Soil Profile and Soil Horizon

Let’s visualize the Soil Profile and Soil Horizon as if we were digging deep into the Earth, layer by layer, like an archaeologist uncovering the past. Each layer tells a unique story of formation, composition, and function.

Imagine standing in a field and digging a pit straight down into the soil. As you go deeper, you’ll notice that the soil doesn’t look the same throughout. The vertical section you see, with distinct layers, is called the Soil Profile.

Each layer, or horizon, has a different texture, color, and composition—like the different floors of a building, each serving a unique purpose. Let’s explore these layers one by one.

Soil Horizons: The Layers of the Soil

Just as a book has different chapters, soil has distinct layers called horizons, each contributing to the overall characteristics of the soil.

1. O Horizon – The Organic Blanket

Think of this as nature’s compost pile. This topmost layer is full of organic material—leaves, twigs, and decomposing matter. In forests, this layer is thick, while in deserts, it may be almost absent.

2. A Horizon – The Surface Soil (Topsoil)

This is the layer where life thrives! It’s where plant roots grow and organisms like earthworms, fungi, and bacteria break down organic matter. It contains a mix of minerals and humus, making it nutrient-rich. However, due to continuous weathering and leaching, nutrients gradually move downward.

• If this layer undergoes intense leaching, a lighter-colored E Horizon may appear below it.

3. E Horizon – The Leached Zone (Eluviation Layer)

Here, elements like iron, clay, and aluminum oxides are washed out (eluviated), leaving behind lighter, sandy material, often rich in quartz. This horizon is mainly found in older, well-developed soils.

4. B Horizon – The Subsoil (Illuviation Zone)

If the E horizon loses minerals, where do they go? The answer lies here! The B Horizon acts as a mineral warehouse, accumulating iron, clay, and organic compounds that seep down from the top layers. This process, called illuviation, is the opposite of eluviation.

Remember: Eluviation is the process where clay, iron, organic matter, and other fine particles are washed out (leached) from the upper soil layers by water whereas Illuviation is the process where leached materials from upper layers accumulate in lower soil layers.

5. C Horizon – The Weathered Parent Material

Going deeper, we reach the C Horizon, made up of weathered parent rock: layer of large unbroken rocks. While it doesn’t support much biological activity, this layer is where soil formation begins, as rocks gradually break down over thousands of years.

6. R Horizon – The Bedrock (Foundation of Soil)

Finally, at the bottom lies the R Horizon, the solid, unweathered bedrock. This is the source material for all the layers above. In some places, soil forms directly from this bedrock, meaning the soil inherits many of its properties.

Final Thoughts

Each soil horizon has a role to play, shaping the fertility, drainage, and structure of the land. Farmers, geologists, and ecologists study these layers to understand soil health, land use, and environmental changes.

Important Concepts to Remember

1. Temperature and Humus Formation

• When temperature decreases, microbial activity reduces, leading to less decomposition of organic matter. As a result, humus content decreases.
• In forests with warm conditions, humus accumulation is higher due to increased biological activity.

2. Waterlogging and Mineral Dissolution

• In waterlogged conditions, reduction reactions dominate, meaning oxygen availability is low.
• This leads to dissolution of minerals in water, influencing soil composition.

3. Temperature and Soil Salinity

• When temperature increases, evaporation intensifies, causing water to move upward through capillary action.
• This leads to calcification (accumulation of calcium carbonate) and increased soil salinity.

4. Soil Colour and Humus Content

• Generally, dark-coloured soils have higher humus content.
• Exception: Black cotton soil is dark not due to humus, but because of its parent rock composition.

5. Impact of Humus on Soil Horizons

• If humus is present, the upper horizon (A-horizon) is light in color due to leaching, while the lower horizon (B-horizon) is darker.

6. Humus, Water Retention, and Acidity

• Even with low humus, if soil retains high organic matter and water, it tends to be acidic.

7. Mineral and Salt Deposition

• If minerals or salts are found in the B-horizon, leaching may have occurred, meaning water was present to carry them downward.
• If minerals or salts accumulate in the A-horizon, evaporation and capillary action were dominant, pulling minerals to the surface.