Cambridge IGCSE0680

Soil composition

Environmental Management 0680 Chapter Notes

What this chapter covers

Soil compositionSoils for plant growthAgriculture typesIncreasing agricultural yieldsImpact of agricultureCauses and impacts of soil erosionManaging soil erosionSustainable agriculture
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In this chapter

  1. 1The Four Components of Soil
  2. 2Soil Texture and Particle Size
  3. 3Properties of Sandy vs. Clay Soils
  4. 4Soil Formation and Weathering
  5. 5Simple Field Tests for Soil Type

1. The Four Components of Soil

Soil is the foundation for most life on land. It's not just 'dirt'; it's a complex mixture of four key components. A typical, healthy soil for growing crops is made up of approximately 45% mineral particles, 5% organic matter, 25% water, and 25% air. The mineral part comes from broken-down rock, while the organic part consists of living organisms and dead plants and animals. The spaces between the solid particles, called pores, are filled with a constantly changing balance of air and water, which is vital for plant roots and soil life.

Key term

Loam: A soil that is a mixture of sand, silt, and clay, often considered ideal for agriculture as it combines the beneficial properties of each.

Examiner insight

Examiners reward answers that clearly state the four main components and explain that the proportions of air and water are variable and interdependent.

Common pitfall

Stating that soil is 50% solid and 50% pores, without explaining that the pore space is filled with a variable mix of air and water depending on conditions like rainfall.

Fun fact

A single teaspoon of healthy soil can contain more microorganisms than there are people on Earth, playing a vital role in decomposition and nutrient cycling.

Worked example 13 marks

A soil sample has a total volume of 200 cm³. It contains 90 cm³ of mineral particles and 10 cm³ of organic matter. Calculate the percentage volume of pore space in the soil.

  1. 1

    Step 1: Calculate the total volume of solid components. Total solids = Volume of minerals + Volume of organic matter = 90 cm³ + 10 cm³ = 100 cm³.

  2. 2

    Step 2: Calculate the volume of pore space. Pore space = Total volume - Total solids volume = 200 cm³ - 100 cm³ = 100 cm³.

  3. 3

    Step 3: Calculate the percentage of pore space. Percentage pore space = (Pore space volume / Total volume) * 100 = (100 cm³ / 200 cm³) * 100 = 50%.

Recap

  • Soil is composed of mineral particles, organic matter, air, and water.
  • A typical cultivated soil is about 45% mineral, 5% organic, 25% air, and 25% water.
  • The spaces between soil particles are called pores.
  • The proportions of air and water in the pores vary depending on rainfall and plant use.

Quick check

  1. List the four main components of soil.2 marks
  2. What fills the pore spaces in soil?1 mark

2. Soil Texture and Particle Size

The mineral part of soil is classified by the size of its particles. This property, known as soil texture, has a huge impact on how a soil behaves. There are three main particle types: sand (largest), silt (medium), and clay (smallest). Sand particles (2.0 - 0.02 mm) feel gritty. Silt particles (0.02 - 0.002 mm) feel smooth or soapy. Clay particles (<0.002 mm) are microscopic and feel sticky when wet. The different sizes mean they pack together differently, creating different pore sizes and affecting water drainage, aeration, and nutrient retention.

Key term

Soil Texture: The relative proportion of sand, silt, and clay particles in a soil, which determines its physical properties.

Examiner insight

Candidates must know the relative sizes of sand, silt, and clay particles and be able to link particle size to pore size and its consequences.

Common pitfall

Mixing up the properties of sand and clay, for example, stating that sand has small pores and poor drainage.

Fun fact

Due to their tiny size and large combined surface area, the clay particles in just one cubic centimetre of soil can have a total surface area equivalent to a tennis court!

Worked example 16 marks

A student is describing three soil mineral particles, A, B, and C. Particle A is 1.5 mm in diameter. Particle B is 0.01 mm. Particle C is 0.001 mm. Identify each particle as sand, silt, or clay and describe the texture of each.

  1. 1

    Particle A (1.5 mm): This falls within the sand range (2.0 - 0.02 mm). It would feel gritty.

  2. 2

    Particle B (0.01 mm): This falls within the silt range (0.02 - 0.002 mm). It would feel silky or soapy.

  3. 3

    Particle C (0.001 mm): This is smaller than 0.002 mm, so it is clay. It would feel sticky when wet.

Recap

  • Soil texture is determined by the proportions of sand, silt, and clay.
  • The particle size order from largest to smallest is Sand > Silt > Clay.
  • Sand feels gritty, silt feels silky, and clay feels sticky when wet.
  • Particle size directly influences the size of pores between particles.

