Soil formation is a slow, continuous process involving weathering of rocks, biological activity, and climatic influence. It lays the foundation for agriculture, ecology, and environmental science.

How Long Does Soil Formation Take?

• Formation of 1 inch of soil takes approximately 800–1000 years.
• Influenced by parent material, climate, organisms, topography, and time.

Scientific Definitions

🔹 Dokuchaev (Russian Pedologist) “Soil is the result of the combined activity and reciprocal influence of parent material, organisms, climate, topography, and age of land.”

🔹 Hans Jenny (1941); “Soil (S) = f(cl, o, r, p, t…)”
Where:

• cl = climate
• o = organisms
• r = relief (topography)
• p = parent material
• t = time

Factors of Soil Formation

Soil formation factors are categorized as:

(A) Passive Factors:

1. Parent Material
2. Topography/Relief

(B) Active Factors:

1. Climate
2. Organisms (Biosphere)

(C) Neutral Factor:

1. Time / Age of Land

🔸 1. Parent Material; The unconsolidated rock or organic material from which soil develops.

Types:

• Residual: Remains in place (e.g., granite, basalt)
• Transported: Moved by water, wind, ice, or gravity
• Biological: Composed of organic matter (e.g., peat)

Influences soil texture, structure, mineral composition, and fertility.

2. Topography / Relief; Affects:

• Drainage and runoff
• Soil erosion
• Sun and wind exposure

Slope Impact:

• Upper slope: Less clay, OM, lighter color, well-drained
• Valley: Deep, clayey, poorly drained

3. Climate; Includes:

• Rainfall
• Temperature
• Humidity
• Wind

🔹 Rainfall affects leaching, OM content, clay mineral formation
🔹 High rainfall → Leaching of salts → Acidic soils
🔹 Low rainfall → Salt accumulation at surface
🔹 Temperature speeds up chemical weathering

🔸 4. Organisms (Biosphere)

• Flora: Roots break rocks, add OM, and form the O horizon
• Fauna: Earthworms, ants mix and aerate the soil
• Microbes: Decompose OM → humus

5. Time / Age of Land

• Refers to how long soil-forming processes have been active
• Young soils: Less developed horizons
• Old soils: Well-developed profiles with distinct layers
• Time allows for horizonation and maturity

Soil Forming Processes (Simonson, 1959)

Fundamental Pedogenic Processes

1. Humification: Decomposition of organic matter into humus
2. Eluviation (Bluviation): Washing out of clay, Fe, Al, OM from upper layers
3. Illuviation (Mhuviation): Deposition/accumulation of eluviated materials in lower horizons
4. Horizonation: Development of distinguishable soil layers

Specific Zonal Pedogenic Processes

🔸 1. Calcification

• Accumulation of CaCO₃ in soil (as Kankar nodules)
• Occurs in arid/semi-arid regions

CO₂ + H₂O → H₂CO₃ (Carbonic Acid)

H₂CO₃ + Ca → Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ ↑

🔸 2. Gypsification; Deposition of gypsum (CaSO₄·2H₂O) in arid zones

🔸 3. Decalcification; Leaching/removal of CaCO₃, Occurs in humid regions, Opposite of calcification

🔸 4. Podzolization; Removal of bases, Fe, Al, OM → acidic soil, Forms bleached A-horizon (ash-colored), Common in cold, moist forest zones

Typical Podzol Profile:

1. Precipitated humus
2. Reddish brown Fe/Al layer
3. Yellowish horizon → parent material

🔸 5. Laterization

• Leaching of silica, accumulation of Fe/Al oxides (sesquioxides)
• Soil becomes brick-hard when dry → Laterite
• Found in humid tropics

Laterites are:

• Rich in Fe/Al oxides
• Poor in silica and bases
• Used for coconut, banana, coffee plantations

Conditions Favoring Laterization

• High rainfall (2000–2500 mm)
• Constant high temperature
• Good drainage
• Tropical vegetation

Summary Table – Zonal Soil Forming Processes

Key Exam Points

• Kankar = Result of calcification
• Podzol = Acidic, ash-like layer
• Laterite = Brick-hard, Fe-rich soil
• Jenny’s equation: S = f(cl, o, r, p, t)
• Simonson (1959): 4 basic soil processes
• Humification = humus formation
• Eluviation vs Illuviation = Outwashing vs Inwashing

5. Gleization

• Origin: From Russian word “Gley” meaning blue-grey clay
• Occurs: In waterlogged and poorly drained conditions
• Soil type: Hydromorphic soils
• Key Features:
  • Formation of gley horizon (grey-blue color)
  • Anaerobic conditions cause Fe and Mn reduction
  • Appearance of mottles and concretions
  • Occurs in wetlands, swamps, or low-lying rice fields

Gleying is due to poor aeration, not climate.

6. Salination

• Definition: Accumulation of soluble salts (Na⁺, Cl⁻, SO₄²⁻, etc.) in the root zone
• Common in: Arid and semi-arid regions

Causes of Salinization:

1. Shallow/brackish groundwater
2. Low-lying topography
3. Poor natural drainage
4. Drying of lake beds (leaves salts on surface)
5. Alluvial deposits near coastal areas
6. Use of saline irrigation water

Results in white crust on soil surface; harmful for most crops.

7. Solonization (Alkalization)

• Definition: Accumulation of exchangeable sodium (Na⁺) in soil colloids
• pH: Often > 8.5
• Soil type: Sodic or black alkali soil

Features:

• Dispersion of soil particles → poor structure
• Formation of black organo-clay coatings
• Hard crusts, poor infiltration
• Toxic to plant roots

Black alkali soils are rich in sodium and organic matter stains.

8. Solodization (Dealkalization)

• Definition: Removal of exchangeable sodium from soil
• Improves soil structure and fertility

Example Reaction: 2NaX + CaSO₄ → Na₂SO₄ (leached) + CaX

Where:

• X = soil exchange complex
• CaX = calcium-rich exchange complex (improves structure)

 This is a reclamation process for sodic soils.

9. Pedoturbation (Soil Mixing)

Pedoturbation = natural or biological mixing of soil layers, affecting horizon development and profile uniformity.

Types of Pedoturbation:

Vertisols show deep cracking and self-mulching due to argillipedoturbation.

Summary Table – Intrazonal Processes

Competitive Exam Quick Pointers

• Gleization → Anaerobic condition → grey/blue layer
• Salination → White salt crusts → affects plant growth
• Solonization → High Na⁺ → dispersion → poor soil structure
• Solodization → Na⁺ removed → Ca improves soil
• Vertisols → Show argillipedoturbation