Soil texture and the textural triangle
What sand, silt and clay are, how they are measured, how to read the textural triangle, and why texture decides water, nutrients and tillage.
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Soil texture is the proportion of sand, silt and clay in the mineral part of a soil. It is the one soil property a farmer cannot change at field scale, and it quietly decides almost everything else: how much water the soil holds, how fast water drains, how many nutrients it can store, how easily it is ploughed and how readily it erodes.
That is why texture is the first thing a soil scientist records and one of the first topics in every soil science syllabus. The textural triangle, which turns three percentages into a named class, is a favourite exam exercise from school level to professional licensing.
Sand, silt and clay: the separates
The mineral particles of soil are grouped by size into soil separates. Only the fraction finer than 2 mm, the fine earth, counts towards texture; gravel and stones are recorded separately.
| Separate | Diameter (USDA system) | Diameter (international system) | Feel when moist |
|---|---|---|---|
| Sand | 2 – 0.05 mm | 2 – 0.02 mm (coarse 2–0.2, fine 0.2–0.02) | gritty |
| Silt | 0.05 – 0.002 mm | 0.02 – 0.002 mm | smooth, floury, like talc |
| Clay | below 0.002 mm (2 µm) | below 0.002 mm | sticky and plastic |
Two systems are in common use. The USDA system is used with the textural triangle in most of the world's soil survey and in many textbooks. The international (Atterberg) system, adopted by the International Society of Soil Science, sets the sand–silt boundary at 0.02 mm. Both agree on the most important limit: clay is smaller than 2 micrometres. Always check which system a question or a laboratory report uses.
Why particle size matters so much
Size controls surface area. Cut a 1 mm cube into cubes 1 µm across and the same volume has a thousand times more surface. Clay particles are also flat plates, and some clay minerals have internal surfaces too, so a gram of clay can carry hundreds of square metres of surface, while a gram of sand carries a fraction of one.
Almost every active property of soil happens on surfaces: holding water films, holding nutrient cations, binding particles into aggregates. So clay dominates soil behaviour far beyond its share of the weight. Sand and silt are mostly quartz and other resistant minerals: they form the skeleton, create the large pores, and hold little water or nutrient themselves.
Measuring texture
In the laboratory
Particle-size analysis first destroys the things that glue particles together, organic matter (usually with hydrogen peroxide) and sometimes carbonates or iron oxides, then disperses the clay with a chemical dispersant, commonly sodium hexametaphosphate, and vigorous shaking. Sand is separated by sieving. Silt and clay are separated by how fast they settle in water, which follows Stokes' law:
Here ρs is particle density (about 2.65 g/cm3 for mineral soil), ρf the density of water, g gravity, r the particle radius and η the viscosity of water. The key point is that velocity rises with the square of the radius: halve the size and settling is four times slower.
Two methods use it. The pipette method draws a small sample from a fixed depth after a calculated time and weighs what is still suspended. The hydrometer method (Bouyoucos) reads the density of the suspension: an early reading, after about 40 seconds, measures silt plus clay still in suspension; a reading after a couple of hours measures clay alone. The pipette is more precise; the hydrometer is faster and cheaper, and is what most routine laboratories use.
How long does clay take to settle?
Take a clay particle 2 µm across (r = 1 × 10-6 m), density 2,650 kg/m3, in water at 20 °C (density 1,000 kg/m3, viscosity 0.001 Pa·s).
v = 2 × (2,650 − 1,000) × 9.81 × (1 × 10-6)2 ⁄ (9 × 0.001) = 3.6 × 10-6 m/s
To fall 10 cm it needs 0.10 ⁄ (3.6 × 10-6) ≈ 27,800 s, almost 8 hours. A silt particle 50 µm across is 25 times wider, so it settles 625 times faster and covers the same 10 cm in about 44 seconds. This is why a hydrometer reading at about 40 seconds separates sand from the rest.
In the field: texture by feel
A trained surveyor estimates texture in a minute. Wet a walnut-sized lump to the consistency of putty, then squeeze it between thumb and finger into a ribbon.
- Gritty, falls apart, no ribbon: sand or loamy sand.
- Smooth and silky, weak ribbon: high in silt.
- Ribbon under 2.5 cm: a loam group; 2.5–5 cm: a clay loam group; over 5 cm and shiny when rubbed: a clay group.
- Then judge whether it feels gritty (sandy), smooth (silty) or neither, to name the class within the group.
With practice the feel method is within one class of the laboratory most of the time. It is how most of the world's soil maps were made.
Reading the textural triangle
The textural triangle puts sand, silt and clay on the three sides of an equilateral triangle. Any point inside it represents one mixture whose three percentages add up to 100, and the triangle is divided into twelve texture classes.
To place a soil:
- Find its clay percentage on the clay side and follow that line parallel to the base.
- Find its sand percentage on the base and follow the line running up and to the left, parallel to the silt side.
