SC1250 Environmental Geology - Soil and Erosion Detailed Notes

SC1250-Environmental Geology

Module III: Planetary Resources - Soil

Overview

  • Focus: Soil, including weathering and erosion.

Importance of Soil

  • UN Perspective: Soil erosion must be halted to ensure a sustainable future.

  • Food Production: Topsoil is critical as it allows for the growth of 95% of the world's food, yet it is rapidly disappearing.

  • Call to Action: Governments are urged to prioritize soil conservation to maintain food security.

Land Use for Food Production

Global Land Statistics

  • Earth’s surface consists of approximately 29% land, approximately 149 million km².

  • Distribution: 71% Ocean and 29% Land

  • Habitable Land: 71% of the land surface is habitable, equating to about 104 million km².

  • Land Usage:

    • Agricultural Land: 50%

    • Forests: 37%

    • Shrubland: 11%

    • 12 million km² classified as barren land.

    • Freshwater: 1% is used for lakes and rivers.

Agricultural Land Breakdown

  • Livestock Production: 77% of agricultural land allocated to livestock (meat and dairy production).

  • Crops: 23% of agricultural land dedicated to crops.

  • Caloric Supply:

    • 18% from meat & dairy

    • 82% from plant-based food

  • Protein Supply:

    • 37% from meat & dairy

    • 63% from plant-based food.

Soil Formation and Composition

Definition of Soil

  • Regolith: Defined as an unconsolidated, heterogeneous layer of rocks, minerals, and organic matter that lies above bedrock.

    • Includes various other materials: beaches, glacial till, stream beds, volcanic ash.

  • Engineer’s Perspective: Soil regarded as any uncemented or weakly cemented accumulation of mineral particles formed from weathered rock inclusively housing void spaces containing water and/or air (Craig, 2001).

  • Soil Scientist’s Perspective: Soil is a mixture of mineral and organic materials, water, and air that sustains plant life.

Difference Between Soil and Sediment

  • Soil remains close to where it was formed, while sediment involves transport and deposition in new locations.

  • Components of Soil:

    • Mineral matter, air, water, organic material, living plants and animals, microorganisms.

    • Soil organic matter (SOM) is essential for maintaining soil fertility.

Weathering and Erosion

Definitions

  • Weathering: The physical and chemical breakdown of rocks that occurs in place. Major factors influencing weathering include climate and water.

  • Erosion: The wearing away and transport of soil and rock material by agents like water, wind, or ice.

  • Erosion vs Weathering:

    • Weathering prepares materials for erosion; both processes are continuous.

Types of Weathering

  1. Mechanical Weathering:

    • Physical breakdown of minerals without chemical alteration.

    • Increases surface area for subsequent weathering processes.

    • Processes:

      • Freeze-thaw cycles (ice wedging)

      • Salt wedging

      • Mechanical abrasion

      • Biological activity (e.g., root growth).

  2. Chemical Weathering:

    • Involves chemical reactions that alter minerals.

    • Dependent on mineral composition and formation conditions.

    • Chemical Processes:

      • Dissolution (e.g., halite, calcite)

      • Hydrolysis (e.g., K-feldspar to kaolinite)

      • Oxidation (e.g., ferromagnesian silicates.)

    • Biological Weathering: Organisms enhance the breakdown of rocks (e.g., lichen).

Factors Affecting Weathering Rates

  • Climate influences weathering type and rates.

  • Higher temperatures and water enhance chemical reactions.

  • Angular fragments generally weather more rapidly than rounded ones.

Soil Profiles and Properties

Soil Horizons

  1. O Horizon: Organic matter only.

  2. A Horizon: Weathered mineral fragments mixed with organic material.

  3. E Horizon: Zone of leaching; water infiltrates and transports minerals.

  4. B Horizon: Zone of accumulation; deposits of leached materials (e.g., clay).

  5. C Horizon: Partly broken rock material.

Properties of Soil

  • Composition and properties influenced largely by the parent material.

  • Color indicates organic content:

    • Dark = more organic matter

    • Light = less organic matter

    • Red/Yellow = presence of iron.

  • Texture Classification (USDA)

    • Clay < 0.002 mm

    • Silt 0.05 - 0.002 mm

    • Sand 0.05 – 2 mm

    • Loam is a balanced mixture of these components.

Soil Drainage

  • Texture Impact: Influences drainage capacity:

    • Sand > Silt > Clay.

Soil Classification

Classification Types

  • Based on composition and environmental factors leading to leaching, e.g.,

    • Pedalfer: High aluminum and iron accumulation, found in wet regions.

    • Pedocal: High calcium carbonate content prevalent in arid regions.

Global Soil Orders

  • FAO/UNESCO Soil Map of the World identifies about 110 soil classifications.

  • USA Soil Classification contains 12 main soil orders including:

    • Alfisols, Entisols, Andisols, Gelisols, Aridisols, Histosols, Inceptisols, Spodosols, Mollisols, Ultisols, Oxisols, Vertisols.

Canadian Soil Orders

  • Classification involves 10 soil orders, e.g., Gleysolic, Luvisolic, Cryosolic.

Soil Degradation and Erosion

Causes of Soil Erosion

  • Farming is the major source; other contributors include:

    • Construction, off-road vehicles, strip mining.

  • Dust Bowl (1930s): A historical example diminished by poor agricultural practices leading to soil loss.

Regional Issues: Sahel Region, Africa

  • Overgrazing caused by population increase.

  • Resulting desertification linked to climatic extremes and poor land management.

Impacts and Consequences of Soil Erosion

  • Organic matter loss can lead to significantly reduced crop yields (e.g., 6 inches of loss can reduce yield by 40%).

  • Increased need for fertilizers: leads to economic burden and nutritional issues.

  • Pollution of waterways due to sediment loss degrades water quality.

Strategies to Reduce Erosion

Erosion Prevention Techniques

  • Vegetation: Sowing roots in the ground post-harvest to stabilize soil and reduce erosion.

  • Contour Plowing: Aligning plowed rows along a land's contour to mitigate runoff.

  • Terracing: Creating steeper slope steps to slow water runoff.

  • Strip Cropping: Alternating crops of varying heights to utilize the landscape effectively.

Policy Considerations

  • Addressing soil erosion may require costly measures that could affect yield, meaning feasible policy compliance is vital.

Conclusion on Soil Erosion

  • Soil erosion remains a pressing global issue, exacerbated by human activities and inadequate management practices. Concerns regarding sustainability of agricultural practices must be addressed urgently to protect this crucial resource.