• From a geological perspective, Indian soils can be categorized into two main groups: those of peninsular India and those of extra-peninsular India.
• The peninsular Indian soils originate primarily through the in-situ decomposition of rocks, directly deriving from the underlying geological formations. 
• These soils, commonly referred to as sedentary soils, experience minimal transportation and redeposition processes.
• On the other hand, the soils of extra-peninsular India result from the depositional actions of rivers and wind. 
• Characterized by significant depth, these soils are often termed as transported or azonal soils due to their origin from external geological processes rather than on-site rock decomposition.

Major groups:

1. Alluvial soils,
2. Black soils,
3. Red soils,
4. Laterite and Lateritic soils,
5. Forest and Mountain soils,
6. Arid and Desert soils,
7. Saline and Alkaline soils and
8. Peaty and Marshy soils.

ALLUVIAL SOIL

• Alluvial soils primarily originate from the silt deposits carried by the Indo-Gangetic-Brahmaputra rivers, with additional formations occurring in coastal regions due to wave action. 
• The parent material for these soils is derived from the rocks of the Himalayas, indicating a transported origin. 
• Constituting the largest soil group, covering approximately 15 lakh sq km or about 46% of the total area, these soils play a crucial role in supporting over 40% of India's population through highly productive agricultural lands.

Characteristics of Alluvial Soils:

Origin and Composition:

• Immature and weak profiles due to recent formation.
• Predominantly sandy with occasional occurrences of clayey soils.
• Pebbly and gravelly soils are uncommon, while kankar (calcareous concretions) beds may be present in specific regions along river terraces.
• Porous nature attributed to a loamy composition (equal proportion of sand and clay).

Drainage and Agricultural Favorability:

• Porosity and texture contribute to excellent drainage, creating favorable conditions for agriculture.
• Regular replenishment through recurrent floods enhances soil fertility.

Chemical Properties:

• Generally low proportion of nitrogen.
• Adequate levels of potash, phosphoric acid, and alkalies.
• Varied proportions of iron oxide and lime.

Distribution of Alluvial Soils in India:

• Found along the Indo-Gangetic-Brahmaputra plains, excluding areas covered by desert sand.
• Present in deltas of rivers such as the Mahanadi, Godavari, Krishna, and Cauvery.
• Occur in the Narmada, Tapi valleys, and northern parts of Gujarat.

Crops Suited for Alluvial Soils:

• Ideal for agriculture, supporting the cultivation of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds, vegetables, and fruits.

Geological Divisions:

• Geologically, the Great Plain of India's alluvium is divided into newer (younger) khadar and older bhangar soils.

Bhabar:

• Located along the Shiwalik foothills, the porous bhabar belt results from the deposition of alluvial fans, consisting of pebbly soils.
• Unsuitable for agriculture due to porosity; supports mainly large trees.

Terai:

• A damp and marshy tract south of Bhabar, characterized by re-emerging underground streams and silty soils.
• Rich in nitrogen and organic matter; suitable for crops like wheat, rice, sugarcane, and jute.

Bhangar:

• Older alluvium forming terraces higher than the floodplain; clayey and dark-colored composition.
• Contains lime nodules known as "Kankar."

Khadar:

• Comprises newer alluvium forming flood plains along river banks.
• Flooded annually, contributing to high fertility; sandy clays and loams, less calcareous and carbonaceous compared to Bhangar.

BLACK SOIL

• The parent material for the majority of black soils is derived from volcanic rocks formed in the Deccan Plateau, including the Deccan and the Rajmahal trap. 
• In Tamil Nadu, gneisses and schists constitute the parent material, with gneisses being sufficiently deep and schists generally shallow. 
• These soils are characteristic of regions with high temperatures and low rainfall, making them typical to the dry and hot areas of the Peninsula.

Characteristics of Black Soils:

Argillaceous Composition:

• Highly argillaceous with a significant clay factor, usually 62% or more.
• Upland black soils generally exhibit lower fertility compared to those in valleys.

Moisture Retention:

• Highly retentive of moisture, swelling significantly in the presence of moisture.
• Becomes sticky and challenging to work on during the rainy season.
• Shrinkage and the development of broad, deep cracks occur in summer, allowing for oxygenation and maintaining extraordinary fertility in the lower layers.

Color of Black Soils:

• Black color results from titaniferous magnetite or iron and black constituents from the parent rock.
• In Tamil Nadu and parts of Andhra Pradesh, the black color is derived from crystalline schists and basic gneisses.
• Various shades of black, including deep black, medium black, shallow black, and mixtures of red and black, may be observed.

