Soil as a system at Higher Level: The same system idea as SL, with different real cases: Scotland's Flow Country peat, clover and its nitrogen-fixing bacteria, and the US Corn Belt's lost topsoil. At HL, use the system idea to explain why a soil is building up or running down.
Practise this as you read
- Sort every process into input, output, storage or flow.
- Judge whether a soil's inputs or outputs are winning, and why.
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Soil is a system: A system has inputs, outputs, storages and flows. Soil has all four, so it can be studied as a system.
The points to remember
- Soil is an open system: matter and energy cross its boundary.
- Inputs: rain, dead leaves and dung, minerals from weathering of rock, gases from the air.
- Outputs: water lost by evaporation, minerals lost by leaching and erosion, nutrients taken up by plants.
- Storages: organic matter (humus), minerals and nutrients such as nitrates, water, air.
- Flows inside the soil: decomposition turns dead matter into nutrients; water moves down the soil.
- In an answer, name the process and what it moves: 'minerals are lost by leaching', not just 'minerals'.
Remember it as: In, out, kept, moved: inputs, outputs, storages, flows.
Name the thing, then the process: An output to the air is the carbon dioxide or the nitrogen gas, not 'respiration' or 'denitrification' on their own. And an input or output is not a transfer: water seeping down to the B horizon stays inside the soil.
Real example: in Scotland's Flow Country, a huge peat bog, dead moss is added faster than it decays in the wet, airless soil. The storage of peat grows by about 1 mm a year.
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An ecosystem has living and non-living parts that interact. Soil has both, so it is an ecosystem in its own right.
Why soil is an ecosystem
- Like any ecosystem, soil has biotic parts: bacteria, fungi, earthworms, insects, plant roots.
- ... and abiotic parts: sand, silt, clay, water, air and heat.
- The parts interact: decomposers recycle minerals; bacteria in bean roots fix nitrogen.
- Soil supports other systems: plants grow in it, animals burrow in it, people farm and build on it.
- For each feature, add an example of it: 'biotic parts, such as earthworms'.
Biotic: living
- Bacteria and fungi (decomposers)
- Earthworms, mites, insects
- Plant roots
Abiotic: non-living
- Sand, silt and clay
- Water and air in the gaps
- Heat from the Sun
Real example: clover and beans have small swellings on their roots called nodules. Inside, Rhizobium bacteria turn nitrogen gas from the soil air into nitrates the plant can use: a plant, a bacterium and the air interacting.
One feature, one example: Write each feature in its own sentence, then give its example: 'soil has abiotic parts, such as clay and water'. A list of examples with no feature named does not show how soil is like an ecosystem.
A system that keeps changing: Soil is a dynamic system: its storages grow or shrink as the balance of inputs and outputs changes. It is also part of a bigger system, the ecosystem above and around it.
The points to remember
- Dynamic: the soil is always changing, as inputs and outputs change its storages.
- If inputs are bigger than outputs, a storage grows: more humus, a deeper soil.
- If outputs are bigger, it shrinks: erosion or leaching drain the soil.
- Soil sits within the larger ecosystem: changes in plants, climate or farming change the soil.
- Soils are a resource for life and they vary widely from place to place.
Remember it as: Inputs bigger: the soil builds. Outputs bigger: the soil drains.
Real example: a 2021 study of satellite images found that about a third of the farmland in the US Corn Belt has lost all of its dark, carbon-rich topsoil since ploughing began, mostly on hilltops, to erosion.
Soils vary widely
- Deep and dark under grassland
- Thin and stony on mountains
- Red and leached in the wet tropics
... and are a resource for life
- Most of our food grows in soil
- Soil stores and cleans water
- Soil is home to countless species
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How this comes up: Paper 2, Section B (a): show how soil is like an ecosystem, one feature per point, each with an example.
Soil scientists in the Amazon rainforest have found thousands of species of bacteria, fungi and insects in a single hectare of forest soil.
Outline how soil can be viewed as an ecosystem.
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