Everything a soil does comes back to particle size. Sand, silt and clay behave differently because they are different sizes, and once you understand that, the whole properties table writes itself. This page also takes you through the soil texture triangle, which is the most commonly lost mark in the whole of Topic 5.
📚 What you need to know
Soil is mineral particles, organic matter, water and air. The mineral particles are sand, silt and clay.
Sand is under 2 mm, silt under 0.02 mm, clay under 0.002 mm.
Cation-exchange capacity (CEC) is a soil’s ability to hold and release nutrient ions. Clay has a high CEC; sand has a low one.
Texture is the proportions of particle sizes. Structure is how those particles are arranged.
Loam is a balanced mix and has the highest potential primary productivity.
A soil texture triangle converts three percentages into a named textural class. The three must total 100%.
The three particle sizes
Particle size controls two things: the size of the gaps between particles, and the total surface area available for holding onto things. Big particles mean big gaps and little surface area. Small particles mean tiny gaps and enormous surface area.
The circles are not to scale — a real clay particle is about a thousand times smaller than a sand grain.
Sand
The largest particles, mostly derived from quartz. Big particles leave big pore spaces, so water runs straight through. Sand also has a low cation-exchange capacity, which means it cannot hold onto nutrient ions, so they leach away with the drainage water.
Silt
Medium-sized and also often from quartz. Silt feels smooth, holds more water than sand but drains better than clay, and has a relatively low CEC. Its weakness is that it compacts easily if it is worked while wet, which wrecks the structure.
Clay
The smallest particles, and the most chemically active. Because clay particles have a huge surface area for their volume, they have a high CEC and can hold and release nutrient ions such as calcium, magnesium and potassium. Clay feels sticky when wet, drains poorly but retains water well.
Cation-exchange capacity is worth a sentence of your own. Nutrient ions carry a positive charge; clay and humus surfaces carry a negative charge, so they hold those ions loosely and release them to roots. That is why a soil rich in clay or humus stays fertile and a sandy soil does not.
Structure is not texture
Texture is what the particles are. Structure is the arrangement or shape they take once they clump together into crumbs and blocks. You cannot change a soil’s texture, but you can change its structure — which is why farmers add organic matter.
Clay soils
High potential for nutrient exchange because of the large surface area relative to volume. But they become waterlogged easily and are often called “cold” or “heavy”. In drought they shrink and crack, which damages structure.
Sandy soils
Excellent drainage and easy to work, which is why they are called “light” soils. The trade-off is poor water and nutrient retention.
Silt soils
Compact very easily if ploughed when wet. Once compacted, the pore spaces close, aeration falls and root growth suffers.
Loam soils
A balanced mix of sand, silt and clay. Easy to work, drains well, retains moisture and nutrients, well aerated. This is why loam has the highest potential primary productivity.
Property
Sand
Loam
Clay
Nutrient status
Poor
Moderate
Good
Water infiltration rate
High
Medium
Low
Water holding capacity
Low
Medium
High
Aeration
Good
Moderate
Poor
Potential to hold organic matter
Low
Medium
High
Ease of working
Good
Moderate
Poor
Learn this table as a shape rather than as words. Sand is high for drainage and low for everything to do with holding on. Clay is the mirror image. Loam is in the middle everywhere, which is exactly why it is the best.
Reading the soil texture triangle
The triangle turns three percentages into one named textural class. Each side is one particle size, and the three values must add up to 100%.
A simplified triangle. The three dashed lines cross at one point, and the zone that point falls in is your textural class.
🧩 How to read it without losing marks
Check the three values total 100%. If they do not, you have misread the question.
Find your clay percentage on the left axis and follow the line horizontally across.
Find your sand percentage on the bottom axis and follow the line parallel to the right-hand side.
Find your silt percentage on the right axis and follow the line parallel to the left-hand side.
All three should meet at one point. If they do not, you have followed a line in the wrong direction.
Name the zone that point sits in. That is your textural class.
EXAM QUESTION
A soil sample contains 40% sand and 20% clay. Determine its silt content and its textural class. [3]
The three must total 100silt = 100 − 40 − 20 = 40%Now read the triangleFollow 20% across from the clay axis, 40% up from the sand axis and 40% in from the silt axis.All three lines meet in the middle of the triangle.Name the zoneThe textural class is loam
The other properties you should know
Texture is the headline, but the syllabus lists several more soil properties. Each one is a short definition plus what it tells you.
Property
What it measures
What it tells you
Organic matter content
How much decomposed plant and animal material is present
Improves structure, water-holding capacity and nutrient availability
Water content
How much water the soil can hold
Depends on texture and organic matter; controls how long plants can go between rains
Infiltration rate
How quickly water enters the soil
Fast in sandy soils, slow in clay-rich ones; slow infiltration means more surface runoff
Bulk density
Mass of soil per unit volume
A measure of compaction. Lower bulk density means better structure and easier root penetration
Colour
The visible shade of the soil
Dark means organic matter and fertility; red or yellow means iron oxides and aeration; greyish-blue means waterlogging
pH
How acidic or alkaline the soil is
Most plants prefer pH 6 to 7. Below pH 6 nutrient availability is limited; above pH 7 nutrient imbalances are common
EXAM QUESTION
Explain why a loam soil generally supports higher primary productivity than a sandy soil or a clay soil. [4]
Define loam
Loam is a balanced mixture of sand, silt and clay, so it combines the advantages of all three.
Compare against sandSandy soils drain too quickly and have a low CEC, so water and nutrients are lost by leaching.Compare against clayClay soils hold nutrients well but waterlog easily and compact, so aeration falls and roots struggle.Then the conclusionLoam drains freely enough to stay aerated while still retaining moisture and nutrients, so plant growth is not limited by any one of the three.Balance is the whole answer
💡 Exam tip
Always check your three percentages sum to 100 before touching the triangle. It is the fastest way to catch a misread.
If a question gives you only two values, the third is 100 minus the other two. That is usually a mark on its own.
Say “cation-exchange capacity” in full at least once, then abbreviate. It reads as though you know what it means.
Link every property back to a consequence for plants. Properties on their own do not earn explanation marks.
Use the sand-loam-clay comparison table as your structure for any “compare these soils” question.
⚠ Common mix-up
Reading the triangle in the wrong direction is the classic error. Each axis is read along a specific direction, and following the wrong one puts you two zones away.
High CEC does not mean high fertility on its own. Clay holds nutrients well but may still be a poor growing medium because of drainage and aeration.
Texture and structure are different words. Texture is fixed; structure can be improved by adding organic matter.
Bulk density is not the same as particle density. Bulk density includes the pore spaces, which is exactly why it indicates compaction.
“Light” and “heavy” soils refer to how easy they are to work, not to their actual weight. Sandy soils are called light; clay soils are called heavy.
Up next: Soil as a Carbon Store — the last section, and the one that links soil straight back to climate change and tipping points.
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