Notes · Environmental ScienceUK · A-Levels

Soils

This chapter treats soil as a living system on which agriculture and ecosystems depend. It covers the composition of soil and the horizons of the soil profile, the properties that determine fertility, the ways human activity degrades soil, and the processes of soil erosion together with the conservation practices that combat it. Throughout it links soil to the biogeochemical cycles, agriculture and sustainability.

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

T·0888 / 16

Exam profile

AO1 · Describe soil composition, the soil profile, the determinants of fertility, degradation and erosionAO2 · Apply soil concepts to interpret texture, pH and erosion dataAO3 · Analyse the causes of soil degradation and evaluate soil-conservation strategies

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basic level

AS-Level expects you to describe what soil is made of, the soil profile, the main causes of soil degradation and erosion and the main conservation methods.

higher level

The full A-Level requires you to relate soil properties to fertility quantitatively, analyse the causes of degradation and evaluate conservation strategies.

Depth

Text

Contents · 4 sections

Soils

What soil is: composition and the soil profile#

FoundationAQA 7447 3.2.5

The soil profile

Fig. 1The soil profile: horizons from the organic litter at the surface down to the bedrock (depths approximate).

Soil is not merely dirt but a complex, living system made of four components: mineral particles derived from the weathering of rock, organic matter from dead and decaying organisms, water held in the pore spaces, and air in the remaining pores. A typical fertile soil is roughly half solid (mostly mineral, with a small but vital fraction of organic matter) and half pore space shared between water and air. The proportions matter: too little air and roots and soil organisms cannot respire, too little water and plants wilt, so a good soil balances the two.

The mineral particles are classified by size into sand (largest), silt and clay (smallest), and the relative proportions give the soil its texture. Sandy soils have large particles and large pores, so they drain freely and are well aerated but hold little water or nutrients; clay soils have tiny particles and tiny pores, so they hold water and nutrients well but drain poorly and can become waterlogged and airless. A loam, with a balance of all three, combines good drainage and aeration with good water and nutrient retention, which is why loams are the most fertile textures.

The organic matter, especially the dark, well-decomposed material called humus, is disproportionately important. It binds mineral particles into crumbs that improve structure and aeration, holds water and nutrients, provides food for the soil organisms, and releases nutrients slowly as it decomposes. The soil biota, from bacteria and fungi to earthworms, drive the decomposition and nutrient cycling met in earlier chapters and mix and aerate the soil, so a soil rich in organic matter and life is fertile and resilient.

A vertical section through the soil reveals horizons, distinct layers that make up the soil profile. At the top is a layer of organic litter; below it the topsoil, dark with humus and full of roots and organisms; then the subsoil, lighter and richer in minerals washed down from above; then weathered parent rock; and finally the unweathered bedrock. The profile records how the soil formed and its depth and quality, and reading a profile is a standard fieldwork and examination task.

The composition of a fertile loam

Fig. 2The approximate composition of a fertile loam by volume (illustrative): about half solids and half pore space.

Worked example

Interpreting a soil profile

A pit dug in a woodland shows a dark, crumbly, root-filled layer over a paler, denser layer, over broken rock. Identify the horizons and explain what the dark layer indicates about fertility.

Identify the horizons
The dark, root-filled layer is the topsoil (A horizon), the paler dense layer is the subsoil (B horizon), and the broken rock is weathered parent material (C horizon).
Interpret the dark layer
The dark colour and crumbly structure indicate abundant humus and organic matter, which improve structure, aeration and nutrient and water retention.
Conclude
A deep, humus-rich topsoil indicates a fertile soil, because it supports roots and soil life and holds nutrients and water.

Result: The horizons are topsoil, subsoil and weathered rock; the deep, humus-rich topsoil indicates good fertility.

Typical mistakes

Forgetting that pore space (water and air) is roughly half of a good soil, and that roots need air as well as water.
Assuming clay soils are always best because they hold nutrients; they can be waterlogged and airless, whereas a loam is the most fertile texture.

Active revision

Explain why a loam soil is generally more fertile than either a pure sand or a pure clay, referring to drainage, aeration and nutrient retention.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Soil properties and fertility#

StandardAQA 7447 3.2.5

What determines soil fertility

Fig. 3Fertility arises from several interacting properties; organic matter improves many of them at once.

Soil fertility is the capacity of a soil to support plant growth, and it depends on several interacting properties. The supply of the mineral nutrients plants need (especially nitrogen, phosphorus and potassium), the availability of water, good aeration for root and microbial respiration, a suitable pH, and a stable structure that resists erosion and allows roots to penetrate all contribute. Because these properties interact, fertility cannot be judged from any one of them alone; a soil rich in nutrients but waterlogged or too acid will still grow a poor crop.

