5.3 Soil formation, profiles, and classification
2 min read•july 24, 2024
Soil formation is a complex process influenced by various factors. From physical and to , these processes work together to create the diverse soils we see today. Understanding how soils form is crucial for agriculture, construction, and environmental management.
Soil profiles reveal the intricate layers that develop over time. These horizons, from the organic-rich topsoil to the underlying bedrock, tell a story of soil development. Classification systems help us organize this complexity, providing a common language for soil scientists and land managers worldwide.
Soil Formation and Components
Process of soil formation
Top images from around the web for Process of soil formation
9.1 Soil Profiles & Processes | Environmental Biology View original
Is this image relevant?
The Soil | OpenStax Biology 2e View original
Is this image relevant?
Lecture 7: Soil Horizon - Nre509 View original
Is this image relevant?
9.1 Soil Profiles & Processes | Environmental Biology View original
Is this image relevant?
The Soil | OpenStax Biology 2e View original
Is this image relevant?
1 of 3
Top images from around the web for Process of soil formation
9.1 Soil Profiles & Processes | Environmental Biology View original
Is this image relevant?
The Soil | OpenStax Biology 2e View original
Is this image relevant?
Lecture 7: Soil Horizon - Nre509 View original
Is this image relevant?
9.1 Soil Profiles & Processes | Environmental Biology View original
Is this image relevant?
The Soil | OpenStax Biology 2e View original
Is this image relevant?
1 of 3
- involves breaking down rocks into smaller particles, chemical weathering altering mineral composition, and biological activity adding and influencing soil structure
- , , , organisms, and time affect soil formation (CLORPT factors)
- Organic matter contribution through decomposition of plant and animal residues forms and drives
- Time allows accumulation of weathered materials, development of distinct , and influences and profile depth (young vs. )
Components of soil
- categorized by size: (2.0 - 0.05 mm), (0.05 - 0.002 mm), (< 0.002 mm)
- Organic matter includes living organisms, dead plant and animal residues, and humus
- Water forms and occupies pore spaces crucial for nutrient transport
- Air in pore spaces not filled with water enables root respiration and microbial activity
- Typical : ~45% mineral matter, ~5% organic matter, ~25% water, ~25% air
Soil Profiles and Classification
Soil horizons and characteristics
- : organic layer at surface with leaf litter and partially decomposed matter
- : topsoil rich in organic matter and zone of maximum biological activity
- : zone of eluviation where materials are leached downward
- : subsoil with accumulation of clay, iron, and aluminum oxides
- : partially weathered parent material or sediments with minimal soil development
- : solid bedrock underlying soil profile
Soil classification systems
- : hierarchical system with 6 levels (Order, Suborder, Great Group, Subgroup, Family, Series)
- 12 soil orders: , , , , , , , , , , ,
- Classification based on soil properties, diagnostic horizons, and soil temperature and moisture regimes
- Guides agricultural practices, aids urban planning, supports conservation, facilitates communication among soil scientists
- Other systems: World Reference Base for Soil Resources (WRB), various national systems
Key Terms to Review (39)
A Horizon: The A horizon, also known as the topsoil, is the uppermost layer of soil in a soil profile. It is crucial for plant growth as it contains a high concentration of organic matter, nutrients, and microorganisms that support life. This layer plays a vital role in soil formation and classification, reflecting the influence of climate, organisms, and human activities on soil characteristics.
Alfisols: Alfisols are a soil order characterized by clay-rich subsoils that have a significant amount of nutrients and fertility, making them suitable for agriculture. These soils typically form in temperate regions with distinct seasonal changes and can support a variety of vegetation, including forests and grasslands. Their formation is influenced by factors such as climate, parent material, and biological activity, resulting in distinct soil profiles that are important for understanding soil classification and land use.
Andisols: Andisols are a type of soil formed from volcanic ash and other volcanic materials, characterized by their high fertility and unique physical properties. These soils often develop in areas with frequent volcanic activity and can support diverse vegetation due to their rich nutrient content. The formation of Andisols is influenced by factors such as climate, topography, and the nature of the parent material.
