The composition and formation of soil, soil horizons and texture, the properties that make a fertile soil, the causes and consequences of soil degradation, and methods of soil conservation.

What this dot point is asking

AQA wants you to describe what soil is made of and how it forms, explain soil horizons and texture, identify the properties of a fertile soil, explain the causes and consequences of soil degradation, and describe methods of soil conservation. Examiners reward candidates who can link a property (such as crumb structure or texture) to a function (such as drainage or nutrient retention) rather than just listing features.

Composition and formation

Soil forms by the interaction of five factors, often remembered as CLORPT: climate, living organisms, relief (topography), parent material and time. Physical weathering (freeze-thaw, expansion and contraction) breaks rock into fragments without changing its chemistry. Chemical weathering alters mineral composition: rainwater dissolves carbon dioxide to form weak carbonic acid that dissolves carbonates, and oxidation rusts iron-bearing minerals. Biological weathering by roots, burrowing animals and the acids released by lichens accelerates breakdown.

Once a layer of mineral fragments exists, pioneer plants colonise, die and are broken down by decomposers (bacteria and fungi) into humus. Earthworms and other fauna mix this organic material through the profile. Because formation depends on weathering and the slow accumulation of organic matter, soils form at only about 0.1 to 1 mm per year, which is why erosion is so serious.

Soil horizons and texture

As soil develops it forms layers called horizons, seen in a vertical cut called a soil profile:

• O horizon: surface litter of leaves and organic debris.
• A horizon (topsoil): dark, humus-rich, biologically active, where most roots and nutrients are concentrated.
• B horizon (subsoil): mineral-rich, lighter coloured, accumulating clays and minerals leached from above.
• C horizon: weathered parent rock fragments, grading into bedrock below.

Texture depends on the proportions of sand, silt and clay, classified using a soil texture triangle. Sand particles (0.06 to 2 mm) create large pores so sandy soils drain freely and warm quickly but hold few nutrients and dry out. Clay particles (smaller than 0.002 mm) have a huge surface area and a negative charge that binds water and cations, so clay soils are nutrient-rich but drain poorly and can waterlog and compact. Loam, a balanced mix of sand, silt and clay with good humus, combines drainage with water and nutrient retention and is generally the most fertile.

Properties of a fertile soil

A fertile soil links several properties to plant function:

• A good crumb (ped) structure in which particles clump into stable aggregates, giving both free-draining macropores and water-holding micropores.
• Adequate plant nutrients (nitrogen, phosphorus, potassium and trace elements) held on clay and humus surfaces, ready for root uptake.
• Enough organic matter (humus) to bind aggregates, store water, feed soil organisms and slowly release nutrients.
• A suitable pH (most crops prefer roughly 6.0 to 7.0); too acidic and aluminium becomes toxic and nutrients lock up, too alkaline and iron and phosphorus become unavailable.
• Enough air in the pore spaces so roots and aerobic soil organisms can respire, and good drainage to prevent waterlogging.

Soil degradation

Causes are mainly human: removing vegetation (deforestation, overgrazing) exposes bare soil to rain splash and wind, ploughing breaks aggregates, monoculture mines nutrients, and irrigation in hot climates concentrates salts. Consequences include falling crop yields, sediment and nutrient runoff that pollutes and eutrophies rivers, dust storms, and in severe cases desertification. The 1930s American Dust Bowl, where deep ploughing of dry grassland followed by drought stripped topsoil across the Great Plains, is the classic case study of structural loss combined with wind erosion.

Soil conservation

Conservation methods slow or reverse degradation by protecting structure and reducing exposure:

• Contour ploughing and terracing slow runoff on slopes and let water infiltrate.
• Crop rotation (especially including legumes that fix nitrogen) and cover crops or stubble retention keep the ground covered and maintain nutrients and structure.
• Adding organic matter (manure, compost, green manures) rebuilds humus, improves crumb structure and water holding.
• Reduced or zero tillage leaves aggregates and root channels intact.
• Shelter belts of trees reduce wind speed and wind erosion, and controlling stocking density prevents overgrazing and compaction.

Try this

Q1. Name the five main components of soil and give the approximate percentage by volume of pore space in a fertile mineral soil. [3 marks]

• Cue. Mineral particles, organic matter (humus), water, air, and living organisms; pore space is about 50 percent.

Q2. Explain why a loam soil is usually more fertile than a pure sandy or pure clay soil. [3 marks]

• Cue. Loam balances sand (drainage, aeration) and clay (water and nutrient retention) plus humus, giving good crumb structure with both drainage and retention.

Q3. A field loses 8 tonnes of topsoil per hectare per year with a bulk density of $1.2 \text{ t m}^{-3}$. Calculate the depth of soil lost per year in millimetres. [3 marks]

• Cue. $8 \text{ t ha}^{-1} = 0.8 \text{ kg m}^{-2}$; depth $= 0.8 / 1200 = 6.7 \times 10^{-4} \text{ m} \approx 0.67 \text{ mm yr}^{-1}$.