Soil formation is a complex process influenced by parent material, climate, organisms, topography, and time. These factors interact to create diverse soil types with unique physical, chemical, and biological properties that shape landscapes and ecosystems.

Understanding soil formation and classification is crucial for managing land use, agriculture, and environmental conservation. By studying soil properties and distribution, we can make informed decisions about sustainable resource management and ecosystem health.

Soil formation process

Factors influencing soil formation

• Soil formation, or pedogenesis, is the process by which soil develops from unconsolidated parent material through the complex interaction of physical, chemical, and biological processes over time
• The five primary factors that influence soil formation are parent material, climate, organisms, topography, and time (ClORPT)
  • Parent material refers to the geologic material from which the soil forms and influences the soil's texture, mineral composition, and chemical properties (granite, limestone, alluvium)
  • Climate, particularly temperature and precipitation, affects the rate of weathering, leaching, and biological activity in the soil (tropical, temperate, arid)
  • Organisms, including plants, animals, and microbes, contribute to soil formation through organic matter additions, nutrient cycling, and physical alterations of the soil (earthworms, fungi, grasses)
  • Topography influences soil formation by affecting water movement, erosion, and deposition processes (hillslopes, floodplains, terraces)
  • Time is required for soil-forming processes to act upon the parent material and develop distinct soil horizons (hundreds to thousands of years)

Weathering and soil-forming processes

• Weathering, which includes physical, chemical, and biological processes, breaks down parent material and contributes to soil formation
  • Physical weathering involves the mechanical breakdown of rocks and minerals through processes such as freeze-thaw cycles, thermal expansion, and abrasion (exfoliation, frost wedging)
  • Chemical weathering involves the alteration of minerals through reactions with water, acids, and other chemical agents (hydrolysis, oxidation, carbonation)
  • Biological weathering occurs when organisms, such as plants and microbes, contribute to the breakdown of rocks and minerals through processes like root growth and acid secretion (lichen, moss, tree roots)
• Soil-forming processes, such as additions, losses, translocations, and transformations, lead to the development of distinct soil horizons with specific properties
  • Additions involve the input of materials to the soil, such as organic matter from plant and animal residues or deposition of sediments (leaf litter, loess, volcanic ash)
  • Losses occur when materials are removed from the soil through processes like leaching, erosion, or volatilization (calcium carbonate, clay particles, nitrogen gas)
  • Translocations involve the movement of materials within the soil profile, such as the downward movement of clay particles or the upward movement of salts (eluviation, illuviation, capillary rise)
  • Transformations involve the alteration of materials within the soil, such as the formation of secondary minerals or the humification of organic matter (clay mineral formation, decomposition, mineralization)

Soil properties

Physical properties

• Physical properties of soils include texture, structure, porosity, bulk density, and color
  • Soil texture refers to the relative proportions of sand, silt, and clay particles in the soil and influences water retention, drainage, and nutrient-holding capacity (sandy loam, clay loam, silty clay)
  • Soil structure describes the arrangement of soil particles into aggregates and affects water movement, aeration, and root growth (granular, blocky, prismatic)
  • Porosity is the volume of soil voids that can be filled by water or air, while bulk density is the mass of dry soil per unit volume (0.3-0.6 for porous soils, 1.2-1.8 g/cm³ for compacted soils)
  • Soil color can indicate the presence of organic matter, iron oxides, and other soil constituents (dark brown for high organic matter, red for iron oxides, gray for poorly drained soils)

Chemical properties

• Chemical properties of soils include pH, cation exchange capacity (CEC), and nutrient content
  • Soil pH measures the acidity or alkalinity of the soil and affects nutrient availability and microbial activity (acidic soils pH < 7, alkaline soils pH > 7, neutral soils pH = 7)
  • CEC is the soil's ability to hold and exchange positively charged ions (cations) and influences nutrient retention and availability (high CEC in clay soils, low CEC in sandy soils)
  • Soil nutrients, such as nitrogen, phosphorus, and potassium, are essential for plant growth and are influenced by soil mineralogy, organic matter content, and management practices (fertilization, crop rotation, cover cropping)

Biological properties

• Biological properties of soils include organic matter content, microbial biomass, and biodiversity
  • Soil organic matter consists of decomposing plant and animal residues and influences soil structure, water retention, and nutrient cycling (humus, particulate organic matter, dissolved organic matter)
  • Soil microorganisms, such as bacteria, fungi, and archaea, play crucial roles in nutrient cycling, organic matter decomposition, and soil aggregation (nitrogen-fixing bacteria, mycorrhizal fungi, actinomycetes)
  • Soil biodiversity encompasses the variety of organisms living in the soil and contributes to ecosystem functioning and resilience (nematodes, protozoa, arthropods)

