3.2 Soil Formation and Properties
7 min read•august 14, 2024
Soil formation is a complex process influenced by various factors like parent material, climate, and organisms. These elements work together over time to create the diverse soils we see today, each with unique physical and chemical properties that affect plant growth and ecosystem health.
Understanding soil composition and properties is crucial for managing this vital resource. From supporting agriculture to filtering water and storing carbon, healthy soils play a key role in sustaining life on Earth and mitigating climate change impacts.
Soil Composition and Components
Soil Definition and Main Components
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- Soil is a mixture of weathered rock, , water, and air that forms the outermost layer of the Earth's surface and supports plant growth
- The four main components of soil are:
- Mineral particles (sand, silt, and clay)
- Organic matter (decomposed plant and animal material)
- Water (held in the spaces between soil particles)
- Air (fills the pore spaces not occupied by water)
Mineral Particles and Organic Matter
- Mineral particles make up the largest portion of soil and are classified by size into sand, silt, and clay
- Sand particles are the largest (0.05-2 mm), followed by silt (0.002-0.05 mm) and clay (<0.002 mm)
- The relative proportions of these particles determine and influence properties such as water retention and nutrient holding capacity
- Organic matter consists of decomposed plant and animal material, which provides nutrients and improves soil structure
- Humus, the stable form of organic matter, enhances and water-holding capacity
- Soil organisms (bacteria, fungi, and invertebrates) break down organic matter and release nutrients for plant uptake
Soil Water and Air
- Soil water is held in the spaces between soil particles and is essential for plant growth and soil organism survival
- Water dissolves nutrients and makes them available for plant uptake
- Soil moisture content affects microbial activity and chemical reactions in the soil
- Soil air fills the pore spaces not occupied by water and is necessary for root respiration and microbial activity
- Adequate soil aeration is crucial for healthy root development and nutrient cycling
- Poor soil aeration can lead to anaerobic conditions, which can harm plant roots and soil organisms
Factors Influencing Soil Formation
Parent Material and Climate
- Soil formation is a slow process that occurs over hundreds to thousands of years and is influenced by five main factors: parent material, climate, topography, organisms, and time
- Parent material refers to the underlying rock or sediment from which soil forms and determines the initial composition and texture of the soil
- Examples of parent materials include granite, limestone, and alluvial deposits
- The mineralogy and chemistry of the parent material influence soil properties such as pH, nutrient content, and clay mineralogy
- Climate, particularly temperature and precipitation, affects the rate of , organic matter accumulation, and of in the soil
- Warm, humid climates accelerate weathering and soil development compared to cold, dry climates
- High rainfall can lead to increased leaching of soluble nutrients (calcium and magnesium) and the formation of acidic soils
Topography, Organisms, and Time
- Topography influences soil formation by affecting drainage, erosion, and deposition of materials
- Steep slopes tend to have thinner, less developed soils compared to flat or gently sloping areas due to increased erosion
- Depressions and lowlands accumulate deposited materials, leading to deeper, more fertile soils
- Organisms, including plants, animals, and microbes, contribute to soil formation by adding organic matter, facilitating weathering, and mixing soil components
- Plants add organic matter through root growth and leaf litter, while their roots help break down rocks and minerals
- Burrowing animals (earthworms and rodents) mix soil layers and improve soil structure and aeration
- Time is a crucial factor in soil formation, as the longer a soil has to develop, the more distinct its horizons and properties become
- Young soils (entisols) have minimal horizon development, while ancient soils (oxisols) have highly weathered profiles
- The degree of soil development can indicate the relative age and stability of the landscape
Soil Properties: Physical and Chemical
Physical Properties
- Physical properties of soils include texture, structure, porosity, and color, which influence soil behavior and plant growth
- Soil texture refers to the relative proportions of sand, silt, and clay particles in the soil and affects water retention, drainage, and nutrient holding capacity
- Sandy soils have high infiltration rates and low water and , while clay soils have the opposite characteristics
- Loamy soils, with balanced proportions of sand, silt, and clay, have favorable properties for plant growth
- Soil structure describes the arrangement of soil particles into aggregates and influences water infiltration, root penetration, and soil aeration
- Well-structured soils have stable aggregates that create a network of pores for water and air movement
- Poor soil structure can lead to compaction, reduced infiltration, and limited root growth
