Chapter 6 Soils

Introduction to Soil

Soil is the vital uppermost layer of the Earth's crust, supporting most terrestrial life forms, including plants and animals. Even aquatic organisms indirectly depend on soil for nutrients absorbed through water. It's a crucial resource, providing the foundation for agriculture, which supplies a significant portion of our food and clothing.

Importance of Soil

Soil is indispensable for plant growth, acting as a medium for anchoring roots and providing essential nutrients, water, and air. The majority of human food and clothing originates from crops cultivated in the soil. It also plays a role in the water cycle and serves as a habitat for numerous organisms.

Factors Affecting Soil Formation

Soil formation is a complex process influenced by several key factors over extended periods:

• Relief: The topography of the land, including its elevation and slope, affects drainage and erosion, thus influencing soil development.
• Parent Material: The underlying rock from which the soil is derived dictates its mineral composition and texture.
• Climate: Temperature and rainfall significantly impact weathering processes, organic matter decomposition, and leaching, all of which are crucial for soil formation.
• Vegetation and Other Life-forms: Plants contribute organic matter (humus) through decomposition, improve soil structure with their roots, and influence nutrient cycling. Microorganisms also play a vital role.
• Time: Soil formation is a slow process, and the age of the soil influences its development and the formation of distinct layers.
• Human Activities: Human interventions like deforestation, agriculture, and urbanization can significantly alter soil formation and degradation rates.

Components of Soil

Soil is a heterogeneous mixture composed of several components:

• Mineral Particles: These are derived from the weathering of rocks and include sand, silt, and clay, which determine the soil's texture and structure.
• Humus: This is the decomposed organic matter from dead plants and animals. It improves soil structure, water retention, and nutrient availability.
• Water: Essential for plant life and soil processes, water is held within the soil pores.
• Air: Soil pores also contain air, which is necessary for the respiration of plant roots and soil organisms.

The proportion of these components varies greatly, leading to different soil types.

Soil Horizons

A vertical cross-section of soil reveals distinct layers known as horizons, each with unique characteristics:

• Horizon A (Topsoil): This is the uppermost layer, rich in organic matter (humus), mineral nutrients, and moisture. It is the most biologically active and fertile zone, crucial for plant growth.
• Horizon B (Subsoil): Located beneath Horizon A, this layer is a transition zone. It contains leached materials from above and weathered parent material from below. It has less organic matter than Horizon A but may contain accumulated minerals.
• Horizon C: This layer consists of loose parent material, the initial stage of soil formation. It is less weathered than the layers above and is directly above the bedrock.

Soil Profile

The complete arrangement of these soil horizons from the surface down to the parent rock is called the soil profile. The soil profile provides insights into the soil's development, age, and fertility.

Underneath these horizons lies the unweathered parent rock or bedrock.

Classification of Soils

Understanding the diversity of soils necessitates their classification. This helps in identifying their properties, suitability for different uses, and the best methods for their management.

Ancient Classification

Historically, soils were broadly categorized based on their fertility. In ancient India, soils were classified into two main types:

• Urvara: Fertile soils suitable for cultivation.
• Usara: Sterile or infertile soils.

Classification Based on External Features

In the 16th century, soils were classified considering their observable characteristics such as:

• Texture: Based on the proportion of sand, silt, and clay (e.g., sandy, clayey, silty, loam).
• Colour: Red, yellow, black, etc.
• Slope of Land: Indicating topography.
• Moisture Content: Reflecting water retention capacity.

Modern Classification (ICAR)

In India, the Indian Council of Agricultural Research (ICAR) has classified soils based on their nature and character, aligning with international standards like the United States Department of Agriculture (USDA) Soil Taxonomy. This classification categorizes Indian soils into eight major orders:

Source: Soils of India, National Bureau of Soil Survey and Land Use Planning.

Classification Based on Genesis, Colour, Composition, and Location

Considering these factors, Indian soils are primarily classified into eight broad types:

1. Alluvial Soils
2. Black Soils (Regur Soil)
3. Red and Yellow Soils
4. Laterite Soils
5. Arid Soils
6. Saline Soils
7. Peaty Soils
8. Forest Soils

Types of Soils in India

India's diverse geographical features, climate, and vegetation have led to the development of various soil types across the country.

