4.1 Primary and secondary soil minerals

Soil minerals form the backbone of our planet's fertile ground. From quartz to clay, these tiny particles shape soil's ability to support life. Understanding their properties and roles is key to unlocking the secrets of soil health and plant growth.

Primary minerals like quartz and feldspar weather into secondary minerals like clay. This transformation changes soil texture, nutrient content, and water-holding capacity. Together, these minerals create the dynamic environment where plants thrive and ecosystems flourish.

Primary Soil Minerals

Common soil minerals and sources

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• Primary minerals
  • Quartz dominates many soils withstands weathering found in sand and silt fractions (beaches, deserts)
    • Source: igneous and metamorphic rocks break down into soil particles
  • Feldspars comprise about 60% of Earth's crust provide essential nutrients (potassium, calcium)
    • Orthoclase releases potassium during weathering
    • Plagioclase contributes calcium and sodium to soil solution
    • Source: igneous rocks form basis for many soil types (granite, basalt)
  • Micas form flat, sheet-like particles influence soil structure
    • Biotite dark-colored mica rich in iron and magnesium
    • Muscovite light-colored mica contains potassium and aluminum
    • Source: igneous and metamorphic rocks weather into soil components
  • Amphiboles chain silicate minerals contribute iron and magnesium
    • Hornblende common in soils derived from igneous rocks
    • Source: igneous and metamorphic rocks break down during soil formation
  • Pyroxenes important source of calcium, magnesium, and iron in soils
    • Augite weathers rapidly releases nutrients
    • Source: igneous rocks especially prevalent in volcanic soils
• Secondary minerals
  • Clay minerals result from weathering of primary minerals crucial for soil fertility
    • Kaolinite low shrink-swell capacity common in highly weathered soils (tropical regions)
    • Illite moderate CEC retains potassium
    • Smectite high shrink-swell capacity forms expansive soils (Vertisols)
    • Vermiculite high CEC important for nutrient retention
    • Source: weathering of primary minerals through various processes
  • Iron and aluminum oxides influence soil color and phosphorus fixation
    • Hematite gives red color to soils (terra rossa)
    • Goethite responsible for yellowish-brown soil colors
    • Gibbsite common in highly weathered tropical soils
    • Source: weathering of iron and aluminum-bearing minerals under oxidizing conditions

Mineral weathering in soil formation

• Physical weathering breaks down rocks without changing chemical composition
  1. Freeze-thaw cycles expand water in cracks causing rock fragmentation
  2. Root pressure exerts force on rocks as plants grow
  3. Thermal expansion and contraction causes rock stress and fracturing
• Chemical weathering alters mineral composition through reactions
  • Hydrolysis breaks down minerals through reaction with water (feldspar to clay)
  • Oxidation changes mineral state through exposure to oxygen (iron rusting)
  • Dissolution removes minerals as they dissolve in water or acids (limestone in caves)
• Biological weathering accelerates mineral breakdown through organism activity
  • Root exudates release organic acids that dissolve minerals
  • Microbial activity produces CO2 and organic acids enhancing weathering
• Mineral transformation alters existing minerals and forms new ones
  • Alteration of primary minerals changes their structure and composition
  • Formation of secondary minerals creates new soil components
    1. Clay mineral synthesis builds complex layered structures
    2. Precipitation of oxides and hydroxides forms distinct mineral phases

Soil Mineral Properties and Functions

Soil minerals' influence on properties

• Cation Exchange Capacity affects soil's ability to hold and release nutrients
  • Clay minerals and organic matter contribute to CEC through negative surface charges
  • Affects nutrient retention and availability for plant uptake
• Soil structure impacts water movement and root growth
  • Clay minerals influence aggregation by binding soil particles
  • Impacts water retention and aeration in soil pores
• Soil pH determines nutrient availability and microbial activity
  • Buffering capacity of clay minerals stabilizes soil pH
  • Affects nutrient availability by influencing solubility and ion exchange
• Water holding capacity determines soil moisture retention for plant use
  • Clay content influences soil moisture retention through small pore spaces
• Nutrient supply provides essential elements for plant growth
  • Weathering of primary minerals releases elements (potassium, calcium, magnesium)
• Soil color indicates soil composition and conditions
  • Iron oxides contribute to soil color (red, yellow, brown hues)
• Soil plasticity and cohesion affect soil workability and erosion resistance
  • Clay minerals affect soil physical properties through particle interactions

Primary vs secondary mineral roles

• Primary minerals form foundation of soil composition
  • Serve as parent material for secondary minerals through weathering processes
  • Slowly release nutrients through gradual breakdown (potassium from feldspars)
  • Contribute to soil texture by forming sand and silt fractions
  • Generally have lower surface area and reactivity compared to secondary minerals
• Secondary minerals dominate soil chemical processes
  • Higher surface area and reactivity due to small particle size and complex structures
  • Dominate soil chemical processes through ion exchange and adsorption
  • Major contributors to CEC enhancing nutrient retention
  • Influence soil pH and buffering capacity stabilizing soil chemistry
  • Play crucial role in nutrient cycling and retention for plant availability
  • Affect soil structure and water-holding capacity improving soil physical properties
  • More susceptible to environmental changes (pH, redox conditions)