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. Capillary action also helps water move up the narrow tubes in a plant stem, along with the pull from water lost through the leaves.
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
- Two seedlings of mint or coriander, one for the comparison
- A second pot or bottle for the hand-watered seedling
- 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. Keep the water tank covered by the soil part and change its water every week so that mosquitoes cannot breed in it.
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.
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.
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.
01What is capillary action?
Water rising in narrow spaces due to adhesion and cohesion.
02Why is cotton used and not plastic thread?
Cotton absorbs water and has many tiny gaps.
03How does this save water?
The plant takes only what it needs and little evaporates.
04Name 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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