31.2 The Soil
4 min read•june 14, 2024
formation is a complex process influenced by , climate, , organisms, and time. These factors interact to create distinct soil layers, each with unique properties that support diverse ecosystems and plant growth.
Understanding soil composition is crucial for managing ecosystems and agriculture. , structure, organic matter, pH, and biodiversity all play vital roles in determining soil health and fertility, impacting plant growth and ecosystem function.
Soil Formation and Composition
Process of soil formation
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- Soil formation () occurs through the interaction of five main factors:
- Parent material: original rock or organic matter from which soil develops (granite, limestone, volcanic ash)
- Climate: temperature and precipitation influence and biological activity (tropical, temperate, arid)
- Topography: slope and aspect affect water retention and (hillsides, valleys, plateaus)
- Organisms: plants, animals, and microorganisms contribute to soil development (trees, grasses, earthworms, fungi)
- Time: duration of soil formation processes (hundreds to thousands of years)
- Weathering breaks down parent material into smaller particles
- Physical weathering: mechanical breakdown due to temperature changes, frost action, or abrasion (freezing and thawing, wind erosion)
- Chemical weathering: alteration of minerals through reactions such as oxidation, hydrolysis, or carbonation (acid rain, dissolution of limestone)
- Soil components include:
- Mineral particles: , , and
- Organic matter: decomposed plant and animal residues ()
- Water: essential for plant growth and soil organism activity
- Air: fills pore spaces and provides oxygen for roots and microorganisms
- Organisms: bacteria, fungi, earthworms, and other soil biota
Layers of soil profiles
- A consists of distinct layers called horizons, formed by vertical processes
- (organic layer):
- Surface layer composed of fresh or partially decomposed organic matter (leaf litter, twigs)
- Thin in grasslands and thicker in forests
- (topsoil):
- Mineral layer with high organic matter content and biological activity
- Dark in color due to (decomposition of organic matter)
- Most fertile layer for plant growth
- (eluviated layer):
- Light-colored layer due to leaching of clay, iron, and aluminum ()
- Not always present in all soil profiles
- (subsoil):
- Accumulation of clay, iron, or organic compounds leached from above layers ()
- Brighter in color and denser than the A
- Less biological activity compared to the A horizon
- (parent material):
- Partially weathered parent material ()
- Minimal biological activity and organic matter
- ():
- Solid, unweathered rock beneath the soil profile (granite, basalt, sandstone)
Soil composition for ecosystems
- Soil texture (relative proportions of sand, silt, and clay) influences:
- Water retention and drainage (sandy soils drain quickly, clay soils retain water)
- Nutrient holding capacity (clay soils have high )
- Aeration and root penetration (sandy soils are well-aerated, clay soils can be compacted)
- Ideal soil texture for most plants is , a balanced mixture of sand, silt, and clay
- (aggregation of soil particles) affects:
- Porosity and water infiltration (granular structure has high )
- Root growth and soil organism movement (blocky structure can impede root growth)
- Resistance to erosion (well-structured soils are less prone to erosion)
- Soil organic matter improves:
- Water and nutrient retention (humus acts as a sponge)
- Soil structure and aggregation (organic matter promotes formation of stable aggregates)
- Microbial activity and biodiversity (organic matter provides energy and nutrients for soil organisms)
- affects nutrient availability and plant growth
- Most plants prefer slightly acidic to neutral soils (pH 6.0-7.5)
- Extreme pH levels can lead to nutrient deficiencies or toxicities (aluminum toxicity in acidic soils, iron deficiency in alkaline soils)
- Soil biodiversity supports:
- Decomposition and nutrient cycling (bacteria and fungi break down organic matter)
- Plant growth and disease suppression (mycorrhizal fungi improve nutrient uptake and protect against pathogens)
- Carbon sequestration and ecosystem resilience (healthy soils store more carbon and are more resilient to disturbances)
Soil Management and Conservation
- is crucial for sustainable agriculture and ecosystem health
- Influenced by nutrient availability, organic matter content, and biological activity
- Can be improved through proper management practices and fertilization
- Erosion is a major threat to soil resources
- Caused by wind, water, and human activities
- Leads to loss of topsoil, reduced fertility, and environmental degradation
- practices aim to protect and improve soil quality
- Include techniques such as contour plowing, terracing, and cover cropping
- Help maintain soil structure, reduce erosion, and enhance soil fertility
- plays a vital role in soil fertility
- Process by which certain bacteria convert atmospheric nitrogen into plant-available forms
- Legumes form symbiotic relationships with nitrogen-fixing bacteria in root nodules
Key Terms to Review (38)
A horizon: The A horizon, commonly referred to as topsoil, is the uppermost layer of soil that is rich in organic material and nutrients, making it essential for plant growth. This layer is typically darker in color due to the presence of decomposed organic matter, which contributes to its fertility and biological activity. The A horizon plays a crucial role in supporting ecosystems, influencing water retention, and providing a habitat for numerous organisms.
