Check nodulation in early spring

01 Sep 2022

Tablelands Telegraph - September 2022

Clare Edwards, Senior Land Services Officer - Pastures

Legumes perform an important function in our pastures. They add diversity to our pasture composition and also supply good quality feed.

One of the critical contributions to our pastures is their ability to supply nitrogen (N) – one of the essential nutrients – to our plants. To do this, legumes have small ‘bumps’ known as nodules on their root systems. These nodules contain a symbiotic bacteria rhizobia which converts atmospheric nitrogen to ammonia.

Pasture legumes can usually fix between 20 and 40 kg of N per tonne of Dry matter they produce. However, above-ground legume appearance and abundance is not always a reliable indication of what might be happening below.

Early springtime is a great time to check your legume nodulation. The question arising is then ‘how do you assess your legume nodulation’?

1. Using a shovel, carefully excavate 15 – 20 plants from a representative part of the paddock. I usually take a 20cm X 20cm square around the plants to a depth of 30cm. I pop these into a bucket of water and carefully wash the soil away from the roots. Note: do not pull the plants out, as this may strip the nodules from the root system. Take care when washing the soil away from the roots, as it can be easy to accidentally knock off the nodules. Lastly, I tend to use warm water, as cold water outside can freeze your hands at this time of year!
2. Use a paper towel to blot up excess water. Then, assess the number and colour of the nodules. Adequate nodulation is greater than 20 nodules on the root system of a single plant. Open the nodules to see if they are pink in colour. A pink to reddish colour indicates the presence of leghemoglobin, meaning that the nodules have been fixing nitrogen. The size, density and frequency of nodules down the root system is important. Note that, if this assessment is late in the season, the colour in the nodules may be more dark red/pink or tending to green. If white inside, they have ineffective strains of rhizobia and may not be fixing much N.
3. Use the chart in this MLA publication to score and assess your legume nodulation level.

Nodulation can be affected by four factors.

1. Nutrient availability and plant growth

   Legume plants (and consequently their associated rhizobia) require specific nutrients for growth, function and persistence. Having adequate phosphorus, sulphur and potassium is essential for plant growth. Molybdenum, a trace or micronutrient, is also important for nodulation and nitrogen fixation.

2. Soil pH and available Aluminium

   Acid soils can not only affect the plant, but also the rhizobia function. Sometimes, plants can tolerate lower pH in soil solution than their rhizobia. For example, the sub-clover plant can perform well in pH down to about 4.2. However, the rhizobia (Group C inoculum) is more sensitive. Coupled with pH, aluminium is often found to be more available in lower pH soils. This can also have an impact on root development and hence nodulation and rhizobia.

3. Herbicide use

   Some herbicides and residuals can adversely impact root development. Check and observe plant-back periods.

4. Other – this includes environmental factors such as waterlogging. The time of assessment and how the samples are collected is important to note, as these can affect the results. Spring is the best time. Also, some legume species – a such as the serradellas – are prolific in nodulation. Nodulation survey results from two different studies revealed that 90% of paddocks had inadequate nodulation. In one survey, across a number of regions, up to 20% of paddocks had no nodules present on the legume plants.

Two recent papers that cover survey work on the Central Tablelands (both papers are available on request) -

1. Soil acidity and nutrient deficiency cause poor legume nodulation in the permanent pasture and mixed farming zones of south-eastern Australia (Hackney et al. 2019)
2. Legume content and sub-optimal nodulation linked to soil acidity and nutrient availability in the Mudgee region of the Central Tablelands, NSW (Edwards and Hackney 2021)

What to do next?

Don’t panic!

It is worth exploring a few questions before we make major decisions. For example, are we seeing low nodulation across the paddock and across different legume species? What are the current soil and environmental conditions? What is the history of legumes in the paddock (never sown, legumes sown with fertiliser in the 1980s, newly sown pasture etc). What herbicides and weed management have you used in the last 5 years?

Depending on the answers, there may be a number of management options which we can talk to you about.

One of the most important steps is to examine the adequacy of nutrients, pH levels, and other soil constraints. We normally do this with soil testing and a check of soil health parameters.

Other management practices might involve using sub-clover with a current inoculum for topdressing pastures in autumn next year.

As always, we are happy to talk to landholders individually about their situation and to see you’re your photos of legume nodulation from across the Central Tablelands.

Contact a pasture officer.