We take it for granted that legumes such as clover provide our pastures with lots of nitrogen. However, this is often not the case. Soil health and chemical contamination can significantly reduce the amount of nitrogen that legumes fix from the atmosphere. So rather than assuming all is well, it’s important to dig up legume roots to see how well they are nodulating.
Legumes can provide nitrogen to a pasture sward or the following crop. But how much depends on how well the legumes are nodulating. Because legume root nodules are fully developed by September, now is a good time to dig some up and take a look. To help, we’re going back to an article published in 2019 to remind producers about nodules and how to keep them healthy.
How well are your legumes fixing nitrogen?
Research suggests that an optimally performing legume can provide about 25 kilograms of nitrogen for every tonne of dry matter grown per hectare, released slowly over three years.
Adequate performance is indicated by a legume root covered with lots of small, and several large, pink nodules. However, pasture legumes often under-perform.
In 2019, South West NRM engaged MALDI-ID Director Dr Sofie De Meyer to help farmers check and improve their nodules. She said that in the NSW’s Riverina region, a study found 93 per cent of samples had unhealthy or ineffective nodules.
Dr Sofie De Meyer at first soils constraints event in Nannup
South West NRM sampled 24 properties under their Soil Constraints project and found similar results, with 42% of sub clover plants found to be poor, 38% moderate and 20% adequate. We concluded that nodulation is a key indicator of soil health.
What’s stopping legumes fixing nitrogen?
The answer lies beneath the soil. Dr De Meyer explained how farmers can carefully dig up and wash several legume roots to assess performance using a nodule scoring system.
If assessment confirms that a problem exists, farmers can start to diagnose the cause by considering the following questions.
Q1: Has the soil been inoculated with the most suitable rhizobium strain in the past four years?
Rhizobia, or root-nodule bacteria, establish mutually beneficial relationships with legume roots to fix nitrogen. Rhizobia occur naturally in soil, but the genera is genetically diverse. Different strains vary in effectiveness, so commercial inoculants have been developed that contain specific strains that optimise nitrogen fixation for specific legumes. Dr Neil Ballard who also spoke at South West NRM’s 2019 Soil Constraints event, explained that crossing breeding of rhizobia can reduce their effectiveness.
Q2: Is the soil pH suitable for the specific strain of rhizobium?
Dr De Meyer said that Soil pH is one of the main factors affecting legumes and rhizobia, with the rhizobia more sensitive to low pH. “One study showed that an increase in pH from 4.5 to 5.0 increased the proportion of plants with nodules from less than 60% to over 80%.”
Q3: Is there enough Molybdenum in the soil?
Molybdenum is an important micronutrient required for nitrogen fixation. South West NRM testing found that around half of the beef properties with sub clover sampled had low or marginal molybdenum levels. Read more about molybdenum, soil pH and how to check micronutrient levels.
Q4: Are legumes suffering from root disease?
Diseased roots typically have less than adequate nodulation – which was observed in our Pasture Challenge Project. Our Soil Constraints project found that two-thirds of sites in the high rainfall South West had severe root disease.
Q5: Are chemical residues killing rhizobia?
Sulfonylureas (SU’s) can have severe impacts on legumes and rhizobia. Dr Ballard explained how the inoculation process can expose rhizobia. “Toxic chemicals, fungicides, solvents, alcohols, heavy metals and disinfectants will all kill rhizobia,” Dr Ballard warned. “When you’re making up a peat slurry or liquid inoculant, don’t use an old chemical container. Use a container and water that is clean, preferably not chlorinated.”
Q6: Is the rhizobia desiccating during inoculation?
Dr Ballard said: “Inoculated peat slurry on the seed needs to be sown as quickly as possible, within 24 hours, and it needs to go into moist soil. You really need to read the instructions on the packet and follow them carefully to maximise the amount of inoculant that you get into the soil.”
For growers with plenty of clover seed in the soil bank, Dr Ballard suggested that direct drilling a granular inoculant into the soil at the break of season could be effective. Alternatively, growers could use a peat slurry inoculant, which is considered the cheapest and most effective method, on small seeded varieties such as canola to introduce the inoculant into the seed bank as it germinates.
Check your legumes before flowering
Dr Ballard finished by recommending all farmers learn what is happening below ground by digging up legumes six to eight weeks after germination and before flowering to score nodules.
“There is a huge saving in nitrogen from pasture legumes, but it could be more if we fix more efficiently,” he said.
For further information, download Inoculating Legumes: Practice and Science” from the GRDC website.