Soil classification

USDA Soil Taxonomy

• The USDA Soil Taxonomy is a hierarchical classification system that groups soils into 12 orders based on diagnostic horizons and other soil properties
  • The 12 soil orders are Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols
    • Alfisols are moderately weathered soils with a subsurface accumulation of clay (argillic horizon)
    • Andisols are soils formed from volcanic materials, characterized by high organic matter content and low bulk density
    • Aridisols are soils of dry climates, often with accumulations of salts, gypsum, or carbonates
    • Entisols are young, poorly developed soils lacking distinct horizons
    • Gelisols are soils with permafrost within 100 cm of the surface
    • Histosols are organic soils with a thick accumulation of organic matter (peat, muck)
    • Inceptisols are young soils with weakly developed horizons
    • Mollisols are grassland soils with a thick, dark surface horizon (mollic epipedon) rich in organic matter
    • Oxisols are highly weathered, low-fertility soils of tropical regions with an oxic horizon
    • Spodosols are acidic soils with a subsurface accumulation of organic matter and aluminum (spodic horizon)
    • Ultisols are strongly weathered, acidic soils with a subsurface accumulation of clay (argillic horizon)
    • Vertisols are clay-rich soils that shrink and swell with changes in moisture content, forming deep cracks when dry
  • Each soil order is further subdivided into suborders, great groups, subgroups, families, and series based on increasingly specific soil properties

Other classification systems and diagnostic horizons

• The World Reference Base for Soil Resources (WRB) is an international soil classification system that emphasizes soil morphology and genesis
• Soil classification systems rely on the identification of diagnostic horizons, such as the mollic epipedon, argillic horizon, and spodic horizon, which are defined by specific soil properties and formation processes
  • A mollic epipedon is a thick, dark surface horizon with high base saturation and organic matter content, characteristic of grassland soils (Mollisols)
  • An argillic horizon is a subsurface horizon with a significant accumulation of illuvial clay, found in Alfisols, Ultisols, and some Mollisols
  • A spodic horizon is a subsurface horizon with an accumulation of organic matter and aluminum, often with iron, characteristic of Spodosols
• Soil classification is essential for understanding soil distribution, land use planning, and management decisions

Soil distribution across landscapes

Spatial variability and soil-landscape relationships

• Soils exhibit spatial variability across landscapes due to the complex interactions of soil-forming factors and processes
• Soil catenas describe the sequence of soils that occur along a hillslope, reflecting the influence of topography and water movement on soil formation (summit, shoulder, backslope, footslope, toeslope)
• Soil-landscape relationships can be used to predict soil properties and distribution based on topographic position, landforms, and other environmental factors (aspect, elevation, curvature)

Digital soil mapping and soil surveys

• Digital soil mapping techniques, such as remote sensing, GIS, and machine learning, are used to create high-resolution soil maps and predict soil properties across landscapes
  • Remote sensing data, such as satellite imagery and aerial photographs, provide information on soil color, moisture, and vegetation cover
  • GIS (Geographic Information Systems) allow for the integration and analysis of spatial data, such as topography, geology, and land use
  • Machine learning algorithms, such as random forests and neural networks, can be trained on field observations and environmental covariates to predict soil properties and classes
• Understanding soil variability is crucial for precision agriculture, land use planning, and ecosystem management
  • Precision agriculture involves the use of site-specific management practices based on soil variability to optimize crop yields and minimize environmental impacts (variable rate fertilization, irrigation, seeding)
  • Land use planning requires knowledge of soil properties and limitations to ensure sustainable development and minimize land degradation (urban expansion, infrastructure projects, conservation areas)
  • Ecosystem management relies on understanding soil variability to maintain biodiversity, productivity, and resilience (habitat restoration, carbon sequestration, nutrient cycling)
• Soil surveys provide detailed information on soil distribution, properties, and interpretations for a given area and are used for land use planning, conservation, and management decisions
  • Soil surveys are conducted by trained soil scientists who describe, sample, and map soils in the field
  • Soil survey reports include maps, descriptions, and interpretations of soil properties, limitations, and suitability for various uses (agriculture, engineering, recreation)
  • Soil survey data is available through national and international databases, such as the USDA Web Soil Survey and the FAO Harmonized World Soil Database