IB Environmental systems and societies Topic 5.1E…

A Soil Profile Reveals How a Soil Formed

Definition

Soil profile

A soil profile is a vertical section of soil that reveals distinct horizons, each formed by long-term interactions of organic and inorganic materials.

A soil profile is the sequence of layers, or horizons, seen in a vertical section through the soil.
Reading a profile reveals the transfers (leaching, biological mixing) and transformations (decomposition, weathering) that built it.
Profiles also link a soil to its biome, since climate and vegetation shape which horizons develop.

Note

A profile makes visible both the movement of water, minerals, and humus and the chemical changes within the soil.

The Main Soil Horizons

Horizon	Description
O horizon (organic)	Surface layer of decomposing plant and animal matter (litter, humus). Common in forests and grasslands.
A horizon (topsoil)	Dark, nutrient-rich layer where most roots grow. High in organic matter.
E horizon (eluviation)	Zone of leaching, where minerals are washed downward. Found in some heavily weathered soils.
B horizon (subsoil)	Accumulates minerals leached from above. Little organic material.
C horizon (parent material)	Weathered rock from which the soil develops.
R layer (bedrock)	Unweathered rock beneath the soil.

Key Distinctions to Remember

The E horizon is a zone of eluviation (leaching) and looks pale because minerals have washed out; it is clearest in podzols.
The B horizon is a zone of illuviation, where those leached minerals accumulate.
The C horizon is weathered parent material, whereas the R layer is solid bedrock and is not a soil horizon.

Common Mistake

Don't confuse parent material (C horizon) with bedrock (R layer).
Bedrock lies beneath the profile and is not counted as a soil horizon.

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Moderately leached, mildly acidic soils with a thick, dark A horizon.
Annual leaf fall and active earthworms mix organic matter deep into the profile.

Tropical Rainforest Soils

Deeply weathered and heavily leached, with thin organic and A horizons despite the huge biomass above.
Nutrients cycle so fast that they pass from litter straight back to roots, largely bypassing the soil.

Tip

Always note that tropical rainforest soils are nutrient-poor, even though the forest itself is highly productive.

Natural Versus Agricultural Profiles

Intensive farming strips the protective O and A horizons through tillage, erosion, and overgrazing.

Horizon	Natural soil system	Intensive agricultural system
O (organic layer)	Present, rich in decomposing plant material, provides nutrients.	Removed by tillage, erosion, or overgrazing.
A (topsoil)	Dark, nutrient-rich, high in organic matter.	Depleted by continuous cropping, fertilisers, and erosion.
B (subsoil)	Collects nutrients leached from the A horizon, supports deep roots.	Remains but may be compacted by machinery.
C (parent material)	Supplies minerals for soil formation over time.	Unaffected but exposed once erosion strips upper layers.

The A Horizon Is the Most Valuable Layer

Definition

Topsoil

Topsoil is the upper mineral-organic layer of soil where most nutrient cycling, decomposer activity, and root growth occur.

The A horizon (topsoil) lies below the O horizon and is dark with humus.
It is the most biologically active layer and holds most of the soil's available nutrients and carbon.
Typical topsoil is only about 12 to 25 cm deep, depending on climate and land use.

Why Losing Topsoil Matters

Sitting at the surface, topsoil is highly vulnerable to wind and water erosion, especially once vegetation is removed.
Losing it cuts fertility and crop yields, forces reliance on fertilisers, and releases stored carbon as CO2.

Example

It can take up to 1,000 years to form 2.5 cm of topsoil, yet intensive farming can destroy it in a few years.

Summary

A profile's horizons record the transfers and transformations that formed the soil.
The organic-rich A horizon is the key to fertility, and it is both slow to form and easily lost.

Active recall

Define a soil profile and explain what it reveals about soil processes.
Distinguish eluviation in the E horizon from illuviation in the B horizon.
Why is parent material (C horizon) not the same as bedrock (R layer)?
Compare brown earths and tropical rainforest soils in horizon depth and nutrient status.
Explain why the A horizon is the most important layer for plant growth.

A Soil Profile Reveals How a Soil Formed

Definition

Soil profile

A soil profile is a vertical section of soil that reveals distinct horizons, each formed by long-term interactions of organic and inorganic materials.

A soil profile is the sequence of layers, or horizons, seen in a vertical section through the soil.
Reading a profile reveals the transfers (leaching, biological mixing) and transformations (decomposition, weathering) that built it.
Profiles also link a soil to its biome, since climate and vegetation shape which horizons develop.

Note

A profile makes visible both the movement of water, minerals, and humus and the chemical changes within the soil.

The Main Soil Horizons

Horizon	Description
O horizon (organic)	Surface layer of decomposing plant and animal matter (litter, humus). Common in forests and grasslands.
A horizon (topsoil)	Dark, nutrient-rich layer where most roots grow. High in organic matter.
E horizon (eluviation)	Zone of leaching, where minerals are washed downward. Found in some heavily weathered soils.
B horizon (subsoil)	Accumulates minerals leached from above. Little organic material.
C horizon (parent material)	Weathered rock from which the soil develops.
R layer (bedrock)	Unweathered rock beneath the soil.

