Shake a Jar of Soil: How Sediment Sorts Itself

Shake a jar of garden soil with water and it looks like brown soup. Forty seconds later there is a gritty layer on the bottom. Two hours later there is a second, finer layer on top of it. Two days later there is a third, and the water above is finally almost clear. Those three bands are sand, silt and clay, sorted by nothing but gravity and particle size, and together they tell you what your soil actually is. This soil layers jar experiment is the classic version of a real soil test, and there is one ingredient almost every set of instructions leaves out.

Key Takeaways

• You’ll shake soil, water and a little dish soap in a tall jar and mark the layer heights at 1 minute, 2 hours and 48 hours.
• It teaches that settling speed depends on particle size, and how a river sorts sediment into graded beds.
• Best for ages 6 to 12, with the two-day wait being the hardest part.
• Without soap, clay particles clump together and settle with the silt, so you will underestimate your clay badly.
• Soil carries bacteria. Hands get washed; nothing goes near a mouth.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Tall, straight-sided glass jar with a lid	1, at least 500 mL	A passata or pickle jar	A clear plastic bottle with the top cut off and taped back on. Straight sides matter for measuring
Garden soil	about 1 cup	Dig from 15 cm down, away from paths	Compost alone will not work; it is nearly all organic matter
Water	to fill the jar	Tap	Rainwater is slightly better, but tap is fine
Liquid dish soap	1 teaspoon	Kitchen sink	Powdered dishwasher detergent, 1 tablespoon. This is the ingredient most versions skip
Masking tape and a marker	—	Stationery	Sticky labels, or a wax pencil on the glass
Ruler	1	—	—
Sieve or colander	1	Kitchen	Pick stones and roots out by hand

Time: prep 20 min, readings at 1 minute, 2 hours and 48 hours, cleanup 5 min Cost: everything is probably in your kitchen Mess level: medium. Do the digging and sieving outside

Do the Experiment

Step 1 — Dig properly and prepare the sample

Take soil from about 15 cm down rather than the surface, because the top few centimetres are mostly leaf litter and roots. Sieve out stones, twigs and roots, and crumble any clods between your fingers. You want roughly a cup of loose mineral soil. Avoid soil from beside a busy road or where pets go.

Step 2 — Fill the jar and add the dispersant

Put the soil in the jar to about one third, add water to about three quarters, and add a teaspoon of liquid dish soap. Leave an air gap so you can shake it. The soap is not for cleanliness. It is a dispersant, and it is the difference between a real measurement and a decorative one.

Step 3 — Take the prediction

Ask: “When this settles, will the big bits be at the top or the bottom, and how long will the whole thing take?” Most children get the order right and the timing very wrong, usually guessing minutes rather than days.

Step 4 — Shake hard for a full three minutes

Lid on tight, and shake until there are no lumps left at all. Three minutes feels absurd and is the correct amount. Thirty seconds leaves clay still stuck to sand grains, and you will read the wrong numbers.

Step 5 — Mark the sand at 60 seconds

Stand the jar somewhere it will not be knocked and start a timer. At one minute, put a strip of tape at the top of the settled layer.

What you should see: a gritty, pale layer on the bottom that has essentially stopped growing by 60 seconds. That is the sand. Above it the water is still thickly clouded. Some dark fragments of organic matter will be floating on the surface, which are not part of the mineral soil and should be ignored.

Step 6 — Mark again at 2 hours, then at 48 hours

Come back twice more and mark the top of the settled material each time, without moving the jar.

What you should see: at two hours a distinctly smoother, often darker layer has formed on top of the sand. That is the silt. By 48 hours a third, very fine layer has appeared above it, usually paler and cracked-looking, and the water is nearly clear. That is the clay. Measure each band with the ruler and divide by the total to get percentages.

What’s Actually Going On in the Soil Layers Jar Experiment

Soil is classified by the proportions of three particle sizes. In the USDA system used by the Natural Resources Conservation Service, particles larger than 2 mm are gravel and are excluded, sand runs from 2 mm down to 0.05 mm, silt from 0.05 mm down to 0.002 mm, and anything finer than 0.002 mm is clay. Feed your three percentages into the NRCS soil texture calculator and it will name your soil: sandy loam, silty clay, clay loam and so on.

