Soil Formation (Pedogenesis)

1. Introduction

• Soil is the natural medium for plant growth and the product of long-term weathering of rocks and minerals, combined with the influence of climate, living organisms, topography, and time.
• The processes that transform rocks into soils and cause the development of distinct layers (horizons) are collectively known as Soil Formation or Pedogenesis.
• In simple terms: Rock → Weathering → Parent Material → Soil → Soil Profile

2. Definition

According to Jenny (1941):

• “Soil formation is the function of five soil-forming factors — climate, organisms, relief, parent material, and time.”
• S=f(Cl,O,R,P,T)S = f(Cl, O, R, P, T)S=f(Cl,O,R,P,T)

where
S = soil,
Cl = climate,
O = organisms,
R = relief (topography),
P = parent material,
T = time.

In simple words: Soil formation is the process by which unconsolidated parent material is transformed into a dynamic, natural body called soil through physical, chemical, and biological processes.

3. Stages of Soil Formation

Soil formation occurs in two main stages:

4. Factors of Soil Formation (Jenny’s Equation)

(i) Parent Material (P)

Definition: Parent material refers to the unconsolidated mineral or organic matter from which soil develops.

Types:

• Residual (In situ): Formed directly from weathering of underlying rocks (e.g., black soil from basalt).
• Transported: Carried and deposited by agents like:
  • Water: Alluvial soil (river)
  • Wind: Aeolian soil (desert)
  • Ice: Glacial soil
  • Gravity: Colluvial soil (at slopes)
• Organic: Formed from accumulation of organic matter (peat, muck).

Influence:

• Determines texture, mineral composition, fertility, and color.
• Example: Basalt → clayey black soils; Granite → sandy soils.

(ii) Climate (Cl)

Role: Climate is the most dominant factor influencing soil formation because it controls:

• Weathering rate (through temperature and rainfall),
• Organic matter decomposition, and
• Leaching and horizon development.

Components:

Examples:

• Humid tropical climate: Intense leaching → lateritic soils.
• Arid regions: Limited leaching → accumulation of salts, calcareous soils.

(iii) Organisms (O)

Definition: All living organisms — plants, animals, and microorganisms — that influence soil formation and development.

Role:

Examples: Grasslands → thick, humus-rich (dark) soils. Forests → acidic soils with distinct leaf litter layer.

(iv) Relief / Topography (R)

Definition: Relief refers to the landscape position, slope, and elevation of an area.

Influence:

• Steep slopes: High erosion, poor soil development.
• Gentle slopes: Better drainage, moderate soil development.
• Flat/lowlands: Deposition, deep and fertile soils.
• Aspect: North- vs. south-facing slopes affect temperature and moisture.

Examples: Valley bottoms → alluvial, fertile soils. Hill slopes → thin, immature soils.

(v) Time (T)

Definition: Time determines the degree of soil development under the combined influence of other factors.

Stages of Soil Development:

Example: Alluvial soils (young) vs. lateritic soils (old and weathered).

5. Soil Forming (Pedogenic) Processes

Once the parent material is exposed to environmental factors, pedogenic processes start modifying it into true soil.
These include additions, losses, transformations, and translocations.

• Fundamental Soil Forming Processes

• Specific Pedogenic Processes

6. Horizon Development (Profile Differentiation); As soil formation proceeds, materials are added, lost, and rearranged vertically, leading to distinct soil horizons:

8. Time Scale of Soil Formation

• Soil formation is a very slow process — it takes hundreds to thousands of years to form a few centimeters of soil.
• On average: 2.5 cm of soil may take 100–500 years to form naturally.
• The rate depends on climate, rock type, and biological activity.

9. Summary Table of Soil Forming Factors and Their Effects