Quick check

  1. Which soil particle type is the smallest, being less than 0.002 mm in diameter?1 mark
  2. Arrange sand, silt, and clay in order of decreasing particle size.1 mark

3. Properties of Sandy vs. Clay Soils

The dominance of either sand or clay particles gives a soil distinct characteristics. Sandy soils, with their large particles and large pores, are 'light' soils. They are free-draining, well-aerated, and warm up quickly in spring. However, this means they don't hold water or nutrients well, as these are easily washed through (leached). Clay soils are 'heavy' soils. Their tiny particles pack tightly, creating small pores. This makes them excellent at holding water and nutrients. The downside is poor drainage, poor aeration (which can harm roots), and they are slow to warm up. They are also difficult to cultivate, being sticky when wet and baking hard when dry.

Key term

Leaching: The process where soluble substances, such as mineral nutrients, are washed out of the soil by water draining through it.

Examiner insight

Marks are often awarded for comparative statements. Instead of just listing properties, explicitly contrast sandy and clay soils, for example, 'Sandy soils have good drainage whereas clay soils have poor drainage'.

Common pitfall

Thinking that high water retention in clay is always a good thing. While it holds water, poor drainage means it can become waterlogged, starving roots of oxygen.

Worked example 13 marks

A farmer is deciding whether to grow carrots on a field with sandy soil or a field with clay soil. Carrots require well-drained soil and can be damaged if they meet resistance while growing. Advise the farmer, explaining two advantages of your chosen soil type for growing carrots.

  1. 1

    Advice: The farmer should choose the sandy soil.

  2. 2

    Advantage 1: Sandy soils are free-draining. This prevents waterlogging, which is important as carrots do not thrive in overly wet conditions.

  3. 3

    Advantage 2: Sandy soils are easier to cultivate and have a looser structure. This means the carrot root can grow straight and long without meeting much resistance, preventing stunted or forked carrots.

Worked example 24 marks

Explain why clay soils are generally more fertile than sandy soils if rainfall is adequate.

  1. 1

    Step 1: Clay particles are very small and have a large total surface area compared to sand particles.

  2. 2

    Step 2: The surfaces of clay particles have a negative electrical charge.

  3. 3

    Step 3: This negative charge attracts and holds onto positively charged mineral ions (nutrients), such as calcium (Ca²⁺) and potassium (K⁺), preventing them from being leached away.

  4. 4

    Step 4: Sandy soils have larger particles with a smaller surface area and no charge, so nutrients are easily washed away, making them less fertile.

Recap

  • Sandy soils have large particles, large pores, good drainage, and good aeration.
  • Clay soils have small particles, small pores, poor drainage, and poor aeration.
  • Sandy soils are quick to warm up but have poor water and nutrient retention.
  • Clay soils are slow to warm up but have excellent water and nutrient retention.
  • Sandy soils are easy to cultivate; clay soils are difficult to cultivate.

Quick check

  1. State one advantage and one disadvantage of a sandy soil for growing crops.2 marks
  2. Which soil type, sand or clay, has a higher risk of nutrients being lost through leaching?1 mark

4. Soil Formation and Weathering

The mineral component of soil originates from a slow, relentless process called weathering, which is the breakdown of underlying parent rock. There are three main types. Physical weathering breaks rocks apart without changing their chemical composition, through forces like the freeze-thaw cycle (water freezing in cracks and expanding), temperature changes causing expansion and contraction, and abrasion by wind or water. Chemical weathering involves chemical reactions that decompose rock, such as when slightly acidic rainwater (carbonic acid) dissolves minerals like limestone. Biological weathering is caused by living organisms, for example, plant roots growing into rock crevices and forcing them apart, or burrowing animals.

CO₂ + H₂O → H₂CO₃ (Carbonic Acid)

Key term

Weathering: The breakdown and alteration of rocks at or near the Earth's surface by physical, chemical, and biological processes.

Examiner insight

Students should be able to describe a specific example for each of the three types of weathering (physical, chemical, biological).

Common pitfall

Confusing weathering with erosion. Weathering is the breakdown of rock in place, while erosion is the movement of the broken fragments to a new location.

Fun fact

The Grand Canyon is a spectacular, large-scale example of weathering and erosion at work over millions of years, carving through layers of rock to a depth of over a mile.

Worked example 13 marks

Describe the process of physical weathering by the freeze-thaw cycle.

  1. 1

    Step 1: Water from rain or melting snow seeps into cracks and pores in a rock.

  2. 2

    Step 2: When temperatures drop below 0°C, the water freezes and expands by about 9% in volume.

  3. 3

    Step 3: This expansion exerts immense pressure on the surrounding rock, widening the crack. Repeated cycles of freezing and thawing eventually cause fragments of the rock to break off.

