After reading this article you will learn about the evolution of soil classification system.
Category # 1. Early Systems of Soil Classification:
Early systems of soils classification were very simple and practical.
1. Economic Classification:
One of the earliest systems of soil classification, adopted by the Revenue Department, for grouping soils according to their productivity was for purpose of taxation. The criteria used were soil colour and texture of the soil combined with irrigation potentialities. Such a system was of little importance and became obsolete when the land use changed.
2. Physical Classification:
It is one of the earliest systems and was based on soil textureâ€”a property associated with soil productivity. This system of soil classification was applicable to zonal soil category.
3. Chemical Classification:
The grouping of soils based on chemical composition has not been used to a great extent in practical purposes. Still soils were grouped as calcareous soils, acid soils and sodic soils etc. But these characteristics do not permit to classify all kinds of soils occurring in nature.
4. Geological Classification:
This system of soil classification was based on the presumed underlying parent material.
According to this system, two broad groups of soils were recognised:
(i) Residual or sedentary soils-developed in situ from the underlying rocks.
(ii) Transported soils-developed on unconsolidated sediments, like alluvium, colluvium or Aeolian materials.
The system of soil classification, however, does not applicable to processes of soil formation controlled by active factors of soils formation (climate and vegetation) which dominate over the influence of parent material and thereby upset the geological soil classification.
5. Physiographic Classification:
According to this system, the characteristics of the landscape were considered and so various geomorphic terms such as levee soils, basin soils, terrace soils, mountain soils, hilly soils, upland soils and lowland soils were introduced to classify soils. A classification based only on physiography may have limited value as two or more soil groups with different properties may be classified in one group.
6. Other Systems:
Several other systems of soil classification based on varying criteria were applied from time-to-time. As for example,
Based on Organic Matter as:
â€”Inorganic or mineral soils, and
Based on Soil structure as:
â€”Single-grained soils, and
Besides this, soil classification based on other parameters like, humidity, vegetation and temperature has also been made.
Category # 2. Recent Systems of Soil Classification:
In the later part of the 19th century and early part of 20th century Dokuchaiev (1900), Joffey (1912), Marbut (1935) classified soil on the basis of zonality, their own properties and morphology emphasizing the need for examination of actual soils for their characteristics like soil colour, texture, structure, consistency, drainage conditions etc. respectively. Marbut also emphasized the concept of pedalfers and pedocals.
The major defects of Marbut Morphogenetic soil classification was that the system was based, in part, on assumptions concerning soil genesis. Many of the soil series recognised in United States (U.S.) could not find place in Marbut systems of classification of soil.
Category # 3. Baldwin and Associate’s Genetic Approach:
Marbut’s morphogenetic soil classification system was revised and elaborated by Baldwin, Kellogg and Thorp (1938).
The important characteristics of the system were:
(i) A return to the zonality concept of Russian school.
(ii) The pedocalâ€”pedalfer concept was not given emphasis.
(iii) More emphasis was given on soil as a three dimensional body and its characteristics.
A new category like soil family was introduced between great soil group and soil Series but neither the Family nor the higher category was defined in relation to soil properties. Serious problems were found when some soil series did not fit in any of the existing great soil groups; whereas others could be placed equally well in two great soil groups.
Realising this problem, the soil classification of Baldwin et al. (1938) was notified by Thorp and Smith (1949) and presented in table 10.2.
Orders of Genetic Approach:
After modification, the soils can be grouped into three orders namely, zonal, intra-zonal and azonal based on the zonality concept developed by Russian soil scientist Dokuchaiev.
Zonal soils were groups of soils developed under similar climate conditions and distributed in a climatic belt. Soils in this group possess well developed profiles reflecting the influence of climate and vegetation. The only differences due to the parent material are rendered subordinate by dominating climate influences. Example, laterite soil, podzol soil, chernozem soil etc.
The soils occur within a zone, but reflect the influence of some local conditions, such as topography and/or the parent material. Under these conditions, the characteristics of soils imparted by the local conditions and dominant viz. excess of water, salt, calcium carbonate etc. Example, saline sodic and saline sodic soil etc.
The soils have poorly developed profiles because of time as a limiting factor. The group includes young soils without horizon differentiation so the soils could not be grouped in any one of these two orders (zonal and intra-zonal) and was placed in a separate order known as azonal soils. Example alluvial soils.
These three orders were further divided into nine (9) suborders on the basis of specific climatic and vegetative regions. Each of the sub-order, in turn, was divided into great soil groups having expressions of more specific conditions. The great soil groups were further sub-divided into numerous soil series and soil types.