B horizon: The B horizon is a soil layer found below the A horizon and above the C horizon. It is characterized by the accumulation of minerals such as iron, aluminum, and clay leached down from the A horizon.
B horizon: The B horizon, often referred to as the subsoil, is the layer of soil found beneath the topsoil (A horizon) and above the parent material. It is characterized by the accumulation of minerals and nutrients leached down from the upper layers, making it essential for plant growth and soil fertility. This layer plays a crucial role in the overall soil profile and influences water drainage and root development.
Bedrock: Bedrock is the solid rock layer beneath soil and regolith. It provides a stable foundation for soil development and influences soil characteristics through weathering processes.
Biological nitrogen fixation: Biological nitrogen fixation is the process by which certain prokaryotes convert atmospheric nitrogen (N₂) into ammonia (NH₃), making it available for use by plants. This process is crucial for the nitrogen cycle and supports plant growth in ecosystems.
C horizon: The C horizon is the layer of soil located below the A and B horizons, consisting mainly of weathered parent material from which the soil is formed. This layer plays a critical role in soil development, as it contains the minerals and nutrients that contribute to the soil's overall composition and fertility. The C horizon is essential for understanding soil profiles and the processes of soil formation, including weathering and organic matter accumulation.
Cation Exchange Capacity: Cation exchange capacity (CEC) is a measure of how well soil can retain and supply cations to plant roots, which are positively charged ions. CEC is crucial for understanding soil fertility, as it influences the availability of essential nutrients like potassium, magnesium, and calcium to plants. A higher CEC typically indicates better nutrient retention and a healthier soil ecosystem, making it an essential aspect of soil science.
Clay: Clay is a fine-grained natural soil material containing clay minerals. It has high plasticity when wet and can hold water and nutrients effectively.
E horizon: The E horizon, also known as the eluviation layer, is a specific layer of soil found beneath the O and A horizons, characterized by the leaching of minerals and organic materials. This horizon is typically lighter in color due to the loss of clay, iron, and other elements, leading to a more sand- or silt-rich composition. It plays a crucial role in soil formation and influences the characteristics of the layers below it.
Erosion: Erosion is the process through which soil, rock, and other surface materials are worn away and transported from one location to another by natural forces such as water, wind, and ice. This process plays a critical role in shaping landscapes and affects soil quality, fertility, and agricultural productivity. Erosion can be both a natural phenomenon and a result of human activities, leading to significant environmental impacts.
Horizon: A horizon is a distinct layer of soil that has unique physical and chemical properties compared to other layers. These horizons are part of the soil profile and influence plant nutrition and soil health.
Humification: Humification is the process by which organic matter in soil is broken down and transformed into humus, a dark, stable form of organic matter that enhances soil fertility and structure. This process is crucial for nutrient cycling and helps to improve soil's ability to retain water, supporting plant growth and maintaining ecosystem health.
Humus: Humus is a dark, organic material formed from the decomposition of plant and animal matter in soil. It plays a crucial role in improving soil fertility, enhancing water retention, and providing essential nutrients for plants. The presence of humus contributes to the overall health of the ecosystem by supporting microbial activity and promoting biodiversity.
Illuviation: Illuviation is the process of deposition or accumulation of materials, such as minerals and organic matter, that have been leached from upper soil layers and transported downward into lower soil horizons. This phenomenon plays a crucial role in soil formation and influences the chemical and physical properties of the soil profile, contributing to its fertility and structure.
Loam: Loam is a fertile soil mixture composed of roughly 40% sand, 40% silt, and 20% clay, making it ideal for agriculture and plant growth. This balanced composition allows for good drainage while retaining necessary moisture and nutrients. Loam is often considered the best type of soil for gardening and farming due to its ability to provide a healthy environment for roots and beneficial microorganisms.
Loams: Loams are soil mixtures composed of roughly equal parts sand, silt, and clay. They are known for their balanced texture and fertility, making them ideal for plant growth.
Mineral soils: Mineral soils are soils primarily composed of inorganic materials such as sand, silt, and clay. They typically have less than 20% organic material by weight.
Nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen ($$N_2$$) is converted into a form that living organisms can use, typically ammonia ($$NH_3$$). This essential biochemical process enables the incorporation of nitrogen into organic compounds, which is crucial for the growth and development of plants and other organisms.
O horizon: The O horizon, also known as the organic layer, is the topmost layer of soil, primarily composed of organic matter such as decomposed leaves, plant materials, and microorganisms. This layer is crucial for soil health as it plays a significant role in nutrient cycling and supports a diverse ecosystem of organisms that contribute to soil fertility and structure.
Organic soils: Organic soils are soils primarily composed of organic matter such as decomposed plant and animal residues. They are typically found in wetlands or areas with poor drainage.
Parent material: Parent material refers to the underlying geological material, such as bedrock or unconsolidated sediments, from which soil forms. It serves as the source of the minerals and nutrients necessary for soil development, influencing its texture, structure, and fertility. Understanding parent material is crucial for comprehending the characteristics of different soil types and their capacity to support various forms of life.
