Soil Processes, Natural Resources, Geologic Materials & Corrosion – Lecture Review

Soil Formation and Pedogenic Processes

• Parent material ("parent rock") undergoes physical and chemical weathering → produces unconsolidated regolith that gradually evolves into soil.
• Weathering proceeds more rapidly when the surface (exposed rock) is fractured, thin, or chemically susceptible.

1. Podzolization (often miss-spelt as “Codzolization”)

• Typical of cold, humid, coniferous‐forest environments.
• Organic acids produced by litter fermentation break down primary silicate minerals.
• Mobilised \text{SiO}_2, \text{Al}^{3+} and \text{Fe}^{3+} form soluble organo-metallic complexes.
• Complexes are eluviated (washed downward) from the surface (A/E) horizon → accumulate in a darker B horizon, creating a bleached E horizon and a spodic B horizon below.
• Result = strongly leached, acidic, nutrient-poor soils (Podzols / Spodosols).

2. Gleying

• Develops under permanently water-logged, anaerobic conditions (e.g. marshes, swamps).
• Reduced iron compounds (Fe²⁺) give grey/blue colours; mottles form where alternating oxidation–reduction occurs.
• Iron is removed or redistributed, structure becomes massive; root aeration is limited.

3. Desilication / Ferrallitisation

• Characteristic of intensely weathered, humid-tropical settings.
• Heavy rainfall + high temperature → extreme leaching of silica and basic cations.
• Residual enrichment of insoluble \text{Fe}2\text{O}3 and \text{Al}2\text{O}3 forms ferralsols / laterites.
• Results in low-nutrient, highly acidic, red soils rich in sesquioxides.

4. Translocation Processes (general vertical movements)

• Leaching: downward removal of soluble ions when precipitation > evapotranspiration.
• Eluviation: mechanical/chemical removal from a horizon (commonly clay, Fe, Al).
• Illuviation: deposition / accumulation in a lower horizon (B) forming clay skins, iron pans, etc.
• Calcification: precipitation of \text{CaCO}_3 where evapotranspiration ≈ precipitation (semi-arid).
• Salinisation: upward or downward concentration of soluble salts in arid / poorly drained zones.
• Laterisation (advanced desilication): almost complete removal of silica; formation of bauxite.

Natural-Resource Conservation

1. Water Resources

• Defined as “complex, interconnected and dynamic ecological systems” supplying domestic, agricultural and hydropower needs.
• Kenya’s high-elevation ‘water-tower’ catchment forests provide >75\% of surface water for urban use and irrigation.

2. Forest Resources

• Serve as ecological power-houses: regulate climate, cycle nutrients, preserve soil, generate biodiversity.
• Act as primary water-catchments—buffering floods and sustaining base-flow during dry seasons.
• Economic roles: timber, fibre, medicinal products, tourism, livelihood support.
• Non-quantifiable values: cultural identity, carbon sequestration, scenic beauty, habitat integrity.
• Sustainable stewardship required for inter-generational equity and national stability.

Engineering Geology

• Road stability, dam siting, foundation design and water-supply schemes depend directly on engineering-geological assessment.
• Critical geological features: faults, joints, bedding planes, dykes and folds—can weaken foundations, cause leakage or induce landslides.
• Early site investigation and geological mapping dramatically reduce project cost overruns.
• Major application fields:
– Mining and mineral extraction.
– Nuclear‐waste disposal & nuclear-explosion seismology.
– Space exploration landing-site analysis.
– Land-use planning, geo-hazard zoning and seismography.

Rocks: Classification & Key Structures

1. Sedimentary Rocks

• Formed by deposition, compaction and lithification of sediments.
• Primary structures preserve depositional environment:
– Horizontal bedding / stratification.
– Cross-bedding (palaeocurrent indicators).
– Graded bedding (turbidity currents, flood events).
– Ripple marks, mudcracks, sole marks, flute casts.
• Common clastic lithologies: conglomerate, sandstone, shale;
chemical/biogenic: limestone, chert, evaporites.

2. Metamorphic Rocks

• Produced when pre-existing rock experiences elevated temperature (>200\,^\circ\text{C}) and/or pressure (>4000 bars) without melting.
• Textural changes: foliation, schistosity, gneissic banding.
• Mineral changes: growth of biotite, garnet, staurolite, amphibole.
• Examples:
– Marble (from limestone/dolostone).
– Quartzite (from quartz-rich sandstone).
– Schist (from mudstone/shale under medium-grade conditions).
– Amphibolite (from basalt / gabbro + water).

Corrosion (Materials Science Review)

• Definition: Gradual destruction or deterioration of a material (usually metals) through electro-chemical or chemical reaction with its environment.
• Basic rate expression:
\text{Corrosion Rate} = \frac{K \times W}{\rho \times A \times t}
where W = mass loss, \rho = density, A = exposed area, t = time, K = unit constant.

Major Types

1. Uniform (general) corrosion – surface attacked evenly; predictable thickness loss.
2. Galvanic corrosion – dissimilar metals electrically connected in electrolyte create an anode/cathode pair.
3. Pitting corrosion – localised attack producing deep cavities; dangerous because mass loss is small but penetration rapid.
4. Crevice (under-deposit) corrosion – occurs in shielded zones with stagnant solution.
5. Inter-granular corrosion – preferential attack along grain boundaries after sensitisation.
6. Stress-corrosion cracking (SCC) – tensile stress + corrosive medium → brittle fracture.
7. Erosion–corrosion – mechanical wear plus electro-chemical attack in high-velocity fluids.
8. Microbial (biologically induced) corrosion – sulphate-reducing or iron-oxidising bacteria accelerate attack.

Summary of Soil-Profile Horizons

• O-Horizon: fresh > partly decomposed organic litter.
• A-Horizon (top-soil): mineral + humus; active biological zone.
• E-Horizon (eluviation layer): bleached, leached of clay/Fe/Al.
• B-Horizon (illuviation): accumulation of translocated material – clay, oxides, carbonates.
• C-Horizon: slightly weathered parent material.
• R-Horizon: consolidated bedrock.

Key Take-Away Connections

• Pedogenic processes (podzolisation, gleying, desilication, etc.) reflect the interplay of climate, parent material, topography, organisms and time.
• Degradation of forest and water resources disrupts these soil-forming balances, undermines engineering works and accelerates corrosion of infrastructure.
• Holistic, sustainable management—integrating geology, ecology and engineering—is essential for economic development and long-term environmental stability.