- Porosity is the volume of soil occupied by air and water and is determined by soil texture and structure
- High porosity facilitates water movement and gas exchange, while low porosity can lead to waterlogging and anaerobic conditions
- Soil color can indicate the presence of organic matter, iron oxides, and drainage conditions
- Dark brown or black soils often have high organic matter content, while pale or light-colored soils may be low in organic matter
- Red or yellow soils may indicate the presence of iron oxides, which form under well-drained, oxidizing conditions
- Grayish or mottled soils suggest poor drainage and prolonged saturation
Chemical Properties
- Chemical properties of soils include pH, cation exchange capacity (CEC), and nutrient availability, which affect soil fertility and plant growth
- Soil pH measures the acidity or alkalinity of the soil and influences nutrient availability and microbial activity
- Most plants prefer slightly acidic to neutral soils (pH 6-7), where essential nutrients are most available
- Extreme pH levels (<5 or >8) can lead to nutrient deficiencies or toxicities and inhibit plant growth
- CEC is the soil's ability to hold and exchange positively charged ions (cations) and is influenced by clay content and organic matter
- High CEC soils have a greater capacity to store and supply nutrients (potassium, calcium, and magnesium) to plants
- Low CEC soils are more prone to nutrient leaching and may require more frequent fertilization
- Nutrient availability refers to the amount of essential plant nutrients, such as nitrogen, phosphorus, and potassium, present in the soil and available for plant uptake
- Nutrient availability is affected by soil pH, CEC, and organic matter content
- Adequate nutrient levels are crucial for optimal plant growth and crop yields
- Soil testing can help determine nutrient deficiencies and guide fertilizer application
Soil as a Natural Resource
Importance of Soil
- Soil is a critical natural resource that supports plant growth, food production, and various ecosystem services
- Soil provides a medium for plant roots to anchor and obtain water and nutrients, which is essential for agriculture and natural vegetation
- Healthy soils with good structure and fertility are crucial for sustainable crop production and food security
- Soil quality directly influences plant health, yield, and nutritional value
- Soil acts as a natural filter, purifying water as it percolates through the soil profile and recharging groundwater aquifers
- and organic matter help break down pollutants and improve water quality
- Well-managed soils can reduce the risk of groundwater contamination and maintain clean water supplies
Soil Biodiversity and Carbon Sequestration
- Soil organisms, such as bacteria, fungi, and invertebrates, play crucial roles in decomposition, nutrient cycling, and maintaining soil health
- Soil biodiversity promotes nutrient availability, disease suppression, and resilience to environmental stresses
- Soil organisms form symbiotic relationships with plant roots (mycorrhizae and nitrogen-fixing bacteria) that enhance nutrient uptake and plant growth
- Soil stores and sequesters carbon, helping to mitigate climate change by reducing atmospheric carbon dioxide levels
- Soil organic matter, derived from plant and animal residues, is a major reservoir of carbon in terrestrial ecosystems
- Sustainable land management practices (reduced tillage, cover cropping, and afforestation) can increase soil carbon storage and mitigate greenhouse gas emissions
Soil Conservation and Management
- Soil is a non-renewable resource on human timescales, as it takes hundreds to thousands of years to form and can be easily degraded or lost through erosion, contamination, or urbanization
- Soil erosion, caused by wind or water, can lead to the loss of topsoil, reduced fertility, and decreased crop yields
- Soil contamination from industrial pollutants, pesticides, or heavy metals can harm soil health and food safety
- Urban expansion and land-use changes can lead to the permanent loss of fertile agricultural soils
- Sustainable soil management practices, such as conservation tillage, , and organic farming, are essential for maintaining soil health and productivity for future generations
- Conservation tillage (reduced or no-till) minimizes soil disturbance, reduces erosion, and improves soil structure and organic matter content
- Crop rotation, involving the alternation of different crops on the same field, promotes soil fertility, pest control, and biodiversity
- Organic farming relies on natural inputs (compost and green manures) and biological pest control, reducing the use of synthetic fertilizers and pesticides that can harm soil health
Key Terms to Review (24)
A Horizon: The A horizon, often referred to as the topsoil layer, is the uppermost layer of soil that is rich in organic material and nutrients. This layer plays a vital role in supporting plant life and is characterized by a darker color due to the accumulation of decomposed organic matter, which is essential for soil fertility and health. The A horizon is crucial for various soil formation processes and significantly influences the overall properties of the soil profile.