Pedogenesis: Pedogenesis is the process of soil formation, involving the weathering of rocks and the breakdown of organic matter to create a soil profile. This process is influenced by various factors including climate, parent material, topography, biological activity, and time, which together shape the characteristics and fertility of soil. Understanding pedogenesis is crucial for managing land use and sustaining agricultural productivity.
Podzolization: Podzolization is a soil formation process characterized by the leaching of minerals and organic matter from the upper layers of soil, resulting in the development of a distinct layer of leached, acidic material beneath. This process typically occurs in cool, humid climates and leads to the formation of podzols, which are commonly found in coniferous forest regions. The significance of podzolization lies in its impact on soil fertility, water retention, and the overall ecosystem dynamics.
R horizon: The R horizon, also known as the bedrock layer, is the lowest layer of soil, composed primarily of unweathered rock. This layer is crucial because it serves as the foundation for all upper soil horizons, influencing their development and characteristics. The R horizon can vary in depth and type depending on the geological processes in a specific area, affecting drainage, nutrient availability, and plant growth in the overlying soil layers.
Sand: Sand is a granular material composed of finely divided rock and mineral particles. It is a crucial component of soil, affecting its texture and drainage properties.
Saprolite: Saprolite is a weathered rock layer that has undergone significant physical and chemical alteration but remains in its original position beneath the soil surface. This layer is crucial for understanding soil formation processes, as it provides insights into the geological history and mineral composition of the underlying bedrock, influencing soil properties and fertility.
Silt: Silt is a granular material composed of fine mineral particles that are larger than clay but smaller than sand. It plays a crucial role in soil fertility and water retention.
Soil: Soil is a natural resource composed of minerals, organic matter, water, and air. It provides essential nutrients and support for plant growth.
Soil conservation: Soil conservation refers to a set of practices aimed at preventing soil erosion, maintaining soil fertility, and protecting the soil's structure to ensure sustainable land use. This concept is vital as it helps preserve the natural resource that supports agriculture, biodiversity, and the overall health of ecosystems. Effective soil conservation techniques play a crucial role in enhancing agricultural productivity while minimizing environmental degradation.
Soil fertility: Soil fertility refers to the ability of soil to provide essential nutrients to plants, supporting their growth and productivity. It is influenced by the soil's physical, chemical, and biological properties, which determine how well it can retain moisture and nutrients. Healthy soil fertility is crucial for agricultural productivity, ecosystem sustainability, and food security.
Soil horizons: Soil horizons are distinct layers within the soil profile, each characterized by different physical and chemical properties. These layers form as a result of soil formation processes like weathering, organic matter accumulation, and leaching, which contribute to the soil's overall structure and fertility. Understanding soil horizons is crucial for comprehending how soils function in ecosystems and their role in supporting plant life.
Soil pH: Soil pH is a measure of the acidity or alkalinity of soil, indicating the concentration of hydrogen ions present. It is crucial for understanding nutrient availability and microbial activity in the soil, affecting how plants absorb essential nutrients and influencing overall soil health.
Soil porosity: Soil porosity refers to the volume of pore spaces within soil that can hold air and water. This characteristic is crucial because it affects water retention, drainage, and the overall health of the soil ecosystem. Higher porosity typically leads to better aeration and drainage, which are essential for plant growth and microbial activity.
Soil profile: A soil profile is a vertical section of soil from the ground surface downwards to where the soil meets the underlying rock. It reveals different layers or horizons, each with distinct characteristics and compositions.
Soil structure: Soil structure refers to the arrangement of soil particles and the spaces between them, which affects the soil's physical properties and its ability to retain water, air, and nutrients. This arrangement is crucial for determining how well the soil can support plant growth and influence various biological processes. Understanding soil structure helps in assessing soil health, water infiltration, and root penetration, ultimately impacting agricultural practices and ecosystem functions.
Soil texture: Soil texture refers to the relative proportions of sand, silt, and clay particles in a given soil sample. This characteristic influences many important aspects of soil behavior, including its ability to retain moisture, nutrients, and support plant growth. Understanding soil texture is essential for various applications such as agriculture, environmental science, and land management, as it directly affects water infiltration, aeration, and overall soil fertility.
Topography: Topography refers to the arrangement of natural and artificial physical features of an area, including its landforms, elevation, and contours. This concept is crucial in understanding soil formation, as it influences drainage patterns, erosion rates, and the distribution of nutrients within soil layers, ultimately affecting ecosystem dynamics.
Weathering: Weathering is the process that breaks down rocks and minerals at the Earth's surface through physical, chemical, and biological mechanisms. This natural process is crucial for soil formation and influences nutrient cycling, impacting ecosystems and biogeochemical cycles. By breaking down materials, weathering plays a fundamental role in shaping landscapes and providing essential minerals for plant growth.