What it shows
The roots of pulse plants such as gram, pea and groundnut carry small lumps called nodules in which bacteria fix nitrogen from the air. That is why pulses are rotated with cereals to keep soil fertile.
The science
Plants need nitrogen to make proteins, but they cannot use nitrogen gas from the air directly. Rhizobium bacteria live in nodules on the roots of legumes and convert nitrogen gas into ammonium, which the plant can use. In return the plant gives the bacteria sugars. Active nodules often look pink inside because they contain leghaemoglobin, a protein that controls oxygen. Farmers use this by growing pulses in rotation so the soil gains nitrogen compounds.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A whole gram, pea, groundnut or moong plant with its roots (with the owner’s permission)
- A wheat or grass plant for comparison
- A trowel and a bucket of water
- A hand lens or a magnifying glass
- A sheet of white paper
- A sharp knife for the adult only
- A notebook and coloured pencils
Safety
For this project: Take plants only with permission, dig carefully, and wash your hands afterwards. An adult must slice any nodule with the knife.
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.
Dig the plant out gently with a trowel so the whole root system comes up with some soil.
Wash the roots in the bucket of water, shaking them softly so the nodules are not torn off.
Lay the roots on white paper and look for rounded lumps along the roots with the hand lens.
Count the nodules on the plant and measure the largest, using the ruler.
Ask an adult to cut one big nodule in half and look for a pink colour inside.
Wash and look at a wheat or grass plant’s roots, and count any nodules; you should find none.
Draw both root systems, labelling the nodules, and note how the two differ.
Write a paragraph on why pulses improve soil and why farmers rotate crops.
What you should see
The pulse roots carry many small round nodules, some pink inside, while the wheat or grass roots have none. Your drawings show the difference.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What lives in root nodules?
Rhizobium bacteria.
2What do these bacteria do?
They fix nitrogen gas into a form the plant can use.
3Why is crop rotation with pulses helpful?
It adds nitrogen compounds to the soil naturally.
4Why do plants need nitrogen?
To make proteins and other body chemicals.
Take it further
Compare nodule number on plants from a fertiliser-rich bed and a poor bed, and discuss what you see.
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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