The science idea
Plants need water, but too little makes them wilt and too much drowns the roots, because roots also need air. Watering by hand often gives either a flood or a drought. A self-watering planter keeps a small supply of water in a reservoir and passes it to the soil slowly, as the soil dries. It is a clever use of an old plastic bottle and a good way to save water.
The movement of water up the wick is called capillary action. Water molecules stick to the walls of narrow gaps (adhesion) and to each other (cohesion). In a very narrow gap this pulls a column of water upwards against gravity. The gaps between the fibres of a cotton cloth are such narrow tubes. In a simple tube of radius r, the water rises to a height h = 2γcosθ ÷ (ρgr), where γ is the surface tension of water, θ is the angle of contact, ρ is the density of water and g is the gravitational field strength. The narrower the tube, the higher the rise. That is why paper towels, cloth and soil soak up water, and why a wick works best when it is short.
In the soil, the same effect passes the water between the soil grains from wet soil to drier soil. The roots take up the water, together with dissolved minerals. The water then travels up the stem to the leaves, where some of it is lost to the air as water vapour through tiny pores called stomata. This loss is called transpiration, and it helps to pull more water up the plant. So a plant that loses more water draws more from the reservoir; the system is self-regulating.
A wick can only feed the soil as fast as capillary action allows, so it is best for small plants such as seedlings, herbs, spinach or coriander, and not for big thirsty plants. You can make this into an experiment: grow identical seeds in three planters, one self-watering, one watered by hand every day and one watered every three days, and measure the height of the seedlings. You can also compare different wicks (cotton, nylon, wool) to see which carries water best.
Using a bottle that would otherwise be thrown away is itself an environment lesson: reduce, reuse and recycle. The planter also saves water, because less is lost by running off or evaporating from the soil surface.
Materials and tools
Cost: Free
A plastic bottle, an old cotton cloth, soil from the garden and a few kitchen seeds (methi or moong) are all free.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Plastic bottle, 1 to 2 litres, with its cap | 1 | A cold-drink or mineral-water bottle. It must be clean and must never have held pesticide, phenyl or any other chemical. |
| Strip of cotton cloth for the wick (old T-shirt, towel or cotton rope) | 1 strip, about 30 cm × 2 cm | Cut from old cotton clothes. Cotton works better than nylon. |
| Garden soil mixed with a little compost or sand | About 2 cups | From the garden or a pot. Sieve out stones. |
| Seeds that grow fast (methi, moong, coriander or mustard) | A pinch (10 to 15 seeds) | From the kitchen. Choose seeds that are whole and unbroken. |
| Water | About 500 mL to start | Tap or rain water. |
| Tape or a rubber band | 1 | To hold the halves together. |
| Newspaper or dark paper | 1 sheet | To wrap the reservoir and stop algae growing in the light. |
| Cork, clay or cap to plug the refill hole | 1 | A bottle cap or a ball of clay. |
Tools
- Scissors and a craft knife (adult for the first cut)
- Marker pen and ruler
- A hot nail or an awl for the refill hole (adult)
- Measuring cup or a syringe to measure water
- Notebook
Safety first
Read this before you start
- An adult should make the first cut in the bottle and the refill hole. Cut edges can be sharp: cover them with tape.
- Use only a bottle that held water or juice. Never use one that held pesticide, cleaning liquid or any other chemical.
- Wash your hands with soap after handling soil. Do not eat seedlings, soil or sprouts grown in a recycled bottle.
- Keep the reservoir covered. Water standing in an open container can breed mosquitoes, so plug the refill hole and change the water in the reservoir each week.
- Do not put the planter on a window ledge where it can fall, and keep it away from electrical sockets.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Wash the bottle and mark the cut
Wash the bottle and take off the label. Draw a line all the way round the bottle about one third of the way up from the base. Check it is level with a ruler.
Tip: Wrap a piece of paper round the bottle, line up the edges, and draw along the edge of the paper for a straight line.
Cut the bottle in two
An adult makes a small cut on the line with a knife. Then cut right round with scissors. You have two parts: the top with the neck, and the bottom with the flat base. Cover the cut edges with tape.
Common mistake: Cutting at a slant. The two halves then do not sit level on each other.
Make the refill hole
In the side of the bottom part, about 3 cm below the cut edge, an adult makes a hole about 1 cm wide with a hot nail or a small knife. This is where you add water. You can plug it with a cork, clay or a bottle cap.
Tip: Make the hole on the side that will face the wall so the plug does not catch.
Make the wick
Cut a strip of cotton cloth about 30 cm long and 2 cm wide. Fold it lengthways in half, or twist it into a rope. Wet it so that it is easy to handle.
Common mistake: Using a thin thread or nylon cloth. It does not take up much water. Use thick cotton.
Thread the wick
Remove the cap from the top part, turn it neck-down, and push the strip through the neck so that about 10 cm hangs out below the neck and the rest lies in the cone above.
Tip: Tie a knot at the top end of the strip or spread the top part out so that it does not slip down through the neck.
Fill the soil
Spread the top end of the wick out in the cone. Fill the cone slowly with soil mixed with a little sand or compost, pressing lightly with your fingers until it is about two thirds full. Do not pack it hard.
Common mistake: Packing the soil too tight. Water and air cannot move through packed soil, and the roots cannot grow.
Fill the reservoir
Pour about 500 mL of water through the refill hole into the bottom part (so that it is half full). The water level should be about 2 cm below the neck of the top part when the planter is put together.
Tip: Check with a stick through the refill hole. The wick must touch the water, but the neck and the soil must not sit in it.
Put the planter together
Push the top part, neck down, into the bottom part so that the wick hangs into the water. Tape the two halves together. Wrap the bottom part in paper to keep out light.
