Causes of soil degradation and erosion — AQA A-Level Environmental Science

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Soil degradation involves the deterioration of soil physical, chemical, and biological quality, whereas soil erosion refers specifically to the mechanical detachment and transport of topsoil by wind or water. Anthropogenic drivers including deforestation, overgrazing, intensive tillage, and agrochemical overuse drastically accelerate natural erosion rates, threatening global food security and hydrological cycles.

Causes of soil degradation and erosion explained

Soil organic matter is the living, dead and decomposed biological material within soil, including roots, microbes, earthworms, leaf litter and humus.

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It is a key cause-linked factor in soil degradation and erosion because it binds mineral particles into stable aggregates, holds water and supplies nutrients. When organic matter declines, for example through continuous arable cropping, residue removal or excessive tillage, aggregates weaken, infiltration falls and the surface becomes more vulnerable to erosion by water and wind. Learners should distinguish organic matter from the mineral fraction and from added inorganic fertiliser, and recognise that maintaining organic matter through manure, compost, cover crops or reduced tillage reduces degradation risk.

Your focus

Describe what soil organic matter comprises
Explain how organic matter influences soil structure and erosion resistance
Distinguish organic matter from mineral and fertiliser inputs

Which command word is this?

Give clear reasons and mechanisms showing cause and effect (e.g., describe how soil compaction reduces infiltration, leading to increased kinetic surface runoff and rill formation).

Causes of soil degradation and erosion exam tips

Topic Overview

Soil degradation encompasses the chemical, biological, and physical decline of soil health, reducing its productive and ecological capacity. Soil erosion specifically involves the detachment and kinetic transport of topsoil by wind, rain splash, sheet wash, and rilling, removing the organic-rich A-horizon.

Understanding these processes is central to AQA Environmental Science as it links lithospheric mechanics, hydrological pathways, and terrestrial food production. Mitigating these processes is vital for sustainable land management, carbon sequestration, and global food security.

Key Concepts

→Types of water erosion: Rain splash (kinetic droplet impact), sheet erosion (uniform surface wash), rill erosion (scouring into small channels), and gully erosion (deep, permanent channel excavation).
→Mechanisms of wind erosion: Deflation (removal of fine particles), saltation (bouncing movement of medium grains between 0.1 and 0.5 mm), and surface creep (rolling of larger grains).
→Physical degradation factors: Compaction from heavy machinery or overgrazing reduces soil macro-porosity, bulk density, and infiltration capacity, escalating overland flow.
→Chemical degradation factors: Secondary salinisation caused by inappropriate irrigation in arid zones, acidification from ammonium fertilisers, and nutrient depletion due to continuous harvest without organic replenishment.

Marking Points

Identifies soil organic matter as living, dead and decomposed biological material including humus
Explains that organic matter binds particles into aggregates and improves soil structure
Links loss of organic matter to weaker structure, reduced infiltration and greater erosion risk
Distinguishes organic matter from the mineral fraction and from inorganic fertiliser

Examiner Tips

💡Read the stem for the process being tested, such as aggregation or infiltration, before choosing
💡Eliminate options that describe the mineral fraction rather than biological material
💡Link the chosen option to a named degradation process such as water erosion or structural decline
💡Always specify the exact physical mechanism (e.g., 'loss of aggregate stability', 'reduction in infiltration capacity', 'exceedance of shear strength') rather than vague statements like 'the soil becomes loose'.
💡Distinguish clearly between sheet, rill, and gully erosion when evaluating the progressive impacts of surface run-off.
💡Reference the Universal Soil Loss Equation (USLE) factors (rainfall erosivity, soil erodibility, slope length/steepness, cover management, support practice) when structuring evaluation questions.

Common Mistakes

Confuses soil organic matter with the mineral or clay fraction; keep the biological and decomposed material separate
Treats inorganic fertiliser as organic matter; recognise that fertiliser adds nutrients but not humus
Assumes organic matter only affects fertility; connect it also to structure, water holding and erosion resistance
Believing that soil erosion only occurs on steep slopes: Wind erosion occurs extensively on flat terrains with dry, loose, fine-textured soils, and rain splash erosion detaches particles regardless of gradient.
Confusing salinisation with fertiliser run-off: Salinisation is the accumulation of water-soluble salts in the root zone caused by high evapotranspiration drawing saline groundwater upwards, not merely synthetic fertiliser application.
Assuming organic matter only provides plant nutrition: Soil organic matter (humus) acts as a physical cementing agent that stabilises soil aggregates against droplet impact and shear detachment.

