What it shows
Water passes through soils at very different rates. Sand lets water through in seconds, clay holds it for a long time, and the result decides where groundwater forms, where floods happen and which crops grow.
The science
Soil is full of pores between the grains. Big grains leave big connected pores that water flows through easily, so gravel and sand are highly permeable. Clay particles are tiny and pack closely, leaving pores so small that water creeps through. Permeability controls how much rain soaks into the ground to recharge wells and how much runs off the surface.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four funnels made from cut bottle tops
- A piece of cotton cloth for each funnel
- Equal volumes of sand, garden soil, clay soil and gravel
- Four measuring cups or marked glasses
- A stopwatch
- A jug of water
Safety
For this project: An adult should cut the bottles; tape the cut edges.
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 rulesStep 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.
Place a piece of cloth in each funnel so soil cannot fall through the neck.
Fill each funnel with the same volume of one soil, level but not pressed.
Stand each funnel in its own measuring cup.
Pour 100 millilitres of water into the first funnel and start the stopwatch at once.
Note the time when the first drop reaches the cup and the volume collected after one, two and five minutes.
Repeat for each soil with the same amount of water.
Draw a bar chart of volume collected after five minutes for each soil.
Write which soil would be best for a recharge pit and which would hold water in a paddy field.
What you should see
Gravel and sand pass nearly all of the water within a minute; garden soil passes most within five minutes; clay passes only a little and keeps the rest.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is permeability?
How easily water can pass through a material.
2Why does clay drain slowly?
Its particles are tiny and the pores between them are very small.
3Which soil suits a rainwater recharge pit?
Sand or gravel, which lets water sink to the groundwater quickly.
4Why is clay soil used for paddy fields?
It holds standing water that rice needs.
Take it further
Mix sand and clay in different ratios and find the mix that drains at a medium rate.
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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