Inoculating Legumes Seeds
Procedure
- Add the inoculant to another medium (eg peat mixed with water and gum arabic) – use 1 part sticking substance (eg gum arabic) to 10 parts water; other sticking materials that can be used include corn syrup, sugar, powdered milk or various commercial stickers
- It is critical to use only fresh inoculant in the appropriate concentration
- Use the appropriate rhizobium for the legume being grown; keep in mind that rhizobia perform better on some legumes (eg alfalfa) when seed is coated with calcium carbonate, while others perform better when left uncoated (eg red clover)
- Check the expiry date – commercially produced, pelleted seed should be sown as soon as possible; at least within four weeks of production, as it does not store well
- Always store inoculant in a cool, dark place
- In dry conditions, inoculant rate may need to be doubled
- If legumes exhibit yellowing of foliage, this may indicate nitrogen deficiency resulting from failure of the inoculant
- Applying some nitrogenous fertiliser when planting a cover crop may actually enhance the nitrogen fixation of the legumes (eg around 30 kg per hectare of starter nitrogen)
- Generally soil pH needs to be over 5.5 for rhizobia to survive
How to Assess If the Inoculant Is Effective
When using an inoculant on a pulse crop, it's beneficial to assess its effectiveness as the crop begins to grow. This is simple to accomplish through an analysis of the nodulation and nitrogen fixation.
When to Perform a Nodule Assessment
Farmers are advised to perform an assessment of nodulation during early flowering. Typically, nodule formation starts around 14 days after the crop emerges. Under some conditions, though, nodules may form at three to four weeks.
The number of nodules formed and the rate of nitrogen fixation are usually at their highest in early to mid-flowering. Nodule efficiency is diminished after flowering.
Three-Step Process of a Nodule Assessment
Choose five places in the field that represent what is typical at early flowering, and then execute the three steps below in each of the five places to determine the nodulation and potential of nitrogen fixation of the crop:
- Review the Quality of the Plant Ideally, the plants in all five locations will be green and appear to be flourishing in their growth. If nitrogen fixation is poor, though, the plants' growth may be stunted or their leaves may exhibit chlorosis (yellowing).
- Check the Position of Nodules Using caution not to harm the plants, dig around at least two of the plants in each of the five predetermined areas of the field, but do not remove them from the soil. When the roots become visible, examine the nodules. Adding a little water may help with the assessment. Ideally, both crown and lateral nodulation will be present. If the seeds are successfully inoculated, there will mostly be crown nodulation present. The crown refers to the soil that surrounds the seed. If there is native rhizobia in the soil, or if a granular inoculant was applied, there will be lateral nodulation. Having mainly lateral presence is unfavorable.
- Examine the Nodules' Interior At each of the five locations, cut the nodules so that their interior is visible. The nodule cluster number and colour will indicate the efficiency of the nitrogen fixation. If there are upwards of five clusters of deep pink nodules, there is strong nitrogen fixation efficiency. The pink shows that the leghemoglobin required for active nitrogen fixation is available. However, if there are fewer than three clusters of pink nodules or if the nodules are closer to white, green, or brown, then the nitrogen fixation is not effective.
Overall, the warning signs that the inoculant is not working effectively include:
- Diminished growth
- Yellow leaves
- Lack of a healthy mix of crown and lateral nodulation
- Less than three clusters of pink nodules
- Discolored (white, green, or brown) nodules
If the majority of these warning signs are present, consider top dressing with nitrogen.