Limitations in Genetic Systems of Soil Classification:
The major defects in the genetic system of soil classification are as follows:
(i) The two highest categories are defined in genetic terms and not on the basis of properties of soils themselves.
(ii) The definitions and concepts of the highest category (order) in relation to soil properties are not clear.
(iii) The concepts and definitions of great soil groups have been outlined on the basis of only environmental factors and not on the basis of soil properties.
(iv) The classification of arable soils under this system becomes ambiguous because of variation under virigin and cultivated soil conditions.
(v) There are possibilities to attempt definition of units in the lower categories in relation to very few soil properties seemed to be important for one interpretation. Where this was done, it was impossible to utilize those results for other interpretations.
(vi) Most emphasis has been given on soil colour or vegetation than that of soil properties under this system for making nomenclature of the highest categories.
(vii) In this system, it was difficult to name intergrades as the nomenclature was evolved from various languages mixing with nouns and adjectives.
The soil classification system should be based on combinations of soil properties known to be significant to formation and behaviour and the classification must be such that can be explained in relation to genesis and behaviour, but that genesis and behaviour should be one step behind the classification itself.
Category # 4. New Comprehensive System of Soil Classification (Soil Taxonomy):
In order to overcome different anomalies in earlier system of soil classification a new comprehensive system has been developed. Initially started in 1951, several approximations were made after taking critical suggestions from pedologists of different countries.
The 7th Approximation was published in 1960 with supplements in 1964 and 1967. Soil Taxonomy a Basic System of Soil Classification for Making and Interpreting Soil Surveys was published in 1975.
1. Advantages of New Comprehensive System of Soil Classification over the Earlier System:
The new comprehensive system has several advantages which are as follows:
(i) Unlike the genetic systems, the new comprehensive system is based on soil properties. An attempt has also been made to define all classes in terms of soils properties.
(ii) The new comprehensive system considers properties affecting soil genesis which is the backbone of this system.
(iii) The general definition of a class of a taxonomic system is type or ortho-type.
(iv) Most of the words used for nomenclature are derived from Greek and Latin which are most logical and it helps in relating the place of taxon in this system and in making its interpretations.
(v) In this new comprehensive system, the sub-group (new category) has been introduced for the expressions and recognition of soils that are a continuum with gradual change in many properties.
(vi) Unlike the genetic system, it is an orderly scheme without prejudices and facilities easy memorizing the objects.
Taxonomy is that part of classification that is concerned primarily with relationships and is the systematic distinguishing, orderly naming of type groups within a subject field. Thus, soil classification includes not only soil taxonomy, but also other information.
Names of soils orders in the new comprehensive soil classification (7th approximation), their derivation etc. are given in Table 10.3.
Note that all orders end in sol (Latin solum, soil).
Orders of Soil Taxonomy:
These twelve orders are differentiated by the presence or absence of diagnostic horizons.
Little horizonation because of youth or because of parent material that is very resistant to weathering. Soils of this order are found under a wide variety of climatic conditions. In India, soils on strong slopes,soils derived from Aeolian parent material as in the deserts of Rajasthan and recent alluvium have been placed in this group.
Little horizonation because of extreme argillipedoturbation. The central concept of vertisols is that of clayey soils that has deep wide cracks at some time of the year and has high bulk density between the cracks. Most of the black soils of Peninsular India have been placed in this order. Soils of this order are sticky and plastic.
Soils of this order have mainly diagnostic horizons that form rapidly (e.g. Cambic horizons). Rare in desert because of moisture regime restrictions. Horizons of marked accumulation of clay and iron and aluminium oxides are absent in this order. Soils of this order are generally found in South-western India and along the Ganges rivers.
These mineral soils are mostly found in dry climates. They have an ochricepipedon (Gk. epi. = over and pedon-soil) generally light in colour and low inorganic matter. They have a horizon of accumulation of calcium carbonate (Calcic), gypsum (Gypsic) or even more soluble salts (salic). Aridisols may be productive if irrigation water is available.
Mollisols are extensive and important agricultural soils. These are very dark coloured, base-rich soils. They have developed on lime-rich parent material in which there has been decomposition and accumulation of large amounts of organic matter.
The formation of mollisols is favoured by semi-arid to sub-humid climates, but Mollisols, particularly Udolls (Sub-order of Mollisol) are in more humid regions. The native vegetation is usually grasses. Mollisols have the mollicepipedon (Gk. epi = over; and pedon = soil, diagnostic surface horizon).
