Questions on legume inoculation answered
GroundCover™ Issue: 127 Mar - Apr 2017 | Author: Nicole Baxter
Legume inoculation specialist Neil Ballard regularly speaks to growers, agronomists, consultants and students from agricultural colleges, high schools and universities about how to successfully inoculate pasture and pulse seed with rhizobia. Here, the GRDC-supported extension specialist who works with Global Pasture Consultants, answers some of the questions he is commonly asked.
How often should a paddock be inoculated with rhizobia?
A lot of people think that once a paddock has been inoculated, that’s enough for life, but this is not the case. Paddocks need to be re-inoculated every four to five years. And with some pulse crops, particularly chickpeas and field peas, they should be inoculated every time they are sown. In WA, there is a lot of background lupin inoculum, so it is sometimes difficult to justify inoculating lupins every time they are sown. But remember that rhizobia break down in genetic purity over time. This is because the commercial rhizobia cross with native rhizobia in the soil and lose their genetic purity, which will reduce how much nitrogen is fixed.
What is the best form of inoculum to use?
The best way to get the most rhizobia into the soil is to treat seed with peat inoculum and sow the seed into moist soil within 24 hours. Seed treated with freeze-dried inoculum has to be put out into moist soil within five hours of application. If legumes are sown into dry soil, the best option is granular inoculum.
How can established and poorly nodulating clover be re-inoculated without resowing?
Inoculate canola seed with peat rhizobia and sow the seed soon after the seasonal break so freshly germinating clover isn’t killed. Not only will this improve clover nodulation, but it also provides a source of extra early feed. The other option is to top-dress the paddock with Alosca® dry rhizobia granules. Research has shown this formulation of rhizobia can tolerate temperatures of up to 60ºC for up to two months.
Is the granular form of rhizobia effective?
Granular protected rhizobia are very effective if used correctly. They can be put out the year before pastures or pulse crops are sown. Some rhizobia have saprophytic competence, which means they can multiply and colonise the soil without a host plant. These rhizobia build up in the 12 months before a pulse is sown or a pasture legume regenerates. Murdoch University’s Dr Ron Yates is searching for a new rhizobia strain with saprophytic competence so that it can be applied as a granular inoculum allowing rhizobia numbers to increase by the time field peas are sown in the following year.
How do I handle peat?
Peat inoculum contains live bacteria, which need to be treated with care. After purchase, keep the inoculum cool, but don’t freeze it. Check the label to ensure the inoculum is not out of date. Mix it thoroughly, apply it to the seed and then sow the seed within 24 hours. About 90 per cent of the rhizobia die within the first 24 hours after application in the drying-out process. But because billions of rhizobia are in the inoculum, there will still be plenty left for effective nodulation. Ninety per cent of the remaining rhizobia will die every 24 hours after that so it is important to get the rhizobia into moist ground quickly for the best results. Avoid mixing peat inoculum with fungicides, insecticides and liquid fertilisers. Also, any solutions with a trace of copper or zinc will kill the rhizobia.
What inoculum group?
Most inoculum products are species specific. Use the correct inoculant group for a particular legume host (Table 1).
|Rhizobia|| Commercial inoculant group
|| Legumes nodulated
|Sinorhizobium species||AL||Lucerne, strand and disc medic|
|AM||All other medics|
|Rhizobium leguminosarum bv. trifolii||B||Perennial clovers|
|C||Most annual clovers|
|Bradyrhizobium species||G1||Lupins, seradella|
|Rhizobium leguminosarum bv. viciae||E2||Field peas and vetch|
|F2||Faba beans and lentil|
|Bradyrhizobium species||I||Cowpeas, mungbeans|
|Mesorhizobium cicero bv. Biserrulae||BS||Biserrula|
1. Both inoculant groups G and S can be used for lupins and serradella.
2. Although Group E is recommended for field peas and vetch and Group F for faba beans and lentils, if required Group E can also be used for faba beans and Group F used for peas and vetch.
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