Soil classification

SOIL CLASSIFICATION – PRINCIPLES AND CONCEPTS, EARLY AND MODERN

SYSTEMS, USDA AND ITS ADVANTAGES

Soils are formed by the assorted combinations of different soil forming factors and processes. Soil is a heterogenous group with multitude of characteristics. To understand differences, similarities and relationships among the different members, it is necessary that these are grouped in some orderly manner. The study of classification of particular objects, also called as categorization is a basic topic in soil inventory.

Soil classification is grouping of objects in orderly and logical manner into classes based on the properties of soils (differentiating characters) for the purpose of studying and identifying them. The soil individuals are grouped into classes of the lower category (soil series) which are grouped into classes of higher categories (soil orders).The lower categories are defined by large number of differentiating characters and higher category by a few differentiating characteristics. This kind of grouping is called a multicategoric or hierarchial system of classification. In India about 550 soil series have been established and in USA, there are more than 10,000 established soil series.

Class / Taxon

Group of individuals which are similar in one or more properties. It is tested by simple statistical methods namely coefficient of variation, frequency and distribution.

BASIC PRINCIPLES (PURPOSE) OF SOIL CLASSIFICATION

1. To organize our knowledge in such a way that it contributes to economy of

thought

2. To remember properties of the objects classified

3. To brings out and understand relationship among individuals and classes of

the population being classified.

4. To learn new relationships and principles in the population being classified

5. To establish groups or subdivisions of the objects under study in a manner

useful for practical and applied purpose in

• Predicting their behavior

• Identifying their best uses

• Estimating their productivity

• Providing objects for research and

• Extrapolating the research findings to other areas

EARLY SYSTEMS OF SOIL CLASSIFICATION

The early systems of soil classification were simple, practical and their aimwas only utilitarian.

(i) Economic classification

It is the grouping of soils based on their productivity for the purpose of taxation, adopted by the Revenue Department. The criteria used were soil colour and texture in combination with the potentialities for irrigation. Eg.. Soils suited for rice, groundnut etc., The system was of little importance and turned obsolete when the land use changed.

(ii) Physical classification

It is grouping of soils based on their texture – a property closely associated with soil productivity and management. Eg.Loamy soil, sandy soil and clayey soil.

(iii) Chemical classification   

The grouping of soils by their chemical composition has not been used to a great extent as the data on soil analysis cannot be classified according to any definite pattern. Still soils were grouped as calcareous soils, gypsiferous soils, acid soils, alkaline soils etc. These characteristics do not permit to classify all kinds of soils occurring in nature.

(iv) Geological classification

In the 18th and 19th centuries, geologists and geographers recognized close relationship between soil and its parent rock and proposed classification system based on the presumed underlying parent material. According to this system, two broad groups of soils are recognized.

Residual or sedentary soils : The soils developed in-situ from the underlying rock, such as granite soils, soils from sandstone, limestone, basalt etc.,

Transported soils: The soils developed on unconsolidated sediments such as alluvium colluvium or Aeolian.

The system failed to recognize the influence of active soil forming factors like climate and vegetation on the parent materials. Eg. Granite rocks in the temperate region results in podzols whereas in tropical areas results in laterite soils.

Physiographic classification

According to this system, the characteristics of the landscape were considered and geomorphic terms, such as levee soils, basin soils, terrace soils, mountain soils, hilly soils were introduced to classify soils. Yet, this classification has limited value as two or more soil groups with different properties are classified in one group.

Other systems

Based on organic matter content as

1.  Inorganic or mineral soils and

2.  Organic soils

Based on soil structure

1.  Single –grained structure

2.  Aggregated soils

Based on moisture or humidity, vegetation and temperature – Arid soils, humid soils, sub humid soils, grassland soils, forest soils etc.,

RECENT SYSTEMS OF SOIL CLASSIFICATION

In the later part of the 19th century, Dokuchaiev a Russian scientist, the founder of modern pedology, established the concept of soil as an independent natural body and that resulted in a series of publications on soil genesis and classification. Therefore, the Russian approach to soil classification is naturally tended to emphasise on soil genesis and hence the term “Genetic system” of soil classification.

(i) Dokuchaev’s Genetic System

Dockuchaev, the Founder of Modern Pedology and the Russian Scientist (1900) divided soil into three categories as normal, transitional and abnormal soils based on soil genesis. These were later categorized as zonal, intrazonal and azonal soils.