Soil structure, the way particles are bound into crumbs and aggregates, is as important as texture. A well-structured soil has a network of pores that let water drain and air enter while retaining enough water for plants; it is created and maintained by organic matter, soil organisms and plant roots. Compaction, by heavy machinery or trampling, destroys structure, closing the pores so that the soil becomes waterlogged, airless and hard for roots to penetrate, which is why maintaining structure is central to soil management.

The ability of a soil to hold nutrients depends on its clay and humus content through the cation-exchange capacity: the tiny, negatively charged clay and humus particles attract and hold positively charged nutrient ions such as potassium, calcium and ammonium, preventing them from being washed away and releasing them to plant roots. Sandy soils with little clay or humus have a low cation-exchange capacity and lose nutrients easily by leaching, whereas clay- and humus-rich soils retain nutrients well. Adding organic matter therefore improves both structure and nutrient retention.

Soil pH strongly affects fertility because it controls the availability of nutrients and the activity of soil organisms. Most crops grow best in slightly acidic to neutral soils; strongly acidic soils lock up some nutrients and can release toxic aluminium, while strongly alkaline soils make other nutrients unavailable. pH can be raised by adding lime to an acid soil, a standard management practice. Being able to link a measured pH, texture or nutrient level to a soil's fertility, and to recommend a corrective treatment, is the applied skill this section develops.

Worked example

Diagnosing and treating a soil

A field has an acidic, sandy soil that grows poor crops despite fertiliser. Explain the likely problems and recommend treatments.

Nutrient loss
The sandy texture gives a low cation-exchange capacity, so applied nutrients are leached away quickly rather than held for the crop.
Acidity
The low pH reduces the availability of some nutrients and the activity of beneficial soil organisms.
Recommend treatments
Add organic matter to raise the cation-exchange capacity and improve water and nutrient retention, and add lime to raise the pH towards neutral; then fertiliser will be used more efficiently.

Result: The soil loses nutrients (low cation-exchange capacity) and is too acid; adding organic matter and lime would correct both.

Typical mistakes

Confusing texture (the proportions of sand, silt and clay, which cannot be changed) with structure (the arrangement of particles, which management affects).
Assuming any acidity is bad; most crops prefer slightly acid soils, and it is strong acidity or alkalinity that reduces nutrient availability.

Active revision

A sandy soil is found to lose nutrients rapidly after fertiliser is applied. Explain why, and suggest how adding organic matter would help.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Soil degradation caused by human activity#

StandardAQA 7447 3.2.5

How irrigation can cause salinisation

Fig. 4Salinisation: in a dry climate, evaporation concentrates the salts left by irrigation water until the soil is too saline to farm.

Soil degradation is the decline in the quality and productivity of soil through human activity, and it takes several forms. Continuous cropping without returning nutrients depletes the soil's fertility; repeated cultivation and the removal of crop residues run down the organic matter that holds the soil together; heavy machinery and livestock compact the soil, destroying its structure; and irrigation in dry climates can cause salinisation. These processes often act together, so that an intensively farmed soil loses organic matter, structure and nutrients at the same time.

Salinisation is a particular problem of irrigated land in arid regions. Irrigation water always contains some dissolved salts; where the climate is dry, water evaporates from the soil surface and the salts it carried are left behind and accumulate, and a rising water table can bring more salt up from below. As salt builds up, it lowers the water potential of the soil so that plants struggle to take up water, and it can become toxic, until the land is too saline to farm. This shows how a well-intentioned intervention can degrade the very resource it was meant to make productive.

Acidification and contamination are further forms of degradation. Acid deposition and the heavy use of some fertilisers can acidify soils, reducing fertility as described earlier; and soils can be contaminated by heavy metals, persistent pesticides and industrial pollutants, which harm soil life and can enter the food chain. Because soil forms extremely slowly, taking centuries to build a few centimetres, degradation is effectively irreversible on human timescales, which is why prevention matters far more than cure.

The consequences of soil degradation reach beyond the field. Loss of fertility reduces food production and can drive the clearing of new land, spreading the problem; degraded, structureless soil is far more vulnerable to the erosion considered next; and in the worst cases, the combination of degradation and erosion in dry regions leads to desertification, the spread of desert-like conditions into once-productive land. Recognising degradation as a slow, cumulative and often irreversible loss of a vital resource is the key to valuing soil conservation.

Worked example

Explaining salinisation

A desert region is irrigated for decades and its crop yields eventually collapse as the soil turns white and crusty. Explain the process.