Common mistake: Letting the neck sit in the water so that the soil becomes a swamp. Leave a 1 cm air gap between the soil and the water.
Wet the soil and sow
Water the soil from the top once, gently, until it is damp (not dripping), so the wick starts working. Sow 10 to 15 seeds about 1 cm deep, and cover them lightly with soil.
Tip: The first watering from the top starts the capillary flow. After that the wick does the work.
Prepare two comparison pots
Make a second pot from another bottle (without a wick) watered by hand with 50 mL a day, and a third watered by hand with 50 mL every three days. Use the same soil, the same seeds and put all three in the same place.
Common mistake: Using different seeds or soil. For a fair test, only the way of watering should differ.
Measure every day
Every day at the same time, note the soil colour and feel, the height of the tallest seedling in millimetres, the number of leaves, and how much the water in the reservoir has fallen. Write them in your table.
Tip: Measure from the soil surface to the tip of the tallest leaf with a ruler held beside the stem.
Compare and conclude
After 14 days, draw a line graph of height against day for the three pots. Say which pot grew best and why. Note how much water each pot used.
Tip: If the wick pot did worse than the others, check whether the wick was in the water and whether the soil was too wet.
How it should look when it is finished
A tidy plastic planter about 20 cm tall: a clear or paper-wrapped lower part that holds water, and an upper cone filled with dark soil, with a tuft of green seedlings growing from the top. A small plug marks the refill hole, and a thin strip of cotton cloth can be seen hanging from the neck into the water.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The bottom half holds the water, and the top half holds the soil. The wick links them: one end sits in the water and the other spreads through the soil. Because water sticks to the cotton fibres and to itself, it creeps up the wick through the tiny gaps between the fibres, just as it creeps up a narrow tube.
When the soil is dry, water leaves the wick and moves into it. When the soil is damp, the wick has nowhere to send water and its flow slows right down. So the soil is kept evenly damp instead of being flooded or dried out, and this is much better for the roots.
The roots take up water, and it moves up the stem to the leaves. Water vapour leaves through pores in the leaves (transpiration), which pulls more water up from the roots. The water in the reservoir slowly falls, and you only have to top it up through the refill hole.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The soil stays dry | The wick does not touch the water, is too thin, or is not touching the soil. | Lengthen the wick, use thicker cotton, and check that the lower end is in the water and the upper end spreads in the soil. |
| The soil is very wet and smells bad | The neck or soil sits in the water, or there is too little air. | Lower the water level so that there is a gap, and loosen the soil. |
| Seeds do not sprout | Old seeds, buried too deep, or soil too dry or too cold. | Use fresh seeds 1 cm deep, and give the first watering from the top. |
| Green slime or algae grows in the reservoir | Light is reaching the water. | Wrap the reservoir with paper or paint it, and change the water. |
| Mosquito larvae in the water | The refill hole was left open. | Plug the hole, empty and refill with fresh water, and cover the planter. |
| The two halves fall apart | The halves are not tight. | Tape them together all the way round, or use a rubber band. |
Test and record
Measure the tallest seedling in each pot at the same time every day. Write the water used from the reservoir (in mL) in the last column for the wick pot.
| Day | Height, wick pot (mm) | Height, watered daily (mm) | Height, watered every 3 days (mm) | Leaves (wick pot) | Water used (mL) |
|---|---|---|---|---|---|
| 2 | |||||
| 4 | |||||
| 6 | |||||
| 8 | |||||
| 10 | |||||
| 12 | |||||
| 14 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- What is capillary action?
The rise of a liquid in a very narrow tube or gap, caused by the adhesion of the liquid to the walls and the cohesion between its own molecules.
- Why does the wick not flood the soil?
Water moves into the soil only as fast as the dry soil pulls it. When the soil is damp, the flow slows, so the soil stays damp but not wet.
- What is transpiration?
The loss of water vapour from the leaves, mainly through the stomata. It helps to pull water up through the plant.
- Why does the plant need air around its roots?
Roots respire and need oxygen. In waterlogged soil there is little air, so the roots can rot.
- Which wick materials would you test and what would you expect?
Cotton, wool and nylon. Cotton and wool absorb water well and should do best; smooth nylon carries less.
- Why wrap the reservoir in paper?
Light lets algae grow in the water. Paper blocks the light.
- How does this project help the environment?
It reuses a plastic bottle and it saves water, because less is lost by run-off and evaporation.
- How did you make the test fair?
Use the same soil, seeds, light and position for all three pots, and change only the way they are watered.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a self-watering planter from a plastic bottle and to compare the growth of seedlings in it with plants watered by hand.
- Apparatus
- A plastic bottle, a cotton cloth wick, soil, seeds, water, scissors, tape, a ruler and a measuring cup.
- Procedure
- Describe how you cut the bottle, made the wick and refill hole, filled the soil and the reservoir, planted the seeds, and how you measured the seedlings every two days. Include the diagrams.
- Observation
- The table and the graph of seedling height against day for the three pots.
- Result
- After 14 days the seedlings in the wick pot were ... mm tall, in the daily-watered pot ... mm and in the pot watered every three days ... mm. The wick pot used ... mL of water.
- Conclusion
- A cotton wick draws water up by capillary action and keeps the soil evenly damp, so seedlings grow well with less effort and little waste of water.
Take it further
- Test different wicks (cotton, wool, nylon, jute rope) and measure how high the water climbs in a fixed time.
- Put a coloured dye or a few drops of food colour in the reservoir and watch it climb up the wick and into the soil.
- Measure the water used per day by the wick pot on a sunny day and on a cloudy day.
- Grow the same seeds in the light and in the dark and compare.
- Make a bigger planter from a 5 litre can and grow a chilli or tomato plant.
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