What it shows
Water climbs up a cotton wick by capillary action, which moves it from the lower tank to the soil without any pump. Plants take what they need and the tank lasts several days.
The science
Cotton fibres have tiny gaps. Water sticks to the fibres and to itself (adhesion and cohesion), so it creeps up through the gaps against gravity, called capillary action. The soil pulls water from the wick as it dries, so the plant gets a steady supply. The same process pulls water up through the narrow tubes in a plant stem.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 1.5 litre plastic bottle
- A strip of cotton cloth or thick cotton string
- Garden soil or potting soil
- A seedling of mint or coriander
- A sharp nail to make a hole (adult only)
- Water
Safety
For this project: An adult must cut the bottle and make the hole in the cap. 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 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.
An adult cuts the bottle in two about one third from the bottom.
Make a hole in the cap and thread a wick through it, leaving about 10 cm on each side.
Turn the top part upside down into the bottom part so the cap points down into the empty base.
Fill the base with water so the wick end dips into it.
Fill the top with soil, making sure the wick runs through the soil.
Plant the seedling and water once from above to start the wick.
Mark the water level on the bottle and check it every day for a week.
Compare with an identical plant watered by hand in the same sunlight.
What you should see
The soil stays damp from below and the water level falls slowly; the plant stays healthy for several days without watering from above.
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 capillary action?
Water rising in narrow spaces due to adhesion and cohesion.
2Why is cotton used and not plastic thread?
Cotton absorbs water and has many tiny gaps.
3How does this save water?
The plant takes only what it needs and little evaporates.
4Name another example of capillary action.
A towel soaking up spilled water.
Take it further
Test a thin wick, a thick wick and a nylon string and measure the daily water used.
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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