Nodulation of Legumes

All legumes can fix nitrogen in the right conditions. Nodules are the swollen, rounded lumps that are formed on the roots of legumes when there is an effective symbiosis in the rootzone with nitrogen-fixing rhizobia. Unless legumes are forming nodules, they are unlikely to be fixing much nitrogen.

N.B. The exact number of nodules you want for a high scoring root depends on the legume. E.g. You want to look for 50+ nodules per plant for vetch, field pea, faba beans and lentils, & 10-30 per plant for chickpea for a score of 2.

Best to test in autumn and/or spring. Avoid carrying-out this test after heavy rain or when very dry.

What to record

  • Score

    • 0 = None. VERY POOR.

    • 0.5 = Very few small nodules high on roots. POOR.

    • 1 = Some small nodules high up on the roots. MODERATE.

    • 1.5 = Nodules across the root profile, that are pink when cut open. GOOD.

    • 2 = Many nodules across the root profile, pink or red in colour when cut. VERY GOOD.

  • A photo of the roots


  • Spade

  • Bucket & water (to wash roots)

How to do the test

1. Locate your chosen sample site – this may be mapped with the GPS function of Sectormentor.

2. Dig out a small section of soil, no wider than your spade or do with other spade tests (20cm x 20cm).

3. Gently break or wash away soil from roots of legumes.

4. Observe if there are any nodules on the roots. If so, select four of the largest nodules to cut to check the colour.

5. Record a score on the scale above in the Sectormentor app.