Question

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

Short answer

The R horizon is C: bedrock.

Step-by-step solution

Understanding the R Horizon

The R horizon is a term used in soil science and refers to the layer of bedrock. This is the deepest layer in a soil profile and consists of unweathered rock. It is not composed of soil but rather solid rock that has not been broken down by weathering or other natural processes.

Reviewing the Options

Let's review each option provided in the question: - **A: topsoil** - This is the uppermost layer of soil where organic matter accumulates and where most plant roots grow. It is not the R horizon. - **B: humus** - This is the organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms. It is part of the topsoil. - **C: bedrock** - This is the unweathered rock layer found beneath the soil layers. It matches the description of the R horizon. - **D: gravel** - This consists of small, rounded stones often used in construction or surface dressing of roads. It does not define a soil horizon.

Identifying the Correct Answer

By understanding the definition of the R horizon and reviewing the options, we can conclude that the correct answer is **C: bedrock**. This is because the R horizon refers to the layer of rock beneath the soil layers, which aligns with the description of bedrock.

Key concepts

Soil Science

Soil science is the study of soil as a natural resource on the surface of the Earth. Its aim is to understand soil formation, classification, and mapping. Moreover, it seeks to comprehend the physical, chemical, and biological factors that affect soil properties.
Soil consists of different layers known as soil horizons. These are formed by the processes of soil formation, which include physical and chemical weathering of rocks, organic matter decomposition, and biological activity. Each horizon can be distinguished by their composition and characteristics.

• Soil provides essential nutrients for plant growth.
• It acts as a water reservoir and a habitat for many organisms.
• Maintaining soil health is important for sustainable agricultural practices and environmental conservation.

Bedrock

Bedrock is the solid rock that lies beneath surface materials like soil and gravel. It is composed of unweathered rock, meaning it has not undergone the processes of weathering that break down rocks into smaller particles.
This unweathered material forms the foundation upon which soil develops. Over time, fragmentation, physical weathering, and chemical reactions contribute to the transformation of bedrock into various soil horizons.

• Bedrock can consist of different rock types, including igneous, sedimentary, or metamorphic rocks.
• It plays a crucial role in the development and composition of overlying soil layers.
• The type of bedrock influences soil pH and mineral content, affecting plant growth.

R Horizon

The R horizon is the deepest layer in a soil profile, sitting just above the solid bedrock. This layer predominantly consists of continuous rock that has not been subject to many of the natural processes affecting the upper layers of soil.
As the foundation of the soil profile, the R horizon is crucial for understanding the geological history and characteristics of an area.

• The R horizon provides a stable foundation for the development of soil layers above it.
• It reflects the unaltered condition of the rock, offering a glimpse into the Earth's geological past.
• The depth and composition of the R horizon can vary widely depending on the geographical area.

Understanding the R horizon is essential for soil scientists and geologists as it provides insights into past climatic conditions and tectonic activities.