Revision Plan

1Day 1-2: Master the terminology and distinguish between biological, chemical, and physical degradation, and water vs. wind erosion mechanisms.
2Day 3-4: Construct comparative flowcharts detailing how human practices (deforestation, overgrazing, tillage, irrigation) drive specific degradation pathways.
3Day 5-6: Practise exam questions evaluating real-world case studies (e.g., the Dust Bowl, Sahel desertification, and intensive arable farming in East Anglia).
4Day 7: Complete timed active-recall flashcard tests and tackle a full 9-mark essay question on integrated soil conservation strategies.

Exam Question Types

📋Data analysis and calculation questions: Calculating erosion rates, runoff volumes, or bulk density changes from provided tables or field data.
📋Explain the mechanism questions (3-4 marks): Detailing step-by-step pathways such as how overgrazing triggers sheet erosion or how irrigation causes salinisation.
📋Evaluation essays (6-9 marks): Critically assessing agricultural methods that degrade soil versus sustainable conservation techniques (e.g., contour ploughing, cover crops, biochar).

Command Word Expectations (AQA)

Explain

Give clear reasons and mechanisms showing cause and effect (e.g., describe how soil compaction reduces infiltration, leading to increased kinetic surface runoff and rill formation).

Evaluate

Weigh up arguments or management methods, presenting balanced evidence of strengths, limitations, and ecological trade-offs, concluding with an evidenced judgment.

Outline

Provide a brief summary of the essential characteristics or principles without unnecessary narrative or deep mechanistic chains.

How Students Lose Marks (Examiner Pitfalls)

Pitfall: Conflating soil degradation with soil erosion rather than distinguishing between broad quality decline and physical transport.

❌ Weak Answer (Loses Marks):Soil erosion and degradation are the same thing, which is when wind blows dry soil away from farm fields.

Example improved answer:Soil degradation is the broader decline in soil fertility, structure, organic matter, and biological activity caused by chemical, physical, or biological processes (e.g., salinisation, compaction). Soil erosion is a specific physical sub-process involving the detachment, transport, and deposition of topsoil particles by erosive agents such as wind and water.

Examiner Tip: Always define degradation as the overall loss of soil health and function, and erosion as the physical removal of soil material.

Pitfall: Failing to explain the precise physical mechanism by which heavy machinery or cattle cause increased surface run-off and erosion.

❌ Weak Answer (Loses Marks):Cattle walk on the soil so the soil gets ruined and washes away when it rains.

Example improved answer:Livestock trampling causes compaction of soil aggregates, reducing macro-pore space and bulk density porosity. This diminishes infiltration capacity, accelerating surface run-off during precipitation events, which causes detachment and transport of topsoil via rain splash, sheet, and rill erosion.

Examiner Tip: Link the anthropogenic activity directly to changes in soil physical structure (e.g., macropores, infiltration rate) before explaining the hydrological mechanism.

Step-by-Step Worked Solutions

Question: Evaluate how continuous arable monoculture leads to accelerated wind erosion compared to mixed rotational farming (6 marks).

1.Step 1: Define the mechanism of wind erosion (deflation/saltation) and identify the vulnerability of arable soils.
2.Step 2: Contrast soil organic matter (SOM) inputs: Continuous arable cropping depletes SOM because crop residues are frequently removed or burned, whereas rotational systems incorporate ley pasture and green manures to bind soil aggregates.
3.Step 3: Contrast surface vegetative cover: Monocultures leave bare fallow windows between harvest and planting, exposing dry topsoil to wind shear stress, whereas mixed rotations maintain ground cover.
4.Step 4: Conclude by linking loss of aggregate stability and lack of boundary windbreaks directly to increased susceptibility to suspension and saltation.

