Nodulation Assessments can start to be conducted approximately 3-4 weeks after crop emergence, corresponding to the 4th true leaf crop stage. In years where there are cooler spring temperatures, it may take longer for the crop to reach the 4th true leaf stage where an assessment can be conducted. Assessments can continue up until the early/mid flowering crop stage where the number of nodules will be maximized along with the amount of nitrogen fixation. The later you conduct the assessment within the given timeframe, the easier it will be to see the nodules.
This is one of those field scouting operations that does require some tools to do the job properly. You can’t just walk out into the field and pull up plants by hand as you’ll likely rip off the very nodules that you want to assess.
Tools of the trade to conduct a proper assessment of nodulation include:
- A shovel and not just a little hand trowel
- A bucket large enough to fit the plants
- Pocket knife
- Notepad or app like Scoutdoc to record results
Now you are geared up for the job select a number of representative areas of the field that you would like to evaluate being mindful to avoid depressions, knolls and field borders.
Take your shovel and carefully dig around selected plants – making sure not to break off any roots. If you are lucky (in this case meaning you do not have a heavy clay soil), you may be able to lightly loosen the soil around the roots with your hands without doing any damage to the roots or nodules. If you cannot easily loosen the soil, then place the roots in a bucket of water for a few minutes and then try again to loosen the soil from the root system.
What To Visually Look For
Effectiveness of the nodules can be determined to some degree by looking at the color of the nodules. Numbers and size of nodules do not necessarily correlate to the effectiveness of the inoculant strain. If you only remember one point – remember that bright red or pinkish color is the color that you want to see. The red pigment leghaemoglobin is associated with active nitrogen fixation in nodules. If you see medium/large white nodules, these are formed by ineffective rhizobia. Gray or greenish nodules can either indicate the nodule is:
1) past it’s prime and already made it’s nitrogen contribution to the plant
2) the nodule is not contributing to N fixation because of other environmental factors
With most pulse crops you should be able to see the red/pinkish hue of the nodules without cutting open the nodule with a knife. In other crops such as soybeans or alfalfa, you’ll need to slice open the nodule to see the nodule coloration.
The type of inoculant formulation used will dictate where you would expect to see nodules. With seed applied inoculants like liquid or peat formulations, you should mostly see nodulation around the crown area of the plant. With a granular formulation that is applied in furrow, you will see nodulation along the lateral root system.
Should there be a lack of effective nodules or no nodules, then an evaluation of inoculation technique AND environmental/soil conditions should be conducted. If what you see is a bright red/pinkish coloration of the nodules, this a good indicator of high nitrogen fixation potential. Remember number and size of nodules vary between inoculants (native, brands) and counting, weighing or sizing is not as important as recognizing if there are good healthy nodules that are contributing to the overall nitrogen status of the plant.