- The point where they cross is the soil. Check it with the silt percentage: the third line must pass through the same point.
- Read the class name of the area the point falls in. A point exactly on a boundary is usually given the finer class.
The triangle is not symmetric, and this is worth noticing. Loam is not one-third of each separate: a typical loam is about 40 per cent sand, 40 per cent silt and 20 per cent clay. Because clay is so active, it takes only about a fifth clay for a soil to behave as an even balance of the three. The clay class starts at 40 per cent clay, and a soil needs more than 85 per cent sand to be called sand.
Naming a soil from a laboratory report
A 50 g sample of fine earth contains 20 g sand, 20 g silt and 10 g clay after dispersion.
Percentages: sand 20 ⁄ 50 = 40%, silt 40%, clay 10 ⁄ 50 = 20%. On the triangle, the 20% clay line and the 40% sand line meet inside loam.
Now suppose the field sample weighed 100 g, of which 15 g was gravel larger than 2 mm. The texture is still calculated on the 85 g of fine earth; the gravel is reported separately as 15 per cent by weight, and the soil is a gravelly loam.
What texture decides
| Property | Sandy soils | Loamy soils | Clayey soils |
|---|---|---|---|
| Water-holding capacity | low | moderate to high | high, but much is held too tightly for plants |
| Drainage and aeration | rapid, good | good | slow, poor when wet |
| Nutrient-holding (CEC) | low | moderate | high |
| Leaching risk | high | moderate | low |
| Ease of tillage | easy at almost any moisture ("light") | moderate | hard; narrow workable moisture range ("heavy") |
| Warming in spring | fast | moderate | slow |
| Erodibility by water | low to moderate (large grains resist detachment) | silty loams most erodible | low once aggregated |
Some practical consequences follow directly:
- Irrigation on sandy soil needs smaller, more frequent applications; on clay, slower application rates to avoid runoff and waterlogging.
- Fertiliser on sandy soil is best split into smaller doses, because nitrate and potassium are easily leached.
- Tillage of clay must wait for the right moisture: too wet and it smears and compacts, too dry and it forms clods.
- Silty soils form surface crusts after rain, which can stop seedlings emerging.
"Light" and "heavy" refer to the effort of ploughing, not weight: a sandy soil actually has a higher bulk density than a clay.
Texture is not structure
Texture is about the size of individual particles; structure is how those particles are grouped into aggregates (granular, blocky, prismatic, platy). Two soils of the same texture can behave very differently if one is well aggregated by organic matter and roots and the other is compacted. Texture is practically permanent; structure can be built up or destroyed by management in a few seasons. Exams often test this distinction directly.
Key points
- Texture is the percentage of sand, silt and clay in the fine earth (particles below 2 mm).
- Clay is below 0.002 mm in every system; the sand–silt limit is 0.05 mm (USDA) or 0.02 mm (international).
- Settling velocity follows Stokes' law and rises with the square of particle radius; this is the basis of the pipette and hydrometer methods.
- The textural triangle has twelve classes; loam is about 40% sand, 40% silt and 20% clay.
- Clay controls water retention, nutrient holding and stickiness out of all proportion to its share of the weight, because of its surface area.
- Texture cannot be changed in practice; structure can.
What the exams ask
- School-leaving agricultural science and university entrance papers: the size limits of the separates, the feel of each, and the meaning of "light" and "heavy" soils.
- Degree and research-fellowship entrances: naming a class from three percentages, Stokes' law and its assumptions, the principle of the hydrometer and pipette methods.
- Licensing examinations for agronomists and engineers: linking texture to available water, infiltration, irrigation design and erodibility; texture versus structure.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can I change my soil's texture by adding sand or clay?
Not at field scale. Changing a clay to a loam would take hundreds of tonnes of sand per hectare, and adding a little sand to clay can make it set harder. Adding organic matter improves the structure of both sandy and clay soils, which is what actually helps.
Why is a silt loam often called the best agricultural soil?
Silt loams hold a large amount of water that plants can actually use, drain reasonably, and are easy to work. They are also among the most erodible soils, so they need cover and careful management.
Does organic matter count in texture?
No. Texture describes only the mineral fraction. Organic matter is removed before particle-size analysis, and soils that are mostly organic (peats and mucks) are described as organic soils rather than given a texture class.
Sources
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 4, Soil architecture and physical properties.
- Soil Science Division Staff. Soil Survey Manual, USDA Handbook 18. United States Department of Agriculture, 2017. Soil texture and the texture-by-feel method.
- Gee, G. W. and Or, D. "Particle-size analysis." In Dane, J. H. and Topp, G. C. (eds), Methods of Soil Analysis, Part 4: Physical Methods. Soil Science Society of America, 2002.
- FAO. Guidelines for Soil Description, 4th edition. Food and Agriculture Organization of the United Nations, 2006.
- Hillel, D. Introduction to Environmental Soil Physics. Academic Press, 2004.
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