Alluvial Soils

Distribution: These soils are extensively found in the northern plains and river valleys, covering approximately 40% of India's total area. They are also present in the deltas of the east coast and river valleys of the Peninsular region, with a narrow extension into Gujarat through Rajasthan.

Formation: Alluvial soils are depositional, meaning they are transported and deposited by rivers and streams.

Characteristics:

• Texture: Varies from sandy loam to clay.
• Fertility: Generally rich in potash but deficient in phosphorus.
• Types: In the Ganga plains, two main types are recognized:
  • Khadar: New alluvium, deposited annually by floods, making it very fertile.
  • Bangar: Older alluvium, deposited away from floodplains, containing calcareous concretions (kankars).
• Composition: More loamy and clayey in the lower and middle Ganga plains and the Brahmaputra valley. Sand content decreases from west to east.
• Colour: Ranges from light grey to ash grey, depending on deposition depth and material.
• Cultivation: Intensely cultivated due to their high fertility.

Black Soils (Regur Soil)

Distribution: These soils cover large parts of the Deccan Plateau, including Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, and parts of Tamil Nadu. They are also found in the northern parts of the Deccan Plateau.

Formation: Black soils are believed to have formed from the weathering of ancient volcanic rocks (basalt).

Characteristics:

• Other Names: Known as ‘Regur Soil’ or ‘Black Cotton Soil’.
• Texture: Generally clayey, deep, and impermeable.
• Behaviour when Wet/Dry: Swell when wet and become sticky, and shrink when dry, developing wide cracks. This cracking is often referred to as "self ploughing."
• Moisture Retention: They retain moisture for a long time, benefiting rain-fed crops during dry periods.
• Chemical Composition: Rich in lime, iron, magnesia, and alumina, and also contain potash. They are, however, deficient in phosphorus, nitrogen, and organic matter.
• Colour: Ranges from deep black to grey.

Red and Yellow Soils

Distribution: These soils develop on crystalline igneous rocks in areas with low rainfall, particularly in the eastern and southern parts of the Deccan Plateau. They are also found along the Western Ghats, in parts of Odisha, Chhattisgarh, and the southern middle Ganga plain.

Formation: Red soils develop from the diffusion of iron in crystalline and metamorphic rocks. They appear yellow when in a hydrated form.

Characteristics:

• Fertility: Fine-grained red and yellow soils are generally fertile. Coarse-grained soils in dry upland areas are less fertile.
• Deficiencies: Typically poor in nitrogen, phosphorus, and humus.

Laterite Soils

Distribution: Laterite soils are found in areas of high temperature and high rainfall, primarily on the higher areas of the Peninsular plateau, including parts of Karnataka, Kerala, Tamil Nadu, Madhya Pradesh, and the hilly regions of Odisha and Assam.

Formation: They are formed due to intense leaching caused by tropical rains. Rainwater washes away lime and silica, leaving behind soils rich in iron oxide and aluminium compounds. High temperatures accelerate the loss of humus by bacteria.

Characteristics:

• Etymology: Derived from the Latin word ‘Later’, meaning brick.
• Nutrient Content: Poor in organic matter, nitrogen, phosphate, and calcium. Rich in iron oxide and potash.
• Cultivation Suitability: Generally not ideal for cultivation without the application of manures and fertilizers. Red laterite soils in some states are suitable for tree crops like cashewnut.
• Usage: Laterite soils are often cut and used as bricks for building construction.

Arid Soils

Distribution: Predominantly found in the western Rajasthan region, characterized by arid topography.

Formation: Developed in dry climates with high temperatures and accelerated evaporation.

Characteristics:

• Colour: Range from red to brown.
• Texture: Generally sandy and saline in nature.
• Moisture and Humus: Lack moisture and humus due to dry conditions.
• Nutrient Content: Nitrogen is insufficient, while phosphate content is normal.
• Structure: Lower horizons often contain ‘kankar’ (calcium carbonate) layers, which restrict water infiltration.
• Cultivation: Can support plant growth when irrigation is available due to moisture retention facilitated by the 'kankar' layer.

