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How to assess legume nodulation

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Figure 1: A pink legume nodule sliced in half indicating efficient nitrogen fixation

The first step to knowing whether legume pastures are fixing nitrogen is to check whether roots are healthy and nodules are present and effective.

How to assess legume nodulation

Under optimal soil and growing conditions, nodulation occurs relatively early in the growing season.

Sampling can generally be carried out 12 weeks after germination (mid to late winter) when soil moisture conditions are favourable. Here’s how to do it:

  1. Assess a minimum of 15-20 plants. Select plants from a representative area of the paddock.
  2. Carefully excavate plants from the soil to a depth of about 30cm using a shovel (don’t pull plants out of the soil as this can pull nodules off the root system).
  3. Wash the soil from the roots and gently separate plants to assess individual root systems.
  4. Assess the number and colour of the nodules using the rating system in Figure 2. An adequately nodulated plant will have a minimum of 20 nodules on the root system and these nodules will be pink in colour due to the presence of leghemoglobin (see Figure 1) if they are effectively fixing nitrogen.

If plants are assessed late in the growing season, effective nodules may appear dark pink to green in colour when cut open.

White nodules can indicate the nodules contain ineffective strains of rhizobia and are poor at fixing nitrogen.

Figure 2: A scoring system can be used to assess legume nodulation for plants growing for a minimum of 12 weeks in paddock situations