Syllabus Edition
First teaching 2024
First exams 2026
Functions & Properties of Soils (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
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
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
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
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