Soil horizon

Soil has many layers.

These layers are called horizons. Each layer looks different. Some are dark and some are light.

The top layer is often made of dead plants. Below that is the surface soil. It has lots of tiny living things.

Next is the subsoil. This layer catches things that wash down from the top. It can be a different color. This happens because of iron.

Deep down, you might find loose rocks. These are in the next layer. At the very bottom is hard bedrock. This is solid rock. You cannot dig it by hand.

Each layer helps make the soil.

It is amazing to look under the ground!

Soil is made of many layers. These layers are called soil horizons.

Each horizon is different from the ones above or below it. They often have different colors or textures. Scientists use letters to name these main layers.

The top layer is the O horizon. This is an organic layer made of dead plants. Below that is the A horizon. This is the surface soil. It has lots of organic matter and tiny living things. Some minerals move down from this layer. This can create a light-colored E horizon.

Next is the B horizon. This is the subsoil. It is where things like clay and iron collect. This collection is called illuviation. Below the subsoil is the C horizon. This layer has loose rocks.

At the very bottom is the R horizon. This is the bedrock. It is a thick mass of hard rock. You cannot dig it up by hand. Some soils do not have clear horizons. This can happen if humans plow the land a lot. Other times, the soil is just very new.

Soil is much more than just dirt under our feet. It is actually made of different layers called soil horizons.

Each horizon sits parallel to the ground. These layers have different physical and chemical traits. They might look different in color or feel different in texture. Some layers are darker or sandier than the ones around them.

Scientists use these differences to understand the ground. Knowing about horizons helps us learn how the earth works.

Soil forms through a special way of working called pedogenesis. This process creates the distinct layers we see in a soil profile.

It often starts with an organic surface layer called the O horizon. This layer contains dead plant matter like leaves. Below that is the A horizon, which is the mineral surface soil. It has lots of organic matter and many living things. Sometimes, minerals like clay or iron move down from this top layer. This movement can create a lighter E horizon underneath.

Scientists use many different systems to name these layers. One major guide is the World Reference Base for Soil Resources. The 4th edition of this manual was released in 2022. These systems use capital letters to name the main or "master" horizons. They also use lowercase letters as suffixes to add more detail. For example, a suffix might show if a layer is buried. Different countries use different rules to classify their soil. You cannot mix the rules from two different systems together.

There are several specific layers to look for in a deep profile. The B horizon is the subsoil where minerals like iron and clay collect. This collection process is called illuviation. Below the subsoil is the C horizon, which is a layer of loose rock. At the very bottom is the R horizon. This is the bedrock, which is a huge mass of hard rock. You cannot dig up this bedrock by hand. Some very new soils, called Entisols, do not have clear horizons at all.

Understanding soil horizons helps us see how the land has changed. Humans can change soil by deep ploughing or moving large amounts of earth. This can make the distinct layers disappear almost completely. When experts study soil, they also look at the shape of the land. They want to know how the land was used in the past. This helps them pick the right names for the horizons they find. Soil tells a story of the plants, weather, and people that lived there.

A soil horizon is a layer parallel to the soil surface. Each horizon has physical, chemical, and biological characteristics that differ from the layers above or below it.

Scientists often identify these horizons by their color or texture. Texture refers to the particle size distribution within the soil. A horizon might be described as darker or sandier than its neighbors. These layers are essential for understanding the ground. They allow researchers to classify different types of soil across the planet.

Soil horizons are created through soil-forming processes called pedogenesis. This process transforms raw material into organized layers over time. If a layer has not undergone these specific processes, it is simply called a soil layer.

A layer becomes a true horizon when at least one of its properties results from pedogenesis. This distinction is important for scientists studying how the Earth changes. Understanding this mechanism helps us see how minerals and organic matter move through the ground.

Most soil profiles follow a specific sequence of layers. The top layer is often an organic surface layer called the O horizon. This layer contains plant litter, such as leaves. The lower parts of the O horizon may be strongly humified, meaning the organic matter is heavily decomposed. Below the organic layer, the mineral soil usually begins with the A horizon. This surface soil contains the most organic matter and supports much of the soil life.

In some cases, minerals like clay or iron move downward through the A horizon. This downward movement is called eluviation. When this happens, a lighter-colored E subsurface horizon may appear below the A horizon.

Below the surface layers lies the B horizon, also known as the subsoil. This layer usually has less organic matter than the A horizon. Its color often comes from iron oxides. The B horizon is where substances that move down from the topsoil collect. This accumulation of clay, iron, aluminum, or organic compounds is called illuviation. Below the subsoil is the C horizon, or substratum. This is a layer of loose, poorly weathered rock. It may contain soluble compounds like calcium carbonate. At the very bottom of the profile is the R horizon. This represents the bedrock, which consists of hard, continuous masses of rock.

Scientists use different systems to name and classify these horizons. One major system is the World Reference Base for Soil Resources (WRB). The 4th edition of the WRB manual was released in 2022. These systems use capital letters to represent "master horizons." For example, the letter A represents a mineral horizon at the surface. To add more detail, scientists use lowercase suffixes. A suffix like "b" can indicate a buried horizon. A suffix like "p" can show that the soil was modified by human ploughing. Because different systems have different definitions, you cannot mix them together.

Classification systems also identify "diagnostic horizons." These are special layers used to define specific soil types. The WRB lists 40 different diagnostic horizons. Some examples include the cambic horizon or the spodic horizon. Other systems, like the USDA soil taxonomy, use different methods. In the USDA system, some soils called Entisols do not have clear horizon development. This shows that soil formation is not always uniform across the Earth.

Human activity can significantly change how soil horizons appear. Major earthworks or regular deep ploughing can disrupt the natural layers. These actions may cause distinct horizons to disappear almost completely. When experts study soil in the field, they must consider the local geomorphology. They also look at how humans have used the land in the past. This historical context ensures they apply the correct names to the observed layers. By studying horizons, we connect the biology of plants to the geology of the Earth.