3.2 Soil horizons and profiles
5 min read•july 30, 2024
Soil horizons and profiles are the building blocks of understanding Earth's skin. They reveal the complex interplay of physical, chemical, and biological processes that shape our planet's surface over time. These layers tell a story of soil formation, offering clues about climate, geology, and land use history.
From the organic-rich at the surface to the weathered near the bedrock, each layer has unique characteristics. By studying these horizons, we can unlock secrets about soil health, fertility, and environmental conditions, crucial for agriculture, engineering, and ecosystem management.
Soil horizons and development
Formation and significance of soil horizons
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- Soil horizons develop through physical, chemical, and biological processes over time
- Represent stages in soil formation providing information about soil history, composition, and environmental conditions
- Crucial for understanding pedogenesis (soil formation and development process)
- Serve as indicators of soil health, fertility, and potential land use capabilities
- Reflect complex interactions between climate, parent material, topography, organisms, and time
- Aid in assessing soil quality and suitability for various agricultural and engineering purposes
- Provide insights into past and present environmental conditions affecting the soil
Processes shaping soil horizons
- breaks down parent material into smaller particles and releases nutrients
- moves dissolved materials and fine particles downward through the soil profile
- Translocation redistributes soil components between horizons (clay particles, iron oxides)
- Organic matter accumulation and decomposition influence the formation of surface horizons
- Bioturbation mixes soil materials through root growth and animal activity
- Mineral transformations alter the chemical and physical properties of soil components
- Redox reactions occur in waterlogged conditions, leading to distinctive soil features (mottling, gleying)
Major soil horizons
Surface and near-surface horizons
- O horizon comprises fresh and partially decomposed organic matter at the soil surface
- Thickness varies depending on vegetation type and decomposition rate
- Subdivided into Oi (litter), Oe (partially decomposed), and Oa (highly decomposed) layers
- (topsoil) rich in organic matter and maximum biological activity
- Dark due to humus content
- Important for nutrient cycling and
- Often has granular or crumb promoting good aeration and root growth
- characterized by (leaching) of clay, iron, and aluminum oxides
- Light-colored layer due to loss of darkening agents
- Often found in forest soils or areas with high precipitation
- May be absent in some soil profiles due to mixing or
Subsurface horizons and parent material
- (subsoil) accumulates materials leached from upper horizons
- Enriched in clay, iron, and aluminum compounds
- Often has blocky or prismatic structure
- May contain distinct features like clay films or iron concretions
- C horizon consists of partially weathered parent material
- Retains much of the original rock structure
- Less affected by soil-forming processes than overlying horizons
- Important for understanding the soil's mineralogical origin
- represents underlying bedrock
- Unweathered and coherent rock material
- Marks the lower boundary of the soil profile
- Influences soil depth and drainage characteristics
Transitional and specialized horizons
- Transitional horizons (AB, BC) exhibit properties of two adjacent horizons
- Important for understanding gradual changes in the soil profile
- Reflect the continuous nature of soil development processes
- Calcic horizons accumulate calcium carbonate in arid or semi-arid environments
- Spodic horizons form in acidic, sandy soils through the accumulation of organic matter, aluminum, and iron
- Argillic horizons show significant clay accumulation through
- Fragipans are dense, brittle subsurface layers that restrict water and root penetration
Interpreting soil profiles
Analyzing horizon characteristics
- Thickness of horizons indicates intensity and duration of soil-forming processes
- Color provides clues about organic matter content, iron oxidation state, and drainage conditions
- Dark colors often indicate high organic matter (A horizon)
- Red or yellow colors suggest iron oxide presence (well-drained conditions)
- Gray or mottled colors indicate poor drainage or reducing conditions
- reflects particle size distribution and influences water-holding capacity and nutrient retention
- Sand (2.0-0.05 mm), silt (0.05-0.002 mm), clay (<0.002 mm)
- Structure describes the arrangement of soil particles into aggregates
- Types include granular, blocky, prismatic, and platy
- Affects water movement, root growth, and soil aeration
Identifying soil-forming processes
- Eluviation and illuviation processes reflected in movement of materials between horizons
- Clay accumulation in B horizon (argillic horizon formation)
- Organic matter and sesquioxide movement in podzolization
- Organic matter accumulation and decomposition rates evident in depth and darkness of A horizon
- Mollic epipedons in grassland soils vs. thinner A horizons in forest soils
- Weathering intensity inferred from degree of parent material alteration in lower horizons
- Development of distinct horizon boundaries
- Presence of secondary minerals (clay minerals, iron oxides)
- Bioturbation effects visible in mixing and homogenization of soil materials
- Root penetration creating channels and pores
- Animal burrowing leading to horizon mixing (krotovinas)
- Redoximorphic features indicate periods of water saturation and oxygen depletion
- Mottling patterns of red, yellow, and gray colors
- Gleying in permanently waterlogged soils
Soil profiles: Climate vs geology
Climate-driven soil profile variations
- Arid and semi-arid regions exhibit minimal horizon development
- Accumulation of salts or carbonates in subsurface horizons (calcic or salic horizons)
