Soil Formation or Pedogenesis

Soil is a complex and dynamic natural resource that forms the outermost layer of the Earth’s crust. It serves as a reservoir of minerals, organic matter, water, air, and living organisms. Soil also acts as a medium for the filtration and breakdown of injurious wastes. It also helps in the cycling of carbon and other elements through the global ecosystem. The formation of soil is a result of the weathering of rocks over time, a process influenced by factors such as climate, topography, organisms, and time.

Regolith and Soil: Any solid but unconsolidated material lying on the surface of the bedrock is known as “Regolith”. It is constituted of soil, alluvium, and rock fragments weathered from the bedrock. Usually, the thickness of this layer varies from nil (over a rock exposed to erosional agents) to very deep (in areas protected from erosion). The portion of regolith, which supports the growth of plants is known as Soil. Thus soil is a combination of minerals, organic matter, water, and air.

Components of Soil

Soil serves essential functions in supporting plant life, filtering and purifying water providing habitat for various organisms, and serving as a foundation for many human activities such as agriculture and construction. Different regions may have different types of soil, and soil quality can vary, but the following components are known to be the basic elements while defining the soil.

1. Mineral particles: These are the inorganic components of soil and include sand, silt, and clay. The relative proportions of these particles determine the soil’s texture. Sandy soils have larger particles, while clayey soils have smaller particles. The mineral particles contribute to the physical structure of the soil and affect its drainage and nutrient-holding capacity.
2. Organic matter including living organisms: Organic matter present in the soil consists of remains of decaying plant and animal materials. Dead plant roots, leaves, stems, and other organic residues are included in it. Organic matter can significantly improve soil structure, water retention, and availability of nutrients. It also serves as a source of energy and nutrients for soil microorganisms, contributing to overall soil fertility.
3. Water: Water is another important component present in the soil. It is found in the spaces between soil particles and is essential for plant growth. The amount of water a soil can hold depends on its texture and organic material. An adequate quantity of moisture in the soil is necessary for nutrient transport, chemical reactions, and the various biological processes occurring in the soil.
4. Air: Soil possesses pores and spaces in it. These spaces are filled with air and are significantly important for the exchange of gases such as oxygen and carbon dioxide. Soil aeration is necessary for the respiration of plant roots and soil organisms. Compact or waterlogged soils may have reduced air circulation, which results in reduced levels of plant growth and microbial activity.

The interaction and balance among these four components determine the physical, chemical, and biological properties of the soil. These components influence the suitability of soil for plant growth and other ecosystem functions.

Soil Profile

If we examine the walls of a trench, they are found to contain a series of horizontal layers. These layers are called “horizons” All horizons together form the “soil profile”. The three basic horizons from top to bottom are A, B, and C.

A-Horizon

It is the uppermost layer of the soil profile. it is also called “surface soil”. This layer contains organic matter and microorganisms. In this layer the greatest biological activity takes place. It significantly impacts plant growth and various ecosystem processes. Practices of soil management, such as organic matter additions, cover cropping, and proper irrigation, can help preserve and improve the fertility of the A-horizon. Some key features of A-horizon are:

Composition: A sufficient amount of organic matter, minerals, and nutrients is found in A-horizon, thus making it the most fertile layer of the soil. Its fertility is because of the presence of decomposed plant and animal matter, as well as weathered rock particles.

Color: The color of the A-horizon depends on its content. The accumulation of the organic components makes it darker as compared to the lower layers. The process of decay and decomposition of leaves and other plant material by soil microorganisms is continued persistently.

Texture: The texture of this layer is determined by the types and sizes of minerals present. It ranges from sandy to loamy to even clayey, thus affecting water retention, drainage, and nutrient availability.

Presence of Plant Roots: In this layer, the roots of most plants grow to absorb water and nutrients. The abundance and distribution of roots can further influence soil structure and composition using mechanical and chemical processes. This layer is enriched with water, food, and nutrients.

Biological Activity: The A-horizon is teeming with microbial life, including bacteria, fungi, and other organisms that play a crucial role in decomposing organic matter and nutrient cycling.

