How does soil form, what is it made of, and how can it be damaged?
Soil formation by weathering; the composition of soil (mineral particles, organic matter, water, air and organisms); soil types and their properties; and soil degradation and its causes.
An SQA National 5 Environmental Science answer on soils, covering soil formation by weathering, the composition of soil from mineral particles, organic matter, water, air and organisms, the main soil types and their properties, and the causes and prevention of soil degradation.
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What this dot point is asking
The SQA wants you to describe how soil forms by the weathering of rock, list the components that make up a fertile soil, recognise the main soil types and their properties, and explain what soil degradation is, what causes it, and how it can be reduced.
How soil forms by weathering
Soil starts with solid rock, which is broken down over long periods by weathering into small mineral particles.
- Physical (mechanical) weathering breaks rock without changing it chemically. The main example is freeze-thaw: water enters cracks, freezes and expands, and over many cycles splits the rock apart.
- Chemical weathering changes the minerals in the rock, for example rainwater (a weak acid) reacting with and dissolving minerals.
- Biological weathering is caused by living things, such as plant roots growing into cracks and prising rock apart, or organisms producing acids.
As mineral particles build up, dead plant and animal material is added and broken down by decomposers into humus (dark organic matter), turning broken rock into living soil. Soil forms very slowly, so it must be treated as a resource that can be lost faster than it is made.
What soil is made of
The humus supplies nutrients and helps the soil hold water; the air spaces let roots and soil organisms get oxygen; the organisms recycle nutrients and improve the structure.
Soil types and their properties
Soils differ mainly in the size of their mineral particles:
- Sandy soil has large particles with big spaces between them. It drains quickly and warms up fast, but holds little water and few nutrients, so it can be dry and infertile.
- Clay soil has very small particles packed tightly. It holds water and nutrients well but drains slowly, can become waterlogged, and is heavy to work.
- Loam is a balanced mixture of sand, silt and clay plus plenty of humus. It drains well yet holds enough water and nutrients, which makes it the ideal soil for growing crops.
Soil degradation
Common causes are human activities:
- Erosion by wind and water, especially where vegetation has been removed (by overgrazing, deforestation or clearing crops) so the topsoil is left bare and exposed.
- Loss of fertility from intensive cropping that removes nutrients and organic matter faster than they are replaced.
- Compaction by heavy machinery or trampling, which squeezes out air spaces.
- Contamination by pollutants such as chemicals.
Soil degradation can be reduced by: keeping plant cover (cover crops, leaving stubble) so roots bind the soil; crop rotation to maintain fertility; adding organic matter (manure or compost); ploughing along the contours; and planting windbreaks such as hedges and trees to slow the wind.
Examples in context
Example 1. Freeze-thaw building a scree slope. On a mountainside, water repeatedly freezes and thaws in cracks, splitting rock into fragments that collect as scree at the foot of the slope. Over time these fragments weather further and mix with organic matter to begin soil formation, showing physical weathering as the first step.
Example 2. The Dust Bowl as soil erosion. When large areas of grassland were ploughed and then suffered drought, the bare topsoil dried out and was blown away in dust storms, leaving the land infertile. This is a clear example of soil degradation caused by removing vegetation and exposing soil to wind erosion.
Try this
Q1. Name the type of weathering in which water freezes in a crack and splits the rock. [1 mark]
- Cue. Physical (mechanical) weathering, specifically freeze-thaw.
Q2. State why loam is the best soil type for growing crops. [2 marks]
- Cue. It is a balanced mixture of particle sizes plus humus, so it drains well yet holds enough water and nutrients.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe how soil forms, and name the main components found in a fertile soil.Show worked answer →
A 4-mark answer needs the formation process plus the components, so plan two formation marks and two component marks.
Soil forms when rock is broken down into small mineral particles by weathering. Weathering can be physical (for example freeze-thaw, where water freezes in cracks and splits the rock), chemical (rainwater reacting with minerals) or biological (plant roots and organisms breaking rock apart). Dead plant and animal material is then added and broken down by decomposers to form humus.
A fertile soil contains: mineral particles (from weathered rock), organic matter (humus), water, air (in the spaces between particles) and living organisms (such as bacteria, fungi and earthworms).
Markers reward describing weathering as the source of mineral particles, the addition of organic matter, and naming the components of a fertile soil.
SQA N5 style3 marksExplain two ways farming can cause soil degradation and suggest one method to reduce it.Show worked answer →
This needs two causes plus a prevention method, so plan two cause marks and one method mark.
Cause 1. Removing vegetation (for example by overgrazing or clearing crops) leaves bare soil exposed, so wind and rain erode the topsoil and wash or blow it away.
Cause 2. Overuse or intensive cropping removes nutrients and organic matter faster than they are replaced, so the soil loses fertility and structure.
Method. Plant cover crops or maintain plant cover so roots bind the soil and protect it from erosion. Other valid methods: rotate crops, add organic matter or manure, plough along contours, or plant windbreaks (hedges and trees).
Markers reward two valid causes of degradation and one sensible prevention method.
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