Two soils can contain exactly the same ingredients and behave completely differently. One grows a rainforest, the other dries out by lunchtime. The difference is texture — how much sand, silt, clay and humus is in the mix. This page covers the jobs soil does, how texture is measured, and why texture decides how productive the land is.
📘 What you need to know
Soils perform key functions in terrestrial ecosystems: they support plant growth, biodiversity and biogeochemical cycles.
Soil is a medium for plant growth — a seed bank and a store of water and of nutrients such as nitrogen, phosphorus and potassium.
Soils provide habitats and niches, supporting high biodiversity of microorganisms, animals and fungi.
Soils recycle elements such as carbon, nitrogen and phosphorus, mainly through decomposition of dead organic matter.
Soils can act as carbon sinks, stores or sources, depending on environmental conditions.
Soil texture is the physical make-up of a soil: the proportions of sand, silt, clay and humus.
Texture is determined using a soil key, a feel test or a laboratory settling test.
Texture affects nutrient availability, water retention and aeration, and therefore primary productivity.
What soil actually does
Soils carry out important functions in terrestrial ecosystems. Three big ones show up in exam questions.
1. A medium for plant growth
Soil acts as a natural seed bank, giving seeds a substrate to germinate in and roots something to grow into.
It stores water, which plants need for hydration, nutrient uptake and photosynthesis.
It stores essential nutrients such as nitrogen, phosphorus and potassium, which support healthy growth.
The Amazon rainforest is the standard example: fertile soils with high nutrient levels support extraordinarily diverse plant life, which is part of why it is the world’s largest tropical rainforest.
2. A contribution to biodiversity
Soils provide habitats and niches for a huge range of species.
Soil communities support high biodiversity — microorganisms, animals and fungi.
Ancient woodlands in the UK, for instance, have soils rich in biodiversity, including rare fungal species that do important work in nutrient cycling.
3. A 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, and when it decomposes it releases nutrients back into the soil for the next generation of plants.
Notice that all three functions lean on the same thing: organic matter. It holds the water, it releases the nutrients, and it feeds the organisms. If you are ever stuck on a soil question, ask what the organic matter is doing — the answer is usually in there.
Soil and carbon
Soils can be carbon sinks, stores or sources, and which one they are depends on the conditions. It comes down to a race between two things: how fast dead organic matter arrives, and how fast decomposers break it down.
The balance
organic matter added faster than it rots → carbon builds up → sink
organic matter rots faster than it is added → carbon released → source
Ecosystem
Carbon in the soil
Why
Tropical forest
Generally low storage
Warm, moist conditions speed up decomposition by microorganisms, so carbon returns to the atmosphere quickly
Tundra, wetlands, temperate grassland
Large amounts accumulated
Cold temperatures and waterlogged conditions slow decomposition, so organic matter builds up without being fully broken down and released as CO2
Why this matters beyond the exam. A soil that is currently a store can flip to a source if conditions change. Warm a frozen peatland or drain a wetland and decomposition speeds up, releasing carbon that has been locked away for thousands of years. That is a positive feedback loop, and you can use it in essay questions on climate change.
Soil texture
Soil texture describes the physical make-up of a soil. It depends on the proportions of sand, silt, clay and humus in the mix, and it influences almost every other soil property, including how well plants grow.
Humus is the odd one out. Sand, silt and clay are mineral; humus is organic matter, formed by the partial decay of dead plant material, and it lies just beneath the leaf litter.
🧩
Feel the difference
Sand is gritty, silt is smooth, clay is sticky. Gritty, smooth, sticky — largest to smallest. Say it in that order and the particle sizes come with it for free.
Working out soil texture
There are three methods, and each tells you something about the soil’s properties and how suitable it is for different plants and crops.
Method 1: using a soil key
A soil key is the more systematic and detailed method.
It is a step-by-step guide that classifies texture against specific criteria.
It works out the proportions of sand, silt and clay by leading the tester through a series of questions or observations, often about how the soil behaves when moistened and worked between the fingers.
Soil keys are used in formal or scientific settings where precise classification is needed.
Method 2: the feel test
The simpler method: rub moistened soil between your fingers and judge the texture.
It is quick and informal, needs no equipment, and can be done in the field.
Commonly used by farmers, gardeners and anyone needing a quick assessment.
Method 3: the laboratory settling test
This is the one that gives you numbers, so it is the one that turns up in calculation questions.
🧩 The settling test, step by step
Clean the sample. Remove any large debris such as rocks, roots or lumps of organic matter.
Add the soil to a transparent container.
Add water and shake the container vigorously, so every particle is separated and suspended.
Leave it undisturbed on a flat surface, typically for 24 hours.
Let it settle into layers. The heaviest, largest particles drop first, so sand forms the bottom layer, then silt, and the very fine clay settles last, on top.
Measure the thickness of each layer with a ruler, then work out each one as a percentage of the total.
The layers give a clear visual picture of the proportions of sand, silt and clay in a sample. Measure from the boundary lines, not from the outside of the jar.
One thing to be careful about: you will occasionally see it written that silt settles first. Think about why that cannot be right. Big, heavy grains fall through water fastest, so sand hits the bottom first, silt follows, and the microscopically fine clay stays suspended longest and lands on top. If a diagram and a sentence ever disagree, trust the physics.
