Question

Characterize the four major horizons of a mature soil.

Short answer

The major soil horizons are O, A, B, and C.

Step-by-step solution

Understanding Soil Horizons

Soil is composed of distinct layers known as horizons. These horizons are created due to various soil-forming processes acting over time. Recognizing these main horizons and their characteristics is crucial to understanding soil composition.

Identifying the O Horizon

The O horizon is the topmost layer of soil, composed primarily of organic matter such as decomposed leaves, plants, and other organic materials. It is usually dark in color due to the high content of decayed plant material.

Characterizing the A Horizon

The A horizon, often referred to as topsoil, is rich in minerals and organic material. It is typically darker than the lower horizons and is where most root activity and biological processes occur. It is also considered crucial for plant growth.

Analyzing the B Horizon

The B horizon, or subsoil, is located beneath the A horizon and contains minerals leached from the upper horizons, such as iron, aluminum, and clay. It is typically denser and less rich in organic material than the A horizon.

Describing the C Horizon

The C horizon consists primarily of partially disintegrated and weathered parent material from which the soil is formed. This layer is relatively unaffected by the processes that influence the layers above it and often resembles the underlying rock.

Key concepts

O horizon

The O horizon is the very top layer of the soil that you see when you look at the ground. It might not always be visibly thick, but it's incredibly important. This layer is made up almost entirely of organic material, including things like decomposing leaves, plants, and sometimes tiny organisms like insects and fungi.
This organic matter is why the O horizon is typically quite dark, as all this decomposed material is rich in nutrients.

• Acts as a barrier, protecting the underlying soil
• Provides essential nutrients to plants
• Hosts many bacteria and organisms that help in decomposing plant material

The O horizon plays a critical role in maintaining healthy soil, which benefits plant life and subsequently the entire ecosystem.

A horizon

The A horizon is often referred to as the topsoil, which is the layer directly beneath the O horizon. It's the layer that's most important for growing food and supporting plant life. The A horizon is rich in both organic materials and minerals.
It is typically darker than the other horizons due to its higher organic content.

• Considered the most fertile part of the soil
• Has the highest concentration of plant roots
• Hosts important biological processes

This is where most of the soil's biological activity happens because it provides an ideal environment for plant roots and microorganisms. Fertility of the A horizon is crucial for agriculture and forestry, supporting plant growth and food supply.

B horizon

Beneath the A horizon lies the B horizon, known as the subsoil. This layer is characterized by its denser texture, as it contains materials like iron, aluminum, and clay that have been leached down from the topsoil above.
While the B horizon is less rich in organic materials, it plays a vital role in storing essential nutrients and minerals.

• Acts as a reservoir of minerals leached from upper layers
• Has a lighter color due to fewer organic materials
• Helps in soil structure stability

This layer is important for supporting plant structures and holding moisture, which is slowly released to the plants. While not as biologically active as the A horizon, the B horizon is integral to the overall soil system.

C horizon

The C horizon rests beneath the B horizon and consists largely of parent material. This is the layer where the soil starts to form, containing pieces of rock and partially disintegrated material from which the soil develops. Unlike the horizons above it, the C horizon remains largely unaffected by soil formation processes such as leaching, weathering, or organic transformation.
Thus, it appears more like bedrock or sediment.

• Acts as a transition between soil and rock
• Provides minerals to other layers through weathering processes
• Contains minimal organic material

The C horizon serves as a foundational layer for the rest of the soil and slowly contributes mineral content through natural weathering. While it plays less of a direct role in supporting plant life, it is crucial for the continuous renewal and formation of soil.