Soil Science — Horizon Identification In a standard soil profile, the R-horizon (also written as the “R layer”) consists of which material?

Introduction / Context:
A soil profile is arranged in horizons from the surface downward, each with distinct properties. Examinations often test quick recognition of horizon labels (O, A, E, B, C, R) and what they contain. The R-horizon is the deepest reference layer in many profiles.

Given Data / Assumptions:

• We are asked specifically about the R-horizon.
• Options mention materials ranging from plant litter to rock.
• Assume standard pedology conventions used in school and competitive exams.

Concept / Approach:

By convention, the R-horizon is consolidated, unweathered bedrock. Above it, the C-horizon contains partially weathered parent material, and higher horizons (O/A/E/B) represent organic matter, leaching, accumulation, and pedogenic transformations. Therefore, the only option that correctly names the R-horizon’s content is “unweathered bedrock.”

Step-by-Step Solution:

Recall soil horizon order: O → A → E → B → C → R.Associate each horizon: O (organic litter), A/E/B (topsoil/subsoil processes), C (weathered parent), R (rock).Select the description matching R: unweathered bedrock.

Verification / Alternative check:

Introductory pedology sources consistently depict R as hard, continuous rock that roots cannot easily penetrate, confirming the choice.

Why Other Options Are Wrong:

weathered parent material: That is the C-horizon, not R.plant litter: That describes the O-horizon.None of the above: Incorrect because “unweathered bedrock” is correct.

Common Pitfalls:

Mixing up C and R horizons. Remember: C = weathered; R = rock (unweathered). The mnemonic “R for Rock” is helpful.

Final Answer:

unweathered bedrock