Classification of Soils
Soil is a valuable resource of India. Much of the Indian agriculture depends upon the extent and qualities of soil. Weathering prepares loose materials on the surface of the earth and mixed with decayed organic matters it forms soil. The
nature of soil in a place is largely influenced by such factors as climate, natural vegetation and rocks. There are various types of soil found in India.
In olden times, soil was classified on the basis of its fertility. The soil was either 'Urvara' i.e. fertile or 'Usara' meaning non-fertile or sterile; but in this modern day various characteristics are taken into consideration
and soil is classified on the basis of its texture, colour, moisture content, etc. In the year 1956, Soil survey of India, an institution was established by The Government of India to study soil and its characteristics.
North Indian Soil: The Northern India plains are mostly formed of deep alluvial soil. The topsoil varies in texture from sand to clay, the greater part being light loam, porous in texture, easily worked and naturally fertile. The great depth of the alluvium keeps down the soil temperature. This soil is supposed to be naturally very rich in the plant-nourishing food, and is consequently very good for our Rabi and Kharif crops. However, the most important advantage from the level character of the plains is that they facilitate a more even distribution of rainfall in Northern India, having no barriers to check the flow of the monsoon currents.
Soils of South India: The Southern India peninsular earth surface is made up of hills and river valleys. Hilly tracts are naturally unsuitable for cultivation. Some highlands are very hot. The river valleys, however, possess important attributes that make them very suitable for agriculture. The black cotton areas are included in them. In the rains, some of these tracts become sticky, in the dry weather hard and crumby, holding the moisture at lower levels. Thicker, dark-colored, and more fertile valleys are rich in chemical properties favorable for plant life.
Soil Classification Systems
In general, soils may be classified by the following classification systems:
- Particle size classification
- Textural classification
- Unified Soil Classification
- Indian Soil Classification
- Indian Standard Classification
Indian Standard Soil Classification
As per Indian Standard Soil Classification the soil is divided into six groups depending on the particle size and the groups are further divided into coarse, medium and fine sub-groups. These are boulders, cobble, gravel, sand, silt and clay. These are explained in the below figures.
The Indian Council of Agricultural Research (ICAR) divided the Indian soils into eight major groups. These are classified on the basis of genesis, colour, composition and location. These eight groups are: Alluvial Soils, Black Soils, Red Soils, Laterite Soils, Forest and Mountain Soils, Arid and Desert Soils, Saline and Alkaline Soils, Peaty and Marshy Soils.
These are formed by the deposition of sediments by rivers. Materials deposited by rivers, winds, glaciers and sea waves are called alluvium and soils made up of alluvium are alluvial soils. They are found in Great Northern plain, lower valleys of Narmada and Tapti and Northern Gujarat. Through a narrow corridor in Rajasthan, they extend into the plains of Gujarat. In the Peninsular region, they are found in deltas of the east coast and in the river valleys.
These soils are renewed every year. In the Upper and Middle Ganga plain, two different types of alluvial soils have developed, viz. Khadar and Bhangar. Khadar is the new alluvium. It is deposited by floods annually, which enriches the soil by depositing fine silts. These are lighter in color and occur in the deltas and the flood plains. Bhangar is the old alluviums. These are deposited away from the flood plains and are clayey and sticky, have a darker color, contain nodules of lime concretions and are found to lie on slightly elevated lands. Both the Khadar and Bhangar soils contain calcareous concretions (Kankars). These soils are more loamy and clayey in the lower and middle Ganga plain and the Brahamaputra valley. The sand content decreases from the west to east
Alluvial soils are by far the largest and most important soil group of India. They are very fertile. Covering about 45.6 per cent of the total land area of the country, these soils contribute the largest share of our agricultural wealth and support the bulk of India’s population.
They are rich in humus and potash but poor in phosphorus. The colour of the alluvial soils varies from the light grey to ash grey. Its shades depend on the depth of the deposition, the texture of the materials, and the time taken
for attaining maturity.
The black soils are also called regur (from the Telugu word Reguda) and black cotton soils because cotton is the most important crop grown on these soils. Most pedologist believe that these soils have been formed due to the solidification of lava spread over large areas during volcanic activity in the Deccan Plateau, thousands of years ago.
Geographically, black soils are spread over 16.6 per cent of the total geographical area of the country. It covers most of the Deccan Plateau which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu. In the upper reaches of the Godavari and the Krishna, and the north western part of the Deccan Plateau, the black soil is very deep.
The black colour of these soils has been attributed by some scientists to the presence of a small proportion of titaniferous magnetite or even to iron and black constituents of the parent rock.
The black soils are generally clayey, deep and impermeable. They swell and become sticky when wet and shrink when dried. So, during the dry season, these soil develop wide cracks leading to a kind of ‘self ploughing’. Due to the character of slow absorption and loss of moisture, the black soil retains the moisture for a very long time, which helps the crops, especially, the rain fed ones, to sustain even during the dry season.
Chemically, the black soils are rich in lime,
iron, magnesia and alumina. They also contain potash. But they lack in phosphorous,
nitrogen and organic matter. The colour of the soil ranges from deep black to grey.
Red and Yellow Soil
Red Soil is a comprehensive term which comprises several minor types. The red soils occupy a vast area of about 10.6 per cent of the total geographical area of the country.
Most of the red soils have come into existence due to weathering of ancient crystalline and metamorphic rocks. It developed on crystalline igneous rocks in areas of low rainfall in the eastern and southern part of the Deccan Plateau. Red colour indicates good drainage.
