Question

Characterize the principal aspects of each soil horizon. Where does the main accumulation of organic material occur? Where does humus form? Explain the difference between the eluviated layer and the illuviated layer. Which horizons constitute the solum?

Short answer

The soil profile is divided into horizons: O, A, E, B, C and R. Organic material primarily accumulates in the O horizon, while humus forms in A horizon. The E horizon is the eluviated layer where leaching occur, and the B horizon, or the illuviated layer, is where these leached materials accumulate. The solum consists of both the A and B horizons.

Step-by-step solution

Understanding Soil Profile and its Horizons

The soil profile can be divided into several layers, known as soil horizons: O, A, E, B, C and R. These horizons have varying properties such as colour, texture, structure, and chemical composition.

Location of Organic Material and Humus Formation

The O horizon, which is closest to the surface, is chiefly composed of organic material, such as decomposing plant matter, and is where much of the organic material accumulates. Humus essentially forms in the A horizon, also called topsoil. It consists of humified organic matter mixed with mineral particles.

Eluviated and Illuviated Layers

The E horizon, the eluviated layer, is an area of maximum leaching. It is the layer from which materials are being removed and is notable for its paler coloration. The B horizon, also known as the illuviated layer, is where the leachate from the E horizon accumulates. This layer accumulates clay, iron, aluminium, or organic compounds which lend it a darker colour.

Understanding Solum

The solum refers to the combination of the A and B horizons. It is the main region of soil biological activity and root development. The A horizon offers organic matter and nutrients while the B horizon accumulates leached minerals and nutrients from above.

Key concepts

Soil Profile

A soil profile represents a vertical section of the ground, showcasing a sequence of distinct layers known as soil horizons. These layers are fundamental to understanding soil composition and development. Each horizon has unique characteristics such as color, texture, and composition, which result from various soil formation processes.

• O Horizon: This is the topmost layer rich in organic material. It primarily consists of decomposed leaves, plants, and other organic debris.
• A Horizon: Known as topsoil, this horizon contains a mixture of organic material with mineral particles, making it crucial for plant growth.
• E Horizon: Characterized by leaching or eluviation, this layer loses minerals like clay and iron, leading to a lighter color.
• B Horizon: Also referred to as the subsoil, it accumulates minerals like iron and aluminum leached from above, known as illuviation.
• C Horizon: This consists of weathered parent material from which the soil has developed.
• R Horizon: This is the unweathered bedrock underlying the soil layers.

Organic Material Accumulation

Organic material accumulation is a critical process in soil formation and fertility. It predominantly occurs in the O horizon, where leaves, plants, and other organic materials collect and decompose. This layer is vital as it supports a host of microorganisms that break down this organic matter, releasing nutrients back into the soil ecosystem.
The accumulation of organic material enriches the soil, providing essential nutrients that aid plant growth and development. It enhances soil structure by increasing porosity, which in turn improves aeration and water retention capacity.

Humus Formation

Humus is a stable, decomposed form of organic matter, and its formation is essential for creating fertile soil. This process occurs mostly in the A horizon, the topsoil, where decomposed organic matter, or humified materials, mix with minerals.
Humus is crucial for soil health due to its ability to retain moisture and improve soil structure. It acts as a reservoir of nutrients like nitrogen, phosphorus, and sulfur, which are slowly released to plants over time. Additionally, humus enhances soil's ability to store and buffer essential plant nutrients, making it fundamental for sustainable plant growth.
The presence of humus also gives the A horizon its darker color, indicating rich soil quality.

Eluviation and Illuviation

Two important processes that affect soil horizons are eluviation and illuviation. Eluviation occurs in the E horizon, where materials such as clay, iron, and aluminum oxides are leached out due to water movement. This process creates a layer depleted of these elements, resulting in its pale appearance.
In contrast, illuviation takes place in the B horizon. It involves the accumulation of materials leached from the E horizon above. This layer is richer in minerals and organic compounds, characterized by a darker color.
Both processes are integral to soil development as they reflect movement within soil layers, significantly affecting soil fertility and structure.

Solum

The solum refers to the active, upper part of the soil profile, primarily consisting of the A and B horizons. It is considered the zone of biological activity and significant root development.
In the A horizon, plentiful organic matter and nutrients are present, supporting plant growth and fostering microbial interactions. The B horizon complements this by accumulating leached minerals and nutrients from the layers above, contributing to the soil's physical resilience and fertility.
Together, these horizons make up the solum, emphasizing its importance as the vibrant core of soil, where both organic and inorganic materials interact to support a thriving ecosystem.