Final Answer: Continuous arable monoculture depletes soil organic matter, destroying soil aggregates into loose, transportable particles. Prolonged periods of bare fallow remove surface roughness and root anchoring, allowing wind detachment (deflation, saltation, suspension), whereas mixed rotational farming preserves organic binders and surface cover.

Question: An agricultural catchment receives 80 mm of rain during a storm event. On an uncompacted woodland plot, infiltration capacity is 60 mm/hr and surface run-off is 5 mm. On an adjacent heavily grazed pasture plot, infiltration capacity is reduced to 15 mm/hr, resulting in 45 mm of surface run-off. Calculate the percentage increase in surface run-off caused by overgrazing and explain the implications for rill formation.

1.Step 1: Calculate the absolute increase in surface run-off: 45 mm - 5 mm = 40 mm.
2.Step 2: Calculate percentage increase: (40 mm / 5 mm) * 100 = 800%.
3.Step 3: Relate increased surface discharge to rill formation: The 800% increase in overland flow concentrates kinetic energy into shallow micro-channels (rills), overcoming soil shear strength and scouring topsoil.

Final Answer: The pasture plot experienced an 800% increase in surface run-off. This heightened volume and velocity of overland flow concentrates into micro-channels, scouring topsoil and initiating rill erosion.

Active Recall Memory Test

What are the three distinct transport mechanisms involved in wind erosion?

Key Fact: Suspension (<0.1 mm), saltation (0.1 to 0.5 mm), and surface creep (>0.5 mm).

How does secondary salinisation occur in irrigated dryland agriculture?

Key Fact: Excess irrigation raises the saline water table; capillary action draws water to the surface where rapid evapotranspiration leaves dissolved mineral salts in the topsoil.

Define rain splash erosion.

Key Fact: The detachment and local movement of soil particles caused by the impact of kinetic energy from falling raindrops.

Why does a reduction in soil organic matter (SOM) directly increase soil erodibility?

Key Fact: SOM produces polysaccharide glues and humus that bind mineral particles into stable peds/aggregates; without it, soils disaggregate into loose, easily transported particles.

Frequently Asked Questions

What is the difference between soil erosion and soil degradation?

Soil degradation is an umbrella term covering any physical, chemical, or biological deterioration of soil function, such as nutrient depletion, acidification, compaction, or salinisation. Soil erosion is a specific sub-category involving the physical removal and transport of topsoil by wind or water. While erosion is a form of degradation, degradation can occur without soil particles being physically moved.

How does deforestation accelerate soil erosion?

Deforestation removes the multi-layered canopy that intercepts rainfall, drastically increasing the kinetic energy of raindrops striking the bare ground (rain splash detachment). Furthermore, tree roots that bind the soil and maintain porous structure decay, while the supply of leaf litter to form humus ceases. The combined effect lowers infiltration rates and exposes loose topsoil to rapid surface run-off and wind action.

Why is overgrazing such a major driver of desertification in drylands?

Overgrazing strips vegetation faster than it can regenerate, removing root anchorage and organic inputs. Repeated trampling by livestock hooves compacts the topsoil, destroying aggregate structure and lowering infiltration rates. When rain does fall, water runs off as sheet wash rather than soaking into aquifers, desiccating the soil profile and leaving it vulnerable to extreme wind deflation.

What is a plough pan and how does it form?

A plough pan is a dense, compacted layer formed in the subsoil directly beneath the working depth of a plough. It occurs when repeated heavy tillage shears and compresses the soil at the same depth, particularly when the soil is wet. This impermeable layer inhibits deep root penetration and drastically restricts vertical drainage, promoting perched water tables and topsoil erosion.

What role does soil organic matter (SOM) play in preventing erosion?

Soil organic matter supplies humus, fungal hyphae, and microbial polysaccharides that cement individual sand, silt, and clay particles into water-stable aggregates. These larger aggregates are far heavier, making them resistant to wind saltation and water detachment. Additionally, organic matter improves macro-porosity and water retention capacity, which enhances rainwater infiltration and reduces erosive overland flow.

Causes of soil degradation and erosion explained

Your focus

Which command word is this?

Causes of soil degradation and erosion exam tips

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