Pedogenic Processes (Soil Forming Processes)

1. Introduction

The term Pedogenesis refers to the formation and development of soil from parent material under the influence of various soil-forming factors and processes.

• The word “pedogenesis” is derived from Greek:
  • Pedon = soil
  • Genesis = origin or formation

Thus, pedogenic processes are the natural processes responsible for the transformation of rock material into soil, leading to the development of distinct soil horizons and a soil profile.

2. Factors of Soil Formation

Before understanding pedogenic processes, it is important to recall the five major factors of soil formation, proposed by Hans Jenny (1941):

The relationship among these is often expressed as: Soil = f (Cl, O, R, P, T)

3. Major Pedogenic Processes

Pedogenesis involves a sequence of physical, chemical, and biological processes acting together over long periods.
Below is a detailed description of each major pedogenic process recognized by the Sixth Deans’ Committee (ICAR).

Weathering

Definition: The disintegration and decomposition of rocks and minerals to form parent material (regolith).

• Physical Weathering: Breaking down of rocks by temperature changes, water, wind, and pressure.
• Chemical Weathering: Alteration of minerals through reactions like oxidation, hydration, hydrolysis, carbonation, and solution.
• Biological Weathering: Action of plants, roots, and microbes.
• Result: Formation of mineral particles (sand, silt, clay) and release of plant nutrients.

Humification

• Definition: The process of decomposition of organic residues (leaves, roots, dead organisms) into humus, a stable dark-colored organic compound.
• Mechanism: Microorganisms (bacteria, fungi, actinomycetes) break down complex organic matter → simpler compounds → humic substances (humic acid, fulvic acid, humin).

Effects:

• Improves soil structure and aggregation.
• Increases cation exchange capacity (CEC).
• Enhances water-holding capacity and nutrient retention.
• Gives soil its dark color (A horizon).

Eluviation

• Definition: The washing out (removal) of fine materials like clay, Fe, Al oxides, or humus from the upper horizon due to percolating water.
• Derived from: Latin “ex” (out) + “lavare” (to wash).
• Result: Formation of an E horizon (zone of eluviation) — typically light-colored and poor in clay and nutrients.
• Common in: Areas with high rainfall and sandy texture.

Illuviation

• Definition: The accumulation or deposition of materials (clay, Fe, Al, humus) leached from upper horizons into lower horizons.
• Derived from: Latin “in” (into) + “lavare” (to wash).
• Result: Formation of B horizon (zone of accumulation) — enriched with clay (argillic horizon) or sesquioxides (Bt horizon).

• Leaching

Definition: The downward movement of soluble salts and nutrients (like Ca²⁺, Mg²⁺, Na⁺, K⁺, NO₃⁻) through percolating water beyond the root zone.

Effects:

• Loss of nutrients → soil infertility.
• Soil becomes acidic (due to removal of basic cations).
• Prominent in humid tropical and subtropical regions.

Calcification

• Definition: The accumulation of calcium carbonate (CaCO₃) in the soil profile due to incomplete leaching under arid and semi-arid conditions.
• Process: Ca²⁺ + CO₃²⁻ → CaCO₃ (precipitated as nodules or layers).
• Result: Formation of caliche layer (calcic horizon) — white or whitish layer.
• Example: Common in desert and grassland soils (Aridisols).

Podzolization

• Definition: The leaching of Fe, Al, and organic matter from the upper horizons under acidic conditions and deposition in lower horizons.
• Process: Organic acids (from conifer litter) chelate Fe and Al → move downward → accumulate in B horizon.

Result:

• Formation of Podzols (acidic, low-fertility soils).
• Upper horizon is ash-gray (E horizon).
• Common in cold, humid climates under coniferous forests.

Laterization

• Definition: The intense weathering and leaching of silica from soil leaving behind iron (Fe) and aluminum (Al) oxides.
• Process: Under hot, humid conditions → heavy rainfall removes silica → Fe/Al oxides accumulate → laterite formation.

Result:

• Reddish to yellowish color.
• Low base saturation and fertility.
• Common in tropical regions (India, Africa, Brazil).

Gleization

• Definition: Formation of gleyed soils under waterlogged or poorly drained conditions, where oxygen is deficient.
• Process: Fe³⁺ (oxidized) → Fe²⁺ (reduced) → bluish-gray or greenish color.

Result:

• Occurs in swamps, marshes, and valley bottoms.
• Poor drainage, sticky texture, and low oxygen.

• Salinization

Definition: Accumulation of soluble salts (NaCl, Na₂SO₄, MgCl₂, etc.) in the soil surface due to high evaporation and low rainfall.

Effects:

• Formation of white salt crusts.
• Poor seed germination, plant stress.
• Common in arid and semi-arid regions or poorly drained irrigated lands.

• Alkalization (Solonization)

Definition: Accumulation of exchangeable sodium (Na⁺) on soil colloids, replacing Ca²⁺ and Mg²⁺.

Effects:

• Soil becomes alkaline (pH > 8.5).
• Dispersion of clay → poor structure → reduced infiltration and aeration.
• Common in black cotton soils and canal-irrigated areas.

• Desalinization / Dealkalization

Definition: The removal of soluble salts or exchangeable Na⁺ from the soil profile through leaching or reclamation.

Methods:

• Good-quality irrigation water
• Gypsum application (CaSO₄·2H₂O) to replace Na⁺ with Ca²⁺
• Proper drainage system

Effect: Improves soil structure, fertility, and plant growth.

13. Pedoturbation

Definition: Mixing of soil materials by organisms (earthworms, burrowing animals, roots) or natural forces (freeze-thaw, wet-dry cycles).

Result:

• Improves aeration and drainage.
• Homogenizes soil layers.
• Common in grasslands and temperate regions.

4. Summary Table of Major Pedogenic Processes