Andisols: Andisols are a unique soil order formed from volcanic ash and tephra, characterized by their high fertility and specific physical and chemical properties. These soils typically develop in areas with significant volcanic activity and are known for their ability to retain moisture and nutrients, making them ideal for agriculture. The formation of Andisols is closely tied to volcanic processes, resulting in distinct horizons that are rich in organic matter and weathered minerals.
B Horizon: The B horizon, also known as the subsoil, is a layer of soil that is typically found beneath the A horizon (topsoil) and above the C horizon (parent material). This layer is characterized by the accumulation of minerals and nutrients leached down from the upper layers, making it crucial for plant growth and soil fertility. The B horizon plays a key role in soil formation and properties by influencing drainage, water retention, and the overall health of the soil ecosystem.
Clay Soil: Clay soil is a type of soil composed mainly of fine particles that are less than 0.002 mm in diameter, giving it a dense, sticky texture when wet. This soil type is known for its ability to hold water and nutrients but can also lead to drainage problems due to its compact nature. Understanding clay soil is essential in assessing soil formation processes and its properties, influencing agriculture, construction, and environmental management.
Crop rotation: Crop rotation is an agricultural practice where different types of crops are planted in the same area across a sequence of seasons. This technique helps maintain soil health, reduce soil erosion, and minimize pest and disease cycles, all of which are vital for sustainable farming and soil properties. By alternating crops, farmers can enhance nutrient availability in the soil, prevent depletion from continuous planting of the same species, and promote biodiversity within the ecosystem.
D. e. e. l. r. m. u.: The term d. e. e. l. r. m. u. refers to a conceptual framework that helps in understanding soil formation and properties through various influencing factors such as climate, organisms, relief, parent material, and time. This acronym encapsulates the key components that contribute to the development of soil profiles and their characteristics, emphasizing how each factor interacts over time to shape the soil we encounter in different environments.
Erosion control: Erosion control refers to the practices and techniques used to prevent or reduce the loss of soil from the land surface due to water, wind, or human activity. Effective erosion control helps maintain soil quality, preserves nutrients, and enhances water infiltration, which are crucial for healthy soil formation and properties. These methods can include vegetation planting, mulching, contour plowing, and the use of barriers or structures to slow down water flow.
Hans Jenny: Hans Jenny was a Swiss soil scientist known for his pioneering work in soil formation and classification. His contributions laid the groundwork for understanding the processes that lead to soil development, particularly the interplay between environmental factors and biological activity. Jenny’s framework for soil formation emphasized the importance of climate, organisms, parent material, topography, and time as fundamental factors influencing soil properties.
Humification: Humification is the process of converting organic matter, such as dead plants and animals, into humus, a dark, organic component of soil. This transformation plays a crucial role in soil formation and health by enhancing soil structure, fertility, and its ability to retain moisture. The humification process contributes to the formation of a rich ecosystem in the soil, supporting plant growth and maintaining ecological balance.
Leaching: Leaching is the process by which soluble substances are removed from soil or other materials through the action of water percolating down through them. This process is crucial in soil formation and properties because it influences nutrient availability, soil composition, and overall soil health. As water moves through the soil, it can dissolve minerals and nutrients, carrying them away and affecting the chemical properties of the soil.
Loamy soil: Loamy soil is a balanced mixture of sand, silt, and clay that provides optimal conditions for plant growth and agriculture. This type of soil is highly valued because it retains moisture and nutrients while also allowing for proper drainage and aeration, making it ideal for cultivating a wide variety of crops.
Minerals: Minerals are naturally occurring inorganic substances with a definite chemical composition and crystal structure. They are the building blocks of rocks and play a crucial role in soil formation, influencing soil texture, fertility, and overall health. Understanding minerals is essential for grasping how soils develop and their properties affect ecosystems.