Saline Soils (Usara Soils)

Distribution: Occur in arid and semi-arid regions, waterlogged areas, and swampy regions. Widespread in western Gujarat, deltas of the eastern coast, and Sundarbans of West Bengal. In areas with intensive cultivation and excessive irrigation, fertile alluvial soils can become saline (e.g., Punjab and Haryana).

Formation: Formed due to the accumulation of salts, largely because of dry climates and poor drainage. In coastal areas, seawater intrusion contributes to salinity. In arid/semi-arid regions, excessive irrigation combined with capillary action can bring salts to the surface.

Characteristics:

• Other Names: Also known as ‘Usara soils’.
• Composition: Contain a higher proportion of sodium, potassium, and magnesium, leading to high salt content.
• Fertility: Infertile and do not support significant vegetative growth.
• Texture: Range from sandy to loamy.
• Nutrient Deficiencies: Lack nitrogen and calcium.
• Remedy: In areas experiencing salinity due to over-irrigation, adding gypsum is advised to improve soil quality.

Peaty Soils

Distribution: Found in areas with heavy rainfall and high humidity, supporting dense vegetation growth.

Formation: Formed from the accumulation of large quantities of dead organic matter due to poor drainage, leading to a high humus and organic content (up to 40-50%).

Characteristics:

• Colour: Normally heavy and black.
• Fertility: Rich in humus and organic matter.
• Alkalinity: Can be alkaline in nature.
• Location: Widely found in the northern part of Bihar, southern Uttarakhand, and coastal areas of West Bengal, Odisha, and Tamil Nadu.

Forest Soils

Distribution: Formed in forest areas with adequate rainfall, varying according to mountain environments.

Formation: Developed in forest regions with sufficient rainfall. Their texture and structure depend on the mountainous terrain where they are located.

Characteristics:

• Texture: Loamy and silty on valley sides, and coarse-grained on upper slopes.
• Snow-bound Areas: In the Himalayas, these soils experience denudation, are acidic, and have low humus content.
• Valley Locations: Soils in lower valleys are fertile.

Soil Degradation

Soil degradation refers to the decline in the quality and productivity of soil. It encompasses a reduction in soil fertility, a decrease in soil depth, and an overall deterioration of its physical and chemical properties.

Definition of Soil Degradation

Soil degradation is defined as the decline in soil fertility and its overall quality, often due to processes like erosion, loss of organic matter, salinization, and nutrient depletion. It leads to a reduction in the soil's capacity to support plant life and fulfill its ecological functions.

Causes of Soil Degradation

The primary causes of soil degradation in India are varied and often interconnected:

• Soil Erosion: The removal of the fertile topsoil by agents like wind and water.
• Misuse of Soil: Improper agricultural practices, such as overgrazing, excessive use of chemical fertilizers without organic manure, and continuous cultivation without adequate rest periods.
• Deforestation: The removal of trees loosens the soil, making it more susceptible to erosion.
• Over-irrigation: In arid and semi-arid regions, excessive irrigation can lead to waterlogging and salinization as salts are brought to the surface by capillary action.
• Unsustainable Land Use: Converting forests and grazing lands into agricultural land without considering the soil's carrying capacity.
• Industrialization and Urbanization: Land taken up for development often leads to soil compaction and pollution.

Approximately half of India's total land area is affected by some degree of soil degradation.

Soil Erosion

Soil erosion is a natural process but is significantly accelerated by human activities, leading to the loss of fertile topsoil.

Definition of Soil Erosion

Soil erosion is the process by which the soil cover is destroyed and removed. It involves the detachment, transport, and deposition of soil particles, primarily by natural agents like wind and water.

Agents of Soil Erosion (Wind and Water)

The primary agents responsible for soil erosion are:

• Wind: Wind erosion is significant in arid and semi-arid regions with sparse vegetation. It lifts and transports fine soil particles.
• Water: Erosion by running water is more prominent in areas with heavy rainfall and steep slopes. It is a major cause of soil loss in India.

