Soils: Formation, Classification

Soils: Formation, Classification

Soil is a mixture of rock debris and organic materials that develop on the surface of the Earth. Mineral particles, humus, water, and air are components of Soil. The soil on the surface of Earth has evolved over a thousand years.

Soil consists of mineral particles, humus, water, and air. The amount of these components varies and depends on the type of soil.

Factors affecting soil formation:

The major factors affecting soil formation are:

• Relief
• Vegetation and other life forms (Organic materials)
• Parent material
• Time
• Climate
• Human activities also influence the formation of soil to a large extent.

Soil Profile

Horizons: The layer of soil is called Horizons.

The arrangement of these layers is known as the soil profile. The three layers of soil are:

1. Horizon – A: It is the upper zone where organic materials have been incorporated with the mineral matter, nutrients, and water that are necessary for the plant’s growth.
2. Horizon – B: It is the transition zone between “Horizon A and Horizon C”.
3. Horizon – C: It is composed of the loose parent material. Horizon C is the first stage in the soil formation process. It led to the formation of “Horizon A” and “Horizon B”.

• Parent rock or the bedrock is fount bellows these three Horizons.

Classification of soils

During ancient times, soils were classified on the basis of their fertility i.e. Fertile (Urvara) and sterile (Usara)

Soils were classified on the basis of their inherent characteristics and external features like texture, color, the slope of the land, etc. in the 16th  Century AD.

Classification of soils on the basis of texture:

• Sandy: This type of soil consists of small particles of weathered rock. Sandy soil is formed by the breakdown of rocks like granite, limestone, and Quartz. It is one of the poorest types of soil. In this type of soil nutrient content for plants is very low because of less observation of water. for drainage sandy soil is good.
• Silty: Its particles are smaller than sandy soils, made up of rock and other mineral particles. It holds water better than sandy soil. Silty soil is more fertile than other types of soil. It improves soil fertility, so used in agriculture.
• Clayey: Clay-type soil has good water storage as its particles are very tightly packed together. Clayey soil is the densest and heaviest type of soil. Its drainage is not good.
• Loamy: This type of soil is a combination of sand, silt, and clay, so it is very beneficial and has similar qualities. It retains moisture and nutrients, so it is suitable for farming. because of its inorganic origin it also possesses higher calcium and pH levels

Classification of soil on the basis of color:

• Red
• Yellow
• Black

Taxonomy of soil on the basis of nature and character as per the United States Department of Agriculture (USDA):

Order	Percentage
Inceptisols	39.74
Entisols	28.08
Alfisols	13.55
Vertisols	8.52
Aridisols	4.28
Ultisols	2.51
Mollisols	0.40
Others	2.92

(Source Soils of India, National Bureau of soil survey and land use planning, Publication Number 94)

Classification of soils of India on the basis of genesis, color, composition, and location:

• Alluvial soils
• Black soils
• Red and yellow soils
• Laterite soils
• Arid soils
• saline soils
• Saline soils
• Peaty Soils
• Forest soils.

Alluvial Soils

Alluvial soils are widespread in the Northern plains and river valleys. This type of soil covers 40% of the total area of the country.

• Alluvial soil is depositional soil formed by sediment brought and deposited by rivers and streams.

The area of alluvial soils: Through a narrow corridor of Rajasthan, they extend into the plains of Gujarat. It is found in deltas of the East coast and river valleys in the Peninsular region.

Characteristics of Alluvial Soils

Characteristics of Alluvial soils are as follows:

• These soils vary in nature from sandy loam to clay.
• The Colour of this soil also varies from light grey to ash grey, depending on the depth of deposition, the texture of the material, and the time is takes for attaining maturity.
• Alluvial soil is rich in Potash and lime but poor in phosphorus.

Types of Alluvial Soil:

Two types of Alluvial soil are found in the Upper Ganga and Middle Ganga plains I.e. Bhangar and Khadar.

• The Khadar soil is new alluvial and more fertile because it is deposited by floods annually.
• Bhangar is an older deposit of alluvium in the Upper valleys away from the flood plains.
• Both Khadar and hangar contain more calcareous concretions.
• In Lower and Middle Ganga and in Brahmaputra valley these soils are loamier and clayey.
• The sand content of the soil decreases from West to East.
• In India, cultivation in these soils is found the most.

Black Soil

Black soils are found in the region of the Deccan plateau including Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, and some parts of Tamil Nadu.