Texture and primary productivity
Soil texture affects primary productivity by influencing three things: nutrient availability, water retention and soil aeration. Each one is a trade-off.
Sand and clay fail in opposite directions: one loses water too fast, the other refuses to let it go. A loam does both jobs adequately, which is why it out-produces both.
Nutrient retention against leaching
Humus contributes a great deal to the nutrient content of soils. It lies beneath the leaf litter, has a loose, crumbly texture, and 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 by leaching.
Forest floors such as those in the New Forest in Hampshire, UK, have rich humus layers that support diverse plant life.
Water retention against drainage
Clay and humus-rich soils retain water well, because the gaps between particles are small and water is held in them.
Sandy soils drain quickly, so they may not hold enough moisture for some plants.
Sandy soils in East Anglia, UK, need more frequent irrigation for crops as a result.
Aeration against compaction and waterlogging
Well-aerated soils support root growth and beneficial microbial activity, because both need oxygen.
Clay soils can become compacted, which limits aeration.
Humus helps improve aeration in clay soils by opening up the structure.
Compacted clay soils in urban areas often need organic matter added to improve their structure and aeration.
Property
Sandy soil
Clay soil
Particle and pore size
Large particles, large pores
Very fine particles, very small pores
Drainage
Fast, sometimes too fast
Slow, can waterlog
Water retention
Poor, dries out quickly
Good, holds water tightly
Nutrient retention
Poor, nutrients leach away
Better, but nutrients can be locked up
Aeration
Good, plenty of air spaces
Can be poor, especially if compacted
Effect of adding humus
Improves water and nutrient retention
Improves structure and aeration
Humus is the fixer. It is the one addition that improves both extremes: it helps a sandy soil hold on to water and nutrients, and it opens up a clay soil so air can get in. If a question asks how to improve a poor soil, adding organic matter is almost always a valid answer.
Worked examples
WE 1
Calculate soil texture from a settling test
A settling test gives a sand layer of 4.4 cm, a silt layer of 2.4 cm and a clay layer of 1.2 cm. Calculate the percentage of each component. (3 marks)
Step 1: find the total4.4 + 2.4 + 1.2 = 8.0 cm of soil in total. Ignore the water layer.
Step 2: divide each layer by the total, then multiply by 100sand = (4.4 ÷ 8.0) × 100 = 55%silt = (2.4 ÷ 8.0) × 100 = 30%clay = (1.2 ÷ 8.0) × 100 = 15%Step 3: check55 + 30 + 15 = 100%55% sand, 30% silt, 15% clayalways add your three answers up — if they do not make 100 you have used the wrong total
WE 2
Explain the effect of texture on productivity
Explain how soil texture influences the primary productivity of an ecosystem. (4 marks)
Point 1: nutrients
Texture affects nutrient availability. Soils with more humus retain nutrients, while soils with little humus lose them by leaching.
Point 2: water
Texture affects water retention. Clay and humus-rich soils hold water well; sandy soils drain quickly and may not keep enough moisture for plants.
Point 3: air
Texture affects aeration. Well-aerated soils support root growth and microbial activity, while compacted clay soils limit the oxygen available.
Point 4: link it back
Plants need water, nutrients and oxygen at the roots, so a soil that supplies all three supports faster growth and therefore higher primary productivity.
Nutrients + water + aeration → rate of plant growththe final link is the mark most people drop — do not stop at describing the soil, connect it to plant growth
WE 3
Compare carbon storage in two soils
Suggest why tropical forest soils generally store less carbon than the soils of temperate wetlands. (3 marks)
Point 1: tropical conditions
Warm, moist conditions accelerate decomposition of organic matter by microorganisms.
Point 2: the consequence
Carbon is released back to the atmosphere quickly as carbon dioxide, so little accumulates in the soil.
Point 3: the contrast
In cold or waterlogged soils, decomposition is slow, so organic matter builds up over time without being fully broken down, and carbon is locked away.
It is the rate of decomposition, not the rate of plant growth, that decides soil carbona rainforest produces huge amounts of biomass — the point is that it rots almost as fast as it falls
💡 Exam tips
Learn the three soil functions as headings: plant growth, biodiversity, biogeochemical cycles.
Use the words sink, store and source correctly when writing about soil carbon.
In texture calculations, divide by the total soil depth and leave the water layer out.
Quote the feel test properly: gritty, smooth, sticky for sand, silt and clay.
Know why each method is used — soil key for precision, feel test for speed, settling test for numbers.
Whenever a question asks about improving a soil, organic matter is a safe and creditworthy answer.
⚠ Common mistakes
Getting the settling order backwards. The largest, heaviest particles sink fastest, so sand is the bottom layer and clay is on top.
Including the water layer in the total when calculating percentages. Only the settled soil counts.
Saying clay soils are simply bad. They hold water and nutrients well; the problem is aeration and drainage.
Saying sandy soils are simply good because they drain. They dry out and lose nutrients by leaching.
Assuming a rainforest must have carbon-rich soil. The biomass is huge, but decomposition is so fast that the soil store stays low.
Forgetting humus in the definition of texture. Texture depends on sand, silt, clay and humus.
That is the whole of 5.1 Soil. The two notes fit together as one argument: soil is a system built from four ingredients, and the proportions of those ingredients decide everything the system can do. Up next: Land Use and Farming Systems — how much land there is to farm, who gets to use it, and the different ways farms are organised around the world.
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