What is the soil made of, grain by grain?: Soil texture is the physical make-up of the mineral soil. Big grains feel gritty, middle-sized grains feel smooth, and the tiniest grains feel sticky.
The points to remember
- Soil texture = the physical make-up of the mineral soil: the relative proportions of sand, silt and clay.
- Particle sizes: sand 2 to 0.05 mm (gritty), silt 0.05 to 0.002 mm (smooth), clay under 0.002 mm (sticky).
- Sand: big particles, big pores: drains fast, well aerated, holds little water, warms quickly in spring.
- Clay: tiny particles, tiny pores: drains slowly, easily waterlogged, holds water, slow to warm, hard when dry.
- Loam: a balanced mix of sand, silt and clay; it drains well AND holds water and nutrients.
- Where it is abundant, humus changes texture too: it makes soil dark, crumbly and spongy.
Sandy soil
- Large particles (2 to 0.05 mm)
- Good drainage and aeration
- Low water and nutrient retention
- Warms up quickly in spring
Clay soil
- Tiny particles (under 0.002 mm)
- Poor drainage, easily waterlogged
- High water and nutrient retention
- Slow to warm, hard when dry
Remember it as: Sand is gritty, silt is silky, clay is sticky.
Real example: China's Loess Plateau is covered in loess, mostly silt. It feels smooth like flour, and because the silt grains do not stick together, it washes away easily in heavy rain.
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The quickest way to find a soil's texture needs no equipment: the feel test.
The points to remember
- Take a teaspoon of soil, remove stones and roots, and wet it until it is like putty.
- Rub it between finger and thumb: gritty = sand, smooth and silky = silt, sticky = clay.
- Try to roll it into a ball: sandy soil falls apart; loam and clay hold together.
- Squeeze it out into a ribbon: the longer the ribbon before it breaks, the more clay.
- Quick, cheap and done in the field, but subjective (it depends on the person) and gives no exact percentages.
Gritty, falls apart
Mostly sand: no ball, no ribbon.
Smooth, short ribbon
Silt or loam: holds a ball, ribbon under about 2.5 cm.
Sticky, long ribbon
Clay: a shiny ribbon more than about 5 cm long.
Real example: the UK's Royal Horticultural Society teaches gardeners this test: a soil that can be rolled into a long, thin sausage and bent into a ring without cracking is a clay soil.
A limitation to name: The feel test is subjective: two people may judge the same soil differently, and it gives a name, not percentages. Practice and a key make it more reliable.
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A texture key turns the feel test into a fixed set of questions, so everyone reaches the same answer in the same way.
The points to remember
- A key is a list of yes or no questions; each answer sends you to the next question.
- The questions come from the feel test: does it hold a ball? Does it make a ribbon? How long? How does it feel?
- The last answer gives a texture name: sand, loamy sand, sandy loam, loam, silt loam, clay loam or clay.
- Keys make the feel test more consistent: everyone asks the same questions in the same order.
| Question | If yes | If no |
|---|---|---|
| 1. Does the moist soil hold together in a ball? | Go to 2 | Sand |
| 2. Can it be pressed into a ribbon? | Go to 3 | Loamy sand |
| 3. Is the ribbon longer than about 5 cm? | Clay or clay loam: go to 4 | Go to 5 |
| 4. Does it feel very sticky? | Clay | Clay loam |
| 5. Does it feel mainly smooth and silky? | Silt loam | Loam or sandy loam (gritty) |
Real example: the famous vineyard of Petrus in Pomerol, France, grows on a patch of heavy blue clay. A moist lump holds a ball (yes), makes a ribbon (yes) longer than 5 cm (yes) and feels very sticky (yes): clay.
Say which method: If asked how texture can be found, name the method (a key, a feel test or a sedimentation test) and say in one line how it works.
Let the water sort the grains: In a sedimentation test, heavy sand sinks first and tiny clay particles sink last. Measure the layers to get percentages.
The points to remember
- Put soil in a tall jar, add water (and a little detergent), shake hard and leave it to settle.
- Particles settle by size: sand first (bottom, within a minute), silt next (hours), clay last (top, a day or more).
- Measure the depth of each layer and the total depth of sediment.
- % of a particle = its layer depth / total depth x 100. Divide by the TOTAL, not by 100 mm.
- In a laboratory, the same idea is done with exact timings and weights: more accurate than a feel test.
Remember it as: Sand sinks first, silt next, clay last.
Real example: the US Department of Agriculture's soil service teaches farmers this jar test: shake, wait a minute and mark the sand, wait a few hours and mark the silt, wait a day or two for the clay.
Divide by the total: If the sediment is 60 mm deep and the sand layer 24 mm, sand = 24 / 60 x 100 = 40%. Show the calculation: a number alone may not score.
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Once you have the three percentages, the soil texture triangle gives the soil its name.
The points to remember
- The soil texture triangle turns the three percentages into a texture name.
- Clay % is read on the left side, silt % on the right side, sand % along the bottom.
- Follow each value into the triangle along its own grid lines; where the lines meet, read the name.
- Two values are enough: the third is 100 minus the other two.
- Write one name only: if you write two, only the first counts.
Real example: the soils of the Nile Delta in Egypt formed from mud left by floods: they are mostly clay and silt, so they plot near the top right of the triangle, in the clay and silty clay areas.
Check the sum: Clay + silt + sand must add up to 100%. If your three readings do not, you followed a line the wrong way.
How this comes up: Paper 2, Section A: using a sedimentation test, calculate percentages [1] each. Show the working.
The figure shows a sedimentation test on a garden soil.
Using the figure, calculate the percentage of sand and the percentage of silt in the soil.
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