Aridisols: Aridisols are a type of soil found in arid regions, characterized by low moisture availability and typically high salinity. They often support desert vegetation and can have varying mineral content, which affects their use in agriculture and land management. Their formation is closely related to climate, topography, and parent material, making them a significant focus in understanding soil classification and profiles.
B Horizon: The B horizon, often referred to as the subsoil, is a layer of soil located beneath the A horizon and above the C horizon. This layer is significant in soil profiles as it is where materials leached from the upper layers accumulate, playing a crucial role in soil fertility and structure.
Biological activity: Biological activity refers to the processes and interactions of living organisms within an ecosystem that influence soil formation, composition, and structure. This includes the role of microorganisms, plants, and animals in breaking down organic matter, cycling nutrients, and altering soil properties. These activities not only impact soil health but also affect weathering and erosion rates through mechanisms such as root growth, burrowing, and decomposition.
C Horizon: The C horizon is the layer of soil that lies beneath the A (topsoil) and B (subsoil) horizons, composed mainly of weathered parent material. This layer plays a crucial role in soil formation as it serves as the source of mineral content for the upper horizons while also containing larger rock fragments and partially disintegrated materials that have yet to undergo significant soil development.
Chemical weathering: Chemical weathering is the process by which rocks and minerals undergo chemical alterations due to interactions with water, air, and biological organisms. This type of weathering breaks down the original minerals and forms new minerals, often leading to soil formation and altering landscapes. Understanding chemical weathering is crucial as it contributes to soil development, influences the characteristics of different soil profiles, and plays a significant role in the formation of unique landforms such as karst landscapes.
Clay: Clay is a fine-grained natural soil material that contains clay minerals and is characterized by its plasticity when wet and hardness when dry. This unique composition is crucial in soil formation, as it influences drainage, nutrient retention, and overall soil fertility. Clay particles are typically less than 0.002 mm in diameter, allowing them to bind tightly together, which impacts the formation of soil horizons and the classification of different soil types.
Climate: Climate refers to the long-term average of weather conditions, including temperature, precipitation, humidity, and wind patterns in a particular region over extended periods, usually 30 years or more. It plays a crucial role in shaping various Earth processes, influencing the rock cycle, soil formation, weathering, erosion, and the hydrologic cycle.
E horizon: The e horizon, also known as the eluviation horizon, is a layer in the soil profile characterized by the leaching of minerals and organic matter, resulting in a lighter color compared to the layers above and below it. This horizon typically occurs between the upper organic layer and the lower illuvial horizon, playing a critical role in soil formation by influencing the nutrient availability and overall fertility of the soil.
Entisols: Entisols are a soil order characterized by their young age and lack of significant horizon development, typically found in areas with recent sediment deposition or disturbances. They are often associated with environments such as river valleys, floodplains, and regions with minimal soil formation processes due to factors like erosion or human activity. Understanding Entisols is essential for studying soil formation, profiles, and classification as they represent the initial stages of soil development.
Gelisols: Gelisols are a soil order characterized by the presence of permafrost within two meters of the soil surface, often found in cold regions such as tundra and polar areas. These soils are significant because their formation and properties are closely linked to climate conditions, parent material, and the biological activity that occurs in these frigid environments. Gelisols play an important role in the global carbon cycle and influence land use and vegetation patterns in the areas they occupy.
Histosols: Histosols are a type of soil characterized by a high organic matter content, specifically composed of decomposed plant material known as peat. These soils typically form in wetland environments where waterlogged conditions slow down the decomposition process, allowing organic material to accumulate. Histosols are significant for their unique properties, which influence soil formation, profiles, and classification within various ecosystems.
Humus: Humus is a dark, organic component of soil formed from the decomposition of plant and animal matter. It plays a crucial role in soil fertility, enhancing its structure, moisture retention, and nutrient availability, which are essential for healthy plant growth.