- Often have thin A horizons due to low organic matter input
- Humid tropical regions display deeply weathered soil profiles
- Thick B horizons rich in clay and oxides (oxisols)
- Often lack distinct A horizons due to rapid organic matter decomposition
- Intense leaching leads to nutrient-poor, acidic soils
- Temperate forest soils show well-developed horizons
- Prominent A horizon with organic matter accumulation
- Evidence of clay translocation in B horizon (alfisols, ultisols)
- Spodosols in coniferous forests with distinct E and spodic horizons
- Permafrost-affected soils in polar regions have cryoturbated profiles
- Disrupted horizon sequences due to freeze-thaw cycles
- Accumulation of organic matter in surface horizons (cryosols)
Geologic influences on soil profiles
- Young landscapes (recently glaciated areas) have weakly developed horizons
- Retain many characteristics of parent material
- Entisols or Inceptisols with minimal
- Volcanic soils exhibit unique profiles with andic properties
- Rapid weathering of volcanic ash forms amorphous minerals (allophane, imogolite)
- High organic matter retention and phosphorus fixation capacity
- Soil profiles on steep slopes may be truncated or have colluvial materials
- Influence of erosion and deposition processes
- Thinner A horizons on upper slopes, thicker accumulations on lower slopes
- Limestone-derived soils often have high clay content and neutral to alkaline pH
- Terra rossa soils with red, clay-rich B horizons
- Potential for karst topography development
- Floodplain soils show stratified profiles due to periodic sediment deposition
- Buried horizons and abrupt textural changes
- Often have high fertility due to nutrient-rich alluvial deposits
Key Terms to Review (26)
A Horizon: The A horizon, often referred to as the topsoil, is the uppermost layer of soil that is rich in organic matter and nutrients. This layer plays a critical role in supporting plant growth, as it contains the majority of soil's biological activity and is where most of the soil-forming processes take place, influenced by factors such as climate, parent material, and organisms. The A horizon is essential for understanding soil profiles, as it directly affects water retention, root development, and overall ecosystem health.
B Horizon: The B horizon, also known as the subsoil, is a layer of soil situated beneath the A horizon and above the C horizon. This layer is crucial for soil development and fertility as it typically accumulates minerals, nutrients, and organic matter leached down from the upper layers. The characteristics of the B horizon can indicate the processes that have occurred during soil formation, reflecting how factors like climate, vegetation, and topography influence its development.
C Horizon: The C horizon is the layer of soil that lies beneath the A and B horizons, consisting primarily of weathered rock and unconsolidated material. This horizon plays a crucial role in the soil profile as it provides the parent material from which the upper layers develop, influencing both the physical and chemical properties of the soil above it.
Color: Color refers to the visual perception of different wavelengths of light reflected from surfaces, which plays a crucial role in identifying and interpreting soil properties. In the context of soil, color is influenced by factors such as organic matter content, mineral composition, moisture levels, and oxidation states. Understanding the color of soil horizons helps in assessing soil fertility, drainage capabilities, and overall health.
Cover Cropping: Cover cropping is an agricultural practice that involves planting specific crops primarily to improve soil health, manage erosion, and suppress weeds rather than for harvest. This method can enhance the organic matter in the soil, influence the formation of soil horizons, and affect soil profiles, thereby promoting a more sustainable farming system and improving long-term soil fertility.
Crop rotation: Crop rotation is an agricultural practice that involves alternating the types of crops grown on a particular piece of land over a sequence of seasons. This method helps to improve soil health, manage pests and diseases, and enhance crop yields by optimizing nutrient use in the soil. By rotating crops, farmers can influence soil-forming processes and create more complex soil horizons that support diverse ecosystems.
E Horizon: The E horizon, also known as the eluviation horizon, is a specific layer in the soil profile characterized by the leaching or removal of minerals and nutrients. This horizon often appears lighter in color compared to the layers above and below it due to the loss of organic matter and minerals, primarily caused by water movement through the soil. The E horizon plays a crucial role in understanding soil composition and fertility as it influences the availability of nutrients for plants.
Eluviation: Eluviation is the process of leaching or the removal of dissolved or suspended materials from the upper layers of soil, primarily through water movement. This process plays a critical role in soil formation and development by transporting minerals and organic matter downward into lower horizons, affecting soil fertility and composition. Eluviation is often contrasted with illuviation, where materials accumulate in lower soil layers.
Erosion: Erosion is the process by which soil, rock, and other surface materials are worn away and removed from their original location by natural forces such as water, wind, ice, or gravity. This process is essential in shaping landscapes and influencing sediment transport, which connects various components of the Earth's surface system.
Horizon Differentiation: Horizon differentiation refers to the distinct layering of soil horizons that develops over time through various soil formation processes. These layers, known as horizons, differ in physical and chemical properties, including texture, color, structure, and organic matter content, reflecting the specific environmental conditions and processes that have acted upon them.
