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The organic materials that decompose on the earth's surface and mix with rock debris are called soil. 

Soil Formation

Soil formation is a gradual and complex process that occurs over geological time scales through the interactions of various factors such as minerals, organic matter, water, air, and living organisms. Geological processes, including the weathering of rocks and minerals, contribute to the raw materials from which soil emerges. Physical forces such as erosion and deposition play a role in transporting and redistributing these materials, further contributing to soil formation.

Factors affecting Soil formation 

Relief

• By affecting drainage, erosion, and sediment deposition, the overall shape of the landscape, which includes hills, valleys, and other landforms, can have an effect on how soil is formed.

Parent material

• The kind of rock or unconsolidated material used to create the soil has a big impact. 
• The presence of various minerals and compositions in the parent material can result in the development of various soil properties.

Climate

• Soil formation is significantly influenced by the climate, including temperature and precipitation. 
• It has an impact on how quickly organic matter weathers, erodes, and decomposes. 
• In comparison to arid or cold regions, regions with higher rainfall and warmer temperatures typically experience faster soil development.

Vegetation 

• The accumulation of organic matter in the soil is facilitated by vegetation through plant growth, shedding, and decomposition. 
• Litter is the term for the layer of organic matter that is made up of dead plants, twigs, and fallen leaves.  
• As this litter decomposes, it transforms into humus, a dark, nutrient-rich substance that enhances soil fertility, structure, and water-holding capacity.

The role of Time 

• Over the course of geological time scales, soil formation takes place gradually. 
• The development of the soil profile increases with the duration of the soil-forming processes in a given area.

In addition to these, human activities have a significant impact on it as well. Mineral shards, humus, water, and air are the soil's constituents. The type of soil determines how much of each of these is present in actuality. While others have different combinations, some soils are deficient in one or more of these.

Components of Soil

• Mineral particles 

• Organic matter 

• Water 

• Air 

Classification of Soils

The relief, landforms, climatic zones, and vegetation types in India are diverse. These have helped Indian soils develop into a variety of soil types.

Classification of Soils in ancient times

• Urvara

• Usara

Source:  ICAR - IARI, New Delhi

Soils were categorised in the 16th century A.D. based on both their inherent qualities and external features like texture, colour, slope of the land, and moisture content in the soil. 

Based on texture, main soil types were identified as 

        Sandy 

• It is made up of tiny fragments of weathered rock. 
• It has very low nutrients and poor water holding capacity. 
  
  Clayey
  
  
• Compared to the other two types of soil, clay has the smallest particles.
• It is difficult for moisture and air to penetrate this soil because of its excellent water-holding capacities.
  
  Loam 

• The advantages of each of the three components—sand, silt, and clay—are combined in this mixture. 
• Given that it contains an equilibrium of all three types of soil materials, this soil is also known as agricultural soil.
  
  Silty (most fertile)
  
  
• The soil holds water better than sand because it is smooth and fine.
• In comparison to the other three types of soil, silt soil is the most fertile. 
  
  

On the basis of colour, soils are

• Red 

• Yellow

• Black

These soil types are explained in detail later in the blog 

Surveys of soils In India

• Numerous agencies have carried out scientific soil surveys since the country's independence. 
• The 1956 establishment of the Soil Survey of India resulted in thorough investigations of soils in a number of locations, including the Damodar Valley. 
• Numerous studies on Indian soils were conducted by the Land Use Planning and the National Bureau of Soil Survey. 
• Later, the Indian soils were categorised by the ICAR using the United States Department of Agriculture's (USDA) Soil Taxonomy based on their nature and characteristics. 

Major Soil types of India

On the basis of genesis, colour, composition and location, the soils of India have been classified in to:

1. Alluvial soils 
2. Black soils
3. Red and Yellow soils 
4. Laterite soils
5. Arid soils
6. Saline soils 
7. Peaty soils 
8. Forest soils

(i) Alluvial soils 

• The North Indian Plains and the river valleys are both covered in alluvial soils. 
• Approximately 40% of the country is made up of these soils. 
• Since they are depositional soils, rivers and streams move and deposit them. 
• They extend into the Gujarati plains through a small passageway in Rajasthan. 
• They can be found in the river valleys and east coast deltas of the Peninsula.
• The types of alluvial soil range from sandy loam to clay. 
• They typically have high potash content but low phosphorus content. 
  
  

Khadar and Bhangar 

• Khadar and Bhangar alluvial soils have developed in the Upper and Middle Ganga plains. 
• Every year, floods deposit the new alluvium known as khadar, which enriches the soil by adding fine silts. 
• A system of older alluvium that was deposited far from the flood plains is represented by Bhangar. 
• Calcareous concretions, or Kankars, are present in both the Khadar and Bhangar soils. 
• In the lower and middle Ganga plain as well as the Brahmaputra valley, these soils are more loamy and clayey.  
• The sand content decreases from the west to east.
• The alluvial soils are various shades of grey, ranging from light grey to ash grey. 

