Genesis of Soils | Geography Optional for UPSC (Notes) PDF Download

Introduction

The top layer of Earth's crust, known as soil, is created through the ongoing process of mountain weathering over thousands of years. Soil is composed of four primary elements: minerals, organic materials, air, and water. Its texture is determined by the presence and proportions of three main components: sand, silt, and clay. The mineral texture of the soil can vary depending on the combination of these three components.
The decomposition of leaves and other organic matter forms an upper organic layer in the soil, which is called humus. The presence of humus in soil is crucial for its fertility, as it provides essential nutrients for plant growth and enhances the soil's overall health.

Soil Genesis

Soil is a crucial component of the natural environment, serving as a connection between climate and vegetation, and significantly influencing human activities due to its varying fertility levels. The scientific study of soil is known as pedology, while the process of soil formation is called pedogenesis or soil genesis.

• Soil refers to the top weathered layer of the Earth's crust, which is influenced by plants and animals. A vertical section of this zone is called a soil profile, consisting of several distinguishable layers or horizons that help identify different types of soil.
• Essentially, soil is a thin layer of mineral matter on the Earth's surface containing a substantial amount of organic material, capable of supporting plant life. It spans from the surface down to the deepest point where living organisms, such as plant roots, can penetrate. Soil is characterized by its ability to produce and store plant nutrients, which results from the interaction of various factors such as water, air, sunlight, rocks, plants, and animals.
• Although soil is thinly spread across the Earth's surface, it plays a critical role as the meeting point for the atmosphere, lithosphere, hydrosphere, and biosphere. The majority of soil consists of inorganic material, classifying it as part of the lithosphere. However, soil is also closely connected to the other three Earth spheres.

Soil development (Genesis of Soil-Structure) begins with the physical and chemical disintegration of rock exposed to the atmosphere and to the action of water percolating down from the surface. This disintegration is called weathering. The basic result of weathering is the weakening and breakdown of solid rock, the fragmentation of coherent rock masses, and the making of little rocks from big ones.

• The primary outcome of weathering processes is a layer of loosely packed inorganic material known as regolith, which is also referred to as "blanket rock" because it covers the unbroken rock beneath it. The regolith typically comprises materials that have weathered from the underlying rock, with the largest and least fragmented pieces located at the bottom, right above the bedrock.
• However, regolith can also be made up of materials transported from other locations by wind, water, or ice, which can cause its composition to differ significantly from one place to another.
• The top half-meter of the regolith is distinct from the lower layers due to the increased intensity of biological and chemical processes occurring within it. This upper layer is known as soil, which is primarily composed of finely fragmented mineral particles and is the ultimate product of weathering. Soil usually contains a mix of living plant roots, decaying plant matter, microscopic plants and animals (both living and dead), as well as varying amounts of air and water. Soil is not a final product but rather a stage in an ongoing cycle of physical, chemical, and biological processes.

Soil-forming Processes
There are four classes of soil-forming processes: soil enrichment, removal, translocation, and transformation.

Soil Enrichment
Soil enrichment involves the addition of organic or inorganic matter to the soil in order to improve its quality. One example of this is when minerals from silt are deposited on the soil's surface by river floods or carried by wind in the form of dust. Organic enrichment can also occur when water transports humus, a nutrient-rich organic material, from the upper layer of soil (O horizon) into the layer beneath it (A horizon). This process helps to enhance the overall health and fertility of the soil.

Removal Process

• In the process of soil removal, materials are taken away from the soil mass. This can happen when erosion transports soil particles into water bodies like streams and rivers. Another significant removal process is leaching, where soil compounds and minerals are dissolved in water and flow to lower levels.
• Cheluviation is a process that is quite similar to leaching, involving the downward movement of materials within the soil. However, cheluviation takes place due to the action of organic substances, known as chelating agents, rather than just water. This process involves the use of plant acids to facilitate the movement of materials, distinguishing it from leaching, which relies solely on water.

Translocation Process

• Translocation describes the movement of materials upward or downward within the soil.

Downward Translocation:

• The process of downward translocation involves the movement of fine particles like clays and colloids from the upper layers of the soil to the lower layers. Eluviation is the term used for this movement, which results in the formation of the E horizon, consisting of sand and coarse silt. The material brought down from the E horizon, such as clay particles, humus, or iron and aluminum sesquioxides, accumulates in the B horizon through a process called illuviation.
• The top layer of the soil consists of a thin deposit of wind-blown silt and dune sand, which adds to the soil profile. As organic matter in the O horizon decays, humus moves downward, enriching the A horizon and giving it a brownish color. The E horizon appears whitened due to the removal of colloids and sesquioxides through eluviation, while the B horizon displays orange-red colors due to the addition of iron sesquioxide through illuviation.
• Calcium carbonate translocation is another significant process in soil formation. In wet climates, excess soil water moves down to the groundwater zone, leaching calcium carbonate from the soil in a process called decalcification. Soils that have lost most of their calcium are usually acidic and low in bases. The addition of lime or pulverized limestone can correct the acidity and replenish the missing calcium, an essential nutrient for plants.

