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

• 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