1.2 Soil-forming factors and processes

Soil formation is a complex process influenced by various factors and processes. Parent material, climate, organisms, topography, and time work together to shape soil characteristics and development over long periods.

Weathering, leaching, humification, translocation, and pedoturbation are key processes in soil formation. These processes break down rocks, move materials, transform organic matter, and mix soil components, resulting in distinct soil profiles and properties.

Soil-Forming Factors

Factors of soil formation

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• Parent material: Original geological material from which soil forms influences soil texture, mineral composition, and initial nutrient content (granite, limestone)
• Climate: Temperature and precipitation patterns affect weathering rates, organic matter decomposition, and mineral transformations (tropical, arid)
• Organisms: Plants, animals, microorganisms, and humans contribute to organic matter input, nutrient cycling, and soil structure development (earthworms, fungi)
• Topography: Landscape position and slope influence water movement, erosion, and deposition processes (hillslope, floodplain)
• Time: Duration of soil formation determines the extent of soil development and horizon differentiation (young vs. old soils)

Influences on soil development

• Parent material determines initial soil texture and mineralogy influences soil fertility and drainage characteristics (sandy soils from quartz-rich rocks)
• Climate affects weathering intensity and rate of soil formation determines vegetation types and organic matter accumulation (faster weathering in humid tropics)
• Organisms: Plants contribute to organic matter input and root development while soil fauna and microorganisms facilitate decomposition and nutrient cycling (mycorrhizal fungi)
• Topography affects soil depth and drainage patterns influences soil erosion and deposition rates (deeper soils in valleys, thinner on slopes)
• Time allows for horizon development and soil profile maturation enables accumulation of organic matter and clay particles (thousands to millions of years)

Soil Formation Processes

Key soil formation processes

• Weathering: Breakdown of rocks and minerals into smaller particles releases nutrients and creates new mineral forms (feldspar to clay minerals)
• Leaching: Downward movement of dissolved materials through soil profile leads to horizon differentiation and nutrient redistribution (calcium carbonate accumulation in arid soils)
• Humification: Transformation of organic matter into stable humus compounds improves soil structure and nutrient retention capacity (dark-colored A horizon)
• Translocation: Movement of soil components from one horizon to another results in clay accumulation in subsurface horizons (clay films in Bt horizons)
• Pedoturbation: Mixing of soil materials by biological or physical processes homogenizes soil profile and disrupts horizon development (burrowing animals, cryoturbation)

Types of weathering processes

• Physical weathering: Mechanical breakdown of rocks and minerals through freeze-thaw cycles, thermal expansion and contraction, root wedging (rock fracturing)
• Chemical weathering: Alteration of mineral composition through chemical reactions including hydrolysis, oxidation, carbonation, dissolution (rust formation)
• Biological weathering: Breakdown of rocks and minerals by living organisms through root exudates, microbial activity, lichen colonization (organic acid production)
• Interactions between weathering types: Physical weathering increases surface area for chemical reactions while biological processes can enhance both physical and chemical weathering (lichen-covered rocks)