Understanding Soil Horizons and Layers

Soil is typically arranged in layers, known as horizons. These horizons make up the soil profile. Each horizon has distinct characteristics that differentiate it from the layers above and below it, such as composition, color, texture, and structure. Matching the common soil horizons (O, A, B, C) to their corresponding layers helps in understanding the basic structure of a soil profile.

Matching Soil Horizons to Layers

Let's examine each soil horizon provided in List I and match it with the appropriate layer description from List II:

• O Horizon: This is the uppermost layer, primarily composed of organic matter in various stages of decomposition, such as leaves, twigs, and other plant and animal residue. This layer is often dark in color due to the presence of humus.
• A Horizon: Located below the O horizon, this layer is often called the topsoil. It's a mineral layer mixed with decomposed organic matter (humus), giving it a darker color than the layers below. It's rich in nutrients and is where most biological activity occurs.
• B Horizon: Also known as the subsoil, this layer is below the A horizon. It's often lighter in color and contains less organic matter. Materials leached from the A horizon, like clay, iron, and aluminum oxides, accumulate here.
• C Horizon: This layer consists of partially weathered parent material, which is the original geological material from which the soil is formed. It contains rock fragments and minerals that have undergone some weathering. Below the C horizon is the R horizon, which is the unweathered parent rock.

Detailed Matching

Based on the characteristics described:

• (a) O Horizon corresponds to (ii) Organic Matter.
• (b) A Horizon corresponds to (iii) Surface Soil (Topsoil).
• (c) B Horizon corresponds to (iv) Subsoil.
• (d) C Horizon corresponds to (i) Parent Rock (or partially weathered parent material leading down to it).

Summary Table of Soil Horizon Matching

Here is a table summarizing the correct matches:

Checking the options, the correct combination that represents these matches is (a) - (ii), (b) - (iii), (c) - (iv), (d) - (i).

Revision Table: Key Soil Horizons

Additional Information on Soil Profile

A soil profile is a vertical cross-section through the soil, showing its horizons. The sequence and properties of these horizons are influenced by soil formation factors: parent material, climate, organisms, topography, and time. The O, A, B, and C horizons are the master horizons commonly observed, though others like E (eluviation) and R (parent rock) are also part of the complete soil profile classification system. Understanding these horizons is fundamental in fields like agriculture, ecology, and environmental science.