Media-shy bacteria plays starring role

But it could make you money, so read on.

We’re talking about rhizobia, the nitrogen-fixing bacteria that rarely gets a plug and is arguably, the last thing thought about as you establish a nitrogen-fixing crop.

After all, if you’re sowing lupins or faba beans, chickpeas or a pasture crop, you’ve inoculated the seed with a Department of Primary Industry (DPI)-approved inoculant. Right?

And to double check everything is ‘kosher’, you’ve scratched a root nodule around July to see if it’s pink.

But that paddock test is only indicative.

Because you may have the wrong rhizobia in the nodules, the root may be accessing soil nitrogen instead of ‘fixing’ it, as you intended it to do.

Are you still with me?

Because it gets more interesting.

According to Murdoch University post doctoral researcher Dr Sofie De Meyer, we are introducing ‘selected’ legume species – mainly from overseas – to our WA soils which contain native rhizobia, which aren’t fixing with introduced species.

And each legume species has its own rhizobia family, so to speak, hence the need to use the right inoculant or ‘family’.

And hence the need to ensure you’re using the correct inoculant group for the right legume.

Get it right and you’re on your way to growing healthy plants with more ability to combat diseases and with more potential to yield well.

Up to 70 per cent yield improvements have been recorded where the correct rhizobia have been introduced through inoculation.

The good news for farmers is that Sofie has developed a world-first test that can identify rhizobia types and provide a diagnostic report (see breakout story below).

The test might reveal, for example, an ‘old’ family of rhizobia that is less productive for N-fixing, or the tests may identify native rhizobia.

Interestingly, when Sofie examines samples, which should come with the paddock’s inoculant history, she discovers many farmers simply don’t inoculate.

Symptoms that can alert farmers to testing for the right rhizobia include less biomass, shorter plant, paler colour or an examination of the roots for nodulation.

“And sometimes you can look at a green and vigorous-growing plant and think everything is okay,” Dr De Meter said.

“But studying the roots, you may find no nodulation because the root is taking in the soil nitrogen, which defeats the purpose of growing these legumes in the first place.

“We have found with introduced species such as biserrulas and chickpeas, the bacteria have changed to such an extent that there is native rhizobia not fixing N in the nodules, allowing it to thrive and outcompete against inoculant rhizobia intended to fix nitrogen in the soil.”

This is a major problem because native rhizobia also are more acid tolerant.

The other problem is that ‘introduced’ rhizobia becomes less effective over time because of genetic changes, which is the reason for checking plant status regularly to ensure roots have the optimum population of inoculant rhizobia.

“I would really like to see rhizobia testing become as normal as soil testing,” Dr De Meter said.

“Farmers are aware of the many benefits that can come from testing their soils and that can be extended to rhizobia testing.”

As an annual management practice, it has a lot of merit, particularly as interest grows in moving towards more diversified farming systems involving grain-graze crops and pastures.

And the importance of rhizobia for root nodulation in a dryland environment is underlined by Murdoch University’s Professor John Howieson’s research trials with the South African legume plant Lebeckia.

One of the plant’s major characteristics is the ability of rhizobia to persist over summer, due to its long tap root and ability to access moisture otherwise unavailable to short-rooted legumes.

Moisture is the key to bacterial survival, which points to the necessity to maintain a structured soil profile (for moisture and air access), along with soil amelioration to bring soil pH more into balance.

I’ve said it so many times this year ... old is new.