Legumes' ability to build soil nitrogen dependent on several critical factors

However, legumes' ability to build soil nitrogen is dependent on a number of important factors, including the most efficient rhizobia strain, how well they are added to legumes and soil factors like pH and fertility.

This estimate is based on decades of research across Australia.

Comprehensive documentation of much of Australian rhizobia research in crops and pastures is recorded in a recently released 119-page publication, 'Inoculating legumes: practice and science'.

It is authored jointly by most of the nation's leading scientists involved in legume fixation research and has been published by GRDC.

Rhizobia are very specific beneficial soil bacteria that live in nodules on legume roots and fix nitrogen from the atmosphere (78 per cent nitrogen) and transfer much of it to the plant.

As plant material matures and dries off, much of this transferred nitrogen, other than what is exported via grain and meat, returns to the soil in forms that will break down and be available for future crops or pasture plants, commonly grasses.

In Australia, there is, on average, around 2.8 million hectares of legume crop and 47 million hectares of pastures with legumes.

Legumes, according to the authors of the document, fix, on average, around 70 kilograms per hectare nitrogen with a range varying from zero to 600kg/ha.

Here lies the challenge, as quite a bit of research has shown that many legume crops and pastures do and can have poor nitrogen building.

An example of how important it is to effectively introduce new species specific rhizobia was highlighted at a recent field day I attended.

Coonabarabran agronomist Callen Thompson, AgSTAR Projects, noted that while we were inspecting a good well nodulated biserrula establishment in a tropical grass pasture on Sam and Megan Clifton's property, Penalva, Purlewaugh, many farmers had given up on the species.

Mr Thompson felt, correctly I feel, that many failures were due to thinking existing paddock rhizobia population suited to medics, serradella and sub clover would be sufficient for biserrula.

This is not the case as biserrula requires a unique rhizobia strain, marketed as "Biserrula special".

Rhizobia application method is also a common cause of failure, no matter what legume is being sown.

For example, seed treated with peat-based slurry inoculums must be treated just prior to sowing and soil moisture levels need to be adequate for immediate germination to ensure effective nodulation.

Pre-coated seed is convenient and popular, but rhizobia numbers decline with time.

If seed has been coated for more than four weeks, viable rhizobium numbers can be very low.

While expensive, appropriate rhizobia strains for a given species, supplied via clay-based granules, so far only via the Alosca brand, is proving flexible and effective.

Rhizobia can survive in the granules for a considerable time, allowing dry sowing ahead of germination to be feasible.

Many other aspects are covered in the publication, such as assessing nodules that are healthy and effective and the importance of using new, updated rhizobia stains for many legume species.

For example, older pastures commonly experience a run down in the population of effective rhizobia.

Also, new, improved strains have commonly been released for a number of legume types.

These tend to compete better with less effective soil rhizobia populations and are effective for more years.

Rhizobia tend to be more sensitive to soil acidity than the species they are suited to.

Hence, the importance of assessing soil pH at not just the top zero to five centimetres, but layers down to at least 30cm.

Rhizobia populations will also generally be better if soil fertility issues like phosphorus, sulphur and, in acid soils, molybdenum have been addressed.

'Inoculating legumes: practice and science' is available via the GRDC website.

Next week: Restoring top-quality pastures to mine sites.