Inceptisols: Inceptisols are a soil order characterized by their weakly developed profiles and a significant degree of horizon differentiation. They often form in a variety of climates and landscapes, typically in areas with young landforms or where the process of soil formation is still ongoing, making them crucial for understanding soil formation and classification.
Mature Soils: Mature soils are developed soils that have undergone significant weathering and horizon development, resulting in distinct layers or horizons with well-defined characteristics. These soils are typically rich in nutrients and have a stable structure, indicating a long period of formation under consistent environmental conditions, which makes them crucial for supporting diverse plant life and ecosystems.
Mineral particles: Mineral particles are the small solid fragments derived from the weathering and breakdown of rocks, which play a crucial role in soil formation and composition. These particles contribute to the physical structure of soil, influencing its texture, drainage, and nutrient retention. Their size and mineral composition significantly impact soil properties and the types of vegetation that can thrive in different environments.
Mollisols: Mollisols are a type of soil characterized by a thick, dark topsoil rich in organic matter, typically found in grassland ecosystems. This unique soil formation results from the accumulation of decomposed plant material and is particularly fertile, making it ideal for agriculture. Mollisols play a critical role in soil profiles and classification, showcasing the importance of organic content in determining soil properties and land use capabilities.
Nutrient Cycling: Nutrient cycling refers to the movement and exchange of organic and inorganic matter back into the production of living matter within ecosystems. This process is crucial for maintaining soil health and fertility, as it involves the recycling of essential nutrients like nitrogen, phosphorus, and potassium through various biogeochemical processes, including decomposition, mineralization, and absorption by plants. Understanding nutrient cycling helps explain how ecosystems function and sustain plant growth, directly linking to soil formation, profiles, and classification.
O Horizon: The O horizon is the uppermost layer of soil, primarily composed of organic matter such as decomposed leaves, plant materials, and other organic debris. This layer plays a crucial role in soil formation and is vital for nutrient cycling, providing a rich habitat for microorganisms and supporting plant growth. Its characteristics greatly influence the overall health and fertility of the soil profile beneath it.
Organic Matter: Organic matter refers to the decomposed remains of plants, animals, and microorganisms found in soil, which plays a crucial role in soil health and fertility. It enriches the soil by providing essential nutrients, improving its structure, and enhancing water retention. Organic matter is vital for maintaining soil biodiversity and promoting the growth of crops and vegetation.
Oxisols: Oxisols are a type of soil found primarily in tropical regions, characterized by their high degree of weathering and low nutrient content. These soils are formed in humid conditions and are typically rich in iron and aluminum oxides, which contribute to their reddish color. Oxisols are significant as they reflect the processes of soil formation and classification, demonstrating how environmental conditions impact soil characteristics over time.
Parent Material: Parent material refers to the underlying geological material from which soil forms. It serves as the source of mineral particles and influences the soil's physical and chemical properties, impacting its formation, profile development, and classification. The type of parent material can significantly determine soil characteristics such as texture, fertility, and drainage, which are crucial for agricultural and ecological processes.
Pedogenesis: Pedogenesis is the process of soil formation that involves the physical, chemical, and biological weathering of parent material, as well as the accumulation and transformation of organic matter. This dynamic process plays a crucial role in the development of distinct soil profiles, which exhibit varying characteristics based on factors like climate, organisms, topography, parent material, and time. Understanding pedogenesis is essential for classifying soils and comprehending their ecological functions.
Physical Weathering: Physical weathering refers to the mechanical breakdown of rocks and minerals into smaller fragments without altering their chemical composition. This process plays a critical role in soil formation and influences soil profiles by contributing to the mineral content and structure of soils, which are essential for plant growth and ecosystem health.
R horizon: The r horizon, also known as the bedrock layer, is the lowest layer of soil in a soil profile, consisting of unweathered rock. This layer is crucial in the context of soil formation, as it provides the mineral foundation from which soils develop through weathering processes. It influences the types of soils that can form above it, the drainage characteristics, and the nutrient availability for plants.
