❄️Earth Surface Processes Unit 3 – Soil Formation and Classification

Key Concepts and Terminology

• Pedogenesis refers to the process of soil formation and development over time
• Weathering breaks down parent material into smaller particles through physical, chemical, and biological processes
• Soil texture describes the relative proportions of sand, silt, and clay particles in a soil
• Soil structure relates to the arrangement of soil particles into aggregates or peds
• Soil horizon is a layer of soil with distinct characteristics that differ from layers above and below
• Soil profile represents a vertical cross-section of soil horizons from the surface to the parent material
• Cation exchange capacity (CEC) measures the soil's ability to hold and exchange positively charged ions
• Soil pH indicates the acidity or alkalinity of a soil, affecting nutrient availability and plant growth

Soil Formation Processes

• Physical weathering involves the breakdown of rocks and minerals through mechanical processes (frost wedging, thermal expansion, and abrasion)
• Chemical weathering occurs when rocks and minerals are altered or dissolved by chemical reactions (hydrolysis, oxidation, and carbonation)
  • Hydrolysis is the reaction between water and minerals, leading to the formation of clays and other secondary minerals
  • Oxidation involves the reaction of oxygen with minerals, often resulting in the formation of iron oxides and hydroxides
• Biological weathering is caused by the actions of living organisms (plant roots, microorganisms, and burrowing animals)
• Humification is the process of converting organic matter into humus, a stable form of organic matter in soil
• Eluviation refers to the downward movement of dissolved or suspended materials through the soil profile
• Illuviation is the accumulation of eluviated materials in lower soil horizons
• Gleization occurs in poorly drained soils, leading to the reduction of iron and the formation of gley colors (bluish-gray or greenish-gray)

Factors Influencing Soil Development

• Climate significantly impacts soil formation through temperature and precipitation patterns
  • Temperature affects the rate of weathering and biological activity in soils
  • Precipitation influences the amount of water available for weathering and leaching processes
• Parent material is the original rock or sediment from which soil develops, determining the initial composition and texture of the soil
• Topography affects soil development by influencing drainage, erosion, and deposition patterns
  • Steep slopes often have shallower and less developed soils due to increased erosion
  • Flat or depressional areas may have deeper soils with poor drainage and gleization
• Biota, including vegetation and soil organisms, contributes to soil formation through organic matter additions and nutrient cycling
• Time is a crucial factor in soil development, as older soils tend to have more distinct horizons and greater profile development
• Human activities (agriculture, urbanization, and deforestation) can significantly alter soil properties and development

Soil Horizons and Profiles

• O horizon is the uppermost layer, consisting of organic matter in various stages of decomposition
• A horizon, or topsoil, is a mineral layer with high biological activity and accumulation of organic matter
• E horizon, or eluvial layer, is a light-colored layer where clay, iron, and aluminum have been leached out
• B horizon, or subsoil, is a layer of accumulation where materials leached from above horizons are deposited
  • Bt horizon is a subsoil layer with significant accumulation of clay particles
  • Bh horizon is a subsoil layer with accumulation of organic matter and sesquioxides
• C horizon is a layer of partially weathered parent material, often showing the original rock structure
• R horizon is the consolidated bedrock layer beneath the soil profile

Soil Classification Systems

• Soil Taxonomy, developed by the USDA, is a hierarchical system based on soil properties and formation processes
  • Soil orders are the highest level of classification, with 12 orders recognized (Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols)
  • Lower levels of classification include suborders, great groups, subgroups, families, and series
• World Reference Base for Soil Resources (WRB) is an international soil classification system endorsed by the International Union of Soil Sciences
  • WRB uses diagnostic horizons and properties to classify soils into 32 reference soil groups
• Soil classification systems help in understanding soil genesis, properties, and management implications

Physical and Chemical Properties of Soil

• Soil texture is determined by the relative proportions of sand (0.05-2 mm), silt (0.002-0.05 mm), and clay (<0.002 mm) particles
  • Soil textural triangle is used to determine soil textural classes based on the percentages of sand, silt, and clay
• Soil structure describes the arrangement of soil particles into aggregates, affecting water and air movement, root growth, and soil stability
  • Common soil structures include granular, blocky, prismatic, columnar, and platy
• Soil porosity refers to the volume of soil occupied by air and water, influencing water retention and drainage
• Soil bulk density is the ratio of the mass of dry soil to its total volume, indicating soil compaction and root penetration
• Soil organic matter consists of decomposed plant and animal residues, providing nutrients, improving soil structure, and increasing water-holding capacity
• Soil pH affects nutrient availability, microbial activity, and plant growth
  • pH values below 7 indicate acidic soils, while values above 7 indicate alkaline soils
• Cation exchange capacity (CEC) represents the soil's ability to hold and exchange positively charged nutrients (potassium, calcium, and magnesium)

Soil Mapping and Survey Techniques

• Soil surveys involve the systematic examination, description, and mapping of soils in a given area
• Soil maps depict the spatial distribution of soil types, properties, and interpretations for land use and management
• Remote sensing techniques (aerial photography and satellite imagery) are used to identify and map soil patterns and properties
• Geographic Information Systems (GIS) are used to store, analyze, and visualize soil survey data and create digital soil maps
• Field observations and soil sampling are essential components of soil surveys, providing ground truth data for mapping and classification
• Soil survey reports provide detailed descriptions of soil properties, interpretations, and management recommendations for each mapped soil unit

Environmental and Agricultural Implications

• Soil erosion is the detachment and transport of soil particles by water or wind, leading to soil degradation and reduced productivity
  • Conservation practices (terracing, contour farming, and cover crops) help reduce soil erosion and maintain soil quality
• Soil contamination occurs when pollutants (heavy metals, pesticides, and hydrocarbons) accumulate in soils, posing risks to human health and the environment
• Soil salinization is the accumulation of soluble salts in soils, often resulting from poor irrigation practices and leading to reduced plant growth
• Soil fertility refers to the soil's ability to supply essential nutrients for plant growth, influenced by factors such as pH, organic matter, and CEC
• Soil management practices (tillage, crop rotation, and nutrient management) aim to maintain or improve soil health and productivity
• Sustainable soil management is crucial for ensuring food security, maintaining ecosystem services, and mitigating climate change impacts