Depth: The depth of the A-horizon can vary depending on many factors, but it generally extends several inches to a few feet below the soil surface.

B-Horizon (Subsoil)

It lies beneath the A-horizon. This layer is also also known as the subsoil. It is a layer in the soil profile that is often characterized by the accumulation of minerals and materials leached down from the overlying A-horizon. The characteristics of the B-horizon can vary widely depending on factors such as climate, parent material, vegetation, and soil-forming processes. The following features can distinguish it from A and C-horizon.

Mineral Illuvation: The B-horizon typically contains minerals and nutrients that have leached down from the A-horizon (the overlying layer). This process is known as leaching and involves the downward movement of dissolved minerals and organic matter through the soil. Accumulation of dissolved or suspended soil materials in one area or layer as a result of leaching (percolation) from another. Usually, clay, iron, or humus wash out and form a line with a different consistency and color. These lines are important for studying the composition and ages of rock strata.

Color Changes: The difference in colors between the two layers is due to the quantity and characteristics of minerals present in the two layers. The B-horizon often exhibits color changes compared to the A-horizon. These changes may be indicative of mineral content and the processes that have affected the soil over time.

Structure: The structure of the B-horizon can vary, and it may exhibit different physical characteristics compared to the overlying soil layer. The texture, compactness, and other structural properties are quite different from A-horizon.

Root Penetration: Generally most of the plant roots are limited to the A-horizon, but still some roots may extend into the B-horizon, but their density is usually lower than in the A-horizon. The B-horizon provides a medium for root growth and supports plant anchorage.

C-Horizon (Parent Material)

The lowest horizon is called the C-horizon. It mainly consists partly of altered parent rock materials. The C-horizon is the layer of soil that represents the unconsolidated, weathered parent material from which the soil has developed. This horizon is not significantly affected by soil-forming processes like leaching or organic matter accumulation. Instead, it serves as the source of mineral particles and nutrients for the overlying horizons.

Composition of C-horizon: The composition of the C-horizon can vary widely, ranging from partially weathered rock to unconsolidated sediments. The characteristics of the C-horizon play a crucial role in determining the overall fertility and physical properties of the soil. Soil scientists often study the C-horizon to understand the geological processes that have influenced the development of soils in a particular region.

Colors of C-horizon: Commonly the C-horizon is found in brown, red, yellow, gray, or even black colors. These colors are often indicative of the presence of certain minerals or the effects of weathering processes on the original rock material.

Texture of C-horizon: The geological characteristics of the parent material determine the color of the C-horizon. It may include a mix of sand, silt, clay, and various-sized rock fragments. The presence of certain minerals and the degree of weathering influence the overall texture. Commonly, C-horizons have a coarser texture than the overlying horizons, but this can vary based on the specific geological history of the region.

Factors Affecting Formation of Soil

Earth scientists attribute soil formation to several factors, among which parent material, biodata (living organisms), climate, time, and topography are prominent and most active. These five factors interact to form thousands of soil forms around the globe. Different types of soils can be managed differently by studying their physical, biological, and chemical properties. Let’s throw a detailed light on these factors.

Parent Material

The parent material, which is also known as “bedrock”, or “regolith” has a significant role in soil formation. Soil formation (pedogenesis) is a complex process influenced by the mineral and chemical composition of the parent material. Different types of minerals and chemicals determine the soil properties differently.

Mineral Composition: The mineral composition of the parent material determines the types and proportions of minerals present in the soil. Different minerals weather in different ways, resulting in variations in soil texture and fertility. For example, granite-based parent material may result in soils with higher quartz content, while basalt may contribute more to soils with higher concentrations of certain minerals.

Chemical Composition: Different types of bedrock contain different chemicals. The chemical composition of the parent material influences the initial nutrient composition of the soil. Certain parent materials may contain minerals rich in essential nutrients, while others may lack these nutrients. With the course processes of weathering break down the minerals, thus releasing nutrients and influencing soil fertility.

Rate of weathering: The rate at which the parent material weathers influences the soil formation. Parent materials are broken down by the processes of physical and chemical weathering, such as freeze-thaw cycles, organic activities, and mineral dissolution. The faster the weathering rate, the more developed is the soil.

