Soil components by the jar settling test, the ribbon test and Stokes’ law

The idea

A soil is a mixture of sand, silt, clay and organic matter whose grain sizes settle at rates set by Stokes’ law, so a shaken jar sorts the soil for you.

What you need

• three soils from different sites, about 200 g each, air-dried and crumbled
• straight-sided jars 500 mL to 1 L with lids, water, a teaspoon of dishwashing detergent as dispersant per jar
• ruler, marker, stopwatch, clock
• squeeze bottle of water for the ribbon test
• soil texture triangle (soilquality.org.au fact sheet)

How to do it

1. Fill each jar one-third with soil, add water to three-quarters and the dispersant, cap and shake for two minutes.
2. Stand the jars and mark the sediment top at 10 minutes (sand: by the model every grain of 0.02 mm or more has fallen 20 cm by 9.3 minutes), 16 hours (silt: grains of 0.002 mm or more by 15.5 hours) and 48 hours (clay); for a water column other than 20 cm, take the times from the model; measure each band in mm.
3. Convert band heights to percentages of the total settled height and read the texture class from the triangle.
4. Ribbon test: moisten a walnut of soil, work it between thumb and finger and record the ribbon length before it breaks and the feel (gritty, silky, sticky).
5. Compare the jar class with the ribbon class and with the Stokes’ law settling times from the model.

What you should see

Coarse sand settles within the first minute, fine sand down to the 0.02 mm sand–silt boundary within 9.3 minutes, silt down to the 0.002 mm silt–clay boundary within 15.5 hours, and clay stays in suspension for a day or more; a sand reading at 1 minute would count grains finer than 0.061 mm as silt, and a silt reading at 2 hours would count silt finer than 0.0056 mm as clay (computed with the model's constants for a 20 cm fall); the sandy soil shows one thick lower band and a short ribbon (under 25 mm), the clay soil a thin lower band and a ribbon over 50 mm (the soilquality.org.au texture table gives no ribbon for sand, 15 to 25 mm for sandy loam and 50 to 75 mm for light clay). Stokes’ law for quartz grains in 20 °C water (ρ = 998.2 kg/m³, μ = 1.002 mPa·s) gives a 0.02 mm silt grain 0.36 mm/s, so 20 cm in 9.3 minutes, and a 0.002 mm clay grain 3.6 µm/s, so 20 cm in 15.5 hours; a 0.2 mm sand grain would fall 3.6 cm/s, but its Reynolds number of 7 is outside the Stokes range, so sand falls somewhat slower than that figure and is on the bottom within the first minute either way.

What changes

What you change

soil sample (site)

What you measure

percentage sand, silt and clay; ribbon length

What you keep the same

• same jar diameter
• same water volume and dispersant
• same shaking time
• same reading times

Common misconceptions

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

• Soil is one substance called dirt.
• Clay is heavier than sand, so it settles first (clay grains are smaller and settle last).

Safety card

Curriculum references

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

• Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-8EES11/12-3EES11/12-4EES11/12-5
• Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-03
• Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
2. www.earthlearningidea.com/PDF/410_Soil_water_shake.pdf
3. www.earthlearningidea.com/PDF/152_Make_own_soil.pdf
4. www.soilquality.org.au/factsheets/soil-texture
5. webbook.nist.gov/chemistry/fluid