What it shows
Legume roots carry nodules full of nitrogen-fixing bacteria, which enrich the soil. Comparing roots of pulses and cereals shows why farmers rotate crops with beans, gram or groundnut.
The science
Rhizobium bacteria live inside the nodules of legume roots and convert nitrogen gas from the air into ammonia the plant can use. In return the plant gives them sugars. Healthy active nodules are pink inside because of a pigment that carries oxygen.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Young plants of gram, pea or groundnut and of wheat or maize (from a farm or garden, with permission)
- A trowel and a bucket of water
- A hand lens or magnifier
- A knife for cutting nodules (adult only)
- A white tray
- A notebook and a camera
Safety
For this project: Ask the landowner for permission before digging. An adult must cut nodules with the knife. Wash hands after handling soil and plants.
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.
With permission, carefully dig up whole plants with the roots intact.
Wash the soil off the roots gently in a bucket of water.
Place each plant on a white tray and count the nodules on the roots.
An adult cuts a few nodules in half and you note the colour inside.
Do the same for roots of a cereal plant and record that there are no nodules.
Measure the size of nodules with a ruler under the lens.
Repeat with at least five plants of each kind.
Make a table and a chart of nodule counts and write your conclusion.
What you should see
Legume roots show many round nodules, some pink inside, while cereal roots show none. The table and chart support the idea that legumes add nitrogen to soil.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What are root nodules?
Swellings on legume roots that house nitrogen-fixing bacteria.
2Why do farmers rotate crops?
To restore nitrogen and break pest cycles.
3What is nitrogen fixation?
Changing nitrogen from the air into a form plants can use.
4Which bacteria are involved?
Rhizobium.
Take it further
Compare nodule counts in fertilised and unfertilised pulse fields.
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
Science projects
Biology Models Guide
Make the heart, lungs, kidney, eye, cell and more, and explain them well
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology