Unit 1: Introduction to Soil Science

1. General Concept of Soil

Definition

Soil is defined as a dynamic natural body on the surface of the earth, composed of mineral and organic materials, harboring living organisms and supporting plant life. It is the product of the destructive and synthetic forces of nature.

The Three-Phase System:
Soil is considered a three-phase system:

1. Solid Phase (50%): Minerals (45%) and Organic Matter (5%).
2. Liquid Phase (25%): Soil solution (water + dissolved ions).
3. Gaseous Phase (25%): Soil air (High , lower compared to atmosphere).

2. Pedological and Edaphological Concepts

Soil science is broadly divided into two distinct approaches regarding how soil is viewed and studied.

A. Pedology (Pedological Approach)

Derived from the Greek word pedon (soil/earth) and logos (study).

• Definition: The study of soil as a natural body, focusing on its origin, formation, classification, and morphology.
• Focus: It treats soil as a distinct entity, independent of its practical utilization.
• Key Areas:
  • Soil genesis (formation).
  • Soil survey and mapping.
  • Soil taxonomy (classification).
• Objective: To understand the soil's internal and external properties and its position in the landscape.

B. Edaphology (Edaphological Approach)

Derived from the Greek word edaphos (soil/ground) and logos (study).

• Definition: The study of soil from the standpoint of higher plants. It considers the physical, chemical, and biological properties of soil as they affect plant growth and crop yield.
• Focus: It treats soil as a medium for plant growth (habitat).
• Key Areas:
  • Soil fertility and nutrient management.
  • Soil physics (water retention for roots).
  • Soil conservation for agriculture.
• Objective: To maximize food and fiber production.

Comparison Table

3. Soil Formation (Weathering)

Soil formation begins with the weathering of rocks and minerals. Weathering is the disintegration and decomposition of rocks and minerals near the earth’s surface.

A. Physical Weathering (Disintegration)

Mechanical breakdown of rocks into smaller fragments (regolith) without changing chemical composition.

• Temperature: Differential expansion and contraction of minerals causes exfoliation and cracking.
• Water: Abrasion by running water; Freeze-thaw cycles (frost wedging) where water expands by 9% upon freezing, splitting rocks.
• Wind: Sandblasting effect acts as an abrasive agent.

B. Chemical Weathering (Decomposition)

Alteration of mineral composition, transforming primary minerals into secondary minerals (clays). This is most active in hot and humid climates.

1. Hydration: Chemical attachment of water molecules to a mineral.
   • (Anhydrite) (Gypsum)
2. Hydrolysis: Reaction with water where water splits into and ions. This is the most important process in silicate weathering.
   • (Orthoclase) (Acid silicate clay)
3. Carbonation: Reaction between carbonate ions and minerals. dissolves in soil water to form carbonic acid (), which solubilizes limestone.
   • (Soluble Calcium Bicarbonate)
4. Oxidation: Gain of oxygen / loss of electrons. Crucial for iron-bearing minerals (causes red/yellow color).
   • (Ferrous oxide) (Ferric oxide/Hematite)
5. Solution: Dissolution of soluble salts (NaCl, gypsum) in water.

C. Biological Weathering

• Physical: Roots growing into cracks exert pressure (turgor pressure) to split rocks.
• Chemical: Lichens, mosses, and microbes secrete organic acids (chelation) that dissolve minerals.

4. Factors of Soil Formation

According to Hans Jenny (1941), soil formation is a function of five independent variables.

Where:

• = Soil
• = Climate (Active Factor)
• = Biosphere/Organisms (Active Factor)
• = Relief/Topography (Passive Factor)
• = Parent Material (Passive Factor)
• = Time (Neutral/Passive Factor)

1. Active Factors

These factors supply the energy (heat, moisture, biological activity) that acts upon the mass.

• Climate: The most influential factor.
  • Precipitation: Drives leaching and chemical weathering. High rain = Acidic soils; Low rain = Alkaline/Saline soils.
  • Temperature: For every 10°C rise, chemical reaction rates double (Van 't Hoff's law). Accelerates organic matter decomposition.
• Biosphere (Organisms):
  • Vegetation adds Organic Matter (OM).
  • Microorganisms drive humification and nutrient cycling.
  • Earthworms/termites mix the soil (bioturbation).

2. Passive Factors

These serve as the source of mass and conditions receiving the active factors.