The sorting happens because settling speed depends very strongly on size. A small particle sinking through still water reaches a steady speed where gravity balances viscous drag, and for small spheres that speed is proportional to the square of the diameter. Double the particle size and it sinks four times as fast. A fine sand grain at 0.1 mm is fifty times the diameter of a clay particle at 0.002 mm, which puts their settling speeds a couple of thousand times apart. That is why sand takes a minute and clay takes days.

Now the soap, because this is the part that separates a real test from a pretty jar. Clay particles carry electrical charge on their surfaces, and in ordinary water they attract one another and stick together into clumps called flocs. A floc of a thousand clay particles settles like a single silt grain, so without a dispersant your clay lands in the silt layer and your reading comes out as a much sandier, siltier soil than you actually have. Dish soap and dishwasher powder contain surfactants and polyphosphates that keep those particles apart. Laboratories use sodium hexametaphosphate for the same reason. If your clay band never appears at all, this is almost always why.

Why does any of this matter beyond gardening? Because rivers do the same experiment continuously. The USGS explains that a stream’s ability to carry sediment depends on flow velocity, so when flowing water slows down it drops its coarse load first and its fine load last. That is why a flood leaves gravel in the channel and silt on the floodplain, and why a delta is made of mud while the riverbed upstream is made of stones. Repeated often enough, the result is the layered rock the National Park Service describes under sedimentary geology: each graded bed a record of one event that started fast and ended slow. Your jar makes one of those beds in two days.

The honest limit: this gives a good rough texture, not a laboratory analysis. Expect to be within about 10 percentage points on each fraction, which is plenty to tell a sandy loam from a clay but not enough to publish.

How to Explain It to Your Kid

Ages 5–8: Big bits sink fast, tiny bits sink slow

Do a side demonstration first. Drop a pebble, a grain of rice and a pinch of flour into a tall glass of water and watch how differently they fall. Then shake the soil jar and tell them the jar is doing that with thousands of pieces at once, which is why the layers come out sorted. Let them put the tape on at each reading, and make the 48-hour mark an event rather than a chore.

Ages 9–12: Turn layer heights into percentages

Have them measure each band in millimetres, add them up, and work out each as a percentage of the total. Then look up the result on the NRCS texture triangle and read off the soil’s name. Naming your own back garden from your own measurements is the moment this experiment earns its two days. Then the control: run a second jar without soap, alongside the first, and compare the clay bands.

Ages 13+: Settling velocity and the square law

Give them the result that settling speed goes as the square of particle diameter and ask them to predict the ratio of settling times for a 0.1 mm sand grain and a 0.002 mm clay particle. They should get a factor of a couple of thousand. Then have them check whether their own timings are consistent with that, and work out why the prediction and the observation do not match exactly: particles are not spheres, the suspension is crowded, and flocculation is never fully suppressed.

The question to ask: “If a river slows down as it reaches a lake, which part of its load drops first, and what would you expect the lake bed to be made of a kilometre out from the river mouth?”

Why Teaching This Early Pays Off

This is one of the few kitchen experiments that produces a genuinely useful answer about the real world. Knowing your garden soil is a clay loam rather than a sandy loam changes what will grow in it and how often it needs watering, and children find it satisfying that an experiment answered a question about their own house rather than a textbook’s.

Conceptually, the payoff is the idea that a process can sort things, and that the sorted result is a record of the process. That idea unlocks far more than soil: it is how geologists read rock layers, how a centrifuge separates blood, how gold panning works, how an air filter grades dust. Once a child has seen sorting by size happen in front of them, a diagram of sedimentary strata stops being decoration and starts being evidence.

Chi and Wylie’s ICAP framework finds that learners who construct and explain outperform those who merely observe (Chi & Wylie, 2014), and the percentage calculation in Step 6 is where that happens here. Without it, this is a jar of mud on a windowsill. With it, your child has measured something. No study claims that one jar improves science grades, and the honest version of the benefit is narrower and more durable than that.

