Agriculture and Food 7–12 · Years 7–8

Soil in a jar: settling out sand, silt and clay

Food and agricultural practices

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Soil particles of different sizes settle through water at different speeds, so a shaken soil sample separates into layers, coarse sand at the bottom and fine clay on top, and particle size is what separates this mixture.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

soil may contain sharp objects and animal droppings
dispersant dust is a mild eye irritant

Controls

wear gloves when collecting and sieving soil and wash hands afterwards
use plastic jars
the teacher weighs and adds the dispersant, with eye protection, and closes the jar before it is handled further
learners handle only closed jars, if the site-specific risk assessment allows it

Note

Sodium hexametaphosphate is not listed in NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update, Volume 2 Appendix D. Under the Appendix D flow chart (Figure D-4) an unlisted chemical "must be regarded as an N category chemical" and must not be used "until a site specific risk assessment is completed by an approved user", with the control strategies approved by a supervisor. The CSIS conditions of use for an N-category chemical are that it is not recommended for use in schools and that its use in teaching and learning is restricted to approved teachers for demonstration purposes only. The hazards and controls listed here are what this entry puts to that assessment, not a substitute for it: the assessment decides whether the dispersant may be used at all, who may weigh and add it, and whether learners may handle the closed jars. Where the assessment does not allow learner handling, the teacher prepares and shakes the jars as a demonstration and learners mark, measure and interpret them. A school that cannot meet those conditions runs the field ribbon test on the same soils instead.

What you need

air-dry soil sieved to remove gravel and roots larger than 2 mm, 100 mL per jar
1 L straight-sided clear plastic jar or 1 L measuring cylinder with a lid or stopper
sodium hexametaphosphate dispersant, 5 g per jar
tap water to 2 cm below the top of the jar
ruler graduated in millimetres taped vertically to the jar
stop clock, permanent marker

How to do it

Sieve the soil and remove the fraction larger than 2 mm, as for a laboratory particle size analysis.
Put 100 mL of soil in the jar. The teacher weighs and adds the 5 g of dispersant and fills with water to 2 cm below the top, under the conditions the site-specific risk assessment sets, then closes the jar.
Close the jar and shake it hard for 2 minutes, then stand it on a level bench and start the clock.
Watch the bottom of the jar and mark the top of the settled layer when it stops visibly thickening: this first layer is mostly sand.
Mark the top of the settled layer again the next day: the layer added since the first mark is finer material, mostly silt with some clay.
After 3 days mark the top of the final layer, which is the finest material (clay), and note whether the water is still cloudy.
Measure each layer thickness, express each as a percentage of the total settled thickness, and compare the result with the field ribbon test on the same soil.
Explain the order of the layers by particle size, and why pouring off the cloudy water (decanting) separates the finest particles from the rest.

What you should see

A coarse, gritty layer of sand forms within minutes, finer material keeps settling for hours, and the clay settles over days; very fine clay can keep the water cloudy for longer. Each layer is mixed: the jar was shaken, so fine particles that started near the bottom reach the floor early and are buried with the coarse ones, and after 3 days some of the finest clay is still in the water. The layer thicknesses are therefore only a rough guide to the sand, silt and clay in the soil, and the layers also pack differently. A soil that ribbons as a sandy loam (15–25 mm ribbon, 10–20% clay) shows a thick bottom layer and a thin fine layer; a clay loam shows a much thicker fine layer. The learner knows it worked when the layers appear in the order coarse to fine from the bottom up and repeat jars of one soil give a similar pattern.

What changes

What you change: soil sample (for example a vegetable bed and a paddock)
What you measure: thickness of each settled layer as a percentage of the total settled thickness
What you keep the same: 100 mL of sieved soil
water volume
5 g of dispersant
2 minutes of shaking
same jar size and water depth

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Soil is all the same material, only different colours.
Clay soils feel heavy because clay particles are the largest.
Once the water looks clear on top, every particle has settled.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agricultural Technology Years 7–10 Syllabus (2019)AG4-6 AG4-11 AG4-12
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9AC9TDE8K04AC9S7U06

Sources

The pages the author read to write this activity.

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