What it shows
The depth and amount of soil limit how far roots can spread. Growing the same bean in a tall cup and a short cup shows how room for roots affects the plant above ground.
The science
Roots anchor the plant and take up water and minerals, and a larger volume of soil holds more of both. In a small pot the roots fill the soil quickly and become crowded, so the plant may be stunted and need watering more often. Clear cups let you see the roots through the side, and the soil depth is the only variable if the width, light and water are 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.
- Two clear plastic cups of the same width but different height (or one cup cut shorter)
- Garden soil, enough to fill each cup to 1 cm below the rim
- 4 soaked bean or gram seeds
- A marker and a ruler
- A measuring cup for water
- A notebook
Safety
For this project: An adult must cut the cups and make drainage holes. Wash your 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.
Ask an adult to cut one cup so it is half as tall as the other, and make four drainage holes in each base.
Fill each cup with soil up to 1 cm below the rim and press it down gently.
Plant two soaked seeds in each cup, 2 cm deep, and label the cups TALL and SHORT.
Place both cups on the same windowsill and water each with the same amount, say 30 mL, every second day.
Wait until both have sprouted, then remove the weaker seedling so one plant remains in each cup.
Measure shoot height and count leaves every three days for three weeks.
Look at the roots through the clear walls and sketch how far they have spread in each cup.
At the end, tip each plant out and compare the length and spread of the roots and the overall size.
What you should see
The plant in the tall cup usually shows longer roots and a taller, leafier shoot, while the short cup plant slows down once roots reach the base. Your sketches and growth table show this.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What do roots do for the plant?
They anchor it and absorb water and minerals.
2Which variable did you change?
The depth of soil, meaning the room for roots.
3Why use clear cups?
So the roots can be seen through the side without disturbing the plant.
4Why is it a problem when roots circle around the pot?
They are crowded, take up less water and minerals and the plant becomes stunted.
Take it further
Make a third cup with soil only half full and see whether the roots reach the bottom sooner and what that does to growth.
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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