Key Distinctions to Remember

The E horizon is a zone of eluviation (leaching) and looks pale because minerals have washed out; it is clearest in podzols.
The B horizon is a zone of illuviation, where those leached minerals accumulate.
The C horizon is weathered parent material, whereas the R layer is solid bedrock and is not a soil horizon.

Common Mistake

Don't confuse parent material (C horizon) with bedrock (R layer).
Bedrock lies beneath the profile and is not counted as a soil horizon.

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Moderately leached, mildly acidic soils with a thick, dark A horizon.
Annual leaf fall and active earthworms mix organic matter deep into the profile.

Tropical Rainforest Soils

Deeply weathered and heavily leached, with thin organic and A horizons despite the huge biomass above.
Nutrients cycle so fast that they pass from litter straight back to roots, largely bypassing the soil.

Tip

Always note that tropical rainforest soils are nutrient-poor, even though the forest itself is highly productive.

Natural Versus Agricultural Profiles

Intensive farming strips the protective O and A horizons through tillage, erosion, and overgrazing.

Horizon	Natural soil system	Intensive agricultural system
O (organic layer)	Present, rich in decomposing plant material, provides nutrients.	Removed by tillage, erosion, or overgrazing.
A (topsoil)	Dark, nutrient-rich, high in organic matter.	Depleted by continuous cropping, fertilisers, and erosion.
B (subsoil)	Collects nutrients leached from the A horizon, supports deep roots.	Remains but may be compacted by machinery.
C (parent material)	Supplies minerals for soil formation over time.	Unaffected but exposed once erosion strips upper layers.

The A Horizon Is the Most Valuable Layer

Definition

Topsoil

Topsoil is the upper mineral-organic layer of soil where most nutrient cycling, decomposer activity, and root growth occur.

The A horizon (topsoil) lies below the O horizon and is dark with humus.
It is the most biologically active layer and holds most of the soil's available nutrients and carbon.
Typical topsoil is only about 12 to 25 cm deep, depending on climate and land use.

Why Losing Topsoil Matters

Sitting at the surface, topsoil is highly vulnerable to wind and water erosion, especially once vegetation is removed.
Losing it cuts fertility and crop yields, forces reliance on fertilisers, and releases stored carbon as CO2.

Example

It can take up to 1,000 years to form 2.5 cm of topsoil, yet intensive farming can destroy it in a few years.

Summary

A profile's horizons record the transfers and transformations that formed the soil.
The organic-rich A horizon is the key to fertility, and it is both slow to form and easily lost.

Active recall

Define a soil profile and explain what it reveals about soil processes.
Distinguish eluviation in the E horizon from illuviation in the B horizon.
Why is parent material (C horizon) not the same as bedrock (R layer)?
Compare brown earths and tropical rainforest soils in horizon depth and nutrient status.
Explain why the A horizon is the most important layer for plant growth.

IB Environmental systems and societies 5.1E Soil Horizons Notes

IB Environmental systems and societies 5.1E Soil Horizons Notes

A Soil Profile Reveals How a Soil Formed

The Main Soil Horizons

Key Distinctions to Remember

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Tropical Rainforest Soils

Natural Versus Agricultural Profiles

The A Horizon Is the Most Valuable Layer

Why Losing Topsoil Matters

Summary

A Soil Profile Reveals How a Soil Formed

The Main Soil Horizons

Key Distinctions to Remember

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Tropical Rainforest Soils

Natural Versus Agricultural Profiles

The A Horizon Is the Most Valuable Layer

Why Losing Topsoil Matters

Summary

Topic 1: Foundation

Topic 2: Ecology

Topic 3: Conservation and biodiversity

Topic 4: Water

Topic 5: Land

Topic 6: Atmosphere and climate change

Topic 7: Natural resources

Topic 8: Human populations and urban systems

HL lenses

Topic 1: Foundation

Topic 2: Ecology

Topic 3: Conservation and biodiversity

Topic 4: Water

Topic 5: Land

Topic 6: Atmosphere and climate change

Topic 7: Natural resources

Topic 8: Human populations and urban systems

HL lenses

IB Environmental systems and societies 5.1E Soil Horizons Notes

IB Environmental systems and societies 5.1E Soil Horizons Notes

A Soil Profile Reveals How a Soil Formed

The Main Soil Horizons

Key Distinctions to Remember

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Tropical Rainforest Soils

Natural Versus Agricultural Profiles

The A Horizon Is the Most Valuable Layer

Why Losing Topsoil Matters

Summary

A Soil Profile Reveals How a Soil Formed

The Main Soil Horizons

Key Distinctions to Remember

Soil Profiles Differ Between Biomes

Brown Earths (Temperate Deciduous Forest)

Tropical Rainforest Soils

Natural Versus Agricultural Profiles

The A Horizon Is the Most Valuable Layer

Why Losing Topsoil Matters

Summary