A new test that measures rhizobia numbers in the soil will help take the guesswork out of inoculation decisions for pulse growers in South Australia and Victoria.

The PREDICTA rNod soil DNA test, which has been developed with GRDC investment, uses PREDICTA® B technology to quantify the numbers of the Group E and F nitrogen-fixing bacteria present in the soil. Group E and F rhizobia nodulate faba beans, lentils, field peas and vetch.

The cost of rhizobial inoculants is relatively cheap and some growers routinely inoculate every time they grow pulses as an ‘insurance policy’. However, seed application methods can be time-consuming and complicate sowing logistics.

The new test informs growers of the number of rhizobia in the soil and whether or not pulses might benefit from inoculation.

Factors such as soil type, soil moisture at sowing, soil pH and the frequency of host pulse crops grown in the paddock all affect rhizobia numbers and the need to inoculate.

Ross Ballard, a researcher from the South Australian Research and Development Institute (SARDI), the research arm of Department of Primary Industries and Regions, says the extent to which each of these factors combine to affect rhizobia numbers is difficult to predict.

Objective measurement of rhizobia

“There are general rules of thumb around the need to inoculate pulses, but the new DNA test now provides an objective way of measuring how many rhizobia are present in the soil and whether inoculation is necessary,” Mr Ballard says.

“Based on the test results, we’re promoting strategic inoculation where numbers are low, to optimise nodulation and nitrogen fixation. If inoculation is needed and stressful conditions are likely, especially in acidic and/or dry soils, then doubling the inoculation rate can enhance nodulation.”

If growers apply inoculant when rhizobia numbers in the soil are below optimum levels, this will on average improve nitrogen fixation by 20 to 30 per cent and increase crop yields by 10 to 12 per cent.

“In the worst cases, insufficient rhizobia numbers can cause 90 per cent reduction in nitrogen fixation and 70 per cent decrease in crop yield,” Mr Ballard says.

“Poor nitrogen fixation also reduces the amount of nitrogen available to future crops.”

Mr Ballard says if rhizobia numbers are high and inoculation is not required, then growers also have the option of using fungicide seed treatments and sowing into dry soil.

The new PREDICTA rNod test for Group E and F rhizobia is available now for growers in SA and Victoria via PREDICTA® B-accredited agronomists and is expected to roll out nationally in 2022.

Further work underway

Mr Ballard says work is also underway to evaluate new tests for lupin and chickpea rhizobia.

Growers often order their inoculant in early summer, so researchers are also looking into whether the rhizobia test can be conducted before February.

Mr Ballard says work is being undertaken to determine whether soils collected in November or December, after the paddocks are harvested, will give growers a reliable estimate of rhizobia numbers in the soil for the following season.

More information: Ross Ballard, 08 8429 2217.

Useful resources: Inoculating Legumes: A practical guide; PREDICTA rNod kits are available from PREDICTA® B-accredited agronomists and Russell.burns@sa.gov.au