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Evaluating inoculation of pulse crops

By Dale Risula, Provincial Specialist, Regina

Are your pulse plants fixing nitrogen? The best way to know for sure is to scout your crops and assess them for nodulation.

Pulse plants should not be evaluated on above-ground looks alone—just like a good book shouldn’t be evaluated based on the cover. Soil attributes and variations coupled with soil nitrogen availability can mislead you to believe that nodules are alive and well.

Proper assessment of nodulation is essential to understand the potential for nitrogen fixation (N-fixation). Examining the plant's root system is the only way to know for sure.

The best time to evaluate nodule development on pulse crops is at early flowering. In most cases, nodules will start to form within about two weeks following crop emergence. Poor growing conditions could lengthen this time, so take that into consideration. At mid-flowering, the plants should have the maximum number of nodules developed; maximum N-fixation should also be occurring at this time. The efficiency of N-fixation begins to diminish after flowering is complete.

Healthy nodules on a pea root
Healthy nodules on a pea root

Get out into the field and walk it in an X pattern, taking samples from various areas in the fields with different growing conditions. By this I mean high spots and low spots – try to obtain an average for the field's conditions. Look at five to 10 plants from two or three of these areas. Taking samples from field edges or from erratic spots in the field is not recommended due to greater potential for conditions to vary from the average field condition.

When sampling plants, use a small trowel to gently dig up each plant, being careful not to strip the nodules off the plant. Remove at least two plants from each representative site, unless you desire more samples, such as when increased variability of plants exists in the sample area. Be sure you have a bucket of water along to wash the roots so you can clearly see the nodules.

A notable symptom associated with poor nodulation will normally include poor plant development. Examine the plant leaves and architecture (standability); look for yellowing leaves at the bottom of the plant prior to flowering. Assign the following grades to the plant based on your observations.

Plant condition

  • Plants are green and vigorous: 5
  • Plants are green and relatively small: 3
  • Plants are slightly chlorotic (less green): 2
  • Plants are very chlorotic: 1

Count the number of clusters of nodules on each plant and carefully slice open the nodules. You should see a nice pinkish hue inside each nodule to indicate the nodules are fixing nitrogen. Off colours like white, green or brown indicate that the nodules are not functioning effectively. Assign the following scores to the nodule assessment based on your observations.

Nodule colour and number

  • Greater than five clusters of pink-pigmented nodules: 5
  • Three to five clusters of predominantly pink nodules: 3
  • Less than three clusters of nodules, or whitish/greenish nodules: 1
  • No nodules, or white/green nodules: 0

Next, observe the crown of the plant, the area of soil surrounding the seed. When seeds are inoculated with liquid or peat, you should see most development in this area. Lateral nodulation is common when granular inoculants are used or where indigenous rhizobium bacteria exist. The crown region varies in pulses; for example, in pea plants it is slightly different, and you will observe a cylindrical formation extending about eight cm to 10 cm deep from the soil surface with an eight cm diameter. Assign the following grades to your plants based on your observations.

Examine nodule position 
  • Crown and lateral nodulation: 3
  • Generally crown nodulation: 2
  • Generally lateral nodulation: 1

Add up your numbers to determine your crop's nitrogen-fixing ability.

Nodulation Evaluation Score

Total Score Nodulation Assessment:
11 to 13

Effective Nodulation Nitrogen Fixing Potential: Good.

No further steps required

7 to 10 Less effective nodulation.

Nitrogen Fixing Potential: Reduced. Check inoculation method for errors; could also be a result of less-than-optimal growing conditions.
1 to 6 Unsatisfactory Nitrogen Fixing Potential: Poor.

Re-evaluate inoculants used, inoculation method and growing conditions. Consider fertilizer intervention to salvage crop.

For more information, see 20/20 Seed Labs' Nodulation Assessments or Real Agriculture's Nodulation Pulse School video.

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