Distribution of the particle size: The soil texture is determined by the size of particles in bedrock. Coarse-grained parent materials, like sand or gravel, result in sandy soils, while fine-grained materials, like clay, contribute to clayey soils. The combination of these particle sizes influences water retention, drainage, and aeration properties of the soil.

Living Organisms (biodata)

Living organisms present in soil play a significant role in soil formation. They act through various biological, physical, and chemical processes. The development of soil horizons, fertility, and the soil structure are largely contributed by these factors. They contribute development of soil in several ways.

Decomposition: Soil fertility is largely dependent on the decomposition of organic matter, such as plant and animal remains. Microorganisms like bacteria and fungi break down these materials to form humus, which improves soil structure, nutrient content, and water retention.

Root Activity: Plant roots exert force on the rocks, physically break down them, and contribute organic matter to the soil as they grow and decay. The process is known as bioturbation, and it helps in soil aeration, promotes the development of soil structure, and facilitates the movement of water and nutrients.

Biological Weathering: Some microorganisms produce organic acids that contribute to the weathering of minerals in the soil. This biological weathering breaks down rocks into smaller particles, contributing to soil formation.

Burrowing Organisms: Soil organisms like earthworms, ants, and termites create channels and pores in the soil through their burrowing activities. This improves soil structure, enhances water infiltration, and facilitates the movement of air and nutrients.

Nitrogen Fixation: Certain bacteria, like rhizobia associated with legume roots, have the ability to convert atmospheric nitrogen into a form that plants can use. This process, known as nitrogen fixation, contributes to the fertility of the soil.

Plant Succession: Over time, different plant species may colonize an area and undergo a process known as plant succession. The types of plants that grow in an area can influence the type and quality of organic matter added to the soil, affecting soil structure and nutrient content.

Mycorrhizal Associations: Many plants form symbiotic relationships with mycorrhizal fungi, which help them in nutrient uptake, especially phosphorus. These fungi extend the reach of plant roots, increasing the plant’s access to soil resources.

Climate

Climate plays a pivotal role in soil formation, by influencing various processes that shape the characteristics of soils. The interplay of several climate factors, along with parent material, topography, and time, leads to the development of different soil types and profiles around the world.

Role of Temperature: Soil formation is influenced by temperature through its impact on chemical and biological processes. Higher temperatures generally accelerate chemical reactions, leading to faster weathering of rocks and minerals. Temperature also affects the rate of organic matter decomposition. Warmer climates tend to speed up microbial activity, breaking down organic materials more quickly.

Precipitation: The amount and distribution of precipitation significantly impact soil formation. Water is a key agent in weathering rocks and transporting minerals within the soil profile. Excessive rainfall can leach minerals from the soil, leading to the formation of leached or laterite soils. Conversely, arid regions may have more accumulation of soluble minerals due to limited leaching.

Humidity: Humidity influences the rate of weathering, affecting the breakdown of rocks and minerals into smaller particles. High humidity can enhance chemical weathering by promoting the dissolution of minerals.

Vegetation: Climate affects the type and density of vegetation, which, in turn, influences soil formation. Plants contribute organic matter to the soil through litter and root material. The decomposition of organic matter contributes to the formation of soil organic horizons. Different climates support different types of vegetation, leading to variations in the organic content and structure of soils.

Freeze-Thaw Cycles: In cold climates, freeze-thaw cycles play a significant role in soil formation. The expansion of water as it freezes and contraction as it thaws contribute to physical weathering, breaking down rocks into smaller particles.

Wind: Wind erosion and transportation of soil particles are more prevalent in arid and semi-arid climates. Wind can lead to the accumulation of fine particles in certain areas, forming windblown deposits such as loess.

Sunlight: Solar radiation influences soil temperature and moisture levels. The intensity and duration of sunlight affect the rate of evaporation and, consequently, soil moisture content.

Topography

Time Factor

• Factors affecting the process of weathering
• Theory of seafloor spreading
• Convection Currents