R horizon

Definition

The r horizon, also known as the bedrock layer, is the lowest layer of soil in a soil profile, consisting of unweathered rock. This layer is crucial in the context of soil formation, as it provides the mineral foundation from which soils develop through weathering processes. It influences the types of soils that can form above it, the drainage characteristics, and the nutrient availability for plants.

5 Must Know Facts For Your Next Test

1. The r horizon is not considered part of the soil in the traditional sense, but it is essential for soil development and provides the underlying material for weathering.
2. This layer often consists of various types of rock, including granite, limestone, or sandstone, depending on local geology.
3. The presence and type of r horizon can significantly affect groundwater movement and availability in an area.
4. Unlike upper soil horizons that may contain organic matter and nutrients, the r horizon typically lacks these features due to its unweathered state.
5. In some cases, the r horizon can be very deep, making it challenging for roots to penetrate or access water and nutrients directly from this layer.

Review Questions

• How does the r horizon influence the formation and characteristics of overlying soil layers?
  • The r horizon plays a significant role in determining the mineral content and drainage characteristics of overlying soil layers. As weathering occurs, minerals from the r horizon contribute to the formation of upper horizons, influencing soil fertility and structure. The type of bedrock present in the r horizon can also affect how water moves through the soil profile, impacting plant growth and ecosystem health.
• Evaluate how different types of bedrock in the r horizon might affect local ecosystems and agricultural practices.
  • Different bedrock types in the r horizon can lead to varying mineral compositions in soils above them. For example, limestone bedrock often leads to alkaline soils rich in calcium, benefiting certain crops but potentially limiting others that thrive in acidic conditions. This variation can affect local ecosystems by supporting different plant species and influencing animal habitats, while also guiding agricultural practices based on what crops will best thrive in those specific soil conditions.
• Analyze the impact of human activities on the integrity of the r horizon and subsequent soil formation processes.
  • Human activities such as mining, construction, and land use changes can significantly disrupt the r horizon's integrity. When bedrock is exposed or removed, it alters natural weathering processes that contribute to soil development. Additionally, these activities can lead to erosion and compaction of underlying materials, affecting water infiltration and nutrient availability for vegetation. Understanding these impacts is crucial for sustainable land management and mitigating negative effects on soil health.

Related terms

Soil Profile: A vertical section of soil that shows the different layers, or horizons, present from the surface down to the bedrock.

Weathering:

The process by which rocks and minerals break down into smaller particles due to environmental factors like water, temperature changes, and biological activity.

Horizon: A distinct layer within a soil profile that has specific physical and chemical properties, including the O (organic), A (topsoil), E (eluviation), B (subsoil), and C (parent material) horizons.