Before You Start: What Parents Should Know

Best age	6–12. From 6 for watching, from 9 for the percentages and the texture triangle
Supervision	Adult chooses where to dig and supervises the glass jar. Children can do everything else
Real hazards	Soil contains bacteria, and sometimes parasites. Wash hands thoroughly afterwards, keep soil away from faces, and cover any cuts. Do not use soil from beside a busy road or an old painted building, which can carry lead, or from anywhere pets use. Detergent irritates eyes. A glass jar on a windowsill can be knocked off
Don’t	Never let a child taste or smell soil deliberately, and do not add bleach or any other cleaner instead of dish soap. Bleach will not disperse clay and creates fumes

When It Doesn’t Work

• No clay layer ever appears. You skipped the soap, or used too little. Clay has flocculated and settled with the silt. Start again with a full teaspoon of dish soap and shake for the full three minutes.
• The water is still murky after a week. Very high clay content, very soft water, or a lot of dissolved organic matter. Mark it as “clay still in suspension” and read the layers you can see; some clay soils genuinely take a week.
• There are no distinct boundaries, just a gradient. Your soil is well graded, with a continuous range of sizes, which is a real result. Mark your readings at the agreed times anyway and let the percentages speak.
• Everything settled in two minutes and there is no fine layer. You have a very sandy soil. Dig a second sample from a different part of the garden and compare the two jars side by side.
• A thick dark layer floats on top. Organic matter. It is not part of the mineral texture. Skim it off or exclude it from your total before calculating percentages.

Take It Further

1. Compare three places. One jar from a vegetable bed, one from under a tree, one from a lawn or a path, all shaken and read at the same times. Different land uses produce measurably different textures and organic fractions, and three jars side by side make the comparison obvious.
2. Run the soap control properly. Two jars from the same soil sample, one with dish soap and one without. Measure both clay bands. This is a clean single-variable experiment and it is the version worth taking to a science fair.
3. Link it to moving water. Build a stream table and watch the same sorting happen along a channel instead of down a jar. A stream table that builds a landscape is the natural companion, and rock and mineral collecting is where the sorted layers eventually end up as stone.

What to Watch For Over the Next 3 Months

• Week 4: they comment on soil as a material, noticing that one patch is sticky and another is gritty.
• Month 2: they predict settling behaviour somewhere else, usually in a muddy puddle or a glass of cloudy water.
• Month 3: they read a cliff face, a road cutting or a riverbank as layers with an order rather than as a pattern. The question worth encouraging is “which layer is oldest?”

Frequently Asked Questions

Why does the soap matter so much?

Because clay particles carry surface charge and clump together in plain water, and a clump settles as fast as a much larger particle. The surfactants and phosphates in dish soap keep them separated so each particle settles at its own rate. Without it you will record far less clay than your soil contains.

How long does it really take?

Sand is done in about a minute, silt within two hours, and clay needs 24 to 48 hours. Some clay-rich soils keep a faint haze for a week. You can read usable percentages at 48 hours even if the water has not completely cleared.

Can I use compost or potting mix?

Not for this. Both are mostly organic matter, which floats or stays suspended rather than settling into mineral bands. Use mineral soil dug from below the top layer.

Is the result accurate enough to be useful?

For gardening, yes. Expect each fraction to be within roughly 10 percentage points of a laboratory result, which is enough to distinguish a sandy loam from a clay loam and to decide how often a bed needs watering. It is not accurate enough for any formal purpose.

What school topic does this prepare?

Rocks, soils and the water cycle in primary and middle school, and sedimentary geology later. The National Research Council’s framework names “planning and carrying out investigations” as a core practice, and a two-day settling test with a control jar is a textbook example (NRC, 2012).

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. USDA Natural Resources Conservation Service. “Soil Texture Calculator.” https://www.nrcs.usda.gov/resources/education-and-teaching-materials/soil-texture-calculator
2. U.S. Geological Survey. “Sediment and Suspended Sediment.” Water Science School. https://www.usgs.gov/special-topics/water-science-school/science/sediment-and-suspended-sediment
3. U.S. Geological Survey. “Turbidity and Water.” Water Science School. https://www.usgs.gov/special-topics/water-science-school/science/turbidity-and-water
4. National Park Service. “Sedimentary Rocks.” Geology. https://www.nps.gov/subjects/geology/sedimentary.htm
5. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
6. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165