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Soil Horizons

Soil Horizons

Introduction: A soil horizon is a layer of soil that has different properties than the layers above and below it. For example, soil horizons will differ in texture, color, composition, and structure. If you dig a hole from the surface of the ground downward and examine the layers of soil, you will be looking at a soil profile. A soil profile is a vertical set of soil horizons. Due to different formation conditions, not all soil profiles will look the same or have every soil horizon. The diagram below represents a soil profile. Each of the five major soil horizons are labeled.
 
Soil Layers With Text Labels
The O horizon is the uppermost layer of soil. It is composed mostly of "leaf litter" or leaves and organic materials that have fallen on the ground. As decomposers, like worms, break down the litter, it turns into a nutrient rich material called humus.

The next layer down in the A horizon. It is made of topsoil. It is a mix of humus, minerals, and sediments and has a high level of biological activity. The A horizon is nutrient rich and vital for crop growth. Poor soil management can lead to erosion of topsoil.

The B horizon is directly under A horizon. It is composed of subsoil. Compared to topsoil, subsoil contains less organic material like humus and more mineral materials like clay. Rainwater washes minerals down from the upper soil horizons and deposits them here.

The C horizon is the first soil layer that forms. Bedrock breaks into smaller pieces as it is exposed to weathering over time. The broken rock pieces or debris is called regolith.

At the bottom of our soil profile is the R Horizon. This is bedrock, or soild rock, upon which soil forms.

Directions for This Lesson: In this lesson, you will learn about soil horizons. First, try the practice questions to determine what you already know. Then, watch the video lesson to learn more. Finally, apply what you learned in the activity and practice sections.

 

Required Video:

Activity:
Learn more about the factors that influence soil horizon formation with this interactive.

Practice:
Practice what you have learned by completing the post-lesson worksheets.

Related Worksheets:

Additional Resources:

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