- The suborders show the kind of properties influenced by soil moisture. They indicate whether or not they are wet or dry soils, with gradation such as moist, somewhat moist, or deficient in soil moisture for plant growth.
- The great groups are soils with similarities in kind, arrangement, and degree of expression of the soil horizons, and similarities in base status.
- The subgroups differentiate whether the soils are (1) typic, (2) intergrades or transitional forms to other soil orders, suborders, or great groups, and (3) extragrades, soils with properties not representative of the great group.
- The soil families commonly describe (1) particle-size classes, whether sandy, loamy, clayey, etc., (2) mineralogy classes, whether kaolinitic, montmorillonitic, oxidic, etc., and (3) soil temperature classes, which are related to air temperature.
- The soil series are the lowest category, describing the properties in great detail. They would have a description of a typical pedon or soil body, including the number and kinds of horizons or layers, soil color, apparent texture, structure, consistence, roots, pores, and other unique characteristics. The particular series would also show georgraphic relationships as well as other competing series.