The A horizon is the upper mineral layer of a soil profile, usually located immediately below the organic O horizon. It is distinct because it contains a mixture of mineral particles and decomposed organic matter, making it especially important for plant growth.
Why Is the A Horizon Distinct?
In IB ESS subtopic 5.1 Soil, the A horizon is also called the mixed layer or topsoil. Organic material from dead organisms is decomposed and incorporated into mineral particles such as sand, silt and clay. This often gives the horizon a darker colour than lower layers.
The A horizon contains much of the soil's humus, water, mineral nutrients, roots and biological activity. Its structure therefore supports primary productivity by providing plants with water, nutrients and physical anchorage.
| Horizon | Distinguishing features |
|---|---|
| O horizon | Upper organic layer dominated by dead and decomposing material |
| A horizon | Mixed mineral and organic topsoil; particularly valuable for plant growth |
| B horizon | Lower mineral soil with less organic matter than the A horizon |
| C horizon | Weathered parent material from which soil develops |
A common misconception is that the A horizon is entirely organic. This is incorrect: the O horizon is predominantly organic, whereas the A horizon combines inorganic mineral particles with organic matter.
The A horizon is also especially vulnerable to soil erosion because it lies close to the surface. Removal of vegetation, overgrazing and intensive cultivation expose it to wind and water erosion. During the 1930s Dust Bowl in the United States, drought and unsuitable farming practices contributed to extensive topsoil loss, reducing agricultural productivity and displacing communities.
Exam Technique
This content is HL only in the 2026 IB ESS syllabus. In a question asking you to distinguish horizons, identify the A horizon's position, mixed composition and importance for plant growth; for an explanation, connect topsoil loss to declining fertility, productivity and social impacts. In practical work, use colour, texture and organic content to identify horizons in a soil profile.