Salts arrive and concentrate
Irrigation water contains dissolved salts; in the hot, dry climate the water evaporates from the surface and leaves the salts behind.
Accumulation
With repeated irrigation and a rising water table, salt accumulates in the topsoil, forming a white crust.
Effect on crops
The high salt content lowers the soil's water potential so plants cannot take up water, and some salts are toxic, so yields collapse.

Result: Evaporation concentrated irrigation salts in the topsoil until the soil became too saline for crops (salinisation).

Typical mistakes

Thinking degraded soil can be quickly rebuilt; soil forms extremely slowly, so degradation is effectively permanent.
Overlooking salinisation as a consequence of irrigation, or explaining it without the evaporation step.

Active revision

Explain how irrigating farmland in a hot, dry climate can, over years, make the soil too salty to grow crops.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Soil erosion and sustainable soil management#

AdvancedAQA 7447 3.2.5

How removing vegetation accelerates erosion

Fig. 5Removing vegetation exposes the soil to wind and water erosion, stripping the fertile topsoil.

Soil erosion is the removal of soil, especially the fertile topsoil, by wind and water, and while it occurs naturally at a slow rate, human activity greatly accelerates it. The chief cause is the removal of the vegetation that protects and binds the soil: ploughing, overgrazing, deforestation and the removal of hedgerows all leave the soil bare, so that rain compacts and washes it away and wind lifts the dry particles. Because erosion strips away the most fertile layer and the soil forms so slowly, accelerated erosion is one of the most serious threats to long-term food production.

The mechanisms are worth understanding. Rain splash breaks up the surface, and water flowing over bare ground carries soil away as sheet, rill and gully erosion, more severely on steep slopes and in heavy rain; dry, bare, fine soil is picked up and carried by wind, as in dust storms. Vegetation combats all of these: the canopy intercepts rain and reduces its impact, roots bind the soil and hold it in place, and the cover reduces wind speed at the surface. This is why keeping the soil covered and rooted is the central principle of erosion control.

The consequences of erosion are felt both on the eroding land and downstream. On the land, the loss of topsoil reduces fertility and productivity and can expose infertile subsoil; downstream, the eroded soil silts up rivers and reservoirs, increasing flood risk and reducing water storage, and the nutrients and any pesticides it carries can pollute water and cause eutrophication. Erosion therefore links the soil chapter to water, pollution and the nitrogen cycle, and its off-site costs strengthen the case for conservation.

Sustainable soil management aims to keep the soil covered, rooted and rich in organic matter. Contour ploughing and terracing slow water running down slopes; windbreaks and shelter belts reduce wind erosion; cover crops and crop residues protect bare soil between crops; crop rotation and adding organic matter maintain fertility and structure; and reduced or zero tillage disturbs the soil less. Evaluating these methods means weighing their effectiveness against their cost and practicality for a given farm, and recognising that the cheapest and most effective approaches usually work by keeping the soil protected rather than by repairing damage after it occurs.

\text{percentage reduction in soil loss} = \frac{\text{loss before} - \text{loss after}}{\text{loss before}} \times 100

Reduction in soil loss

Used to compare the effectiveness of conservation measures; a higher percentage means a more effective measure.

The effect of management on soil loss

Fig. 6Soil loss under different management (illustrative values): keeping the soil covered and slowing runoff greatly reduces loss.

Worked example

Quantifying the effect of conservation

A bare, ploughed slope loses 40 tonnes of soil per hectare per year; after contour ploughing and terracing the loss falls to 5 tonnes per hectare per year. Calculate the percentage reduction and comment on the method.

Find the reduction
Reduction = $40 - 5 = 35$ tonnes per hectare per year.
Express as a percentage
$\dfrac{35}{40} \times 100$ .
$\frac{35}{40} \times 100 = 87.5\%$
Comment
Reducing soil loss by 87.5% shows the measures are highly effective, because they slow the water running down the slope and keep the soil in place.

Result: The measures cut soil loss by 87.5%, showing that slowing runoff on slopes is very effective.

Typical mistakes

Describing erosion only as loss of soil and forgetting the downstream siltation and pollution it causes.
Listing conservation methods without explaining how each one keeps the soil covered or slows water and wind.

Active revision

A farmer on a sloping field suffers gully erosion after heavy rain. Recommend two conservation measures and explain how each would reduce the erosion.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Soils

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

Soils

Key points

Interpreting a soil profile

Key points

Diagnosing and treating a soil

Key points

Explaining salinisation

Key points

Quantifying the effect of conservation

Sources

Soils

Key points

Interpreting a soil profile

Key points

Diagnosing and treating a soil

Key points

Explaining salinisation

Key points

Quantifying the effect of conservation

Sources