• Parent Material: The unconsolidated mineral material from which soil develops.
  • Determines soil texture (e.g., Sandstone yields sandy soil).
  • Determines initial chemical composition.
• Relief (Topography):
  • Slope: Steep slopes = rapid erosion, thin soils. Flat lands = deeper soils.
  • Aspect: In the Northern Hemisphere, south-facing slopes are warmer/drier; north-facing are cooler/moister.

3. Time

Soil formation is a slow process.

• Young Soils: Retain features of parent material (e.g., Entisols).
• Mature Soils: Show distinct horizons and equilibrium with the environment.

5. Pedogenic Processes

Pedogenic processes refer to the specific physical and chemical changes that transform the parent material into distinct soil horizons.

A. The Four Basic Processes (Simonson, 1959)

1. Additions: Inputs to the soil (e.g., organic matter from leaves, dust from atmosphere, salts from groundwater).
2. Losses: Removals from the soil (e.g., leaching of nitrates/calcium, erosion of surface soil, evapotranspiration).
3. Translocations: Movement of materials within the soil profile (e.g., clay moving from A to B horizon).
4. Transformations: Chemical or physical modification (e.g., decomposition of leaves into humus, weathering of feldspar into clay).

B. Specific Soil Forming Processes

1. Podzolization:

   • Conditions: Cool, humid climates; coniferous forests; acidic parent material.
   • Process: Intense leaching of bases and iron/aluminum oxides from the surface.
   • Result: An ashy-grey siliceous horizon ( horizon) creates a distinct "Podzol" (Spodosol).

2. Laterization:

   • Conditions: Hot, humid tropical climates (rainforests).
   • Process: Rapid removal of silica (Desilication) and accumulation of Iron and Aluminum oxides (Sesquioxides).
   • Result: Red, hard, iron-rich soils (Oxisols/Laterites).

3. Calcification:

   • Conditions: Semi-arid to arid climates; grassland vegetation.
   • Process: Limited rainfall prevents complete leaching. Calcium Carbonate () precipitates in the B horizon.
   • Result: Formation of a hard "Caliche" layer or kankar nodules.

4. Gleization:

   • Conditions: Poor drainage, waterlogged soils (anaerobic conditions).
   • Process: Reduction of Iron ().
   • Result: Soil turns bluish-grey or greenish-grey (Gleying). Mottling occurs where oxygen is present.

5. Salinization:

   • Conditions: Arid/Semi-arid regions with high water tables or poor irrigation.
   • Process: Accumulation of soluble salts (chlorides, sulfates) at the surface due to high evaporation.
   • Result: White crusts on soil surface (Saline soils).

6. Humification:

   • The process of converting raw organic matter into humus (stable, dark, complex organic material).

6. The Soil Profile

A vertical section of the soil through all its horizons and extending into the parent material is called the Soil Profile.

• Solum: The true soil (A + B Horizons).
• Regolith: Unconsolidated debris overlaying hard rock (A + B + C).

Master Horizons

The profile is divided into layers called horizons, designated by capital letters.

1. O Horizon (Organic Layer)

• Surface layer composed of organic debris (leaves, twigs).
• Common in forests, rare in cultivated fields (plowed under).
• O1: Undecomposed litter.
• O2: Decomposed humus.

2. A Horizon (Topsoil / Eluvial Zone)

• Topmost mineral horizon.
• Darker color due to accumulation of organic matter mixed with minerals.
• Zone of maximum biological activity.
• Eluviation: The "washing out" of fine clay and colloids occurs here.

3. E Horizon (Eluvial / Leaching Zone)

• Found between A and B (mostly in forests/podzols).
• Zone of maximum eluviation (exit).
• Light colored (bleached) due to loss of iron, aluminum, and clay.

4. B Horizon (Subsoil / Illuvial Zone)

• Zone of illuviation (washing in).
• Accumulates materials lost from A and E (Clays, Iron, Aluminum, Humus, Carbonates).
• Usually harder and more compact than topsoil.

5. C Horizon (Parent Material)

• Unconsolidated material from which the solum developed.
• Least weathered; biological activity is negligible.
• Retains structural features of the parent rock.

6. R Horizon (Bedrock)

• Consolidated, hard rock (Granite, Basalt, Limestone, etc.).
• Cannot be dug by a spade.

Transitional Horizons

• Horizons dominated by properties of one master horizon but having subordinate properties of another.
• Notation: AB (More like A, some B), BA (More like B, some A).