The colour of these soils is generally red, often grading into brown, chocolate, yellow, grey or even black. The soil develops a reddish colour due to a wide diffusion of iron in crystalline and metamorphic rocks. Iron found within the soil is oxidised more readily due to the higher oxygen content.
Yellow soil is nothing but a red soil in a hydrated form.
The word ‘laterite’ (from Latin letter meaning brick) was first applied by Buchanan in 1810 to a clayey rock, hardening on exposure, observed in Malabar. Laterite is a kind of clayey rock or soil formed under high temperature and high rainfall. These are the result of intense leaching due to tropical rains.
With rain, lime
and silica are leached away, and soils rich in
iron oxide and aluminium compound are left behind. Humus content of the soil is removed fast by bacteria that thrives well in high temperature. These soils are poor in organic
matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess.
Hence, laterites are not suitable for cultivation these soils are generally infertile; however, application of manures and fertilisers
are required for making the soils fertile for
cultivation. These soils are more suitable for tree
crops like cashew nut. These soils are cut as bricks for
use in construction.
Laterite soils have
mainly developed in the higher areas of the
Peninsular plateau. They are found in South Maharashtra, the
Western Ghats in Kerala and Karnataka, at places on the Eastern Ghat, Tamil Nadu, Karnataka, and in western West Bengal (particularly in Birbhum district). They are also found in hilly areas of
Odisha and Assam.
Arid and Desert Soils
The soils of Rajasthan, Haryana and the South Punjab are sandy. In the absence of
sufficient wash by rain water soils have become saline and rather unfit for cultivation. In some areas, the salt content
is so high that common salt is obtained by
evaporating the saline water.
Due to the dry climate, high temperature and accelerated evaporation, they lack moisture and humus. Nitrogen is insufficient and the phosphate content is normal. Some of these soils contain high percentages of soluble salts, are alkaline with varying degree of calcium carbonate and are poor in organic matter.
Over large parts, the calcium content increases downwards leading to the formation of ‘kankar’ layers at the lower horizons of the soil. The
‘kankar’ layer restricts the infiltration of water. But when irrigation is made available, the soil
moisture is readily available for a sustainable
plant growth. So, cultivation can be carried on with the help of modern irrigation.
Saline and Alkaline Soils
These soils are liable to saline and alkaline efflorescence and are known by different names such as reh, kallar, usar, thur, rakar, karl and chopan.
structure ranges from sandy to loamy. They
lack in nitrogen and calcium. They contain a larger proportion of sodium, potassium, magnesium, calcium salts and sulphurous acid which are liberated by the undecomposed rock and mineral fragments due to weathering. The accumulation of these salts makes the soil infertile and renders it unfit for agriculture.
They have more salts, largely because
of dry climate and poor drainage. Hence, they occur
in arid and semi-arid regions, and in
waterlogged and swampy areas. They are widespread in western Gujarat, deltas of
the eastern coast and in Sunderban areas of
West Bengal. In the Rann of Kuchchh, the
Southwest Monsoon brings salt particles and
deposits there as a crust. Seawater intrusions
in the deltas promote the occurrence of saline
In the areas of intensive cultivation with
excessive use of irrigation, especially in areas
of green revolution, the fertile alluvial soils are
becoming saline. Excessive irrigation with dry
climatic conditions promotes capillary action,
which results in the deposition of salt on the
top layer of the soil. In such areas, especially
in Punjab and Haryana, farmers are advised
to add gypsum to solve the problem of salinity
in the soil.
Peaty and Marshy Soils
They originate in humid regions due to accumulation of organic matter in the soils. This gives a rich humus and organic content to the soil. The peaty soils are black, heavy and highly acidic. Most of the peaty soils are under water during the rainy season but as soon the rains cease, they are put under paddy cultivation. At many places, they are alkaline also.
These soils contain considerable amount of soluble salts
and 10-40 percent of organic matter.
widely in the northern part of Bihar, southern
part of Uttarakhand and the coastal areas of
West Bengal, Odisha and Tamil Nadu
Forest and Mountain Soils
These soils are mainly found on the hill slopes covered by forests. The formation of these soils is mainly governed by the characteristic deposition of organic matter derived from forest growth. These soils are heterogeneous in nature and their characteristics like structure and texture changes with parent rocks, ground-configuration and climate.
They are loamy and silty on
valley sides and coarse-grained in the upper
slopes. In the Himalayan region, such soils are mainly
found in valley basins, depressions, and less steeply inclined slopes. Generally, it is the
north facing slopes which support soil cover; the southern slopes being too precipitous and exposed
to denudation to be covered with soil. Here, they experience denudation, and
are acidic with low humus content. The soils
found in the lower valleys are fertile.
The forest soils are generally very rich in humus but are deficient in
potash, phosphorus and lime.
These soils occupy about 8.67 percent of the total land area of India. This soil is distributed around Karnataka, Tamil Nadu and Kerala. They support wheat, maize, barley and temperate fruits in Jammu and Kashmir, Himachal Pradesh and Uttarakhand.
It is evident from the foregoing discussions
that soils, their texture, quality and nature are
vital for the germination and growth of plant
and vegetation including crops. Soils are living
systems. Like any other organism, they too
develop and decay, get degraded, respond to
proper treatment if administered in time. These
have serious repercussions on other
components of the system of which they
themselves are important parts.