The Soil System

IB ESS - 5.1 Soil

The Soil Profile

O - organic horizon

A - mineral horizon

B - Illuvial horizon (deposited horizon)

C - Weathered bedrock

Wilsonbiggs Vector: EssensStrassen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Image: Richard Webb https://www.geograph.org.uk/photo/529437 CC BY-SA 2.0

Wilsonbiggs Vector: EssensStrassen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

The Importance of Soil

• A medium for plant growth
• Food production
• A habitat for animals
• Mineral store

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• Water store
• Water chemistry
• Affects heat transfers

TASK: Outline the importance of soil with a mind map

The Important of Soil

The Important of Soil

Almost all plants nutrients are taken from the soil. Can you think of the one major exception?

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The Important of Soil

Almost all plants nutrients are taken from the soil. Can you think of the one major exception?

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Carbon! They take it from atmospheric carbon dioxide.

(Bonus point if you mentioned nitrogen from insects by insectivorous plants!)

The Important of Soil

Soils contribute to biodiversity by providing a habitat and niche for many species including animals, fungi and microorganisms.

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Recycling of nutrients

Soil is an essential system for recycling nutrients (biogeochemical cycles).

Leaf litter and other organic matter enters the soil and is broken down by detritivores (e.g. earthworms) then decomposed (e.g. fungi and bacteria).

The soil system - Storages

Organic: Animals, roots, dead organic matter, decayed material

Inorganic: minerals, sand, silt, clay, rock fragments

Also: water and air

The soil system - Inputs and outputs

Inputs

Dead organic matter: plant litter, dead animal biomass, manure.

Organic mineral inputs: weathering, deposition or decomposition, precipitation (water with dissolved minerals), gases, air, humidity and solar energy.

Note: natural inputs may originate from within the ecosystem (weathering of underlying parental rock, litter from above-ground vegetation, decomposition) or be derived from other ecosystems (wind-blown and waterborne deposition, guano).

Outputs

Mineral component outputs: wind or water erosion, water and mineral absorption by plant roots, leaching of dissolved plant nutrients and water, diffusion of gases and evaporation of water.

These outputs can cause the loss or modification of soil components and are different from total loss of soil by erosion; however, they can also lead to degradation of productive soil.

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The soil system - Transfers and transformation

Transfers

• infiltration
• percolation
• groundwater flow
• biological mixing
• aeration
• erosion
• leaching

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Transformations

• decomposition
• Weathering
• nutrient cycling
• salinization.

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TASK - System Diagram

Research the processes on the previous slide.

Draw a system diagram to represent the soil system.

Soil Texture

The soil texture is determined by its relevant proportions of:

• Sand (larger particles)
• Silt (medium-sized particles)
• Clay (smaller particles)
• Humus (organic material)

Soil Texture

Sand

0.05 - 2.00 mm

Silt

0.002 - 0.05 mm

Clay

< 0.002 mm

Soil Texture

The soil texture can be identified using:

• A key
• A feel test
• By mixing with water and separating the layers

The Soil Texture Triangle

The relative proportion of sand, silt and clay can be measured and then the soil texture triangle will help identify the type of soil.

Christopher Aragón, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

TASK

Use the soil texture triangle to identify the soil type in each of the samples, A-F.

Christopher Aragón, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Printable version here

Christopher Aragón, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

ANSWERS

1. Clay soil
2. Silty clay loam
3. Sandy loam
4. Clay loam
5. Clay soil
6. Silty clay loam

Soil and productivity

Soil texture affects primary productivity through the differing influences of sand, silt, clay and dead organic matter, including humus…

Soil and productivity - Humus

• Humus contributes significantly to the texture of soils in which it is abundant.
• It is a dark brown or black substance lying beneath the leaf litter.
• It has a loose, crumbly texture formed by the partial decay of dead plant material.
• It influences mineral nutrient retention versus leaching, water retention versus drainage, and aeration versus compaction or waterlogging;

these influences affect primary productivity.

Soil and productivity - Soil particles

Sandy soils: too much drainage (poor water retention)

Clay soils: particles stick together (few air spaces, poor drainage)

Loam soils: a good mix of particles (good for plant growth)

Carbon storage

Soils can be a carbon sink, source or storage.

Consider:

Tropical forest soil have very little carbon compared to tundras, wetlands and temperate grasslands. Why?

Carbon storage

Tropical forest soil have very little carbon compared to tundras, wetlands and temperate grasslands. Why?

Tropical soils have ideal conditions for decomposition (warm temperatures and moderate moisture), so organic material decomposes quickly and is released to the atmosphere.