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Science project idea

Jar settling test that finds the sand, silt and clay in garden soil

Soil is a mixture of sand, silt and clay particles of different sizes. Shaken in water, the heavy sand sinks in a minute, silt in a few hours and clay over days, so the layers in the jar show the soil's texture.

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What it shows

Soil is a mixture of sand, silt and clay particles of different sizes. Shaken in water, the heavy sand sinks in a minute, silt in a few hours and clay over days, so the layers in the jar show the soil's texture.

The science

Particles sinking in water settle at a speed that rises with their size. Sand grains, larger than 0.05 millimetre, fall within a minute; silt between 0.002 and 0.05 millimetre takes hours; clay below 0.002 millimetre may stay cloudy for days. Measuring the thickness of each settled band and dividing by the total gives percentages that locate the soil on the texture triangle, which tells a farmer how well it drains and holds nutrients.

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • Two or three clear jars with lids
  • Soil samples from a garden, a field and a roadside
  • Water
  • A pinch of salt or a spoon of detergent to help clay separate
  • A ruler
  • A soil texture triangle from a textbook

Safety

For this project: Wash hands after handling soil.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Fill each jar one third with soil after removing stones and roots.

  2. Add water to three quarters full and a pinch of salt or detergent.

  3. Shake hard for two minutes until no lumps remain, then stand the jar still.

  4. After one minute mark the top of the settled layer on the jar; this is sand.

  5. After two hours mark again; the new band is silt.

  6. After two days mark the final level; the band above silt is clay, and anything floating is organic matter.

  7. Measure each band with the ruler and work out its percentage of the total.

  8. Place each sample on the texture triangle and name its type, such as sandy loam or clay loam.

What you should see

Each jar shows two or three distinct bands. Garden soil often comes out as a loam with roughly equal sand and silt and some clay; river sand is nearly all sand.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1Which particles settle first and why?

Sand, because larger particles sink faster.

2What is loam?

A balanced mix of sand, silt and clay that holds water and drains well.

3Why add salt or detergent?

It helps clay particles separate from one another so they settle as a clean layer.

4Which soil holds water longest?

Clay, because its tiny particles leave tiny spaces.

Free ebook: Viva Questions for Science Projects

Take it further

Grow seeds in each soil type and compare growth after two weeks.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

Free ebooks for this project

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