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These are some questions I was asked by a farmer in the Tsitsikamma the other day about legumes and root nodules.

How long does it take for nodules to form?

The time taken for nodules to form is species-specific, but you can generally start to see nodule formation within the first 2 weeks of planting.  Lucerne will show visible nodulation within the first 10 days and clovers within 7 days.

What does the number and mass of nodules per plant tell you about the activity of the nodules?

Irrespective of plant species, more than 15 nodules on roots are indicative of good plant performance and health. A soil fertility test should confirm whether the plant is under any stress from nutrient deficiency or toxicity. Remember, a plant that is under stress will not form healthy, active nodules.

Are the symbiotic microbes, which fix nitrogen, inherent in all soils?

Generally, rhizobia are found naturally in most soils. If they are not inherent in the soil, they can easily be introduced via inoculation and will adapt to any soil, provided the soil conditions are conducive to their growth.

What soil conditions are favourable to rhizobium growth?

There are many factors to consider, but the most sensitive is soil pH. When the pH drops to below 5 (acidic conditions), it will completely inhibit the growth of rhizobia, irrespective of whether it is in the soil or in the nodule. Most legume roots grow below the literature-defined 15cm “rooting depth”. It is therefore important to fix subsurface acidity as this will prevent rhizobium growth and hence nodulation on the subsurface roots.

NB: When nodules only appear in the upper root zone, this may also be indicative of a shortage of oxygen in the lower root zone. In such cases, a farmer should test if there is subsurface compaction. If this is the case, make sure to improve oxygen circulation of the lower root zone in order to induce development of nodules in deeper soil layers.

Extra reading material

Without oxygen in the soil, nothing works

Tapping into the tons of nitrogen above the soil

Regulation of legume nodulation by acidic growth conditions

Portia Phohlo
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