Syllabus Edition

First teaching 2024

First exams 2026

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Functions & Properties of Soils (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Soil Functions

  • Soils carry out important functions in terrestrial ecosystems

  • Soils support plant growth, biodiversity and biogeochemical cycles

Medium for plant growth

  • Soils act as a natural seed bank, providing a substrate for germination and root development

  • They store water crucial for plant hydration, nutrient uptake and photosynthesis

  • They store essential nutrients for plants such as nitrogen, phosphorus and potassium

  • These essential nutrients support healthy plant growth

    • For example, in the Amazon rainforest, the fertile soils contain high levels of nutrients

    • This allows these soils to support diverse plant life

    • This has led to the Amazon's status as the world's largest tropical rainforest

Contribution to biodiversity

  • Soils provide habitats and niches for a wide range of species

  • Soil communities support high biodiversity, including microorganisms, animals and fungi

    • For example, in the UK, ancient woodlands are rich in soil biodiversity

    • Their soils support rare fungal species that play important roles in nutrient cycling

Role in biogeochemical cycles

  • Soils allow the recycling of elements essential for life, such as carbon, nitrogen and phosphorus

  • Dead organic matter from plants is a major input into soils, where it decomposes and releases nutrients

Carbon storage dynamics

  • Soils can function as carbon sinks, stores, or sources, depending on environmental conditions

  • For example, tropical forest soils generally have low carbon storage due to rapid decomposition rates

    • This is because the warm and moist conditions accelerate the decomposition of organic matter by microorganisms

    • This causes carbon to be released back into the atmosphere quickly

  • Tundra, wetlands and temperate grasslands can accumulate large amounts of carbon in their soils

    • This is because colder temperatures and waterlogged conditions slow down the decomposition process

    • This allows organic matter to build up in the soil over time without being fully decomposed and released as CO2

Soil Texture

What is soil texture?

  • Soil texture describes the physical make-up of soils

  • It depends on the proportions of sand, silt, clay and humus within the soil

  • Soil texture influences various soil properties and plant growth

Components of soil texture

  • Sand: larger particles that feel gritty

  • Silt: medium-sized particles that feel smooth

  • Clay: very fine particles that feel sticky when wet

  • Humus: organic matter, dark brown or black, crumbly texture from partially decayed plant material

Determining soil texture

  • Soil texture can be determined using several methods

  • Each method provides insight into:

    • The soil’s properties

    • How suitable the soil is for different plants and crops

  1. Using a soil key:

    • A soil key is a more systematic and detailed method

    • It uses a step-by-step guide to classify soil texture based on specific criteria

    • The key helps identify the proportions of sand, silt, and clay by guiding the soil tester (the user) through a series of questions or observations

    • It often includes descriptions of soil behaviour when moistened and manipulated

    • Soil keys are often used in more formal or scientific settings where precise classification is needed

  2. Feel test:

    • The feel test is a simpler method

    • It involves rubbing moistened soil between the fingers to assess its texture

    • Sand feels gritty, silt feels smooth and clay feels sticky

    • It is a quick, informal assessment that can be done in the field without additional tools

    • The feel test is commonly used by farmers, gardeners, and others needing a quick assessment

  3. Laboratory test:

    • The laboratory test involves mixing soil with water and allowing it to settle into distinct layers

    • This method provides a clear visual representation of the proportions of sand, silt and clay

    • Any large debris like rocks, roots, or organic matter, are first removed from the sample

    • The sample is added to a transparent container

    • Water is added and the container is shaken vigorously

    • The container is left on a flat surface and left undisturbed (e.g. for 24 hours)

    • Silt settles first, then clay, and finally sand

    • The thickness of these layers can be measured to determine their proportions

Diagram of a transparent container with layers of water, clay (2.5 cm), silt (2 cm), and sand (2.5 cm). A ruler beside it indicates the measurements.
After shaking, soil components settle at different speeds, leading to clear layers that can be measured
  • In the example above:

    • The sand layer is 2.5 cm

    • The silt layer is 2 cm

    • The clay layer is 2.5 cm

    • The total thickness is 7 cm

  • These measurements can be used to calculate the percentage of each soil component:

    • The percentage of sand is (2.5 ÷ 7) × 100 = 35.7%

    • The percentage of silt is (2 ÷ 7) × 100 = 28.6%

    • The percentage of clay is (2.5 ÷ 7) × 100 = 35.7%

Influence of soil texture on primary productivity

  • Soil texture affects primary productivity by influencing:

    • Nutrient availability

    • Water retention

    • Soil aeration

  • Nutrient retention vs. leaching:

    • Humus contributes significantly to the nutrient content of soils

    • It lies beneath leaf litter and has a loose, crumbly texture

    • It is formed by the partial decay of dead plant material

    • Soils with more humus retain nutrients better

    • Less humus means nutrients are more likely to be washed away

      • For example, forest floors, like those in the New Forest in Hampshire, UK, have rich humus layers that support diverse plant life

  • Water retention vs. drainage:

    • Clay and humus-rich soils retain water well

    • Sandy soils drain quickly but may not retain enough moisture for some plants

      • For example, sandy soils in East Anglia, UK, require more frequent irrigation for crops

  • Aeration vs. compaction or waterlogging:

    • Well-aerated soils support root growth and beneficial microbial activity

    • Clay soils can become compacted, limiting aeration

    • Humus helps improve aeration in clay soils

      • For example, compacted clay soils in urban areas often need organic matter added to improve their structure and aeration

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

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