Research producing sweet outlook for pulse crops on acidic soils
Author: Sharon Watt | Date: 17 Apr 2018
Research is offering hope to grain growers who have been limited in their ability to successfully grow pulse crops on acidic soils in the southern cropping region.
New strains of rhizobia with improved acid tolerance have shown promise in field trials, lifting the prospects of commercialisation and a subsequent expansion of faba bean and lentil crops into areas with acidic soils.
Profitable production of pulses is dependent on successful nodulation of the crop, so growers inoculate seed with rhizobia bacteria that stimulate the production of nodules on the plant roots which fix atmospheric nitrogen for the pulse crop. This free source of nitrogen ensures a healthy pulse plant with little or no reliance of nitrogen from the soil, and also delivers excess nitrogen that supports crop production in subsequent years.
However, the current commercial strains of rhizobia are typically much more sensitive to acid soils than the host legume plant, so a Grains Research and Development Corporation (GRDC) research investment has been testing the performance of alternative strains of rhizobia that have been identified with improved acidity tolerance.
Research leader Ross Ballard, from the South Australian Research and Development Institute (SARDI), a division of Primary Industries and Regions SA, says 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 percent 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 says.
Mr Ballard, who has been undertaking the rhizobia trials with fellow SARDI researcher Liz Farquharson, says further evaluation of the strains will be undertaken this year and that a new strain for faba bean (and possibly lentil) could be commercially available in 2022, subject to further positive results.
It is envisaged that these new strains may enable expansion of pulses onto more acidic soils than has previously been possible. “We are aiming for reliable nodulation where soil pH is 4.5 when measured in calcium chloride,” Mr Ballard says.
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 52% and grain yield by 41% 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.”
Mr Ballard encourages growers to test a small batch of seed inoculated with peat slurry to ensure seeder blockages don’t occur, especially with any increase in inoculation rate.
Collaborating with SARDI on the acid-tolerant rhizobia studies have been The University of Adelaide, Agriculture Victoria, AgGrow Agronomy in New South Wales, and the Western Australian Department of Primary Industries and Regional Development.
In other related research, the compatibility of rhizobial inoculants with some commonly used herbicides, fungicides, insecticides and fertilisers is being tested in the laboratory to determine the effects of these additives on rhizobial survival.
Mr Ballard says particular care needs to be taken where rhizobia are applied with pesticides on seed, especially where it is to be sown into acidic soils.
“Rhizobia are best applied last and as close as possible to sowing. And where pesticide application is necessary, granular rhizobial inoculant may provide a better option, reducing direct exposure of the rhizobia to the pesticide.”
Improving the survival and viability of rhizobia in current farming systems is an issue identified as a high priority by the GRDC’s Southern Regional Cropping Solutions Network.
Further information on pulse crop inoculation is contained in a number of practical GRDC resources, including the popular Inoculating Legumes guide and a GrowNotes™ Legumes and Nitrogen Fixation Tips and Tactics fact sheet.
Ross Ballard, SARDI
08 8429 2217
Sharon Watt, Porter Novelli
GRDC Project code: UA000138, DAS00128
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