Soil formation is a complex process influenced by various factors like climate, organisms, and parent material. Understanding how soils form and their composition is crucial for grasping the broader concepts of weathering, erosion, and soil development.

Soil classification helps us organize and understand the diverse types of soils found worldwide. By examining soil properties and formation processes, we can better appreciate the role soils play in supporting ecosystems and human activities.

Soil Formation and Composition

Parent Material and Organic Matter

• Parent material is the underlying geological material (bedrock, loose sediments) in which soil forms and is the source of most inorganic material in the soil
• Composition of parent material influences soil properties such as texture, structure, mineral content, and fertility
• Organic matter consists of decomposing plant and animal residues, microorganisms, and humus (stable organic compounds resistant to further decomposition)
• Organic matter improves soil structure, increases water and nutrient retention, and provides nutrients for plants and soil organisms
• Examples of parent materials include granite, limestone, and glacial till
• Soil organic matter content typically ranges from 1-6% in mineral soils (mollisols) and can exceed 20% in organic soils (histosols)

Clay Minerals and Pedogenesis

• Clay minerals are secondary minerals formed through weathering of primary minerals and are important for soil fertility and structure
• Clay minerals have high surface area and charge, allowing them to hold nutrients and water
• Examples of clay minerals include kaolinite, smectite, and illite
• Pedogenesis is the process of soil formation and development over time, influenced by climate, organisms, relief, parent material, and time (ClORPT factors)
• Pedogenic processes include additions (organic matter accumulation), losses (leaching), transformations (mineral weathering), and translocations (eluviation and illuviation) within the soil profile
• Soil formation rates vary but are generally slow, ranging from hundreds to thousands of years for significant development

Soil Physical Properties

Soil Horizons and Texture

• Soil horizons are distinct layers within a soil profile that differ in physical, chemical, and biological properties as a result of pedogenic processes
• Typical soil horizons include O (organic), A (surface), E (eluviated), B (subsoil), C (parent material), and R (bedrock) horizons
• Soil texture refers to the relative proportions of sand (0.05-2 mm), silt (0.002-0.05 mm), and clay (<0.002 mm) particles in a soil
• Soil texture influences soil properties such as water retention, drainage, aeration, and workability
• Soil texture classes include sandy (sand, loamy sand), loamy (loam, silt loam, sandy loam), and clayey (clay, clay loam, silty clay) soils
• Soil texture can be determined by feel (ribbon test) or through laboratory analysis (hydrometer method)

Soil Structure

• Soil structure refers to the arrangement of soil particles into aggregates or peds
• Soil structure affects water and air movement, root growth, and erosion resistance
• Types of soil structure include granular (spherical), blocky (angular or subangular), prismatic (vertically elongated), and platy (horizontally layered)
• Factors influencing soil structure include soil texture, organic matter content, biological activity, and management practices (tillage, compaction)
• Well-structured soils have a balance of macropores (large pores between aggregates) and micropores (small pores within aggregates) for optimal water and air movement

Soil Chemical Properties

Soil pH and Nutrient Availability

• Soil pH is a measure of the acidity or alkalinity of a soil, typically ranging from 3.5 to 9.5
• Soil pH influences nutrient availability, microbial activity, and plant growth
• Most plants prefer slightly acidic to neutral soils (pH 6.0-7.5) for optimal nutrient availability
• Acidic soils (pH <6.0) may have increased solubility of aluminum and manganese, which can be toxic to plants
• Alkaline soils (pH >7.5) may have reduced availability of iron, manganese, and phosphorus due to precipitation or adsorption
• Soil pH can be modified through liming (to raise pH) or addition of sulfur or organic matter (to lower pH)

Soil Classification

Soil Taxonomy and Orders

• Soil taxonomy is a hierarchical system for classifying soils based on their properties and formation
• The USDA soil taxonomy system has six levels: order, suborder, great group, subgroup, family, and series
• There are 12 soil orders, each reflecting dominant soil-forming processes and properties
• Examples of soil orders include:
  • Alfisols: moderately weathered soils with clay accumulation in the subsoil (deciduous forests)
  • Mollisols: grassland soils with thick, dark surface horizons rich in organic matter (prairies)
  • Aridisols: desert soils with limited moisture and organic matter (deserts)
  • Ultisols: highly weathered, acidic soils with low base saturation (humid tropics)
• Soil orders are further divided into suborders based on soil moisture and temperature regimes, great groups based on diagnostic horizons and features, and lower levels based on more specific properties