Question

Which soil horizon often has a dark color because of the presence of humus? A. E horizon C. B horizon B. Chorizon D. A horizon

Short answer

D. A horizon

Step-by-step solution

Understanding Soil Horizons

To determine which soil horizon has a dark color due to humus, we must understand the characteristics of each horizon. Generally, soil is categorized into several layers or 'horizons' that differ in color, texture, structure, and composition.

Defining the A Horizon

The A horizon is also known as the topsoil. This layer is rich in organic material and nutrients, leading to a darker color compared to other horizons. This dark coloration is primarily due to the presence of decomposed plant and animal material known as humus.

Exploring Other Horizons

The E horizon is typically lighter in color and is where leaching or subtracting of minerals occurs. The B horizon, also known as the subsoil, is often composed of minerals that have leached from the A horizon. The C horizon consists of partially disintegrated and weathered parent material.

Conclusion

Given the information above, the A horizon is the most likely candidate to have a dark color due to the presence of humus. This humus gives the soil its rich, nutrient-filled property and dark appearance.

Key concepts

Humus

Humus is the dark, organic material in soil formed by the decomposition of plant and animal matter. This process, called humification, enriches the soil with essential nutrients, helping plants grow better. Humus holds moisture well, which is beneficial for retaining water in soil environments. This retention is crucial for plant health as it reduces the need for additional watering.
Humus also improves soil structure, making it easier for roots to penetrate and seedlings to establish. Its presence in the soil prevents erosion by binding soil particles into aggregates, which enhances stability. Furthermore, humus increases soil fertility by releasing nutrients slowly into the soil over time. Thus, humus not only imparts a dark shade to the A horizon but also plays a vital role in maintaining optimal conditions for plant growth.

Soil Composition

Soil composition refers to the mix of minerals, organic matter, gases, liquids, and organisms that together make the soil. The composition affects how the soil looks, feels, and supports plant growth.

• Minerals: These are derived from weathered rocks and are present in various sizes, from sand to silt and clay particles. They make up about 45% of soil.
• Organic Matter: This includes plant and animal residues at various stages of decomposition, such as humus. It makes up roughly 5% of soil but plays a huge role in fertility.
• Water and Air: These fill the spaces between the particles. About 25% of soil consists of water, and another 25% is air. Both are essential for providing nutrients and oxygen to plants and microorganisms.

The balance of these components influences the soil's texture, drainage capacity, and nutrient availability. A soil rich in organic material, like humus, is particularly beneficial for plant life as it supplies nutrients and improves soil structure.

Soil Layers

Soil is organized into layers, also known as horizons, which differ in color, texture, and composition. Each layer serves a unique function and together they form the soil profile.

• Topsoil (A horizon): This is the uppermost layer, rich in organic material like humus, and is crucial for plant growth.
• E horizon: Often lighter in color as minerals are leached out, leaving mostly sandy particles.
• Subsoil (B horizon): Contains minerals like iron and clay that are washed down from the A horizon.
• C horizon: Composed of larger pieces of rocks and weathered material, it is less affected by soil formation processes.

These layers are developed over thousands of years and indicate the soil's history and its capacity to support ecosystems. They are essential for soil scientists when assessing soil health and land usability.

Topsoil

Topsoil, also called the A horizon, is the most nutrient-rich part of the soil profile essential for agricultural and gardening purposes. Enriched with humus, topsoil is darker than the layers beneath it—a sign of its richness in organic material. Its dark color can range from brown to black, depending on the humus content.
These nutrients feed plants and help maintain a healthy ecosystem for underground organisms like earthworms and beneficial bacteria. Topsoil is crucial for root development and nutrient absorption, making it the key layer in which plants thrive.
Maintaining good topsoil through practices like crop rotation, cover cropping, and adding organic matter is essential for sustainable agriculture and preventing soil erosion. Due to its importance, topsoil conservation is a priority worldwide to ensure food security and environmental protection.