Chemical Composition:

• Alumina: 10%
• Iron Oxide: 9-10%
• Lime and Magnesium Carbonates: 6-8%
• Variable Potash (less than 0.5%)
• Low levels of phosphates, nitrogen, and humus.

Distribution of Black Soils:

• Spread over 46 lakh sq km, constituting 16.6% of the total area.
• Found in Maharashtra, Madhya Pradesh, parts of Karnataka, Telangana, Andhra Pradesh, Gujarat, and Tamil Nadu.

Crops in Black Soils:

• Best suited for cotton cultivation, leading to the designation of regur and black cotton soils.
• Other major crops include wheat, jowar, linseed, Virginia tobacco, castor, sunflower, millets, rice, and sugarcane where irrigation is available.
• Successfully supports a variety of vegetables and fruits.
• These soils have historically sustained diverse crops for centuries without significant reliance on fertilizers and manures, demonstrating little to no evidence of exhaustion.

RED SOIL 

Red soils, along with their minor groups, constitute the largest soil group in India. These soils primarily originate from crystalline and metamorphic rocks, such as acid granites, gneisses, and quartzites.

Characteristics of Red Soils:

Texture:

• Varying from sand to clay, with the majority being loams.
• On uplands, red soils are poor, gravelly, and porous, while in lower areas, they are rich, deep, dark, and fertile.

Chemical Composition:

• Acidic nature primarily due to the characteristics of the parent rocks.
• Fair alkali content.
• Poor in lime, magnesia, phosphates, nitrogen, and humus.
• Fairly rich in potash and potassium.

Color of Red Soils:

• Red color attributed to the presence of iron oxide.
• Erosion of limestone, granites, gneisses, and quartzites leaves clay enclosed within rocks intact with non-soluble materials.
• In oxidizing conditions, rust or iron oxide develops in the clay, giving the soil a characteristic red color above the water table.
• The color is more a result of wide diffusion than a high percentage of iron oxide content.

Distribution of Red Soils:

• Predominantly found in regions with low rainfall.
• Occupying about 3.5 lakh sq km (10.6%) of the total area of the country.
• Extensively spread across Tamil Nadu, with other regions including parts of Karnataka, southeast Maharashtra, Telangana, Andhra Pradesh, Madhya Pradesh, Chhattisgarh, Odisha, Chota Nagpur plateau, parts of south Bihar, West Bengal, Uttar Pradesh, Aravalis, and the eastern half of Rajasthan (Mewar or Marwar Plateau), along with parts of northeastern states.

Crops in Red Soils:

• Red soils, being mostly loamy, do not retain water as effectively as black soils.
• With proper fertilizer use and irrigation techniques, red soils yield good crops such as cotton, wheat, rice, pulses, millets, tobacco, oilseeds, potatoes, and various fruits.

LATERITE SOIL

Laterite soils are primarily the result of extensive weathering processes, forming under conditions of high temperature and heavy rainfall with alternating wet and dry periods. The term "Laterite" is derived from Latin, meaning brick, reflecting their ability to harden significantly upon losing moisture.

Formation and Characteristics of Laterite Soils:

Weathering Process:

• Formed under conditions of high temperature and heavy rainfall, promoting leaching of lime and silica.
• Result in a soil rich in oxides of iron and aluminum compounds.

Color and Composition:

• Red color due to a low clay content and a higher proportion of gravel from red sandstones.
• Rich in bauxite or ferric oxides.
• Poor in lime, magnesia, potash, and nitrogen, with potential high phosphate content in the form of iron phosphate.
• Humus content may be higher in wetter places.

Distribution of Laterite Soils:

• Cover an area of 2.48 lakh sq km.
• Continuous stretches found on the summits of the Western Ghats (1000 to 1500 m above mean sea level), Eastern Ghats, Rajmahal Hills, Vindhyan, Satpuras, and Malwa Plateau.
• Also occur at lower levels and in valleys across various regions.
• Well-developed in South Maharashtra, parts of Karnataka, and scattered in other regions.

Crops in Laterite Soils:

• Generally, lack fertility due to intensive leaching.
• With proper manuring and irrigation, some laterites are suitable for growing plantation crops such as tea, coffee, rubber, cinchona, coconut, and arecanut.
• In certain areas, these soils may support grazing grounds and scrub forests.

