Soil texture by sedimentation: a jar test timed by Stokes' law

The idea

A soil's sand, silt and clay fractions settle out of water at very different speeds because settling speed rises with the square of grain size, and the fractions decide how a soil holds water and supports a biome's productivity.

Safety card

What you need

• 2 clear straight-sided jars (about 1 L) with lids, one per soil
• Soil from two contrasting sites (for example a garden bed and a sandy creek bank), air-dried and pushed through a 2 mm kitchen sieve; not potting mix
• 1 teaspoon of powdered laundry detergent per jar as a dispersant
• Tap water, a ruler with mm marks taped to each jar, a stopwatch, a marker
• Kitchen scales (1 g) and a 1 L measuring jug, to find each jar's soil mass and suspension volume
• The soil texture triangle the class uses, with its sand, silt and clay size boundaries read from its key

How to do it

1. Weigh each empty jar, fill it one-third with sieved soil and weigh again (soil mass, g); add the detergent, then water to three-quarters full. Find the suspension volume by filling a spare jar of the same kind to the same mark from the measuring jug (mL). Solid fraction φ = (soil mass ÷ 2.65 g/cm³) ÷ suspension volume. Measure the water height above the jar base (the fall height, about 10 cm).
2. Shake hard for 3 minutes, set the jar down and start the stopwatch.
3. Enter φ, the fall height and the sand–silt boundary size in the calculator; at the hindered settling time it gives, mark the top of the settled layer: this is the sand layer.
4. At the hindered time computed for the silt–clay boundary (hours), mark the new top of the settled layer: the layer between the two marks is silt.
5. After 48 hours, mark the final top: the layer above the silt mark is clay (the water above may still be cloudy).
6. Measure the three layer thicknesses and convert them to percentages of the total settled depth. These are settled-volume shares, not the mass fractions of the sub-2 mm soil that the texture triangle is defined on: settled clay holds far more water between its particles than settled sand does, so a split by thickness over-reads clay and can name the wrong class. Read the triangle with that in mind, label the class it gives as indicative and as read from settled volume, and use the two soils' figures to compare them with each other rather than as absolute fractions. Repeat for the second soil.

What you should see

Three layers with measured thicknesses whose percentages sum to 100, reported as settled-volume shares, and an indicative texture class for each soil, labelled as read from settled volume rather than from the mass fractions the triangle is defined on. Grains settle more slowly in the crowded jar than a single grain in clear water (hindered settling). For a 10 cm fall, Stokes' law gives single-grain times of 44.6 s for a 0.05 mm grain, 4.6 min for 0.02 mm and 7.7 h for 0.002 mm; at a solid fraction of 0.20 (for example 400 g of soil in 750 mL of suspension: (400 ÷ 2.65) ÷ 750 = 0.20) each time is 2.82 times longer, 2.1 min, 13.1 min and 21.8 h. The sand layer forms within minutes while the water stays cloudy for hours. The two soils give different fractions, and the sandier one is expected to drain faster in the ring infiltrometer practical. It worked if the marks were made at the hindered times computed from the class's own φ rather than by eye.

What changes

What you change

soil sample site

What you measure

percentage sand, silt and clay; texture class

What you keep the same

• soil and water volumes (so φ matches)
• dispersant amount
• shaking time
• fall height and room temperature

Common misconceptions

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

• Soil is just dirt and all soil is the same.
• Clay settles quickly because it is sticky.
• Doubling a grain's size doubles its settling speed.
• The thickness of the settled clay layer is the soil's clay percentage.

Curriculum references

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

• Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE5-1GE5-2GE5-7
• Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE5-TAP-01
• Australian Curriculum v9AC9HG9K01AC9HG9K03AC9HG9S02

Sources

The pages the author read to write this activity.

1. hyperphysics.gsu.edu/hbase/Tables/viscosity.html
2. webmineral.com/data/Quartz.shtml
3. education.nsw.gov.au/teaching-and-learning/curriculum/hsie/hsie-curriculum-resources-k-12/hsie-7-10-curriculum-resources/geography-s5-biomes-and-sustainable-agriculture
4. curriculum.nsw.edu.au/learning-areas/hsie/geography-7-10-2024/outcomes
5. repository.tudelft.nl/record/uuid:9fc4115b-5624-4a1b-a434-b71b53e62cce