Characteristics of Black Soils:

• The black soils are generally clayey, deep, and impermeable.
• It is very deep in the upper reaches of Godavari and Krishna and the North-Western part of the Deccan Plateau.
• In a wet position, it swells and becomes sticky, and in the dry position, it shrinks which is why wide cracks are found in this type of soil. The wide cracks result in the self-plowing of soil.
• This type of soil can retain moisture for a very long time due to slow absorption and less loss of moisture.
• The black soil is rich in lime, Potash, iron, magnesia, and alumina, but the soil lacks phosphorus, Nitrogen, and Organic matter.
• The Colour of the soil varies from deep black to grey.
• It is also known as Regur and Black cotton soil.

Red and Yellow soils

Red soil develops on crystalline igneous rocks in areas of low rainfall in the Eastern and Southern parts of the Deccan Plateau.

• A long stretch of the area along the Piedmont zone of western Ghat is occupied by Red Loamy soil.
• Red and yellow soils are found in parts of Odisha, and Chhattisgarh and in Southern parts of the middle Ganga plain.

Characteristics of Red and Yellow soils are:

• These soils develop a reddish color due to a wide diffusion of iron in crystalline and metamorphic rocks.
• Its color seems yellow when it occurs in a hydrated form.
• Generally, it is poor in Nitrogen, phosphorus, and humus.
• The fine-grained Red and Yellow soils are fertile and coarse-grained soils in dry upland areas are poorly fertile.

Laterite Soils

Laterite soils develop in areas of high temperature and heavy rainfall. These soils are formed as the result of intense leaching due to tropical rains.

• Leaching is washing off the top layer of soil due to excessive rains.

The laterite soils have mainly developed in higher areas of the Peninsular Plateau. These are commonly found in the hilly areas of Odisha and Assam, Madhya Pradesh, Andhra Pradesh, Tamil Nadu, Karnataka, and Kerala.

Characteristics of Laterite soils:

• This soil is rich in iron oxide and aluminum as lime and Silica are leached away by rain.
• Laterite soil is deficient in Phosphate, calcium, nitrogen, and organic matter.
• Iron oxide and Potash are in excess in this soil.
• The Humus content of the soil is removed fast by bacteria that thrive well in high temperatures.
• Laterite soils are not suitable for cultivation but can be made fertile by the use of manures and fertilizers.
• Crops like cashew nuts are cultivated in this soil in Tamil Nadu, Andhra Pradesh, and Kerala.
• Use of the soil is widely in house construction in the form of bricks.

Arid soils

Arid soils are characteristically developed in Western Rajasthan which exhibits characteristic arid topography.

Characteristics of Arid soils:

• The color of Arid soils varies from red to brown in color.
• They are sandy in structure and saline in nature.
• The arid soils lack moisture and humus due to dry climate, high temperature, and accelerated evaporation.
• These soils are poor in nitrogen and organic matter, and phosphate content is normal.
• The kankar layers are found in the bottom horizons due to increased calcium content downwards which restricts the infiltration of water.
• Soil moisture is readily available for sustainable plant growth when irrigation is available.

Saline Soils

These soils are also known as Usara soils. Saline salt consists of more salt due to dry climate and poor drainage. These soils occur in arid and semi-arid regions and in swampy areas.

The Saline soils are widespread in Western Gujarat, deltas on the Eastern coast, and in areas of West Bengal. These soils are formed in the delta regions due to seawater instructions.

Characteristics of Saline soils:

• Saline soils contain a larger proportion of sodium, potassium, and magnesium.
• These soils are sandy to loamy in structure and lack nitrogen and calcium.
• Saline soils are infertile, so they do not support any vegetative growth.
• The fertile soils are also becoming infertile, and saline due to intensive cultivation and excessive irrigation.

Peaty soils

Peaty Soils are found in areas of heavy rainfall and high humidity, where there is a good growth of vegetation.

Peaty soils widely occur in the Northern part of Bihar, the Southern Part of Uttarakhand, and the coastal areas of West Bengal, Odisha, and Tamil Nadu.

Characteristics of Peaty soils:

• They have a large amount of dead organic matter which gives rich humus and organic content to the soil.
• Organic matter in these soils may go even up to 40-50%.
• The color of peaty soils is black and alkaline in nature.

Forest Soils

Forest soils are formed in heavy rainfall forest areas, and it also depends on the mountain environment where they are formed.

Characteristics of forest soils:

• Forest soils vary in structure and texture.
• It is loamy and silty on the valley sides and coarse-grained on the upper slopes. The soil is fertile in the lower valley.
• At higher altitudes and snow-bound areas of the Himalayas, they experience denudation and are acidic in nature with low humus content.

You can also read:

• Landforms and their evolution
• List of main volcanos of the world
• The Rocks- Igneous Rocks 
• Sedimentary rocks
• Metamorphic rocks
• Location and Geological Divisions of India

Thank you 🙂