HIGHLY acidic soils in the Adelaide Hills are not considered an optimum environment for growing lucerne but research at Pewsey Vale has shown that a bit of liming or the right combination of lucerne cultivar and rhizobia can produce phenomenal results.
The acid-tolerant lucerne trial by SARDI, funded by Meat & Livestock Australia, was sown in early October last year.
Two lucerne varieties were used - winter-active SARDI 7 series 2 and a new variety bred by SARDI with improved nodulation and tolerance to aluminum in acidic soils.
"SARDI 7 series 2 was released recently and is a variety we would recommend for cold, wet areas such as the Adelaide Hills," SARDI breeding program leader Alan Humphries said.
"It is the result of more than 30 years of breeding and incorporates five to eight cycles of selection for improved persistence on commercial farms with acidic soils in south-eastern Australia."
The new experimental lucerne variety was developed specifically for improved root growth and nodulation in acidic cultures.
"The results to-date have been encouraging - the experimental line definitely has improved nodulation, which should lead to greater nitrogen fixation," Dr Humphries said.
Two strains of rhizobia inoculant were compared in the trial, with the aim of finding how much progress a new strain of rhizobia might deliver.
"The commercial inoculant available to farmers, RR1128, is being compared to a new rhizobia, SARDI 736, which is a strain we selected out of very acid soils in NSW," Dr Humphries said.
"SARDI eventually aims to release that new strain of rhizobia, once we have learned more about its adaptation and its compatibility with our new lucerne varieties down the track."
Soil properties at the Pewsey Vale site were highly acidic, with pH levels at 4.14 in the 0-10 centimetre area, 4.28 in the 10-20cm section, and 4.4 in the 20-30cm region.
The trial also looked at various lime rates - 0 kilograms a hectare, 700kg/ha, 1400kg/ha and 2800kg/ha - to improve pH levels to understand where the new lucerne varieties and rhizobia strains were likely to be most effective.
The lime was incorporated into the soil about six months before the trial.
SARDI senior scientist Ross Ballard said at the low lime rate of 700kg/ha, which had slightly increased soil pH to about 4.3, the new lucerne/rhizobia combination was performing well.
But in the absence of lime, the pH was probably too low, even for the new lucerne and rhizobia.
"There is a strong relationship between pH and the number of lucerne rhizobia in soils," Mr Ballard said.
"And the older commercial strain of rhizobia is likely to struggle below pH 4.8.
"Lucerne rhizobia are the most acid-sensitive of the rhizobial species and so it is often difficult to find lucerne rhizobia in very acidic soils, which leads to minimal nodulation."
Mr Ballard said it made the performance of the new strain more remarkable.
Inoculation of lucerne in acid soils is critically important for nodulation, particularly longevity of the stands.