Classification of Soils : Geography

The classification of soils in India is based on the factors causing their formation, colour, thickness, texture, age, chemical and physical properties.

Soil development is an undertaking involving natural forces – weathering and erosion by running water (rivers), waves (seashore), ice (glacier), wind etc., the changes in these forces render soils of different types in all parts of India.

1. Alluvial Soils

The entire northern plains are made of alluvial soil.

The Alluvial Soil is deposited by 3 important Himalayan river systems — the Indus, the Ganga and the Brahmaputra.

It is also found in Rajasthan, Gujarat and eastern coastal plains particularly in the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri rivers.

The alluvial soil consists of various proportions of sand, silt and clay. As we move inlands towards the rlver valleys, soil particles appear to be bigger in size whereas in the upper side of the river valley, the soils are coarse.

Based on age, Alluvial soils can be classified as:

• Old Alluvial (Bangar): The Bangar soil has a higher concentration of kanker nodules than the Khadar.
• New Alluvial (Khadar): It has more fine particles and is more fertile than the Bangar.

Alluvial soils are very fertile. These soils contain an adequate proportion of potash, phosphoric acid and lime, which are ideal for the growth of sugarcane, paddy, wheat and other cereal and pulse crops.

2. Black Soil

The soils developed on schists and gneisses and are moderately shallow (50-75 cm) to moderately deep (75-100 cm) where as those developed on basalt are deep (100-150 cm) to very deep (>150 cm). These soils are highly argillaceous with clay content varying from 30-80%.

This soil is black in colour and is also known as regur soil. Climatic conditions along with the parent rock material are the important factors for the formation of black soil. The soil is ideal for growing cotton and is also known as black cotton soil.

This type of soil is typical of the Deccan trap (Basalt) region spread over northwest Deccan plateau and is made up of lava flows.

The soil covers the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh and extends in the south-east direction along the Godavari and the Krishna valleys.

The black soils are made up of extremely fine i.e. clayey material and well-known for their capacity to hold moisture. Black soil is nutrients rich and contains calcium carbonate, magnesium, potash and lime.

The soil is sticky when wet and difficult to work on unless tilled immediately after the first shower or during the pre-monsoon period.

3. Red and Yellow Soil

This type of soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the Deccan plateau.

These soils develop a reddish colour due to diffusion of iron in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form.

Found in pans of Odisha, Chhattisgarh, southern parts of the middle Ganga plain and along the piedmont zone of the Western Ghats.

Those soils are encountered over extensive nonalluvial tracts of peninsular India and are made up of such acidic rocks as granite, gneiss, and schist.

They develop in areas in which rainfall leaches soluble minerals out of the ground and results in a loss of chemically basic constituents; a corresponding proportional increase in oxidized iron imparts a reddish hue to many such soils.

4. Laterite Soil

The laterite soil develops under tropical and subtropical climate with the alternate wet and dry season. This soil is the result of intense leaching due to heavy rain.

Lateritic soils are acidic (pH<6.0) in nature and generally deficient in plant nutrients. This type of soil is found mostly in Southern states, Western Ghats region of Maharashtra, Odisha, some parts of West Bengal and North-east regions.

The soil supports deciduous and evergreen forests but humus poor. This soil is very useful for growing tea and coffee.

Laterite has commonly been referred to as a soil type as well as being a rock type. This and further variation in the modes of conceptualizing about laterite (e.g. also as a complete weathering profile or theory about weathering) has led to calls for the term to be abandoned altogether.

At least a few researchers specializing in regolith development have considered that hopeless confusion has evolved around the name. Material that looks highly similar to the Indian laterite occurs abundantly worldwide.

5. Arid Soils

Arid soils range from red to brown in colour.

This soil is generally sandy in texture and saline in nature. In some areas, the salt content is very high and common salt is obtained by evaporating the water. Arid soil lacks humus and moisture.

The lower horizons of the soil are occupied by Kankar because of the increasing calcium content downwards. The Kankar layer formations in the bottom horizons restrict the infiltration of water.

The three main characteristics of arid soil of India are

• Arid soils are the soils of desert or semi-desert regions and colours varies from red to brown.
  • The soil has sandy texture and salinity. As precipitation is very low the temperature is high and evaporation is faster making it lack in moisture and humus.
  • The calcium content increase downward and forms kankar layers downward and sometimes the kankar layer in the bottom layers restrict the infiltration of water. Sometimes, after irrigation its productivity is increased and it becomes suitable for cultivation.

6. Forest Soils

These soils are found in the hilly and mountainous areas. The soil texture is loamy and silty in valley sides and coarse grained in the upper slopes.

In the snow covered areas of Himalayas, these soils experience denudation and are acidic with low humus content.

The soil is fertile on the river terraces and alluvial fans. The map below shows the different types of soils found in India.

Forest soil variability is enormous, even within a single monitoring plot. A soil profile, consisting of different layers and genetic horizons, is the result of a developing process, which continuously evolves and never stops.