Sand: Sand is a granular material composed of finely divided rock and mineral particles, typically ranging in size from 0.0625 mm to 2 mm in diameter. This vital component of soil plays a key role in soil formation, influencing properties such as drainage, aeration, and nutrient retention, which are essential for plant growth and soil health.
Silt: Silt is a granular material with particles that are smaller than sand but larger than clay, typically ranging from 0.002 to 0.05 millimeters in diameter. It plays a crucial role in soil formation and affects the characteristics of soil profiles, influencing properties such as fertility, drainage, and aeration. Silt is important for creating fertile soils, especially in river valleys, due to its ability to retain moisture and nutrients.
Soil composition: Soil composition refers to the specific mixture of organic and inorganic materials that make up soil, including minerals, organic matter, water, and air. This mixture plays a critical role in determining soil properties, such as fertility, drainage, and texture, which in turn influence plant growth and ecosystem health. Understanding soil composition is essential for analyzing soil formation processes, interpreting soil profiles, and classifying different types of soils.
Soil Horizons: Soil horizons are distinct layers of soil that develop as a result of soil formation processes and are characterized by different physical, chemical, and biological properties. These layers, which include the O, A, E, B, and C horizons, reflect the influence of weathering processes on the parent material and contribute to soil classification and fertility. Understanding soil horizons is crucial for determining land use, agriculture, and environmental management.
Soil maturity: Soil maturity refers to the degree of development and differentiation in a soil profile, characterized by the presence of distinct horizons and the accumulation of organic matter and nutrients over time. As soils evolve, they undergo various physical, chemical, and biological processes that contribute to their stability and fertility, which are crucial for supporting plant growth and sustaining ecosystems.
Soil solution: Soil solution refers to the liquid phase of soil, composed of water and dissolved substances such as minerals, nutrients, and organic matter. This solution is crucial for plant growth as it provides essential nutrients that plants absorb through their roots. Understanding soil solution helps in analyzing soil formation, profiles, and classification as it plays a vital role in determining soil fertility and health.
Spodosols: Spodosols are a type of soil that typically forms under coniferous forests in cool, humid regions. Characterized by their distinctive layers, spodosols are notable for their leached upper horizon and accumulation of iron, aluminum, and organic matter in the subsoil, which results in a unique profile. These features make spodosols particularly important for understanding soil formation, profiles, and classification in forested ecosystems.
Topography: Topography refers to the arrangement of the natural and artificial physical features of an area, including its terrain, elevation, and landforms. Understanding topography is essential as it influences geological processes, the formation of soil, weathering, erosion rates, and the movement of water through drainage systems.
Ultisols: Ultisols are a type of soil found primarily in humid temperate and tropical regions, characterized by their weathered nature and low nutrient content. These soils typically have a clay-rich subsoil and are highly leached, leading to the depletion of essential minerals like calcium and magnesium. Ultisols often support forests and other vegetation but require careful management for agricultural use due to their acidity and nutrient deficiency.
USDA Soil Taxonomy: USDA Soil Taxonomy is a comprehensive classification system developed by the United States Department of Agriculture that categorizes soils based on their physical and chemical properties, horizon development, and genesis. This system helps to organize soil types into a hierarchical structure, making it easier to understand soil behavior, manage agricultural practices, and assess environmental impacts. It plays a crucial role in soil conservation and land-use planning.
Vertisols: Vertisols are a type of soil characterized by a high content of clay, which expands when wet and contracts when dry, leading to significant cracking and unique structural features. This soil type is typically found in regions with distinct wet and dry seasons, and it plays a crucial role in understanding soil formation, profiles, and classification due to its physical properties and behavior under varying moisture conditions.
Young soils: Young soils are soils that have not yet fully developed, often characterized by limited horizons and a lack of distinct layers due to their recent formation. These soils typically arise in environments where weathering processes are still actively shaping the landscape, and they exhibit properties that reflect their parent material rather than having undergone significant alteration from biological activity or leaching.