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Key Considerations in Legume Inoculation This Season

Applying inoculants at sowing time is critical for nodule formation on the roots of legumes.

Inoculants contain microscopic rhizobia that convert atmospheric nitrogen into plant available soil nitrogen through nodules on the roots of legume plants. This nodulation process is important for the legume plant to produce its own nitrogen requirement as well increase the soil N for future crop rotations. This extra nitrogen is of great benefit to crops grown in the following year and can reduce the need to apply inorganic fertilisers.


Rhizobia contained in inoculant must be placed near the seed in the furrow to allow communication with early roots to start the nodulation process. This is especially important in paddocks that have no previous inoculation history, as it is unlikely that there would be enough naturally occurring rhizobia in the paddock to effectively start the nodulation process to fix nitrogen during the season. 


The most common forms of inoculant for broadacre cropping is peat and granular. Peat inoculant that is applied as a slurry directly onto the seed provides consistent coverage, ensuring high rhizobia availability, thus achieving good nodulation rates. However, the application of peat slurry to seed is a bit of a messy job and demands a bit more time of the grower. Peat slurry inoculation has a 24 hour turnaround time for sowing, which growers can also find difficult to achieve at times. 


Self-sticking peat formulations are also available, which allow peat to be applied dry to seed as it moves up the auger into seeding equipment. This does simplify the application process, although it should be noted that the inclusion of moisture, either by pre moistening seed or creating a slurry, will encourage better nodulation as rhizobia need moisture to survive in the soil. This will also help increase the sticking potential of the peat to the seed. 


Using granular inoculant instead of peat can alleviate some of the application issues, as it doesn’t need to be made into a slurry and hence is easier to move through machinery. However, it is recommended that the granules are completely separated from the seed for application accuracy. This is usually achieved by sowing the granules through the small seed box.


Acidic soils are another constraint to nodulation in legume crops and affect rhizobia survival in both granular and peat inoculant situations. Recent studies by Ross Ballard of SARDI have found that survival and efficacy of rhizobia in acidic soils is much lower compared to soils with a higher pH. The effectiveness of the rhizobia has been shown to improve by increasing inoculant above label rates, especially when a paddock is being sown to legumes for the first time and has no background rhizobia.

In response to this issue, the SARDI research team have been developing several strains of acid-tolerant rhizobia for future release in commercial inoculants. These products are still in the trial phase, with experimentation to conclude at the end of the year. Western Ag will provide more information on the progress of these as they become available. For any further information regarding inoculation of legumes in sub-optimal soil conditions and product availability for this season, please don’t hesitate to call myself or the Western AG team.

Source: Maarten Ryder. University of Adelaide
Photo 1: Adequate nodulation on Faba bean plants is considered to be between 50-100 nodules per plant, while poor nodulation is considered to be less than 20 nodules per plant
Article produced by - Claudia Higgins, Western AG Hamilton
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