Economic Value of Laterite Soils:

Provide valuable building material.

Can be easily cut into cakes but hardens like iron when exposed to air.

Being the end-product of weathering, they have limited further weathering potential, rendering them durable.

FOREST- MOUNTAIN SOIL

• Forest-Mountain soils cover approximately 2.85 lakh sq km or 8.67% of India's total land area. 
• These soils are primarily heterogeneous and are commonly found on hill slopes covered by forests. 
• The formation of these soils is influenced by the deposition of organic matter from forests, with characteristics varying based on parent rocks, ground configuration, and climate. 
• As a result, even soils in close proximity may differ significantly.

Distribution of Forest-Mountain Soils:

• In the Himalayan region, these soils are mainly found in valleys, less steep slopes, and north-facing slopes. South-facing slopes, being steep and exposed to denudation, do not support soil formation.
• Forest soils also occur in the Western and Eastern Ghats.

Chemical Properties of Forest-Mountain Soils:

• Very rich in humus, derived from organic matter in forests.
• Deficient in potash, phosphorus, and lime.
• Requires a significant amount of fertilizers for achieving high yields.

Crops in Forest-Mountain Soils:

• Suitable for plantations of tea, coffee, spices, and tropical fruits in the peninsular forest region.
• In the Himalayan forest region, crops such as wheat, maize, barley, and temperate fruits are cultivated.
• The unique characteristics and distribution of Forest-Mountain soils make them well-suited for specific crops, and their fertility often depends on the organic matter contributed by the surrounding forests.

ARID -DESERT SOIL

• Desert soils, primarily composed of Aeolian sand (90 to 95%) and clay (5 to 10%), cover a total area of 1.42 lakh sq km (4.32%). 
• The dominance of sand inhibits soil growth, and the intrusion of desert sand under the influence of wind, known as Aeolian sand, often leads to the desertification of neighboring soils.

Distribution of Arid-Desert Soils:

• Found in arid and semi-arid regions of Rajasthan, Punjab, and Haryana, where the sand is blown from the Indus basin and the coast by prevailing southwest monsoon winds.
• Sandy soils without a significant clay factor are also common in coastal regions of Odisha, Tamil Nadu, and Kerala.

Chemical Properties of Arid-Desert Soils:

• Generally poor in organic matter.
• Some desert soils are alkaline with varying degrees of soluble salts like calcium carbonate.
• Calcium content increases downwards, with the subsoil having ten times more calcium.
• Phosphate content is as high as in normal alluvial soils.
• Nitrogen is initially low, but some is available in the form of nitrates.

Crops in Arid-Desert Soils:

• The presence of phosphates and nitrates makes these soils fertile wherever moisture is available.
• Reclamation is possible with proper irrigation facilities.
• In large areas, drought-resistant and salt-tolerant crops such as barley, cotton, millets, maize, and pulses are grown.
• Despite the challenging conditions, the fertility of arid-desert soils can be harnessed with adequate irrigation, enabling the cultivation of specific crops that are adapted to the unique characteristics of these soils.

SALINE SOIL

• Saline soils, also known as Usara soils, exhibit a higher concentration of sodium, potassium, and magnesium, and they lack vegetative maturity. 
• These soils are commonly found in arid and semi-arid areas as well as waterlogged and swampy regions. 
• Characterized by a deficiency in nitrogen and calcium, saline soils pose challenges to vegetation.

Distribution of Saline Soils:

• Saline soils are prevalent in arid and semi-arid regions, waterlogged areas, and swampy regions.
• Extensive occurrences are noted in western Gujarat, the deltas of the eastern coast, and the Sundarbans region in West Bengal.
• In regions with intensive cultivation and excessive irrigation, especially in areas influenced by the green revolution, fertile alluvial soils are becoming saline.

Formation and Causes:

Extreme irrigation in conjunction with dry climatic conditions promotes capillary action, leading to the deposition of salt on the topmost layer of the soil.

Mitigation Measures:

• In regions facing salinity issues, particularly in Punjab and Haryana, farmers are often recommended to add gypsum to address the challenge of soil salinity. 
• Gypsum can help improve soil structure and reduce the impact of sodium on soil permeability.
• Addressing salinity in soils is crucial for sustaining agricultural productivity, and appropriate measures, such as gypsum application, can contribute to mitigating the adverse effects of soil salinity.