Mollisols: Mollisols are a type of soil characterized by a thick, dark topsoil rich in organic matter and nutrients, making them highly fertile. These soils primarily develop under grassland vegetation and are typically found in temperate regions, which contribute to their distinct properties such as good moisture retention and a high cation exchange capacity. This combination of features makes mollisols particularly valuable for agriculture and land use.
Nutrient Retention: Nutrient retention refers to the ability of soil to hold and store essential nutrients that plants need for growth. This process is crucial for maintaining soil fertility, as it ensures that nutrients like nitrogen, phosphorus, and potassium remain available in the root zone for plants to absorb. Good nutrient retention is influenced by factors such as soil texture, organic matter content, and microbial activity, which all play a role in enhancing the soil's capacity to retain nutrients over time.
O Horizon: The O horizon, also known as the organic horizon, is the topmost layer of soil that consists primarily of organic material such as decomposed leaves, plant matter, and other organic debris. This layer plays a critical role in soil formation and properties, as it influences nutrient cycling, water retention, and overall soil fertility, making it essential for healthy ecosystems and agriculture.
Organic Matter: Organic matter refers to the decomposed remains of plants and animals, as well as organic compounds produced by soil organisms. It plays a crucial role in soil formation and properties by enhancing soil structure, improving water retention, and providing essential nutrients for plant growth. The presence of organic matter is vital for maintaining soil health, fertility, and the overall ecosystem balance.
PH Level: The pH level is a measure of the acidity or alkalinity of a solution, represented on a scale from 0 to 14, with 7 being neutral. In soil, pH levels influence nutrient availability, microbial activity, and overall soil health. Understanding the pH level is crucial as it affects the types of plants that can thrive in a particular soil and plays a vital role in soil formation and properties.
Sandy Soil: Sandy soil is a type of soil characterized by its coarse texture, made up primarily of sand particles. It has larger particle sizes compared to other soil types, which results in excellent drainage and aeration but also lower nutrient and moisture retention. This texture influences how plants grow and the overall fertility of the soil, making sandy soil an important aspect of soil formation and properties.
Soil degradation: Soil degradation is the decline in soil quality and productivity, often caused by various factors such as erosion, nutrient depletion, and contamination. This process significantly impacts soil formation and properties by reducing the ability of soil to support plant growth and maintain ecological balance. Healthy soil is crucial for agriculture, water filtration, and carbon storage, and when it degrades, it can lead to a range of environmental issues like reduced crop yields and increased greenhouse gas emissions.
Soil fertility: Soil fertility refers to the ability of soil to provide essential nutrients to plants, supporting their growth and development. It involves the presence and availability of macro and micronutrients, as well as the soil's physical and biological properties that enhance nutrient retention and microbial activity. Healthy, fertile soil is crucial for agricultural productivity and ecosystem sustainability.
Soil microorganisms: Soil microorganisms are tiny living organisms, including bacteria, fungi, protozoa, and nematodes, that inhabit the soil environment and play a vital role in soil health and fertility. These organisms are essential for breaking down organic matter, recycling nutrients, and maintaining the overall ecosystem balance within the soil. Their activities enhance soil structure, contribute to plant growth, and help in the formation of humus.
Soil Texture: Soil texture refers to the relative proportions of different particle sizes in soil, specifically sand, silt, and clay. This composition influences various soil properties, including drainage, nutrient retention, and aeration, which are critical for plant growth and soil health. Understanding soil texture is essential for evaluating soil formation processes and the impacts of weathering, erosion, and deposition on the land.
Spodosols: Spodosols are a specific type of soil characterized by a leached, acidic upper horizon that is typically low in nutrients and a subsurface layer rich in iron, aluminum, and organic matter. These soils often form under coniferous forests in cool, humid climates and play a significant role in understanding soil formation and properties due to their unique characteristics and the environmental conditions under which they develop.
Weathering: Weathering is the process that breaks down rocks into smaller particles through physical, chemical, and biological means. This process is crucial as it contributes to soil formation, influences landform development, and plays a significant role in the rock cycle, affecting how rocks change over time and interact with the environment.