Upward translocation

• In desert environments, upward translocation can also take place. This occurs when groundwater is situated close to the surface in low-lying areas, leading to a flat and poorly drained region. The evaporation of water near the soil surface causes groundwater to rise and replace it through capillary action, similar to how a cotton wick draws oil in an oil lamp. This groundwater often contains high levels of dissolved salts.
• As the water containing these salts evaporates, the salts are left behind and accumulate in the soil. This process is known as salinization. The presence of large quantities of salts can be harmful to various plant species. When salinization affects irrigated lands in desert areas, the soil can become severely damaged, with little possibility of recovery.

Transformation Process
One of the key stages in soil formation is the transformation process, which involves changes occurring within the soil material itself. For example, primary minerals can be converted into secondary minerals, and organic matter can be decomposed by microorganisms to produce humus through a process known as humification. In warm and moist climates, this transformation of organic matter into carbon dioxide and water can be so efficient that it leaves virtually no organic matter remaining in the soil.

Factors Affecting Soil Formation
Soil is a dynamic and constantly changing substance. It can be compared to a sponge, as it absorbs various inputs and is influenced by the surrounding environment, causing it to change over time. When these inputs or environmental conditions change, the soil also undergoes transformations.

Factors that are responsible for soil development are

• Climate
• Organisms
• Relief
• Parent Material
• Time
• Human Activity

1. Climate

• Climate, specifically precipitation and temperature, plays a significant role in determining soil properties. Precipitation influences the movement of nutrients and other chemical compounds in the soil through a process called translocation. High levels of precipitation can cause water to wash nutrients deeper into the soil, making them less accessible to plant roots. On the other hand, low precipitation can lead to a buildup of salts in the soil, which can negatively affect fertility.
• Temperature also has an impact on the chemical development of soils and the formation of different layers, known as horizons. When temperatures are below 10°C, biological activities slow down, and at or below freezing point (0°C; 32°F), they cease altogether, rendering chemical processes affecting minerals inactive. As a result, decomposition occurs slowly in cold climates, leading to the accumulation of organic matter and the formation of a thick O horizon. This organic matter turns into humus, which enriches the A horizon.
• In warm, moist climates found at lower latitudes, bacteria rapidly decompose plant material, resulting in a lack of O horizons and minimal organic matter throughout the soil profile.
• Precipitation plays a crucial role in determining soil properties. In areas with heavy rainfall, nutrients and organic matter can be leached from the upper layers of soil unless plant roots or other soil components intervene. For example, soils beneath tropical rainforests tend to be nutrient-poor due to the intense leaching caused by heavy rains, with most nutrients stored in the abundant vegetation. In contrast, arid regions with low annual precipitation experience high evaporation rates, leading to the accumulation of salts in the soil.
• Temperature also affects soil properties by controlling the form of water present on the soil surface and within the soil itself. Additionally, it influences the rate of chemical reactions, evapotranspiration, and biological processes. Large temperature fluctuations, especially when water is present, can cause soil to shrink and swell, undergo frost action, and experience general weathering. For instance, laterite soils form in climates with alternating wet and dry periods, while sandy soils develop in Rajasthan, India, regardless of parent rock type, due to high temperatures and wind erosion.

2. Organisms

• Soil is significantly impacted by living plants and animals, as well as their nonliving organic products. The growth of plant roots, for example, contributes to the mixing and disturbance of soil while also supplying organic matter to the upper layers of the soil.
• There is a wide variety of organisms that inhabit the soil, ranging from bacteria to burrowing mammals. Earthworms are particularly important for soil health as they not only create burrows, but also process soil through their digestive systems. Larger burrowing animals, such as moles, gophers, rabbits, badgers, and prairie dogs, create more extensive, tube-like openings in the soil.
• The activities of earthworms, such as cultivation and mixing, greatly enhance soil structure, increase fertility, reduce the risk of erosion, and deepen the soil profile. The presence of many well-nourished earthworms is often an indicator of a productive, or potentially productive, soil.

3. Relief

• The shape and structure of the ground's surface, known as relief, plays a significant role in soil formation. Generally, soil layers tend to be thicker on gentle slopes and thinner on steep slopes. This is because erosion happens more quickly on steeper slopes, causing the soil to be removed faster. Furthermore, slopes that face away from the Sun usually have cooler and more humid soils since they are protected from direct sunlight. On the other hand, slopes that face the Sun experience higher soil temperatures and increased evapotranspiration due to direct exposure to solar rays.
• Topography also affects the distribution of water on the soil's surface. Water runoff from higher ground leads to wetter conditions in lower areas, sometimes resulting in saline sloughs or organic soils. As a result, topography influences various factors such as soil processes, soil distribution, and the type of vegetation found in a particular location by redistributing climatic features.

