Late winter and early spring is the best time to check legume nodules. To fix nitrogen, legumes need the symbiotic soil bacteria rhizobia in their root nodules. Legumes without effective nodules will deplete soil N and can perform poorly.
Lack of rhizobia causes poor nodulation. Rhizobia come from the soil or from seed coated inoculums. Inoculate seed on acid soils or where that type of legume has not been grown recently. Storing inoculated seed in contact with some crop protection chemicals can reduce nodulation.
How to check nodules
- Collect about 10 plants from three different spots in a paddock. Keep samples separate.
- Wash roots to remove soil. Plants from heavy soil may need soaking for up to 30 minutes.
- Count the number of nodules on each plant. 50 per plant is good for field peas, vetch, and faba bean.
- Soil type affects nodule numbers. There are usually fewer nodules on sandier soils.
- Check nodule location. Inoculated seed should have more nodules around the crown. If rhizobia were already in the soil, non-inoculated legumes will have nodules across the root system.
- Cut open nodules to check colour:
- a red/pink colour means the nodules are effective and are fixing nitrogen
- white or green nodules mean they are ineffective.
Check nodulation regardless of whether inoculants were applied or not.
Common causes of poor nodules include:
- Soil acidity. Most rhizobia strains are sensitive to pH below 5.5 and above 7.0. A crop grown in a soil with a pH of 5.2 will have almost no nodulation and will use more N than it will fix.
- Seeds were not inoculated and rhizobia was not available in the soil.
- Timing. Seeds were inoculated too early before planting. Ideally, sow seed within a day of inoculating.
- Poor quality inoculant. Use a quality assured inoculant with the green tick logo
- Wrong inoculant. Different groups of rhizobia suit different legume varieties.
- Crop sown into dry soil.
Adjusting crop nutrition
While you can’t fix poor nodulation mid-season, you can refine your N calcs and N carryover decisions. Fewer nodules mean less N fixed, and if nodulation is very poor the crop may be a N user. Re-check N stocks before sowing next season.