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.