(ii) Black soils

• The Deccan Plateau, which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, and some of Tamil Nadu, is mostly covered in black soil.
• The black soil is very deep in the upper reaches of the Godavari and Krishna rivers as well as the northwestern portion of the Deccan Plateau. 
• These soils are also referred to as "Regur Soil" or "Black Cotton Soil." 
• The majority of the black soils are deep, clayey, and impermeable. 
• They swell and become sticky when wet and shrink when dry. As a result, during the dry season, these soils develop large cracks. 
• The black soils are high in lime, iron, magnesia, and alumina. 
• They also have potash in them. However, they are deficient in phosphorus, nitrogen, and organic matter. 
• The soil's colour ranges from deep black to grey.

As a result, there is some "self ploughing." Due to its slow absorption and loss of moisture, black soil retains moisture for a very long time, allowing crops—especially those that are rain-fed—to thrive even during the dry season.

(iii) Red and Yellow soils 

• Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern part of the Deccan Plateau. 

• Long stretches of the Western Ghats piedmont zone are covered in red loamy soil. 

Additionally, parts of Orissa, Chattisgarh, and the southern portions of the middle Ganga plain have yellow and red soils. 

• Iron diffuses widely in crystalline and metamorphic rocks, giving the soil a reddish hue. 
• When it is present in hydrated form, it appears yellow. 
• In contrast to coarse-grained soils found in dry upland areas, the red and yellow soils have finer grains and are typically more fertile. 
• They typically have low levels of humus, nitrogen, and phosphorus.

(iv) Laterite soils

The Latin word "Later," which means "brick," is where the word "laterite" comes from.  

• The laterite soils develop in areas with high temperature and high rainfall. 
• The result of severe leaching caused by tropical rains, these are the end product. 
• Rain causes soils rich in iron oxide and aluminium compound to be left behind as lime and silica are washed away. 
• Bacteria that do well in hot environments quickly deplete the soil's humus content. 
• While iron oxide and potash are in excess, these soils are deficient in organic matter, nitrogen, phosphate, and calcium. 
• Laterites cannot be farmed as a result of this; however, soils can be farmed if manures and fertilisers are applied to them.
• For tree crops like cashew nuts, red laterite soils in Tamil Nadu, Andhra Pradesh, and Kerala are better suited.
• Bricks made from laterite soil are frequently used in the construction of houses. 
• The higher portions of the Peninsular plateau are where these soils have primarily grown. 
• The hilly regions of Orissa and Assam as well as Karnataka, Kerala, Tamil Nadu, and Madhya Pradesh frequently have laterite soils.

 (v) Arid soils

• The colour of arid soils ranges from red to brown. 
• Most of the time, they have a sand-like structure and are salty in composition. 
• Common salt can sometimes be made by evaporating saline water in some regions due to the high salt content. Nitrogen is insufficient and the phosphate content is normal. 
• They lack moisture and humus as a result of the dry weather, high temperatures, and accelerated evaporation. 
• Because the soil's calcium content rises as it descends, the lower horizons are home to "kankar" layers. 
• When irrigation is made available, the soil moisture is readily available for a sustainable plant growth because the "Kankar" layer formation in the bottom horizons restricts water infiltration. 
• Rajasthan's western region, which has typical arid topography, has developed arid soils in particular. 
• Poor in humus and organic matter, these soils are deficient in both.

(vi) Saline soils 

• Usara soils is another name for them. 
• Saline soils are devoid of fertility and incapable of supporting any vegetative growth because they have higher concentrations of sodium, potassium, and magnesium. 
• Because of the dry climate and inadequate drainage, they have more salts. 
• They can be found in wetlands and swamps as well as arid and semi-arid regions.
• Their composition varies, from sandy to loamy.  
• They lack nitrogen and calcium. 
• Western Gujarat, the deltas of the eastern coast, and the Sunderban regions of West Bengal are where saline soils are more prevalent. 
• The Southwest Monsoon transports salt flakes to the Rann of Kachchh, where it deposits as a crust. 
• Saline soils are more common in the deltas because of seawater intrusions.
• The fertile alluvial soils are becoming salinized in areas of intensive cultivation and excessive irrigation, particularly in areas of the green revolution. 
• Salt deposits on the top layer of soil as a result of excessive irrigation and dry weather conditions that encourage capillary action.
• Farmers in these regions, particularly in Punjab and Haryana, are advised to add gypsum to the soil to reduce salinity.

(vii) Peaty soils 

• Where there is a healthy growth of vegetation, they can be found in regions with high humidity and heavy rainfall. 
• As a result, these areas accumulate a significant amount of decomposing organic matter, which enriches the soil's organic and humus content. 
• Organic matter in these soils may go even up to 40-50 per cent.
• Typically, these soils are dark and heavy. 
• Additionally, they are alkaline in many places. 
• The southern portion of Uttaranchal, the northern part of Bihar, and the coastal regions of West Bengal, Orissa, and Tamil Nadu all have significant populations of it.

(viii) Forest soils.

• Where there is enough rainfall, forest soils develop. 
• On the valley sides and in the upper slopes, they are loamy and silty. 
• They suffer from denudation, are acidic, and have little humus in the Himalayan regions covered in snow. 
• The lower valleys' soils are rich in fertility.
  
  

Key Terms Soil degradation The term "soil degradation" refers to the loss of soil fertility that occurs when nutrient levels drop and soil depth decreases as a result of erosion and improper use.  Soil Erosion    Soil erosion is the term used to describe the destruction of the soil cover. Soil conservation Maintaining soil fertility, preventing erosion and exhaustion, and restoring the degraded state of the soil are all goals of soil conservation.