Soil Taxonomy – Structure and Differentiating Characters

Structure of soil taxonomy

Higher categories

1. Order: The number of taxon is 12 (AVAGAMIHOUSE). Aridisol, Vertisol, Alfisol, Andisol, Mollisol, Inceptisol, Histosol, Oxisol, Ultisol, Spodosol, Entisol and Gellisol. Differentiating character is based on morphology as indicated by the presence and absence of diagnostic horizons.

2. Suborder: It is the sub division of the order based on genetic homogeneity, wetness, climatic environment, parent material and vegetation effects. There are 47 suborders

3. Great groups: They are 230 in number. They are based on similarities in soil moisture, temperature and base status. It also involves the diagnostic sub surface horizons

Lower categories

4. Sub groups : A new category designed to define the central concept of a great group. They are 1200 in number. Three types of subgroups are typic, intergrade and extragrade.

5. Family : Number of families in soil is 6600. Families are based on texture, mineralogy, depth, temperature and calcareousness. It is a practical category for making predictions and land use plans.

6. Series – The lowest and the most specific category. The series is a collection of soil individuals, essentially uniform in differentiating characteristics (like colour, texture, structure, consistency, pH and EC) and its arrangement of horizons. The series are named after the geographic name of the place where it was first recognized.

7. Soil phase – Based on differences in surface soil texture, solum thickness, percentage slope, the stoniness, the saltiness, the extent of erosion damage and other conditions.

SOIL TAXONOMY- STRUCTURE, DIFFERENTIATING CHARACTERS AND LIMITATIONS

The system has six categories of classification. From the highest to the lowest levels of generalization, these are grouped under two broad categories viz., higher and lower.

Higher categories

1. Order – the highest category in the system.

2. Sub order – comparable to great soil groups of the genetic system.

3. Great group – a basic category, based on diagnostic subsurface horizons.

Lower categories

1. Subgroup – a new category designed to define the central concepts of great groups.

2. Family – a practical category for making predictions and land use plans.

3. Series – the lowest and the most specific category.

Application of differentiating criteria used in Soil Taxonomy

Order

The names of the orders are coined words and have a common ending, sol (L.solum, soil) with the connecting vowel “O” for Greek and “i” for Latin. A formative element starts with first vowel (O) and ends with the last consonant (L) proceeding the connecting vowel (i), eg. oll from Mollisol.

Suborder

Each suborder consists of two syllables. The first is suggestive of a property of the class (i.e. suborder) and second, the name of the order as reflected by the formative element. For instance the names ; Udoll, Xerert, Fluent, first suggest the property of the sub order (Ud from udic, xer from xeric, flu from fluviatile) and second is the name of the order (oll from Mollisol, ert from Vertisol and ent from Entisol).

Great group

The names of great groups are coined by prefixing one additional prefix. For instance, Hapludoll, Chromustert. If a great group is distinguished from other within the same sub order by a particular diagnostic horizon or property, the name of that property is used as prefix to form other great groups. E.g. Argiudoll.

Subgroup

Subgroup name consist of the name of the approximate great group modified by one or more adjective, e.g. Typic Hapludoll, Typic Ustifluvent. The adjective Typic is used to typify the central concept of the great group. E.g. the taxonomic name of a soil is given below. From this various categories of soil taxonomy can be identified. The fine silty mixed calcareous mesic family of Ustic Torri Orthents.

 Ent- entisol (order), orthent – suborder, torri orthents – great group.

 Ustic torri orthents – sub group

 Calcareous mesic family of Ustic Torri Orthents – Family

 The fine silty mixed calcareous mesic family of Ustic Torri Orthents – Soil texture with geographic name is series.

Criticism and appreciation of Soil Taxonomy

a. Criticism

• The recently developed system of soil classification viz., soil taxonomy (1975), apparently departs from the genetic approach.

• The system does not have strong geographic bias towards the four order, viz., Entisol, Vertisol, Inceptisol and Histosol.

• The soils with a different genesis but having identical properties are classified within the same unit.

• There is no particular order for the strictly hydromorphic and saline –sodic soils, as is the case in other systems.

b. Appreciation

• It is the most elaborate system marked by great care and precision

• The primary basis for identifying different classes in the system is properties of soils

• as they exist in the field.

• The nomenclature (with Latin and Greek origin) gives a definite connotation of the major soil characteristics.

• The systems has in built mechanism to permit addition of new soil groups. For e.g. the new order Andisol and Gelisols have been included in the system recently.  

Other advantages of the system (Smith, 1963) are;

•  It permits classification of soils rather than soil – forming process

•  It focuses attention on the soil

•  It permits the classification of soils of unknown genesis.

• Salient characteristics and differentiating properties of different soil orders

In soil taxonomy, there are 12 soil orders including the newly proposed Gelisols. A basic system of soil classification for making and interpreting soil survey was published in 1975. The 1975 edition of soil taxonomy recognized 10 soil orders. The 1996 edition incorporated one more soil taxonomic order i.e, Andisols that included volcanic ash soils. In 1998, again taxonomy was revised to include a total of 12 soil orders. The 12th order covered soils that have a dark organic surface layer and mineral layers underlain by permafrost. These soils are commonly found in tundra regions and are termed as Gelisols in soil taxonomy.