These soils have been designed in soil taxonomy to include those soils that were classified as podzols and groundwater Podzols of earlier classification systems. These are soils with accumulation of amorphous materials (humus and sesquioxides) in subsurface horizons.
They are characterized by a bleached, wood ash-coloured A2 horizon and an illuvial horizon of humus and free sesquioxide accumulation. Most spodosols are coarse textured low in phyllosilicate clay, and naturally quite acid in addition to having a spodic or placic horizon.
Alfisols are soils with grey to brown surface horizon, medium to high base supply, and subsurface horizons of clay accumulation (Argillic), usually moist but may be dry during warm season. An alfisol may also have fragipan, duripan, natric horizon, petrocalcic horizon, plinthite or other features, and these features are used in defining the various great groups within the order.
The soils have marks of processes that translocate silicate clays without excessive depletion of bases and without dominance of the processes that lead to the formation of a mollicepipedon. Some of the red and lateritic soils of India have been classified as Alfisols.
The ultisols are soils of mid to low latitudes that have an argillic or kandic horizon with a low base supply. They are characterized by low (<35%) base saturation, and accumulation of clay in the sub-soil. Base saturation in most ultisols decreases with depth because the vegetation has cycled the bases. Cultivation, therefore, is a shifting cultivation unless soil amendments are applied.
The ultisols are most extensive in warm-humid climates that have a seasonal deficit of precipitation. Because of the low-fertility and low base-status, these soils pose limitations for agricultural use. Generally these soils are under forests, but may produce good agricultural crops in case they are adequately limed and fertilized.
Oxisols are soils with oxic horizons or a few kandic horizons that meet the weatherable minerals requirements of oxic horizons. They are the most highly weathered soils. Oxic horizons have very low cation exchange capacity per unit of clay and are very low in 2: 1 layer (phyllo) silicate clays.
Oxisols are reddish, yellowish or greyish soils of tropical and subtropical regions. Oxisols are low in fertility. Weathering and intense leaching are responsible for the removal of a large part of silica from silicate minerals leaving behind a high proportion of the oxides of iron and aluminium. In India these soils are generally found in Kerala, Tamil Nadu, Karnataka and Orissa states.
Histosols are organic soilsâ€”composed mainly of high quantities of plant, but also sometimes of animal residues in various stages of decomposition. They are soils that are commonly called bogs, moors or peats and mucks. Most Histosols are saturated or nearly saturated with water most of the year unless they have been drained.
Histosols, saturated with water, contain at least 12 or 18 per cent organic carbon by weight and Histosols that are not saturated with water contain 20 per cent or more organic carbon by weight.
Soils of this order developed from volcanic eruption. Incomplete weathering of geological materials in found. However, the soils of the surface layer are dark in colour with low bulk density.
During weathering of volcanic materials, amorphous or poorly crystallized minerals like allophane, imogoliteand ferrihydrite are produced on the surface layer. It was earlier classified as sub-orders of the Inceptisols, with minimum profile development.
It is a recently introduced soil order which accommodate soils exhibiting the evidence of Cryoturbation (frost mixing or churning) through which this soil has developed. It is still not recognised in India, but may be found in the higher altitude of Himalayas like Jammu and Kashmir; and Sikkim with perma frost conditions. It has three sub-orders. Such soils are not cultivated due to climatic limitations.
All twelve (12) orders have been classified into different sub-orders primarily on the basis of morphological, chemical and physical properties the reflect either the presence or absence of waterlogging or genetic differences due to climate and its partially associated variable, vegetation. All sub-orders in respect of individual order have been given.
The names of sub-orders are formed by combining two syllables, the first syllable signifying the formative elements of the sub-order (Table 10.4) and the second syllable that of the formative elements of the order (Table 10.3) e.g. the sub-order Argids consists of two syllables, arg for argilla and id for Aridisol.
This connotes that the soil in the sub-order (Argids) has the presence of an argillic horizon at some depth of the pedon.
2. Great Groups:
In the Great-Group, the basis of subdivisions is the consideration of the whole soil, the assemblage of horizons, and the most significant properties of the whole soil, selected on the basis of the number and importance of accessory properties.
Differentiate in the great group category place soils together that have in common the following properties:
(i) Close similarities in kind, arrangement and degree of expression of horizons;
(ii) Close similarities in soil temperature and moisture regimes;
(iii) Close similarities in base status. The name of a great group consists of the name of a sub-order and a prefix that consists of one or two formative elements suggesting something of the diagnostic properties. Names of the great groups may, therefore, have three or more syllables.
