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Difficulty: MediumRocks, Minerals, and Soil

A soil scientist collects core samples from a mature, undisturbed forest soil profile to analyze its vertical stratification. Order the standard soil horizons sequentially from the uppermost surface layer down to the partially weathered parent material.

  1. 1O Horizon: A surface layer consisting primarily of newly deposited and decomposing organic litter such as leaves and twigs.
  2. 2A Horizon (Topsoil): A mineral-rich layer mixed with thoroughly decomposed dark organic matter (humus) with high biological activity.
  3. 3B Horizon (Subsoil): A zone of accumulation (illuviation) where leached minerals, clays, and iron oxides collect.
  4. 4C Horizon: A layer composed of partially weathered and mechanically broken bedrock fragments with minimal organic influence.

Answer

The correct sequence from surface to depth is the O Horizon (surface organic litter), followed by the A Horizon (topsoil), the B Horizon (subsoil zone of accumulation), and finally the C Horizon (partially weathered parent material).
In a well-developed soil profile, soil develops in distinct horizontal layers known as horizons from the surface down to bedrock. The correct sequence starts at the surface with the organic O Horizon, descends into the mineral-rich A Horizon (topsoil), continues into the B Horizon (subsoil/illuviation zone), and terminates at the C Horizon (partially weathered parent material).

Step-by-Step Solution

1
Identify the uppermost surface boundary layer of organic debris
The O horizon consists of organic matter deposited directly on the surface, making it the topmost layer.
Plant leaves and animal remains accumulate directly on the ground before undergoing incorporation into mineral layers.
2
Determine the topmost mineral soil layer beneath the organic debris
The A horizon (topsoil) lies immediately beneath the O horizon.
Decomposed organic humus combines with weathered mineral particles in the zone of active plant rooting.
3
Identify the subsurface zone where percolating water deposits minerals and clay
The B horizon (subsoil) forms below the A horizon.
Water moving downward through the soil column leaches clay, iron, and soluble compounds from the upper layers and deposits them in the B horizon.
4
Identify the basal layer directly overlying unweathered solid rock
The C horizon forms the lowest layer of the active soil profile.
This layer consists of mechanically fractured and weakly weathered parent material prior to developing soil structure.

Key Concept

Vertical soil horizon stratification in a mature soil profile
Estimated Time:1m 0s
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Topics

Rocks, Minerals, and Soil

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