Best practice options for rhizobial inoculation can help southern growers lift the supply of fixed soil nitrogen that underpins high value pulses and cropping system diversity.
Highlighting the importance of this nutrient supply to Australian farming systems, pulses are estimated to fix about 100 kilograms of nitrogen per hectare, worth about $220 million a year to the grains industry.
Dr Matthew Denton from the University of Adelaide says GRDC-invested research shows rhizobia inoculation with peat slurry generally delivers the highest rate of nodulation, compared with granular and liquid inoculants.
This is because peat slurry inoculants generally contain high rhizobia numbers, which more consistently achieve inoculation responses than other inoculant types, plus applying inoculant on seed promotes early nodulation of emerging roots.
"Peat slurry inoculant applications often provide the best nodulation when sown into moist soil due to the high number of rhizobia they contain and the protective properties of peat," Dr Denton says.
However, peat slurry inoculation can be inconvenient to apply to large volumes of pulse seed, particularly when there is time pressure at sowing.
As such, more convenient application options, such as granular and liquid inoculants have become increasingly popular in the past two decades, also in part, because they allow growers to separate rhizobia from toxic chemicals.
Dr Denton says mixing peat inoculants with herbicide, fungicide, insecticide or fertiliser can see a rapid decline in the nodulation potential of pulse crops.
Ongoing laboratory and field testing aims to measure the effects of these crop inputs on rhizobial survival, but preliminary findings suggest that mixing zinc sulphate seed treatments is highly toxic to rhizobia.
"Use of additives with rhizobia, such as fertilisers, seed-applied fungicides and organic products can lead to a reduction in rhizobial survival, nodulation, nitrogen fixation and grain yield," Dr Denton says.
New acid-tolerant rhizobia
Another component of the research, in partnership with the South Australian Research and Development Institute, is examining how new rhizobial strains can improve faba bean and lentil nodulation in acid soils.
To date, trials show two acid-tolerant rhizobia strains for faba beans - SRDI954 and SRDI969 - increased nodulation and nitrogen fixation compared with the current commercial rhizobial strain, WSM1455.
Dr Denton says it is hoped the new acid-tolerant rhizobia might also help improve lentil symbiosis and assist with expansion of the legume cropping area on more acidic soils.
"But growers should still consider liming to ameliorate low pH soils," he says.
GRDC Research Codes UA00138, DAS1805-004RTX
More information: Matthew Denton, firstname.lastname@example.org, 08 8313 1098