Illuviation: Illuviation is the process by which materials, such as nutrients, minerals, and organic matter, are leached from one soil horizon and deposited in another, typically lower horizon. This movement occurs due to the downward movement of water, which carries dissolved substances from the upper layers of soil to the lower layers, affecting soil composition and fertility.
Leaching: Leaching is the process by which soluble substances are washed out from soil or rock due to the movement of water. This natural phenomenon plays a crucial role in shaping soil composition, influencing nutrient availability, and determining the types of minerals that are present in different layers of soil.
Mature Soil Profile: A mature soil profile is a well-developed layer of soil that has distinct horizons, exhibiting significant development due to the processes of weathering, leaching, and organic matter accumulation over time. This profile typically contains all major soil horizons, including O (organic), A (topsoil), E (eluviation), B (subsoil), C (parent material), and R (bedrock), illustrating the dynamic interactions between soil components and environmental factors.
Moisture Content: Moisture content refers to the amount of water present in soil, typically expressed as a percentage of the soil's dry weight. This term is crucial for understanding soil behavior, as it affects plant growth, nutrient availability, and the physical properties of the soil. Variations in moisture content can influence soil stability, erosion potential, and the overall health of the ecosystem.
Nutrient Storage: Nutrient storage refers to the capacity of soil to retain essential nutrients that are crucial for plant growth and development. This concept is closely tied to the composition and characteristics of soil horizons, where different layers can accumulate and store nutrients based on their mineral content, organic matter, and biological activity, impacting the overall fertility and productivity of the soil.
O Horizon: The O horizon, also known as the organic horizon, is the topmost layer of soil primarily composed of organic matter such as decomposed leaves, plant material, and other organic materials. This layer is crucial for soil health and fertility, as it provides essential nutrients to plants and supports a diverse ecosystem of microorganisms and fauna that contribute to soil formation processes and overall soil development.
R horizon: The r horizon, also known as the bedrock layer, is the deepest soil layer found beneath the soil profile and is composed of unweathered rock. This layer plays a critical role in the formation of soil above it by influencing drainage, mineral composition, and root penetration. Understanding the r horizon is essential for comprehending soil formation processes and how different horizons interact with one another in the soil profile.
Salinization: Salinization is the process by which water-soluble salts accumulate in the soil, often due to irrigation practices and evaporation. This buildup can lead to adverse effects on soil health, agricultural productivity, and ecosystem balance, especially in arid and semi-arid regions where water evaporation rates are high and rainfall is limited.
Soil Morphology: Soil morphology refers to the study of the physical characteristics and properties of soil, including its structure, color, texture, and layering. Understanding soil morphology is crucial for analyzing soil horizons and profiles, as these features provide insight into the formation processes and environmental conditions that shaped the soil over time. By examining the morphological characteristics of soil, one can determine its classification, fertility, and suitability for various land uses.
Soil Taxonomy: Soil taxonomy is a systematic classification of soils based on their physical and chemical properties, aimed at understanding soil types and their characteristics. It provides a framework for categorizing soils into hierarchical units, which helps in studying soil formation, distribution, and management. This classification system includes various categories such as orders, suborders, great groups, subgroups, families, and series that reflect different soil horizons and profiles.
Structure: In the context of soil science, structure refers to the arrangement and organization of soil particles and the spaces between them, which influence water movement, root penetration, and overall soil health. This organization can take various forms, such as granular, blocky, or platy structures, which are shaped by soil-forming factors and processes, as well as the development of distinct layers in a soil profile.
Texture: In soil science, texture refers to the relative proportions of different particle sizes within a soil, including sand, silt, and clay. The texture influences the soil's physical properties, such as drainage, nutrient retention, and aeration, making it a key aspect of understanding soil behavior and function. Variations in texture result from soil-forming factors like parent material and weathering processes, leading to distinct soil horizons and profiles that can be classified and mapped based on their unique textures.
USDA Soil Classification: The USDA Soil Classification is a systematic approach to categorizing soils based on their physical and chemical properties, helping to understand their behavior and suitability for various uses. This classification system allows scientists and land managers to identify soil types and their respective horizons, which are distinct layers in the soil profile that differ in composition and characteristics. By utilizing this framework, better land management practices and agricultural strategies can be developed based on the specific needs of the soil.
Water retention: Water retention refers to the ability of soil to hold water within its structure, making it available for plants and organisms. This property is influenced by various factors, including soil texture, structure, and organic matter content, which all play a crucial role in determining how much moisture the soil can retain over time. The distribution of soil horizons affects water retention as different layers have varying capacities and characteristics that influence overall moisture availability.
Weathering: Weathering is the process of breaking down rocks and minerals at the Earth's surface through physical, chemical, and biological means. This process plays a critical role in shaping landscapes, influencing soil formation, and affecting ecosystems by contributing to the material that makes up soils and sediment.
Young soil profile: A young soil profile refers to a relatively undeveloped layer of soil that has not yet undergone significant horizonation or differentiation. These profiles often exhibit minimal layering and limited organic matter, making them distinct from older, more mature soils. The characteristics of a young soil profile are influenced by recent parent material, weathering processes, and the surrounding environmental conditions.