These are sub-divisions of Great Groups.
Three kinds of sub-groups have been recognized viz.
(i) Typic group;
(ii) Integrades and
The typic sub-group is thought to typify the Great Group and does not show any transition to another sub-group e.g. TypicNatrargids.
Intergrade-sub groups are those that belong to one Great Group, but have some properties of another Order, Sub-order, or Great Group, e.g. Ustochrepts.
Extragrade subgroups have important properties that are not representative of the Great Group, but that do not indicate transitions to any other known kind of soil, e.g. PergellicCryorhent (a permanently frozen layer).
The family category of soil classification is the least well-defined. However, properties important to the growth of plants are used to differentiate families. This category permits the grouping of members of sub-groups having common similar properties such as particle size distribution, mineralogy, temperature regime, pH and thickness of the soil permeable by roots.
Soil series represents the properties of a contiguous pedon (smallest volume of soil material) in the recognised polypedon (many pedons). It is the most specific category. The soils of any one series have similar profile characteristics.
In more precise terms, a series is a group of soils developed from the same kind of parent material by the same genetic combination of processes and whose horizons are quite similar in their arrangement and general characteristics. The series are named after the geographic name of the place where it was first recognised or where they have wide extent of distribution, e.g. Jodhan loam.
6. Soil Phase:
A phase is a sub-division on the basis of some important deviation such as surface texture, erosion, slope, stoniness, or soluble salt content.
7. Soil Associations and Catenas:
In the field, soils or different kinds are commonly found together. Such an association of soils may consist of a combination from different soil orders. Soil associations are important in a practical way since they help to determine combinations of land-use patterns which must be utilized to support a profitable agriculture.
Well-drained, imperfectly drained, and poorly drained soils, all of which have developed from the same parent materials under the same climatic conditions, are often found closely associated under field conditions. This association on the basis of drainage or of differences in relief is known as a Catena and is very helpful in practical classification of soils in a given region.
The concept of soil catena has been developed by Milne to indicate a regular repetition of a sequence of soil properties in association with certain topography; the parent material may or may not be the same.
8. Epipedons or Diagnostic Surface Horizons:
Among the most significant of soil properties used as a basis for classification is the presence or absence of certain diagnostic soil horizons. Epipedons (Gk. epi = over; and pedon = soil) are defined in soil Taxonomy as diagnostic surface horizons of mineral soils. The epipedon includes the upper part of the soil darkened by organic matter, the upper eluvial horizons or both.
It may include part of the B horizon if the latter is significantly darkened by organic matter. Six epipedons are recognised i.e. Mollicepipedon, Umbricepipedon, Anthropic epipedon, Histicepipedon, Plaggenepipedon and Ochric-epipedon.
Besides these surface horizons, there are various diagnostic sub-surface horizons that have been recognized in soil taxonomy. Major features of diagnostic horizons (surface and sub-surface) are given in Table 10.5.
Duripan (L. durus = hard, plus pan = hard pan): A sub-surface horizon that is cemented mostly by silica. Although carbonates may be present, duripans would not slake in water nor in 18 per cent hydrochloric acid (HCl) but will disintegrade in hot concentrated potassium hydroxide (KOH) solution or alternating acidic and basic solutions, both of which dissolve silica.
Fragipan (modified from L. fragilus = brittle, and pan = brittle pan) : A natural subsurface horizon with high bulk density relative to A and B horizons (the solum) above, seemingly cemented when dry but showing a moderate to weak brittleness when moist. It has a very low content of organic matter.
It is mottled, slowly or very slowly permeable to water, and usually shows occasional or frequent bleached cracks, forming polygons. Base saturation and pH of soil are normally low, at least in the upper part.
Plinthite (Gk. plinthos = brick): Plinthite is an iron-rich, humus-poor mixture of clay with quartz and other diluents. It commonly occurs as dark red mottles, which usually are in platy, polygonal or reticulate patterns. Plinthite changes irreversibly to an ironstone hardpan or to irregular aggregates on exposure to repeated wetting and drying, especially if it is exposed also to heat from the sun.
Petro Calcic Horizon:
An indurated (hardened) sub-surface horizon cemented by carbonates and not penetrable by spade or auger.
Petro Gypsic Horizon:
A surface or sub-surface horizon that is cemented so strongly by gypsum those dry fragments will not slake in water. Cementation restricts the penetration of roots.
A subsurface horizon cemented by iron, iron and manganese, or by iron and organic matter. Forms most readily in both humid tropics and humid cold regions.