Question

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

Short answer

The A horizon is often dark due to the presence of humus.

Step-by-step solution

Understand Soil Horizons

Soil is composed of different layers known as horizons, each with distinct characteristics. The main horizons include O, A, E, B, C, and occasionally R.

Identify Characteristics of Each Horizon

The A horizon, also known as topsoil, contains a lot of organic material and humus, giving it a darker color. The E horizon is characterized by leaching, which removes materials, often making it lighter. The B horizon is usually where materials leached from above accumulate and is often lighter than the A horizon. The C horizon consists of weathered parent material and is often the least affected by soil-forming processes.

Determine Which Horizon Is Dark Due to Humus

Humus is decayed organic matter found primarily in the A horizon of soil. This horizon is rich in organic material, giving it a dark color due to the presence of humus.

Key concepts

humus

Humus is a crucial component in the structure of soil, contributing to its fertility and health. It consists of decomposed organic matter, such as plant and animal residues, broken down by microorganisms.
This process of decomposition transforms the material into a dark, nutrient-rich substance. Humus is known for its ability to retain moisture and nutrients, making them available to plants.
Due to its dark color, humus imparts a rich, deep hue to the soil, particularly the A horizon or topsoil. Its presence enhances soil structure, improves aeration, and increases its capacity to hold water.

• Enhances soil fertility
• Improves moisture retention
• Promotes plant health and growth

topsoil

Topsoil, technically referred to as the A horizon, is an essential layer in the soil profile. This surface layer is where most plant roots reside and the majority of biological soil activity occurs.
It is rich in organic material and minerals, providing the nutrients needed for plants to grow robustly. The topsoil supports a wide range of soil organisms, from bacteria to earthworms, which help in breaking down organic waste into useful nutrients.
As a result of these processes, topsoil often appears darker due to the accumulation of humus and other organic matter. This darkness often indicates a fertile soil, which is ideal for agricultural activities.

• Supports plant roots
• Contains nutrients and minerals
• Key for plant health and ecosystem balance

organic material

Organic material in soil consists of plant residues, manure, and a myriad of other natural substances left in the environment.
This organic matter is a major source of nutrients for plants, which are released as the material breaks down.
When organic material decomposes, it contributes to the formation of humus, enhancing soil fertility.
Organic material plays a dynamic role in maintaining soil health. It fosters a hospitable environment for soil organisms, which in turn, aid in the decomposition process.

• Source of nutrients
• Enhances soil structure
• Crucial for ecosystem sustainability