Legume root nodules: counting, cutting open and inoculation · Concept Studio

The idea

Rhizobia bacteria in legume root nodules fix nitrogen from the air, and nodules that are fixing are pink to red inside, so inspecting nodules shows whether a legume is supplying its own nitrogen.

What you need

faba (broad) bean or field pea seed, 24
commercial rhizobial inoculant labelled for that legume, used as the label directs
12 pots of heat-treated seed-raising mix or washed sand
clover or lucerne plants dug from a school paddock
bucket of water to wash roots, hand lens, scalpel, white tile, balance and a drying oven set to 90–95 °C (the SAPS radish dry-mass method)

How to do it

Inoculate half the seed as the product label directs and sow it in six pots; sow uninoculated seed in the other six pots, keeping tools separate so the treatments do not mix.
Grow the plants in the same conditions for 5 to 6 weeks, watering equally.
Wash the roots gently, count nodules per plant and note where they sit (crown or side roots).
Cut up to 10 nodules from each treatment in half on the tile (all of them where a treatment has fewer) and record the inside colour: pink or red, white, or green.
Compare leaf colour and shoot dry mass between treatments, and inspect nodules on the paddock plants the same way.

What you should see

Inoculated plants carry nodules, many of them pink to red inside, the colour of the leghaemoglobin in nodules that are fixing nitrogen. Uninoculated plants in heat-treated mix have few or no nodules, and white or green nodules indicate little fixation; poorly nodulated legumes short of nitrogen can show yellowing of the oldest leaves, as the NSW soil acidity fact sheet notes. The legumes fact sheet advises inoculating when a paddock has not grown a host legume for four years, and warns that acid soils (pH below 5) reduce nodulation. The learner knows it worked when the two treatments differ clearly in nodule number.

What changes

What you change: inoculation (inoculated or not)
What you measure: nodules per plant, percentage of pink nodules, shoot dry mass (g)
What you keep the same: legume variety
mix and pot size
water and light
age at harvest

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Legumes take nitrogen from the soil like other crops and give nothing back.
Nodules are a disease or insect damage on the roots.
All the nitrogen a legume fixes stays in the paddock after harvest.

Safety card

Low riskLearners carry it out

Hazards

scalpel cuts
inoculant and potting mix dust

Controls

cut on a white tile away from fingers
gloves and a P2 mask when handling inoculant and mix
wash hands afterwards

Note

No hazardous chemicals. General laboratory rules: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agriculture Stage 6 Syllabus (2013)H2.1
Agriculture 11–12 Syllabus (2026), new syllabus, implementation from 2028AG-12-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.