Soil Science (STT1022) - Soil Concepts and Origins

Soil Science (STT1022) - Concept and Origin of Soil

• Soil science is sometimes divided into two parts:
  • Edaphology: The study of soils as mediums for plant production.
  • Pedology: The study of soils as biochemically synthesized bodies in nature.
• Geological definition of soil: Loose surface of the Earth, distinguished from solid bedrock, that supports plant life.
• Soil Taxonomy definition: A collection of natural bodies on the Earth's surface, possibly modified by humans, containing living matter and supporting or capable of supporting plants. Its upper limit is the air or shallow water, its lower limit is where soil weathering is effective.
• Soil defined: The unconsolidated mineral and organic material on the immediate surface of the Earth that serves as a natural medium for plant growth. Alternatively, unconsolidated mineral and organic material on the surface, influenced by parent material, climate, macro/microorganisms and topography over time. This differs from its origin in many physical, chemical, biological and morphological properties.
• Soil composition: A mixture of mineral matter (45%), organic matter (5%), water (25%), and air (25%).
• Soil profile: A vertical section through a soil, showing the layers (horizons).
• Soil formation (genesis) involves two processes:
  • Weathering: The process of breaking down rocks and minerals into smaller particles.
  • Soil development: The changes within the unconsolidated material over time (processes include additions, losses, transformations, and translocations).
• Soil horizons: Layers within the soil, different in composition or characteristics from other layers; formed from weathering differences with depth, accumulation of humus and translocation of colloids by water. Commonly labeled as O, A, E, B, C, and R horizons.

Soil Forming Factors

• Five (5) soil forming factors that affect the rate and type of soil formation according to Hans Jenny (1940) are:
  • Parent material (original materials forming the soil).
  • Climate (particularly temperature and precipitation).
  • Living organisms (biota) and organic matter.
  • Topography (slope and elevation).
  • Time (length of processes).

Weathering

• Weathering: The breaking down of rocks and minerals, into smaller particles (disintegration and change). Two main forms:
  • Physical weathering: The breaking or grinding of particles into smaller sizes, by agents such as water, ice, wind, temperature change and pressure.
  • Chemical weathering: The dissolving or alteration of minerals through various chemical processes such as hydrolysis, hydration, carbonation, oxidation-reduction and complexation. These processes produce dissolved ions and leave resistant products.

Biogeochemical Processes (examples)

• Hydrolysis: The reaction of minerals with water, breaking down the minerals and forming new compounds.
• Hydration: The absorption of water by minerals, altering their structure.
• Oxidation: The reaction of minerals with oxygen, causing changes in their chemical composition.
• Complexation: The bonding of minerals with other elements or molecules, forming new, stable compounds.

What Happens to Weathering Products?

• Dissolved ions: Some are retained in the soil (mainly cations); others are leached out of the soil profile.
• Least soluble materials: These often resolidify and remain part of the soil solids.

Weathering Scheme for Clay-Sized Minerals

• Minerals are classified by how easily they weather; some weather quickly (e.g., gypsum, calcite, feldspars, and quartz), releasing primary soil minerals such as illite, montmorillonite, kaolinite, gibbsite, and hematite. The most resistant minerals include anatase, zircon, and corundum.

Soil Forming Processes (Summary)

• These processes are additions, losses, transformations, and translocations. These processes involve gravity pulling water downward, organic matter decomposing into humus, and water dissolving minerals for transport.

Soil Horizons

• Horizonal layers (usually roughly parallel) in a soil. Occurring due to different weathering rates, humus amounts, colloid translocation, and other processes. Typical letter horizons are O, A, E, B, C, and R horizons

What Soil Provides for Plants

• All soils provide the same fundamental requirements:
  • Physical support.
  • Oxygen supply (requires proper porosity).
  • Water.
  • Temperature moderation.
  • Nutrients (13 of 16 important elements).
  • Protection from toxic substances.

Degradation and Destruction of Soils

• Degradation causes a reduction in soil productivity due to:
  • Intensive and excessive use, depleting minerals and organics.
  • Soil erosion.
  • Salinization.
  • Dumping of toxic wastes/chemicals.
  • Burning organic soils. Heavy metal accumulation or salt accumulation.

Minerals and Rocks

• Minerals: Inorganic substances that are homogenous, with a defined chemical composition, and characteristic physical properties (shape, colour, melting point, and hardness).
  • Primary minerals: Formed from cooled molten rock. Examples include quartz, feldspars and micas.
  • Secondary minerals: Formed from the weathering of primary materials. Examples includeclays, calcite, and gypsum.
• Rocks: Combinations of minerals into solid masses.
  • Igneous rocks: Formed from cooled molten rock (e.g., granite, basalt).
  • Sedimentary rocks: Formed from sediments consolidated into a solid mass (e.g., sandstone, shale).
  • Metamorphic rocks: Formed from pre-existing rocks through changes in heat or pressure (e.g., slate, marble).

This quiz explores key concepts in Soil Science, specifically focusing on the definitions and classifications of soil, including edaphology and pedology. Learn about the geological and soil taxonomy aspects that define soil as a crucial component for plant life. Test your knowledge on how soil functions as a medium for plant growth and its environmental influences.