Types of Water Erosion (Sheet and Gully Erosion)

Water erosion occurs in several forms:

• Sheet Erosion: This occurs on level lands after heavy rainfall, where a thin layer of topsoil is removed uniformly. The process is often subtle, but its cumulative effect is significant due to the loss of fertile topsoil.
• Gully Erosion: This happens on steep slopes when rainwater cuts deep channels (gullies) into the soil. These gullies deepen with rainfall, fragmenting agricultural land and making it unfit for cultivation. Regions with numerous deep gullies are called badland topography (e.g., Chambal basin).

Causes of Soil Erosion (Natural and Human)

Soil erosion is caused by both natural and human factors:

• Natural Causes:
• Heavy rainfall and steep slopes (accelerate water erosion).
• Wind velocity (drives wind erosion).
• Natural topography.
• Human Causes:
• Deforestation: Removal of trees weakens soil structure and exposes it to erosion.
• Overgrazing: Livestock remove vegetation cover, leaving the soil vulnerable.
• Shifting Cultivation: Clearing land by burning vegetation leads to soil degradation and erosion.
• Unsustainable Agricultural Practices: Inefficient irrigation, lack of crop rotation, and monoculture can deplete soil nutrients and structure.
• Construction and Mining: These activities disturb the land surface and can lead to significant erosion.

Effects of Soil Erosion

Soil erosion has detrimental effects on agriculture and the environment:

• Loss of Fertile Topsoil: Reduces agricultural productivity and crop yields.
• Reduced Water Carrying Capacity of Rivers: Eroded soil deposited in rivers can lead to siltation, reducing their depth and increasing the frequency and severity of floods.
• Damage to Agricultural Lands: Gully formation makes land unusable for cultivation.
• Increased Sedimentation: Affects reservoirs and water bodies.
• Desertification: In severe cases, continuous erosion can lead to the formation of desert-like conditions.

Soil Conservation

Soil conservation is essential to maintain soil fertility, prevent erosion, and restore degraded lands. It involves adopting practices that protect and improve the soil resource.

Definition and Importance of Soil Conservation

Soil conservation is a set of practices and methodologies aimed at maintaining soil fertility, preventing soil erosion and exhaustion, and improving the condition of degraded soils. Its importance lies in preserving the productivity of agricultural land, protecting ecosystems, and ensuring sustainable resource management for future generations.

Measures for Soil Conservation

Various measures can be adopted to conserve soil:

• Contour Bunding and Terracing: Building bunds or terraces along contour lines helps to slow down water flow and reduce erosion on slopes. Terrace farming is particularly effective on steep slopes.
• Regulated Forestry: Proper management of forest resources, including afforestation and preventing deforestation, helps maintain soil cover.
• Controlled Grazing: Managing livestock grazing to prevent overgrazing and allow vegetation to recover is crucial.
• Cover Cropping: Planting crops that cover the soil surface, such as legumes or grasses, protects the soil from direct impact of rain and wind.
• Mixed Farming and Crop Rotation: Diversifying crops and rotating them in a planned sequence helps maintain soil fertility and structure, and reduces pest and disease build-up.
• Preventing Gully Erosion: Implementing measures like terracing, constructing check dams in gullies to reduce water velocity, and planting cover vegetation can control gully formation and expansion.
• Shelter Belts: Planting rows of trees in arid and semi-arid areas acts as windbreaks, protecting agricultural lands from sand dune encroachment.
• Agro-forestry: Integrating trees with crops and livestock can improve soil health and provide multiple benefits.

Integrated Land Use Planning

Integrated land use planning is considered the most effective technique for proper soil conservation. This involves:

• Land Capability Classification: Assessing land based on its potential and limitations for different uses.
• Land Use Maps: Preparing detailed maps to guide land utilization.
• Right Use of Land: Allocating land for appropriate purposes (e.g., agriculture, forestry, grazing) based on its capability to prevent degradation.

The ultimate responsibility for soil conservation rests with the people who utilize and benefit from the land.