4. Parent Material

• The chemistry of soil is significantly influenced by the original source of its parent material. This is because soil inherits various properties from the parent material it forms from, including mineral composition, color, particle size, and chemical elements.
• For instance, the soils in the peninsular regions are highly reflective of their parent rocks. These rocks, which are primarily composed of ancient crystalline and metamorphic materials like granite, gneiss, and schist, form red soils when weathered due to their iron oxide content. On the other hand, soils derived from lava rocks exhibit a black color, while sandy soils originate from sandstone.
• However, it is important to note that soils in the northern plains have a different relationship with their parent material. These soils are transported and deposited from the Himalayan and peninsular blocks, resulting in a less direct connection to the in-situ rock material. This demonstrates the significant role that the parent material plays in determining the properties and characteristics of the soil that develops from it.

5. Time

• The development of soil characteristics and properties takes a significant amount of time. For instance, it may take hundreds or even thousands of years for a mineral deposit, such as the clean sand found in a dune, to develop into sandy soil. Soil formation processes are generally quite slow, often requiring centuries for even a thin layer of soil to form on a newly exposed surface.
• A warm and moist environment can facilitate soil development, but more crucial factors are the attributes of the parent material from which the soil forms. For example, soil forms relatively quickly from sediments, but much more slowly from bedrock. As a general guideline, soil scientists estimate that it takes approximately 500 years to form 2.5 cm (1 inch) of topsoil.

6. Human Activity

• Human activities have a significant impact on the physical and chemical properties of soil. For example, clearing native vegetation for agricultural purposes can lead to erosion, which removes the upper layers of soil that are rich in organic matter. Over time, the structure and composition of these agricultural soils have changed significantly, creating distinct soil classes that are just as important as natural soils.
• Soil is a critical component of an ecosystem, containing both living (biotic) and non-living (abiotic) elements. Biotic factors in the soil include all living and once-living organisms, such as plants and insects. Abiotic factors, on the other hand, consist of non-living elements like minerals, water, and air.
• Essential minerals that support plant growth, such as phosphorus, potassium, and nitrogen gas, are commonly found in soil. Other less common but still important minerals include calcium, magnesium, and sulfur. On average, soil consists of approximately 25% air, 25% water, 45% minerals, and 5% organic matter. This organic matter includes humus (decomposed plant and animal material), microscopic living organisms, and sometimes plant residues.

Conclusion

Soil is a crucial component of Earth's environment and plays a vital role in supporting plant life and influencing human activities. Soil formation, or pedogenesis, is an ongoing process influenced by various factors, including climate, organisms, relief, parent material, time, and human activity. The study of soil, known as pedology, helps us understand the complex interactions between these factors and their impact on soil properties and fertility. It is essential to recognize the importance of soil and its formation processes in order to manage and maintain its health, ensuring the continued support of Earth's ecosystems and human livelihoods.

Frequently Asked Questions (FAQs) of Genesis of Soils

What is the main difference between soil and regolith?

Regolith is a layer of loosely packed inorganic material that covers the unbroken rock beneath it, resulting from the weathering of rocks. Soil is the upper layer of regolith, which contains a mix of living plant roots, decaying plant matter, microscopic plants and animals, as well as varying amounts of air and water. Soil is primarily composed of finely fragmented mineral particles and is the ultimate product of weathering.

What are the four classes of soil-forming processes?

The four classes of soil-forming processes are soil enrichment, removal, translocation, and transformation. Enrichment involves the addition of organic or inorganic matter to the soil, removal involves the loss of materials from the soil mass, translocation refers to the movement of materials upward or downward within the soil, and transformation involves changes occurring within the soil material itself.

How does climate affect soil formation?

Climate, specifically precipitation and temperature, plays a significant role in determining soil properties. Precipitation influences the movement of nutrients and other chemical compounds in the soil, while temperature controls the form of water present on and within the soil and affects the rate of chemical reactions, evapotranspiration, and biological processes.

How do organisms contribute to soil formation?

Organisms, such as plants, animals, and microorganisms, significantly impact soil formation by contributing to the mixing and disturbance of soil, supplying organic matter, and creating burrows that enhance soil structure. For example, earthworms play a crucial role in improving soil fertility and reducing the risk of erosion.

Why is the parent material important in soil formation?

The chemistry of soil is significantly influenced by the original source of its parent material, as soil inherits various properties from the parent material it forms from, including mineral composition, color, particle size, and chemical elements. This means that the parent material plays a crucial role in determining the properties and characteristics of the soil that develops from it.