Soil does four jobs for an ecosystem, and how well it does them comes down to one thing: what it is made of. This page links the components you met last time to the way a soil actually behaves — and shows you the settling test, which is a favourite for data questions.
📚 What you need to know
Soils are a medium for plant growth, a habitat for huge biodiversity, part of the biogeochemical cycles, and a carbon store.
A soil can act as a carbon sink, store or source depending on conditions.
Soil texture is the proportion of sand, silt, clay and humus.
Texture can be judged with a soil key, a feel test or a laboratory settling test.
Texture controls nutrient retention, water retention and aeration, and so controls primary productivity.
You may be asked to calculate percentages from settling-test layer thicknesses. Practise it.
The four jobs soil does
When a question asks about the functions of soil, do not just say “plants grow in it”. There are four distinct roles, and each one earns its own mark.
Four functions, four different kinds of evidence you can use in an answer.
A medium for plant growth
Soil physically holds a plant up, and it acts as a natural seed bank — a store of seeds waiting for the right conditions. It also holds the water a plant needs for photosynthesis and transport, and the nutrients it needs to build tissue. Nitrogen, phosphorus and potassium are the three to name.
A habitat
Soil is one of the most species-rich habitats on Earth, and almost all of it is invisible. Bacteria, fungi, mites, worms and insects live in the pore spaces and in the organic layer. Ancient woodland soils in the UK are a good example: they support rare fungi that took centuries to establish and that do not come back if the woodland is cleared.
Part of the biogeochemical cycles
Carbon, nitrogen and phosphorus all pass through soil. Dead plant material falls in, decomposers break it down, and the elements are released in a form living things can use again. Without that step the cycles stall.
A carbon store — or a carbon source
This one is worth understanding properly because it comes back at HL. Whether a soil gains or loses carbon depends on the balance between how fast dead matter arrives and how fast it decomposes.
Soils that build carbon up
Tundra, wetlands and temperate grassland.
Cold or waterlogged, so decomposers work slowly.
Organic matter accumulates faster than it breaks down.
Soils that release carbon
Tropical rainforest soils, despite the forest above them.
Warm and moist, so decomposition is very fast.
Carbon returns to the atmosphere almost as fast as it arrives.
The rainforest example catches people out. Students assume a huge forest must sit on huge soil carbon. It does not — the carbon is in the trees, and the soil turns organic matter over so quickly that little accumulates.
Soil texture
Soil texture means the proportions of sand, silt, clay and humus in a soil. It is a physical description, and it is the single most useful thing to know about a soil because almost every other property follows from it.
Sand feels gritty. Silt feels smooth, almost like flour. Clay feels sticky when wet and can be rolled into a thread. Humus is dark, crumbly and light, formed from partly decayed plant material.
Three ways to work out texture
A soil key. A step-by-step set of questions about how the soil behaves when you moisten it and work it in your hands. Systematic and repeatable, so it is used where a precise classification matters.
The feel test. Rub moistened soil between your fingers and judge it by touch. Quick, needs no equipment, done in the field. Farmers and gardeners use it. Less reliable because it depends on the person.
The laboratory settling test. Shake the soil with water, let it settle into layers, and measure them. Slower, but it gives you numbers you can work with.
The settling test, step by step
🧩 Method
Remove any large debris — stones, roots, twigs, lumps of undecayed matter.
Put the soil sample into a transparent container with a lid.
Add water and shake vigorously so all the particles separate.
Stand the container on a flat surface and leave it undisturbed, usually for about 24 hours.
The particles settle by size. Sand settles first at the bottom because it is heaviest, then silt, then clay on top.
Measure the thickness of each layer and convert to percentages.
Sand is heaviest so it settles first. Clay is finest and stays suspended longest, so it ends up on top.
EXAM QUESTION
A settled soil sample has a sand layer 2.5 cm thick, a silt layer 2.0 cm thick and a clay layer 2.5 cm thick. Calculate the percentage of each component. [3]
Find the total first2.5 + 2.0 + 2.5 = 7.0 cmThen each layer as a share of the totalsand = (2.5 ÷ 7.0) × 100 = 35.7%silt = (2.0 ÷ 7.0) × 100 = 28.6%clay = (2.5 ÷ 7.0) × 100 = 35.7%Check35.7 + 28.6 + 35.7 = 100.0, so the answer is consistent.Always check the three add to 100
Why texture changes what grows
Texture affects primary productivity through three routes. Learn them as a set, because a good answer walks through all three.
Route
What texture does
Effect on plants
Nutrient retention
Humus and clay hold onto nutrients; sandy soils with little humus do not
Low-humus soils lose nutrients to leaching, so growth is limited
Water retention
Clay and humus hold water; sand drains fast
Sandy soils may dry out; clay soils may waterlog
Aeration
Pore spaces let oxygen reach roots; clay compacts and closes them
Compacted or waterlogged soils suffocate roots and slow microbes
Real places to use as examples
Forest floors such as the New Forest in Hampshire build up thick humus layers, which hold nutrients and support varied plant life.
The sandy soils of East Anglia drain so freely that crops there need irrigating more often.
Compacted clay soils in urban areas often have to have organic matter dug in before anything will grow well, because the structure has closed up.
A named place turns a generic point into a marked one. You only need three or four of these for the whole topic, so pick ones you can spell and remember.
EXAM QUESTION
Explain how soil texture influences primary productivity. [4]
Say what texture is
Texture is the proportion of sand, silt, clay and humus in the soil.
Then take the three routes in turnNutrients: clay and humus hold nutrient ions, so plants have a steady supply; sandy soils lose them to leaching.Water: clay and humus retain water for uptake, while sandy soils drain too quickly to sustain some crops.Air: well-aerated soils supply oxygen to roots and microbes, but compacted clay limits this and slows growth.Finish with the linkLoam, a balance of all three particle sizes, therefore supports the highest productivity.Texture → nutrients, water, air → productivity
💡 Exam tip
Show your working in settling-test calculations. The total is often worth a mark on its own.
Round consistently, usually to one decimal place, and check your percentages sum to about 100.
If a question says explain, give the mechanism. “Sandy soil is bad” earns nothing; “sandy soil drains fast so nutrients leach out of the root zone” earns the mark.
Learn one named example per function of soil. It is the difference between a level 2 and a level 3 answer on the longer questions.
Sink, store and source are three different words. A store just holds carbon; a sink is gaining it; a source is losing it.
⚠ Common mix-up
Clay settles last, not first. The finest particles stay suspended longest, so clay ends up as the top layer.
Texture is not structure. Texture is the proportions of particle sizes; structure is how those particles are arranged into crumbs and clumps.
Humus is not the same as dead organic matter. Humus is what dead organic matter becomes after partial decay.
A rainforest soil is not a big carbon store. The carbon is in the biomass; the soil turns over too fast.
Up next: Reading a Soil Profile — the HL section, where you cut down through a soil and learn to name and interpret each layer.
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