Two new, high-performing, commercial rhizobia inoculants for lentils, field peas and vetch (Group E) and faba beans and broad beans (Group F) have been developed for use in 2024, supporting and expanding options for growing nitrogen-fixing pulses in acid soils.

Inoculating pulses with rhizobia – soil bacteria that nodulate legume roots – is widely recommended and considered essential in acidic soils to achieve good nodulation of acid-sensitive legumes.

Good nodulation is important as that is where nitrogen from the atmosphere is converted into a form that can be taken up by a plant, and higher levels of nitrogen lead to more biomass and – ultimately – greater yields.

In acidic soils, adequate numbers of rhizobia are unlikely to occur naturally. So inoculating legumes with the bacteria not only helps to make these crops a more-viable option on marginal land, but it also provides a nitrogen boost for subsequent cereal crops.

The new rhizobia strains have been commercialised after eight years of collaborative research across Australia’s growing regions, involving 60 replicated trails sites and multiple partner institutions, with support from GRDC.

Presenting findings at the GRDC Grains Research Update in Adelaide, South Australian Research and Development Institute research scientist Dr Liz Farquharson outlined the significant benefits associated with the new rhizobia strains.

Traditionally, field peas, faba beans, lentils and vetch perform “very poorly” on acidic soils as compatible rhizobia for these crops does not persist below pH 6.0 (calcium chloride), Dr Farquharson said.

Optimal nodulation

While performing equally to existing rhizobia products in neutral soils (above pH 5.5), the new products provided optimal nodulation down to pH 5.0 and improved nodulation to pH 4.5, she said.

“We have two inoculation groups (E and F) and a new rhizobia strain for each group, that are both more acidic-tolerant than what we had previously (WSM1455),” she said. “These strains give a big boost to legumes in soils below pH 5.5 – that is where we’ve got the big gains over the previous strain.”

In field trials in acid soils across seven sites in key grain growing regions, for example, the new strain (SRDI-969) for faba beans showed an average improvement of 65 per cent in nodulation and 24 per cent in nitrogen fixation.

For field peas, lentils and vetch, the new rhizobia strain (WSM-4643), in trials over 10 sites in New South Wales and Western Australia, showed an average improvement of 30 per cent nodulation and an increase of 15 per cent in yield. Yield responses were observed at one-third of all sites.

But while the new strains were an improvement on the past, particularly around improved nodulation, Dr Farquharson cautioned there were limits. Rhizobia numbers would still decline quickly in acid soils, so reinoculation would be required for subsequent pulse crops. And liming would remain essential in helping to improve rhizobia survival and legume plant growth.

“So rhizobia inoculation is just one tool in the toolbox for growing pulses in acid soils. It’s by no means a fix and liming is absolutely still required to remediate acidic soils,” she said.

Building on the positives, Dr Farquharson is now taking part in a new national GRDC investment looking further into nitrogen fixation. With research scientists from Queensland, NSW, SA and WA, she will be looking into improved rhizobia strains for chickpeas and mungbeans as well as broadly addressing constraints to nitrogen fixation and nodulation.

“Those constraints vary across the country, so it is very exciting work,” she said.