Worked example 24 marks

Explain how the activities of living organisms can cause both biological and chemical weathering.

  1. 1

    Biological weathering: A tree root can grow into a crack in a rock. As the root grows thicker, it exerts pressure, forcing the crack to widen and eventually splitting the rock.

  2. 2

    Chemical weathering: Microorganisms in the soil, and plant roots, respire and release carbon dioxide (CO₂). This CO₂ dissolves in soil water to form weak carbonic acid.

  3. 3

    This carbonic acid then chemically reacts with minerals in the rock, causing them to dissolve and the rock to crumble.

  4. 4

    Therefore, one organism (like a plant) can contribute to both types of weathering.

Recap

  • The mineral part of soil comes from the weathering of parent rock.
  • Physical weathering breaks rock without chemical change, e.g., freeze-thaw.
  • Chemical weathering decomposes rock through chemical reactions, e.g., acid rain.
  • Biological weathering is the breakdown of rock by living organisms, e.g., plant roots.
  • Weathering is the breakdown of rock, while erosion is the transport of the broken pieces.

Quick check

  1. Name the three main types of weathering.3 marks
  2. Explain how carbonic acid is formed in soil and contributes to weathering.2 marks

5. Simple Field Tests for Soil Type

You can get a good idea of a soil's texture without needing a laboratory. Two simple field tests are the 'feel test' and the 'moulding test'. For the feel test, take a small, moist sample of soil and rub it between your thumb and forefinger. A gritty texture indicates a sandy soil, a smooth and soapy or silky feel suggests a silty soil, and a sticky feel points to a clay soil. The moulding test gives more detail. By taking a ball of moist soil and trying to roll it into a sausage and bend it into a circle, you can determine the dominant texture. A soil that cannot be rolled is sandy. A soil that can be rolled into a sausage but breaks when bent is a loam. A soil that can be rolled and bent into a circle without breaking has a high clay content.

Key term

Soil Analysis: The process of determining the physical and chemical properties of a soil sample, such as its texture, pH, and nutrient content.

Examiner insight

For questions on practical investigations, examiners look for a logical sequence of steps and a clear link between the observation and the conclusion drawn.

Common pitfall

Describing the feel of the soil as 'hard' or 'soft'. Use the specific descriptive terms: gritty, silky/soapy, or sticky.

Worked example 13 marks

A student performs a moulding test on a moist soil sample. They can easily roll it into a ball and then into a thin 'sausage' shape. When they try to bend the sausage into a ring, it cracks and breaks apart. Using this information, what is the likely soil type? Justify your answer.

  1. 1

    Likely soil type: The soil is likely a loam.

  2. 2

    Justification 1: The ability to roll it into a sausage indicates it has enough clay to be cohesive.

  3. 3

    Justification 2: The fact that it breaks when bent into a ring shows it does not have a very high clay content, and contains significant amounts of sand and/or silt. This mixture of properties is characteristic of a loam.

Recap

  • Simple field tests can be used to estimate soil texture.
  • The 'feel test' involves rubbing moist soil to feel for grittiness (sand), silkiness (silt), or stickiness (clay).
  • The 'moulding test' assesses how well moist soil can be shaped.
  • A soil that can be rolled but not bent into a ring is likely a loam.
  • A soil that can be bent into a ring without cracking is high in clay.

Quick check

  1. You rub a moist soil sample and it feels very gritty. What is the dominant particle type?1 mark
  2. During a moulding test, a soil sample will not form a ball at all and just crumbles. What soil type is this?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. What are the four main components of soil?4 marks
  2. Describe the texture of sand, silt, and clay when rubbed between the fingers.3 marks
  3. Explain two reasons why a farmer might find a clay soil difficult to manage.4 marks
  4. Compare the water-holding capacity and nutrient retention of sandy and clay soils, explaining the reasons for the differences.6 marks
  5. Describe one example of physical weathering and one example of biological weathering.4 marks
  6. A diagram shows the composition of a typical loam soil as 45% minerals, 5% organic matter, 25% water, 25% air. Explain why the percentages of air and water might be different in a soil sample taken after a week of no rain.3 marks
  7. A gardener wants to improve their heavy clay soil to make it better for growing vegetables. Suggest and explain two ways they could improve the soil structure.4 marks
  8. Explain in terms of particle size and surface area why clay soils are better at retaining nutrients than sandy soils.5 marks
  9. Distinguish clearly between the terms 'weathering' and 'erosion'.2 marks
  10. A soil is found to be 60% sand, 10% silt, and 30% clay. Describe the likely properties of this soil in terms of its drainage, aeration, and suitability for cultivation. Justify your answer.6 marks

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