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Difficulty: EasySoil Formation, Profiles, and Major Vegetation Types

When examining a vertical cross-section of a mature soil profile, distinct layers develop from the ground surface down toward the underlying parent rock. Place the following soil horizons in correct sequence, starting from the uppermost surface layer and moving downward toward the parent rock material.

  1. 1O Horizon (Surface layer consisting mainly of organic material and leaf litter)
  2. 2A Horizon (Topsoil layer rich in humus where mineral leaching and eluviation occur)
  3. 3B Horizon (Subsoil layer where fine clays and leached minerals accumulate via illuviation)
  4. 4C Horizon (Layer of partially altered and weathered parent rock material)

Answer

The correct sequence from the surface downward is the O Horizon, followed by the A Horizon, B Horizon, and C Horizon.
A mature soil profile naturally organizes from top to bottom based on organic accumulation and weathering processes. The surface starts with the organic O Horizon, followed by the mineral topsoil A Horizon, the subsoil B Horizon of accumulation, and finally the weathered rock material of the C Horizon resting above bedrock.

Step-by-Step Solution

1
Identify the uppermost layer at the soil surface.
The O Horizon forms at the top from organic debris such as leaf litter.
Organic accumulation occurs directly at the soil-atmosphere boundary.
2
Identify the topsoil layer beneath the organic material.
The A Horizon lies directly beneath the O Horizon.
This layer combines mineral particles with decomposed humus.
3
Identify the subsoil layer where leached minerals collect.
The B Horizon lies beneath the A Horizon.
Minerals and clays leached from the upper horizon accumulate here through illuviation.
4
Identify the weathered parent rock horizon.
The C Horizon forms the base layer above bedrock.
It contains broken, partially weathered rock fragments that serve as parent material.

Key Concept

Soil Profile Horizonation
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Topics

Soil Formation, Profiles, and Major Vegetation Types

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