Horizon Layers of Soil - Soil Horizon Explanation

Soil horizons are distinctive layers that form naturally over time on the Earth's surface. These layers are composed of different materials and have specific characteristics that distinguish them from each other. Each soil horizon can reveal clues to the geological history and environmental conditions of the area in which it is found.

The formation of soil horizons is influenced by a variety of complex and interrelated factors. These factors include biological processes such as the breakdown of organic matter by microorganisms and the activity of plant roots. Other important factors include climate, with precipitation and temperature being very important in soil composition. Physical processes such as erosion and sedimentation also play an important role soil horizon formation.

Over time, these factors work together to shape soil layers. They generate unique features in each horizon. Some soil layer horizons may contain high levels of decaying organic matter, making them rich in nutrients and suitable for agriculture. Other horizons may be mineral dominated and less fertile. The presence of different soil horizons can affect water holding capacity, aeration, soil structure and nutrient availability for plants.

In a soil profile, up to five different horizons can be observed. Not all soil types exhibit all five. Younger soils have fewer horizons and are poorly distinguished, while more mature soils have a greater number of horizons. The profile of a mature soil typically consists of the following horizons:

• O Horizon
• A Horizon
• E Horizon
• B Horizon
• C Horizon
• R Horizon

A soil profile can be changed and influenced by humans. Fertilizer is a common form of soil modification. Learn more with our guide to the differences between compost and fertilizer.

Now we know about soil horizon layers more generally, we look at each horizon layer individually. It should be noted that understanding each layer's characteristics can provide valuable insights into soil management and conservation practices, ensuring sustainable use of soil resources.

The O horizon consists primarily of decaying plant materials such as leaves. It is therefore classified as a surface horizon. Its relevance is greater in wooded areas, since a greater mass of plant matter can accumulate there. In more arid areas with less vegetation, the horizon layer O may be extremely thin or not exist at all. This layer is crucial for the recycling of nutrients back into the ecosystem and plays a significant role in carbon storage, acting as a buffer against climate change.

The A horizon is the top layer of soil and is where the roots of grasses become firmly established. Sometimes this layer is obscured by a O horizon layer. Bioturbation can occur in this section of soil. This is the reworking of soils by plants or animals.

This arable soil layer is composed of a combination of clay and sand particles. It is usually relatively thin compared to other soil horizons. The A horizon typically has a dark hue due to the presence of humus or decaying organic matter, although mineral matter is the main component.

Some pedologists (scientists who study soil) consider another level known as the P soil horizon. This is the case with waterlogged soils such as peat.

Learn how to use bioturbation in your garden with our guide to making your own worm castings at home. Additionally, the A horizon is vital for agricultural productivity, as it contains the majority of plant roots and soil organisms that contribute to nutrient cycling and soil health.

The E horizon is characterized by a lack of organic matter and a light color due to the process of eluviation, leaching or vertical washing. As we go deeper into the soil, a layer of organic matter, iron, aluminum or clay is generated. The structure is often underdeveloped and sometimes completely absent. This horizon is important in soil profiles where distinct leaching occurs, indicating movement of materials such as silicate clay, iron, or aluminum oxide to lower layers, which can affect soil fertility and structure.

The B horizon lacks humus altogether, resulting in a lighter coloration. It usually takes on shades of brown or red due to the absence of organic matter. Sediments and materials from the upper horizons (O, A, and E) are transported and deposited in this horizon. These materials can include clays, metal oxides and hydroxides, salts, carbonates, and various other substances.

The B horizon is rich in iron oxide which has accumulated through the transport of more soluble materials from the upper horizon by water. Its characteristics are determined by the composition of the substances that have been gathered. The accumulation of these materials can lead to the formation of clay pans or hardpans, which may impact water movement and root penetration, influencing plant growth.

The C horizon is a layer that marks the transition to bedrock. As we go deeper into the soil, this layer becomes less disturbed until it is indistinguishable from bedrock. The C horizon is composed of fragments of mechanical weathering, including coarse pebbles, sand and clay. It is also the recipient of the solutions carried by water from the upper layers. This horizon provides insights into the geological history of the area, as it often reflects the parent material from which the soil developed, influencing its mineral content and potential fertility.

This is the underlying source rock that remains intact and unfragmented. It serves as the foundation for the remainder of the soil profile. It may not necessarily be bedrock on the ground and could only partially represent a paleosol. Understanding the R horizon is important for construction and land use planning, as it affects the stability and drainage characteristics of the land.

As we have explained, the soil profile is determined by the different horizon layers. However, these layers themselves are created by various factors. They include the following:

• Climate: one of the main factors that affects the soil profile. Temperature and the amount of precipitation influence the rate of decomposition of organic materials, the leaching of minerals and the formation of soil structures. Soils in hot, humid regions tend to have higher organic breakdown and nutrient leaching. This can result in thinner, less fertile soils. Cold and dry regions may have deeper soils rich in organic matter due to less decomposition and leaching. Climate change can alter these patterns, impacting soil properties and agricultural potential.
  
  
• Topographical relief: plays an important role in the formation of the soil profile. The slope of the land influences erosion and the accumulation of sediments. On mountain slopes, soils tend to be thinner due to erosion caused by gravity and water runoff. In flat areas or in depressions, sediments can accumulate and give rise to deeper soils. Human activities such as deforestation and construction can exacerbate erosion, affecting soil profiles.
  
  
• Living organisms: such as plants, animals and microorganisms. They play an essential role in the formation and development of the soil profile. Plant roots help break up rocks and create spaces for water infiltration and soil aeration. In addition, the activities of decomposing organisms contribute to the formation of organic matter and nutrients available to plants. Biodiversity within the soil ecosystem is crucial for maintaining healthy soil functions.
  
  
• The undisturbed rock or sediment from which the soil is formed: the mineralogical and chemical composition of the parent material determines the chemical and physical characteristics of the resulting soil. For example, soils derived from mineral-rich rocks such as feldspars may have a higher nutrient content, while soils derived from calcareous rocks may have a more alkaline pH. The parent material influences soil texture and drainage properties.
  
  

Soils profiles are much more complex in nature. However, even the soils we use for our own homes and gardens will have a type of soil profile. Understanding these factors helps in making informed decisions about land management, agriculture, and conservation practices. Learn about some different types with our articles on what is akadama soil and what is kiryuzuna soil for bonsai.

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