What's beneath your car tires?
Beneath an asphalt road is this incredible soil profile. Can you identify the horizons? There are some clues in the photo that can help you identify the climate type. What type of climate would produce this soil?
Soil Horizons and Profiles
Soil develops over time and forms soil horizons. Soil horizons are different layers of soil with depth. The most weathering occurs in the top layer. This layer is most exposed to weather! It is where fresh water comes into contact with the soil. Each layer lower is weathered just a little bit less than the layer above. As water moves down through the layers, it is able to do less work to change the soil. This is because the chemical reactions have already occurred.
If you dig a deep hole in the ground, you may see each of the different layers of soil. All together, the layers are a soil profile. Each horizon has its own set of characteristics (Figure below). In the simplest soil profile, a soil has three horizons.
In this diagram, a cut through soil shows different soil layers.
The first horizon is the “A“ horizon. It is more commonly called the topsoil. The topsoil is usually the darkest layer of the soil. It is the layer with the most organic material. Humus forms from all the plant and animal debris that falls to or grows on the ground. The topsoil is also the region with the most biological activity. Many organisms live within this layer. Plant roots stretch down into this layer. The roots help to hold the topsoil in place. Topsoil is needed to grow most crops (See Figure below).
Good topsoil is great for growing plants.
Topsoil usually does not have very small particles like clay. Clay-sized particles are carried to lower layers as water seeps down into the ground. Many minerals dissolve in the fresh water that moves through the topsoil. These minerals are carried down to the lower layers of soil.
Below the topsoil is the “B“ horizon. This is also called the subsoil. Soluble minerals and clays accumulate in the subsoil. Because it has less organic material, this layer is lighter brown in color than topsoil. It also holds more water due to the presence of iron and clay. There is less organic material in this layer.
The next layer down is the “C” horizon. The C horizon is made of partially altered bedrock. There is evidence of weathering in this layer. Still, it is possible to identify the original rock type from which this soil formed (Figure below).
This image shows the various soil horizons.
Not all climate regions develop soils. Arid regions are poor at soil development. Not all regions develop the same soil horizons. Some areas develop as many as five or six distinct layers. Others develop only a few.
- An animation of soil profile development can be viewed here: http://courses.soil.ncsu.edu/resources/soil_classification_genesis/soil_formation/soil_transform.swf.
- C-horizon: The lowest layer of soil; partially altered bedrock.
- soil horizon: An individual layer of a complete soil profile; examples include A, B & C horizons.
- soil profile: The entire set of soil layers or horizons for a particular soil.
- subsoil (B-horizon): The subsoil; the zone where iron oxides and clay minerals accumulate.
- topsoil (A-horizon): The A horizon; the most fertile layer with humus, plant roots and living organisms.
- Soil horizons are layers within a soil. Different soil horizons show different amounts of alteration.
- Soil profiles reveal the different layers of soil
- Soil layers include topsoil, subsoil and the C horizon.
- Topsoil has the highest proportion of organic material. Topsoil is essential for farming.
Use this resource to answer the questions that follow.
- What is a horizon?
- How many horizons does typical soil contain?
- What does the O-horizon contain?
- What is duff?
- What is the A-horizon? What does it contain?
- What are the characteristics of the B-horizon?
- What is the C-horizon? How does it differ from the other horizons?
- What is topsoil? Why is topsoil so important for growing plants?
- How does weathering produce soil?
- Why does the C horizon most resemble the parent material?