Legume inoculation is a crucial step in ensuring optimal nitrogen fixation and plant growth. This guide answers common questions about rhizobia, inoculant selection, storage, and application techniques. Whether you’re new to inoculation or looking to refine your approach, these expert insights will help you maximize nodulation and improve soil health.
What are legume inoculants?
Legume inoculants contain live rhizobia bacteria, which play a crucial role in nitrogen fixation. These products are perishable and sensitive to environmental stressors like high temperatures and drying, which can reduce their effectiveness.
What is the purpose of inoculation?
Inoculating legume seeds or soil at sowing introduces beneficial nitrogen-fixing bacteria near emerging roots. This promotes effective nodulation, ensuring optimal nitrogen fixation.
Why are there different inoculant groups?
Each inoculant group contains a specific strain of rhizobia tailored to certain legume species. Selecting the correct inoculant for your legume ensures effective nodulation and nitrogen fixation.
Can soil be tested for rhizobia?
Currently, there are no commercial tests available to identify specific rhizobia in soil. However, if a legume has been previously grown successfully in a paddock with strong nodulation, rhizobia are likely present.
How do I determine if inoculation is needed?
The need for inoculation depends on the legume species, paddock history, and soil conditions. If the soil lacks compatible rhizobia, inoculation is necessary to support nodulation.
Which inoculant formulation is best?
Peat inoculants are commonly used due to their reliability and cost-effectiveness. However, other formulations may be more convenient or suited to specific conditions. Always check for certification to ensure quality.
What are the benefits of inoculation?
Inoculation is crucial for legumes that have never been grown in a paddock before. Even in soils with existing rhizobia, applying high-quality inoculants improves nodulation by introducing superior strains.
How do I assess nodulation?
Nodulation should be evaluated approximately eight weeks after sowing. To accurately assess root development, carefully extract plants from the soil, ensuring the root system remains intact, and gently wash away any soil. While nodulation patterns vary among legume species, the presence of multiple pink nodules near the upper roots typically signifies successful and timely nodulation.
When should I inoculate bare seed or re-inoculate pre-coated seed?
You should inoculate or re-inoculate legume seed in the following situations:
- First-time planting or long gap – If the specific legume hasn’t been grown in the paddock before, or if more than four years have passed since it was last sown.
- New rhizobia strains available – When improved rhizobia strains with better nitrogen fixation and survival rates are available compared to those used previously.
- Old pre-inoculated seed – If the batch date of the pre-inoculated seed is older than five weeks before sowing.
- Challenging soil conditions – If the paddock has highly acidic (pHCa <5.5) or highly alkaline soils that could reduce rhizobia survival.
Can inoculation rates be reduced?
Lowering inoculation rates is not recommended, as insufficient rhizobia can lead to poor nodulation. Using the correct amount ensures strong root development and effective nitrogen fixation.
What is the role of lime pelleting in legume seed treatment?
Lime pelleting helps manage moisture, prevents seed clumping, and improves rhizobia survival, especially when seeds are sown into acidic soils or fertilisers that may affect rhizobia viability.
How long can inoculated seeds be stored?
Freshly inoculated seed should be sown as soon as possible, ideally within 24 hours. Over time, rhizobial numbers decline, reducing the effectiveness of inoculation
Should starter nitrogen be used?
Generally, legumes do not require starter nitrogen, as excess soil nitrogen can suppress nodulation. However, in soils with very low nitrogen levels, a small application may benefit early growth.
What fertilisers do legumes need?
Legumes require phosphorus and potassium, along with trace elements like molybdenum, which is essential for nitrogen fixation. Care should be taken with molybdenum fertilisers, as some can harm rhizobia.
Can inoculated legumes be dry sown?
Dry sowing is not ideal, but if unavoidable, sowing deeper into moisture and using paddocks with a history of successful legume growth can reduce the risk of nodulation failure.
How long will rhizobia survive in dry soil if it doesn’t rain?
Survival depends on factors like soil type, depth, humidity, rhizobial strain, and inoculant formulation. Inoculants are best applied to seed or directly into moist soil.
Can different inoculated pasture species be sown together?
Yes, inoculated pasture species can be mixed without affecting nodulation. If using granular inoculants, apply the full recommended rate for each species in the mix.
How is nodulation assessed?
Nodulation should be checked about eight weeks after sowing by carefully digging up plants and washing their roots. Effective nodulation is indicated by numerous pink nodules near the upper root system.
What if I forgot to inoculate?
If inoculation was missed, it is difficult to fix later. One option is applying a granular inoculant near the original sowing furrow as soon as possible. However, results decline over time as plants mature.
Can inoculant be sprayed onto soil or crops?
No, spraying inoculants onto soil or plants is not effective and is not recommended.
Are rhizobia affected by pesticides and fertilisers?
Yes, many chemicals and fertilisers can harm rhizobia. Fertilisers containing zinc or those with high acidity can be particularly damaging. If pesticides must be applied, consider direct soil inoculation instead.
Can an opened packet of inoculant be reused?
Yes, but it must be stored properly. Seal the packet tightly, remove air, and store it in a refrigerator at 4°C. Exposure to air and moisture can reduce the viability of the bacteria, so use it as soon as possible and always before the expiry date.
