Question

Which of the following is the R horizon? A. topsoil C. bedrock B. humus D. gravel

Short answer

C. bedrock

Step-by-step solution

Understanding the Horizons

The layers of soil are categorized into different horizons. Each horizon has distinct characteristics. The typical horizons are O (organic layer), A (topsoil), E (eluviation, leaching layer), B (subsoil), C (weathered parent material), and R (bedrock).

Identify the R Horizon

The R horizon is known as the bedrock. This is the solid rock layer that is not weathered and lies beneath all the other layers. It is typically located at the deepest part of the soil profile.

Match with Options

Among the given options, 'C: bedrock' corresponds to the R horizon, as the R horizon is defined by its characteristic as the solid rock layer.

Key concepts

topsoil

The topsoil, also known as the A horizon, is one of the most crucial layers in the soil profile. It is the uppermost layer that is rich in nutrients and organic matter. This layer is where most plant roots thrive and is essential for plant growth and agricultural productivity.
Key components of topsoil include:

• Minerals: Essential for supporting plant health, obtained from weathering of rocks.
• Nutrients: Includes nitrogen, phosphorus, potassium, important for plant nutrition.
• Water retention: The ability of topsoil to hold water, which is vital for plants.

The presence of organic matter in topsoil makes it dark and fertile, helping to sustain life above ground and maintain ecological balance. Its texture and composition can greatly affect how plants absorb water and nutrients.

bedrock

Bedrock, also referred to as the R horizon, forms the foundation of the soil profile. Lying deep under all other soil layers, it is composed of solid rock. Unlike the layers above, bedrock is not weathered and remains intact over long periods.
Understanding the bedrock is crucial because:

• Provides stability: Acts as a natural barrier, preventing erosion and land movement.
• Influences soil formation: Over time, weathering of bedrock contributes minerals to the soil above.
• Geological information: Offers insights into the geological history of an area.

Although it doesn’t directly support plant life, as it’s too deep and hard for roots to penetrate, bedrock's interaction with the environment over millennia shapes the soil profile and affects all layers above it.

humus

Humus is the dark, organic component of soil, created from decayed plant and animal material. Found predominantly in the upper layers like the topsoil, it is essential for soil fertility and structure.
Significant attributes of humus include:

• Nutrient supply: Releases nutrients slowly, providing a steady supply to plants.
• Soil structure: Improves the soil texture, making it airy and easy for roots to grow.
• Water retention: Increases the soil's ability to retain moisture.

Humus is vital for sustaining plant life as it enhances the soil’s capability to retain nutrients and water. It acts like a sponge, holding in moisture and making it available for plants to use, thus playing a significant role in the ecosystem's health and productivity.