Accurate field measurements of nitrogen fixation responses to inoculation with Rhizobium are often difficult, undependable, and expensive. However, nitrogen fixation can be estimated through an assessment of nodulation and plant growth characteristics.
This information will help growers and agronomist learn how to assess nodulation and nitrogen fixing potential in the field.
Nodule Assessment Timing
Nodulation assessments should be done during early flowering. Nodule formation begins approximately 14 days after crop emergence, but under certain conditions formation may take three to four weeks.
Nodule numbers and nitrogen fixation rates generally are at a maximum during early-to mid-flowering. After flowering, nodule efficiency is reduced and they begin to shut down.
Assessment Procedure
To assess the nodulation and nitrogen fixation potential of a pulse crop, select five areas that are typical of that field at early flowering. Follow the steps listed below in each of the five areas:
- Evaluate plant growth and vigour of the area according to the assessment codes shown below.
- With a shovel, carefully dig up a minimum of two plants per area. Do not pull plants out of soil as nodules are delicately attached to roots and can be easily lost.
- Carefully examine plant roots to assess the nodules. Depending on the soil type and condition, this may require gently agitating the roots in water.
- Assess the overall nodulation by comparing the calculated scores to those provided for the three categories in the assessment guide.
Assessment Codes
1. Plant and growth vigour
| Plants green and vigorous | 5 |
| Plants green and relatively small | 3 |
| Plants slightly chlorotic | 2 |
| Plants very chlorotic | 1 |
Poor nitrogen fixation can cause nitrogen deficiency symptoms such as yellowing of the leaves at the base of the plant prior to flowering and poor plant development.
2. Colour of interior of the nodules/number
| More than five clusters or groups of pink pigmented nodules (interior of nodules) | 5 |
| Three to five clusters or groups of predominantly pink nodules | 3 |
| Less than three groups of nodules OR nodules whitish or greenish in colour | 1 |
| No nodules OR nodules white or green in colour | 0 |
Nitrogen fixation efficiency can be estimated with nodule colour and the number of nodule clusters present. Carefully slice open the nodules. The strong pink colour of the nodules is caused by the presence of leghemoglobin, which must be present for active nitrogen fixation. If a nodule is brown, white or green it is considered non-effective.
3. Nodule position
| Crown and lateral nodulation | 3 |
| Generally crown nodulation | 2 |
| Generally lateral nodulation | 1 |
Predominantly crown nodulation is observed when seed is inoculated. Lateral nodulation is prevalent when native Rhizobia species exist in the soil or when granular inoculants are used. The crown region of a plant is generally the area of soil surrounding the seed. The approximate size of this region varies according to the crop.
Total score
11 - 13 Effective nodulation. Good nitrogen fixation potential.
7 - 10 Nodulation less effective. Fixation potential reduced. Was inoculation or growing conditions less than ideal?
1 - 6 generally unsatisfactory nodulation. Requires evaluation of inoculants used, inoculation methods and of growing conditions on site. Top dressing with Nitrogen may be required for this year of production.
In the field, a healthy plant does not always reflect effective nodulation and active nitrogen fixation. Localized soil environments, particularly with variations in soil nitrogen, may stimulate vigorous growth of the plant. Such situations are only apparent when the plants are excavated and examined for the presence of active nitrogen-fixing nodules.