ONE OF the major constraints in successfully growing pulse crops on acid soils has been the lack of rhizobia with acid tolerance.
Rhizobium bacteria have a symbiotic relationship with legume plants, ‘infecting’ the roots with nodules that fix atmospheric nitrogen for the plant to grow.
It is the reason pulse crops mean there is excess nitrogen in the soil for the following year’s crop, a key factor in many farmers’ decision to plant pulse crops.
Growers will inoculate the seed with rhizobia prior to planting to promote nodulation.
Rates of nodulation on the acid soils of southern Australia have generally been poor and work has gone into finding new rhizobia strains with improved acid tolerance.
It is hoped the new strains of rhizobia will lead to farmers planting pulse crops on paddocks with pH as low as 4.5.
Farmers at present will generally be reluctant to plant pulses on paddocks with pH below 5.
Research work conducted by the South Australian Research and Development Institute (SARDI), funded by the Grains Research and Development Corporation (GRDC) has made some promising findings in terms of rhizobia suitable for inoculating faba bean and lentil crops.
Research leader Ross Ballard, from the South Australian Research and Development Institute (SARDI) said field trials over the past three years have demonstrated the effectiveness of the new strains in nodulating faba beans and lentils on acidic soils.
Mr Ballard says the new strains of rhizobia have resulted in a dramatic improvement in nodulation compared to commercial strains when applied at recommended rates of inoculation in acid conditions.
Increases in faba bean nodulation of about 30 per cent have been consistently measured with the new strains where soil pH is less than 5.0 when measured in calcium chloride.
“The new strains are improving nodulation and ultimately that results in better crop vigour early in the growing season, as well as improved nitrogen fixation and yield,” he said.
Mr Ballard said the rhizobia could be available commercially as early as 2022, subject to further positive trial results.
However, the development of new rhizobia strains will be a stop-gap measure to manage soil acidity, and should not be seen as a replacement for liming, according to Mr Ballard.
“Even with good inoculation practices on acid soils, nodulation can remain below potential and rhizobial colonisation and survival in the soil is limited, so the addition of lime is still needed.”
Liming to raise soil pH also corrects nutritional deficiencies and toxicities that more broadly limit crop performance.
Until the improved strains are commercially available, growers are also advised they can improve their chances of successful nodulation in acidic soils by doubling the rate of inoculant.
Doubling the inoculant applied as a peat slurry increased nodulation by 52pc and grain yield by 41pc in a faba bean trial at Wanilla on SA’s Eyre Peninsula.
“Doubling the rate also provides a practical way of improving nodulation where pulses are sown for the first time, especially on hostile soils.”