PEATY SOIL

• Peaty soils are commonly found in areas with abundant rainfall and high humidity, where there is vigorous vegetation growth. 
• The accumulation of a substantial amount of dead organic matter in these regions contributes to the development of rich humus and organic content in the soil. 
• Organic matter in peaty soils can range from 40% to 50%, making them nutrient-rich.

Characteristics of Peaty Soils:

Texture and Color:

• These soils are typically heavy and exhibit a black color, indicative of their high organic content.
• In some areas, peaty soils may also have a saline nature.

Geographical Distribution:

• Extensively found in the northern part of Bihar, the southern part of Uttaranchal, and the waterfront areas of West Bengal, Orissa, and Tamil Nadu.

Formation and Composition:

• The presence of abundant vegetation leads to the accumulation of dead organic matter, resulting in the high humus and organic content characteristic of peaty soils.
• Peaty soils are valuable for agriculture due to their high nutrient content, but their management requires careful consideration to prevent issues such as soil subsidence and drainage problems. 
• These soils are particularly suitable for crops that thrive in nutrient-rich environments, and their conservation is important for sustainable agricultural practices in the regions where they are prevalent.

FAQs - Indian Soils from a Geological Perspective

1. What are the main categories of Indian soils from a geological perspective?

Ans. Indian soils are broadly categorized into two groups: Peninsular India soils and Extra-Peninsular India soils.

2. How do Peninsular Indian soils differ from Extra-Peninsular Indian soils?

Ans. Peninsular Indian soils result from in-situ decomposition of rocks, while Extra-Peninsular soils are formed through depositional actions of rivers and wind, characterized by significant depth.

3. What are the major groups of Indian soils?

Ans. The major soil groups in India include Alluvial, Black, Red, Laterite and Lateritic, Forest and Mountain, Arid and Desert, Saline and Alkaline, and Peaty and Marshy soils.

4. What is the origin of Alluvial soils, and where are they predominantly found?

Ans. Alluvial soils originate from silt deposits carried by Indo-Gangetic-Brahmaputra rivers. They are found along the plains of these rivers, covering about 46% of India's total area.

5. What are the characteristics of Alluvial soils?

Ans. Alluvial soils have immature profiles, are predominantly sandy, and exhibit good drainage. They support various crops and are regularly replenished by floods.

6. Where are Black soils found, and what is their parent material?

Ans. Black soils are found in Maharashtra, Madhya Pradesh, parts of Karnataka, Telangana, Andhra Pradesh, Gujarat, and Tamil Nadu. Their parent material includes volcanic rocks from the Deccan Plateau.

7. What is the unique characteristic of Black soils, and what crops do they support?

Ans. Black soils are highly argillaceous, with good moisture retention. They are best suited for cotton cultivation and support crops like wheat, jowar, tobacco, and various vegetables and fruits.

8. Which parent rocks contribute to the formation of Red soils?

Ans. Red soils originate from crystalline and metamorphic rocks like acid granites, gneisses, and quartzites.

9. Where are Red soils predominantly found, and what crops do they support?

Ans. Red soils are widespread in regions with low rainfall, covering about 10.6% of India's total area. They support crops like cotton, wheat, rice, and various pulses.

10. What is the origin of Laterite soils, and where are they found in India?

Ans. Laterite soils result from extensive weathering processes under high temperature and heavy rainfall. They are found on the summits of Western Ghats, Eastern Ghats, Rajmahal Hills, and various plateaus.

11. What is the economic value of Laterite soils?

Ans. Laterite soils provide valuable building material and are durable due to being the end-product of weathering.

12. What characterizes Forest-Mountain soils, and where are they located?

Ans. Forest-Mountain soils cover about 8.67% of India's total land area. They are heterogeneous and are mainly found in the Himalayan region, Western and Eastern Ghats.

13. What are the primary components of Arid-Desert soils, and where are they prevalent?

Ans. Arid-Desert soils are composed of Aeolian sand (90-95%) and clay (5-10%). They are found in arid and semi-arid regions of Rajasthan, Punjab, Haryana, and coastal areas.

14. How do saline soils develop, and what regions in India are affected by them?

Ans. Saline soils develop due to excessive irrigation, especially in arid and semi-arid regions. They are prevalent in western Gujarat, deltas of the eastern coast, and the Sundarbans region.

15. Where are Peaty soils found, and what characterizes their composition?

Ans. Peaty soils are found in areas with abundant rainfall, such as the northern part of Bihar, southern Uttaranchal, and waterfront areas of West Bengal, Orissa, and Tamil Nadu. They are rich in humus and organic content.