Earth Science & Ecology
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Week 9Geosphere ยท Term 1

Soil: Earth's Living Layer

Horizons, composition, types & conservation

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Reading: Soil: Earth's Living Layer

Soil: Earth's Living Layer

A teaspoon of healthy soil contains more living organisms than there are humans on Earth. It has bacteria, fungi, protozoa, nematodes, and in a larger sample, earthworms, insects, and mites. Soil is not dead dirt. It is one of the most complex ecosystems on the planet.

What Is Soil?

Soil = weathered rock minerals + organic matter + water + air + living organisms

Healthy soil is roughly:

  • 45% mineral particles (from weathered rock)
  • 25% water
  • 25% air
  • 5% organic matter (humus and living organisms)

    • The Soil Profile: Layers from Surface to Bedrock

    A cross-section through soil reveals distinct layers called horizons. Each horizon has a different composition, color, and role:

    O Horizon (Organic Layer): Partially decomposed leaves, plant matter, fungi, and bacteria. Dark and rich. Thickest under forests.

    A Horizon (Topsoil): Minerals mixed with humus (decomposed organic matter). The darkest, most fertile layer. Where most plant roots grow. Most biologically active layer in the soil. Takes approximately 500 years to form 1 inch of topsoil under natural conditions.

    B Horizon (Subsoil): Minerals that have been leached (washed down) from the A horizon. Less organic matter, lighter color. Clay, iron, and aluminum oxides accumulate here.

    C Horizon (Parent Material): Partially weathered bedrock. Little to no organic matter. The original rock being slowly broken down, the source of all minerals in the soil above.

    R Horizon (Bedrock): Solid, unweathered rock. Not yet soil, but the foundation from which everything above derives.

    Soil Types and Texture

    Soil Type
    Particle Size
    Drainage
    Best For
    Sandy
    Large
    Fast, drains quickly
    Cacti, some root vegetables
    Clay
    Very small
    Slow, waterlogged
    Poor aeration; holds nutrients
    Silt
    Medium
    Moderate
    Good for many crops
    Loam
    Mix of all three
    Well-balanced
    Ideal for most agriculture

    Loam is the agricultural ideal because it balances drainage, aeration, and nutrient retention.

    Humus is decomposed organic matter created by decomposers (bacteria, fungi, earthworms). It gives topsoil its dark color, spongy texture, and fertility.

    Why Soil Color Matters

  • Dark: high organic matter / humus
  • Red/orange: iron oxidation
  • Pale gray: leaching of minerals
  • Dark blue-gray: waterlogged, oxygen-poor conditions

    • Soil Conservation

    95% of the food humans eat comes from soil, directly or indirectly. No soil = no agriculture = no civilization.

    The US loses approximately 1.7 billion tons of topsoil per year to erosion, primarily from plowing, overgrazing, and deforestation.

    Conservation practices:

  • Cover crops: plant roots hold soil in place
  • Contour plowing: farming across slopes, not down them
  • No-till farming: leaving soil undisturbed to preserve structure
  • Windbreaks: trees that block wind erosion
  • Terracing: building level steps on steep hillsides

    • Key Vocabulary

  • Soil horizon: A distinct layer in a soil profile
  • Topsoil: The A horizon; most fertile layer
  • Subsoil: The B horizon; mineral-rich, less organic
  • Parent material: The C horizon; partially weathered rock
  • Humus: Decomposed organic matter that fertilizes soil
  • Leaching: Downward movement of dissolved minerals through soil
  • Loam: Ideal agricultural soil mixing sand, silt, and clay
  • Decomposers: Organisms (bacteria, fungi, worms) that break down organic matter
  • Soil conservation: Practices that protect topsoil from loss