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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

Portia is a Trace and Save researcher and has been part of the team that works on the Woodlands Dairy sustainability project for the past 4 years. She studied B.Sc in Agriculture where she majored in crop and soil science at the University of Fort Hare. She went on to do her honors and master’s degree in soil science at the University of the Free State. She is very passionate about soil health and soil microbiology and believes that applying soil health principles will rehabilitate degraded soils.

In her free time, Portia loves catching up on House of Cards and The Walking Dead series. The latter she says she finds it fascinating how a dead decomposed organic material can still be conscious, this actually breaks all rules of microbiology according to her. When she’s not watching that, she enjoys watching motivational videos from Ted, especially ones by her idols Chimamanda Ngozi Adichie and Brene Brown.
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Portia Phohlo

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