5.1E Soil horizons (HL) Notes

1. Soils are classified by examining the appearance of the entire soil profile, including colour, texture, structure, horizon thickness, and transitions between layers.
2. A soil profile diagram visually represents these layers and helps explain transfers (e.g., leaching, illuviation) and transformations (e.g., decomposition, humification).
3. Soil scientists commonly use classification systems such as the USDA Soil Taxonomy, which groups soils based on observable characteristics rather than only climate or biome.
4. Soil profiles can be used to link soil types to specific biomes, demonstrating predictable relationships between climate, vegetation, and soil properties.

Why Profile Diagrams Are Useful

1. Soil profile diagrams visually show the arrangement of horizons and highlight key transfers and transformations occurring within the soil.
2. Transfers in profiles include leaching, capillary movement, and organic matter input from above.
3. Transformations include decomposition, mineralization, weathering, and humus formation.
4. Diagrams help students understand how soil processes interact with climate, vegetation, and parent rock.

Linking Soil Profiles to Biomes

1. Different biomes have characteristic soils due to differences in temperature, precipitation, vegetation type, and decomposition rates.
2. Temperate deciduous forests typically have brown earth soils, which show moderate leaching, well-developed A horizons, and active mixing by earthworms.
3. Tropical rainforests often have oxisols, which are deeply weathered, nutrient-poor, and heavily leached because decomposition is rapid in warm, wet climates.
4. Grasslands develop soils with thick, nutrient-rich A horizons due to deep root systems and seasonal organic matter inputs.
5. Deserts generally show thin organic layers and accumulations of salts or calcium due to limited rainfall.

1. Brown Earths - Temperate Deciduous Forests

1. Moderately leached soils with mild acidity.
2. Well-mixed A horizon due to high activity of earthworms and other soil fauna.
3. Thick, dark A horizon enriched with organic matter from annual leaf fall.
4. Lighter B horizon due to downward movement of humus, clay, and iron.
5. Support deciduous forests where seasonal leaf litter provides steady nutrient inputs.

2. Oxisols - Tropical Rainforests

1. Deep, highly weathered soils formed over millions of years in hot, wet climates.
2. Strong leaching removes nutrients and leaves behind iron and aluminium oxides (sesquioxides), giving soils their red/yellow colour.
3. Very thin organic and A horizons, despite high biomass above ground.
4. Rapid decomposition and nutrient uptake by trees means nutrients bypass the soil, cycling directly from litter → roots.

Key Soil Horizons in a Soil Profile

O Horizon (Organic Layer)

1. Formed from freshly fallen plant litter such as leaves, twigs, flowers, bark and animal waste.
2. Contains high levels of undecomposed and partially decomposed organic matter.
3. Supports intense biological activity and begins the nutrient cycling process.
4. Most pronounced in forest ecosystems.

A Horizon (Topsoil)

1. A mixture of humus and mineral particles.
2. Dark in colour due to high humus content.
3. Supports high levels of decomposers such as earthworms, fungi and bacteria.
4. Contains most of the soil’s biologically available nutrients.
5. Critical for plant growth and agricultural productivity.

E Horizon (Eluviation Layer)

1. Present in some soils, especially podzols and forest soils.
2. Characterised by loss of clay, iron, organic matter and minerals through leaching.
3. Pale or bleached appearance because many colouring minerals have been removed.
4. Represents intense downward movement of water.

B Horizon (Subsoil)

1. Zone of accumulation where leached minerals such as iron, aluminum oxides, clay, and humus deposit.
2. Denser and more compact than A horizon.
3. Lower biological activity.
4. Acts as a major zone of water storage.

C Horizon (Parent Material)

1. Composed of partially weathered parent rock.
2. Contains large rock fragments and very little organic matter.
3. Determines the chemical composition and texture of the developing soil.
4. Represents the transition between soil and unweathered bedrock.

Comparison of Natural vs. Agricultural Soil Profiles

The A Horizon (Topsoil)

1. The A horizon, also known as topsoil, lies directly below the O horizon.
2. It contains high organic matter, giving it a dark colour.
3. This layer is the most biologically active, containing bacteria, fungi, protozoa, earthworms, and plant roots.
4. Topsoil has abundant oxygen, nutrients, and recycled organic matter, making it the most valuable layer for plant growth.
5. The typical depth ranges from 12–25 cm, depending on climate and land use.

Importance of the A Horizon

1. Supports the highest density of plant roots, enabling uptake of water and nutrients.
2. Acts as the main site of nutrient recycling (especially nitrogen and phosphorus).
3. Stores significant amounts of carbon compared to lower horizons.
4. Maintains soil structure, porosity, and water-holding capacity.
5. Essential for sustainable agriculture, natural plant growth, and ecosystem resilience.

Vulnerability to Erosion and Degradation

1. Because of its position at the soil surface, topsoil is highly vulnerable to erosion by wind and water.
2. Removal of vegetation increases exposure and accelerates topsoil loss.
3. Intensive farming practices such as:
   1. Monocropping
   2. Frequent tilling
   3. Heavy machinery use
   4. Use of chemical fertilizers and pesticides

Consequences of Losing the A Horizon

1. Reduced soil fertility and microbial activity.
2. Increased risk of compaction in lower layers.
3. Greater reliance on synthetic fertilizers.
4. Declining crop yields, especially in regions with erosion-prone soils.
5. Loss of carbon storage, contributing to increased atmospheric CO₂.