Short interview (circa 2016) describing GRDC‑funded trials to improve biological nitrogen fixation in beans, peas and faba beans on acid soils in southern Australia. Researchers isolated acid‑tolerant Rhizobium strains (from WA, Sardinia and other acidic soils), screened them in glasshouse hydroponic tests at low pH (4.2), and progressed top strains into paddock trials. The project also ranks field pea cultivars and is developing a nitrogen fixation index (due mid‑2017). Economic and agronomic context (area grown, current fixation benchmarks, potential nitrogen and dollar gains) are presented.
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Grain growers with very acid soils face a real challenge growing bean and pea crops. However, new strains of Rhizobia bacteria, which can tolerate acid soil conditions, are showing real promise. Inoculation of beans and peas is strongly recommended on acid soils in southern Australia to build the necessary background levels of symbiotic bacteria to promote soil nitrogen fixation and root nodulation. These are soils that have a ph of less than 6.
This is actually the first test of these strains in the paddock.
The southern trials are part of the GRDC funded nitrogen fixation program and complement trials being run in the western region. On field P, where acid soils are the main constraint, suitable rhizobia had to be found.
The strains have come out of soils from Australia and overseas that are quite acid in nature and we've isolated the rhizobium from those soils and we've tested them in a hydroponic system in the glass house.
Field peas were used for the glasshouse test, which rigorously selected strains of acid tolerant rhizobia by adding them to a solution with a low pH of 4.2. The international material came from the program's Western Australian team, who have already established that their new rhizobia can survive in WA soils.
They've isolated some pea strains from where peas originate over in Sardinia, and they've come back with a selection of potentially acid tolerant pea strains also.
A couple of years on and the fieldwork is well underway. Three species make up 85% of locally grown pulse crops in the southern region. Faber bean, lentil and field pea. But all three can be nodulated by the same Rhizobium strains.
So what we've done in this field trials this year is we've taken the three best strains from Sardi and the three best strains from our West Australian colleagues, and we're now testing them in the field on fava, bean and shield peat.
Sardi's team, the last, believes 25% of bean and pea crops in the southern region are producing around 15 kilos of fixed nitrogen per tonne of dry matter. One of their objectives is to significantly increase the nitrogen fixation of those underperforming crops.
The benchmark for nitrogen fixation is about 22 kg of fixed nitrogen per tonne of dry matter that the pulse produces. The field pea, for example, are about 5 kg per tonne of dry matter off the pace and in some instances it's down as low as 13kg. So that's sort of the opportunity to improve things.
Some 600,000 hectares of pulse crop are grown annually in the southern region, generating about 70,000 tonnes of fixed nitrogen. So optimising nitrogen fixation could significantly lessen the amount growers need to spend on artificial fertilisers. What should also help achieve input cost savings is another of the research project's objectives. The ranking of field pea cultivars.
We've been measuring nitrogen fixation and nodulation of those crops in order to come up with an index that growers will be able to use to understand which crops are putting more nitrogen in and I guess rank those crops and help them make decisions around nitrogen inputs into their systems.
The nitrogen fixation index will be available from mid 2017. The new acid tolerant strains of Rhizobium for pea and bean need more trialling before being made available to growers.
We've certainly hopeful that we will produce a new acid strain of Rhizobium and that would likely be commercialised within the next three years if it proves to be successful, so that will be a significant outcome for growers.
Nitrogen fixation from pulses in the southern region is estimated to deliver a benefit of $55 million worth of nitrogen per annum. Optimising the symbiosis has the potential to deliver an extra $4 million worth of nitrogen with similar success in the western and northern regions. The national gain has been estimated to be in excess of $10 million per year. Add to that the potential to improve crop health by the use of a pulse as a disease break in the rotation and reduce chemical input costs, the financial return on this R and D investment could be even greater.
