Soil Formation - Lecture

Overview

  • Parent material: weathered rock

  • Climate: soil forms faster in warm wet climates

  • Organisms: add organic matter

  • Topography: hills and valley affect exposure to sun, wind, water

  • Time: forms through weathering of rocks and accumulation of organic matter

Defining Soil

  • Rocks help determine soil characteristics

    • influences plants and animals

  • Soil is not dirt

    • dirt is what you get on your clothes

    • soil is a complex plant supporting system

  • Soil is technically renewable, but can be depleted

  • “Ideal” soil composition

    • 5% organic matter, 25% air, 25% water, 45% mineral nutrients (sand, silt, clay)

Weathering and Soil

  • Breakdown of larger rock into smaller particles (sand, silt, clay) or chemical alteration/decomposition of rock/minerals

  • Rock material derived: rock, lava, volcanic ash, surface rocks, dunes

Types of Weathering

  • Physical/Mechanical

    • driven by wind, water, ice, gravity

      • joints: cracks in rock

      • crystal growth: salt or quarts veins

      • thermal expansion: heating and cooling of rock

      • frost wedging: freeze them

  • Biological

    • plants and animals breakdown rock

      • root wedging

      • animal burrows

      • living on rock

  • Chemical

    • chemical alteration or decomposition of rock

    • hydrolysis: H+ or OH- replace minerals through ion exchange

    • oxidation: oxygen reacts with minerals

      • ex: rust, iron oxide

    • dissolution: mineral is dissolved by water and acid

      • ex: carbonic acid dissolves calcite

Takeaways

  • Most rocks and minerals form deep within Earth, when they reach surface via up lost and erosion, they encounter different conditions

    • accelerates weathering rates

    • lower temperatures, lower pressure, more water, more oxygen (free)

  • Both physical and chemical weathering work together to break down rock