Zonal soils

The zonal soils are those with fully developed soil profiles which are in equilibrium with the environmental conditions such as climate and vegetation e.g. Podzol and Laterites. Boreal – Tundra soils, Taiga – Light-grey podzolised soils, Steppe - Chernozem

Azonal soils

The soils where time has been limiting factor to produce horizonation are termed as azonal soils e.g. most of the recent alluvial deposits or dunes or eroded area.

Intrazonal soils

The soils occurring within zonal areas and having characteristics that are determined by the local conditions, like topography, specific parent material are termed as Intrazonal soils e.g. calcimorphic soils and hydromorphic soils. Although the Russian approach was based on sound principles of soil genesis, yet the approach had some inherent weakness of having undue emphasis on climate and vegetation rather than on the properties of soils. Such formations were reflected in the nomenclature used to describe them. E.g. alluvial soils and Aeolian soils. These attempts however, failed to recognize the dynamic processes of soil genesis, controlled by climate and vegetation the two active factors of soil formation-which overcome the influence of parent material with the passage of time and hence upset the geological system of classification. For instance, a granite in Himachal Pradesh and in Tamil Nadu as acted upon by different climate and vegetation, produces two different soils, that is podzolic and lateritic soil, respectively. This is called Ectodynomorphic soils (soils develop under the influence of climate and vegetation). There are soils wherein the composition of parent material subdues the effects of climate and vegetation, such soils are called endodynamorphic soils. E.g. Basalt in Madhya Pradesh and in Kerala gives rice to black cotton soils and laterite soils respectively.

Coffey’s system

Coffey (1912) emphasized that soils, as independent natural bodies, should be classified on the basis of their own properties. He proposed five major classes of soils under.

•  Arid soils

•  Dark coloured prairie soils

•  Light coloured timbered soils

•  Black swamp soils

•  Organic soils

Each class was subdivided into series on the basis of parent material and the series into types on the basis of surface soil texture.

Marbut’s morphogenetic system

Marbut was the central figure in the evolution of soil taxonomy in USA. He was the first to advocate classification of soils on the basis of their own properties rather than on the basis of soil forming factors. He reduced emphasis on geology or parent rock. Marbut evolved his scheme of soil classification in successive steps and published in the Atlas of American Agriculture in 1935. His scheme of soil classification was based on the iron-alumina and lime content. At the highest categoric level, he divided zonal soils into two classes pedalfers and pedocals.

•  Pedalfers – Accumulation of iron and aluminium oxides, occuring in areas of high rainfall having surplus water for leaching.

•  Pedocals – Accumulation of calcium or calcium carbonate, occurring in areas of high evaporation having water deficit.

Marbut emphazied that soil classification should be based on soil morphology and stressed the need for examination of actual soils for their characteristics, like soil colour, texture, structure, consistency, thickness and arrangement of horizons and kind of organic matter. The major limitation of this system was that, it was based, in part, on assumptions concerning soil genesis. As such, many of the soil series, recognized in USA, could not find a place in the Marbut’s system.

Baldwin and associates genetic approach

The morphogenetic system of Marbut was revised and elaborated by Baldwin, Kellogg and Thorp in 1938. The system marked the start of truly comprehensive approach. The salient features of the system are:

1. A return to the zonality concept of Russian school

2. The pedocal - pedalfer concept was deemphasized

3. More emphasis was laid on soil as a three dimensional body and its characteristics

A new category viz., soil family, was introduced between great soil group and soil series, but neither the soil families nor the higher categories were defined in terms of soil properties.

Serious problems arose when soil series did not fit in any of the existing great soil groups. As a stop-gap strategy, Thorp and Smith (1949) revised the system. According to the revised system, the soils were grouped in 3 orders, viz., zonal, intrazonal and azonal following the Russians zonality concept.

Order                       Suborder                                          Great soil groups

Zonal soils         1. Soils of the cold zone                                  Tundra soils

                           2. Light coloured soils of arid regions            Sierozem soils, Desert                                                                                                               soils 

                          3. Dark coloured soils of semi arid,                 Prairie soils, Chestnut soils                                                            sub humid and humid grasslands

                         4. Soils of the forest grassland transition          Degraded Chernozem                                                                                      soils

                         5. Light coloured podzolised soils                       Podzolic soils

                         6.Lateritic soils of warm-temperate and topical regions Laterite soils

Intrazonal soils

1. Halomorphic soils       Solonchak or saline soils

2.Hydromorphic soils           Bog soils

3.Calcimorphic soils         Rendzina soils

Azonal soils

No suborder        Alluvial soils, Regosols

Limitations in the genetic systems of soil classification

1. The two highest categories are defined in genetic terms and not on the basis of properties of soils

2. The definitions and concepts of the highest category viz., order, interms of soil properties is not clear

3. The great soil group concepts and definitions are based on environmental factors, rather than on the soil properties; their definitions are comparative and qualitative and as such it is difficult to obtain agreement among different workers.

