Soil Profile

Introduction

• A soil profile is the vertical section of soil that shows its various layers or horizons — from the surface down to the unaltered parent material.
• It represents the natural sequence of layers that have developed through soil-forming processes (pedogenesis) over time.
• In simple terms: A soil profile is like a “slice” of soil from the top to bottom showing different horizons (O, A, B, C, and R).

Definition

• According to Soil Science Society of America (SSSA):
  “A soil profile is a vertical section of soil extending through all its horizons and into the parent material.”
• In Simple Words:
  A soil profile is a cross-sectional view of the soil that reveals distinct layers (horizons) formed due to weathering, organic accumulation, and leaching.

Importance of Soil Profile

Understanding the soil profile helps to:

• Identify soil type and fertility.
• Understand soil formation and age.
• Know water-holding capacity and root penetration depth.
• Classify soils for agricultural and engineering uses.
• Assess land suitability and soil management practices.

Components of a Soil Profile

A typical soil profile consists of five major horizons:

Detailed Description of Each Horizon

Let’s understand each layer in depth, from the surface downward 👇

(i) O Horizon – Organic Layer

• Location: Uppermost layer (surface).
• Composition: Mostly organic matter — undecomposed or partially decomposed leaves, twigs, roots, and animal residues.
• Color: Dark brown or black due to humus content.
• Processes: Accumulation of litter and humus. Active microbial decomposition.
• Subdivisions:
  • Oi: Slightly decomposed organic material (litter layer).
  • Oe: Moderately decomposed (fermentation layer).
  • Oa: Highly decomposed (humus layer).
• Importance: Source of nutrients. Improves soil fertility and structure.

(ii) A Horizon – Topsoil

• Location: Immediately below O horizon (or surface if O is absent).
• Composition: Mixture of mineral particles (sand, silt, clay) and organic matter. Rich in microorganisms and roots.
• Processes:
  • Leaching (Eluviation): Removal of clay, iron, and soluble minerals due to percolating water.
  • Biological activity: Root growth, burrowing, and organic matter decomposition.
• Color: Dark brown to grayish brown.
• Importance: Most fertile and productive part of the soil. Major zone for plant growth and nutrient uptake.
• Designation: A1 (humus-rich), A2 (leached or eluviated).

(iii) E Horizon – Eluviation Zone (if present)

• Location: Between A and B horizons (not always distinct).
• Meaning: “E” stands for eluviation (washing out).
• Composition: Zone of maximum leaching (loss) of clay, iron, aluminum, and organic matter. Often lighter in color (ash-gray).
• Importance: Indicates strong leaching and advanced soil development.
• Common in: Forest soils and humid climates.

(iv) B Horizon – Subsoil

• Location: Below A or E horizon.
• Composition:
  • Zone of accumulation (illuviation) — materials leached from upper horizons (clay, iron, aluminum, humus) are deposited here.
  • Contains less organic matter but more minerals.
• Color: Reddish, yellowish, or brown (due to iron oxides and clay).
• Texture: Denser and harder than A horizon.
• Processes:
  • Illuviation: Deposition of materials from above.
  • Clay accumulation (argillic horizon).
  • Iron/aluminum enrichment (spodic horizon).
• Importance: Determines drainage, root depth, and nutrient movement. Acts as a storage zone for nutrients.

(v) C Horizon – Parent Material

• Location: Beneath B horizon.
• Composition:
  • Partially disintegrated and weathered rock fragments.
  • Very little biological activity or organic matter.
• Color: Variable, often lighter.
• Processes: Physical weathering dominates. Source of minerals for upper layers.
• Importance: Determines texture, structure, and mineral composition of overlying soil horizons.

(vi) R Horizon – Bedrock

• Location: Lowest layer of the soil profile.
• Composition: Unweathered parent rock (granite, basalt, limestone, etc.).
• Characteristics: Hard and compact. No biological activity.
• Importance: Provides base material for soil formation through weathering.