What it shows
A plant needs the right amount of water, not the most. Roots in soggy soil lack air and plants in dry soil lack water, so only the middle pot grows well.
The science
Seeds and young plants need water to swell, to carry minerals and to make food by photosynthesis. Roots also need air, because they respire and take in oxygen from tiny gaps in the soil. In very wet soil these gaps fill with water, so roots cannot respire well and may rot, while in too little water the plant wilts. A fair test changes only the amount of water and keeps soil, seeds, light and temperature the same.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three identical plastic cups, two with drainage holes poked at the bottom and one without
- Garden soil or compost, the same amount in each cup
- 9 green gram (moong) or gram seeds that have been soaked overnight
- A measuring spoon or a medicine cup for water
- A saucer for each cup
- A ruler and a notebook
Safety
For this project: An adult must make the drainage holes with a hot nail or a sharp tool. Wash your hands after touching 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.
Fill all three cups with the same weight of soil and plant three soaked seeds about 1 cm deep in each.
Label the cups: dry (5 mL each day), enough (25 mL each day, so the soil is damp) and soggy (100 mL each day, in the cup with no drainage hole).
Place all the cups side by side on a sunny windowsill so they get the same light.
Water each cup at the same time every day with its own amount and stand the two drained cups on saucers.
Every day measure the tallest shoot in each cup with the ruler and count the leaves.
Write the readings in a table, with one column for each cup, and note the colour of the leaves.
After two weeks draw a line graph of height against day for all three cups on the same axes.
Gently tip each plant out and compare the roots, looking for brown, soft or smelly roots in the soggy cup.
What you should see
The enough cup shows tall, green seedlings. The dry cup has short, pale or wilted plants, and the soggy cup has slow growth, yellow leaves or soft brown roots. The graph lines separate clearly after about one week.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What did you change in the experiment?
Only the amount of water given each day.
2Name two things you kept the same.
The soil, the seeds, the light and the temperature.
3Why can too much water harm roots?
It fills the air spaces in the soil, so roots cannot get oxygen to respire.
4Why did you plant three seeds in each cup?
So that one seed that fails to sprout does not spoil the result.
Take it further
Try the same test with a sandy soil and a clay soil and find out which one drains fastest and which plant copes best with the same water.
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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