4. The properties of some soils were obvious under virgin soil conditions and were destroyed during cultivation and hence the classification of such arable soils became ambiguous.

5. There are tendencies to attempt definition of units in the lower categories in terms of too few properties thought to be important for one interpretation.

6. Nomenclature in the higher categories laid emphasis on colour or vegetation rather than on the salient properties of the soils.

7. Nomenclature was evolved from several languages and it was difficult to name the intergrades According to Kellogg, most systems of soil classification, as used is 1950’s including that of the USA had one or more serious limitations as most of these over emphasized the virgin soils. The conclusions is that different schools of thought, inevitably, need some compromise on their divergent opinions to meet our greatest need to classify arable soils.

In order to overcome the short comings and to achieve the above objectives the US soil survey staff in cooperation with many other institutions, under the leadership of Guy D. Smith, have been working since 1951. Each year, starting from 1953, an approximation was produced and tested. In 1960, a comprehensive system of soil classification popularly known as the “7” Approximation” (Soil Survey Staff 1960), was published. A supplement to 7th approximation appeared in 1964 and another in 1967. Ultimately in 1975, the system brought out as Soil Taxonomy (Soil survey staff, 1975).

MODERN SYSTEM OF SOIL CLASSIFICATION OR SOIL TAXONOMY (SEVENTH

APPROXIMATION)

To overcome the shortcomings of the earlier systems of soil classification, the U.S. Soil Survey Staff under the leadership of Guy D.Smith has developed a Comprehensive System of Soil Classification. Initially started in 1951, several approximations were made and a comprehensive system of soil classification, popularly called the 7th approximation was published in 1960 with supplements in 1964 and 1967. In 1975, the system was brought out as soil taxonomy (Soil Survey Staff, 1975) and the 2nd edition was published during 1994 and the same is being followed till today.

Soil Taxonomy

It is the basic system of soil classification for interpreting soil survey. It is a multi categorical system based on the concept of real bodies of soil.

Salient Features of Soil Taxonomy

•  Unlike the Genetic Systems, the Comprehensive System is based on measurable soil properties. Efforts have been made to define all classes in terms of soil properties that exist today

•  It considers all such properties which affect soil genesis or are the outcome of soil genesis. Soil genesis forms the backbone of the comprehensive system. But it does not appear in the definition of the taxa.

•  The common definition of a class of taxonomic system is type or orthotype.

•  The nomenclature used in coining words is derived from Greek and Latin languages which are the most logical system.

•  A new category i.e., sub group has been introduced to define the central concepts of great groups and their intergrades

•  Unlike the Genetic System, it is an orderly scheme without prejudices and facilitates easy remembering of the objects.

Nomenclature

The basic principles followed in coining names, according to Heller (1963) are that the name 

• Should be most easily remembered

•  Should suggest some properties of the object

•  Should suggest the place of a taxon in the system

•  Should be as short as possible

•  Should be as euphonic as possible and

•  Should fit readily in as many languages as possible.

Criticism of Soil Taxonomy

•  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 orders viz., Entisol, Vertisol, Inceptisol and Histosol

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

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

Appreciation of Soil Taxonomy

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

•  The primary basis for identifying different classes in the system are properties of soils as they exist in the field

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

•  The system has in-built mechanism to permit addition of new soil groups. E.g. the new orders Andisols and Gellisols have been included in the system recently

•  It permits classification of soils rather than soil forming process

•  It permits the classification of soils of unknown genesis.

Applications of Soil Taxonomy

The main aim of soil classification is to provide information that helps to serve the cause of soil survey for correlating soils and making predictions by assessing the potential and constraints for making interpretations and agro technology transfer The greatest advantage of Soil Taxonomy is it forms the basis for soil correlation and for separating the mapping units in a soil survey and mapping project. Earlier transfer of agro technology was made by conducting field experiments .With advancement in technology and grouping of soils, transfer of technology is analogous. The soils classified into the same groups have a common response to management practices; the basic assumption is that the properties of the soil are similar. Soil Taxonomy also helps to bring together soil and crop experts to provide basic information relating to crop requirement and soil management and to assess the suitability of soil for various uses.