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 By Dr Liz Drew and Ross Ballard (South Australian Research and Development Institute (SARDI)) and Dr Matthew Denton (University of Adelaide)

Article courtesy of GRDC Ground Cover

The nitrogen that is fixed by a pulse crop and becomes available for the next cereal crop is one of the unseen and perhaps undervalued benefits of growing legumes. The amount of fixed nitrogen returned to the soil can be substantially increased by inoculation, even when grain yield is not greatly improved.

Pulse crops such as field peas, faba beans, lentils and chickpeas may appear green and healthy throughout the growing season and yield well, but they may in part be relying on mineral nitrogen in the soil rather than contributing nitrogen to the system via biological nitrogen fixation.

Figure 1 summarises results from two trial sites in Victoria, one with low numbers of host-compatible rhizobia in the soil at sowing and one with high (3000 cells per gram of soil) numbers of rhizobia.

Inoculation with a peat-based rhizobia inoculant at varying rates increased nodulation of faba beans. While yield increased in response to increased nodulation, it was nitrogen fixation that increased the most from increased nodulation.

Although it makes sense to top up rhizobia numbers by inoculating at sites where there are low or moderate background populations of rhizobia, the practice can be just as beneficial where rhizobia are already present in soil, but have changed over time and become suboptimal in their capacity to fix nitrogen.

At the South Australian Research and Development Institute (SARDI), our research under the auspices of the GRDC-funded National Rhizobium Program aims to better understand where rhizobial inoculants are best applied and how their impact can be maximised.

Field peas

There is at present no commercial test available to measure the number of rhizobia in soils and because the rhizobia for different pulses are adapted to different soil conditions it can be difficult to predict if and when rhizobia inoculation is needed.

In 2009-10 field soils were collected from across Victoria, South Australia and Western Australia where peas had been sown in the past 10 years. Soils were collected in autumn, prior to the start of the growing season, and the number of pea rhizobia present in soil and their ability to nodulate and fix nitrogen with peas determined.
 
Many growers assume that once the legume has been grown, inoculation is probably not worth the hassle, but the survey results showed this is not true for many soils.

On average 62 per cent of soils tested from WA and 18 per cent of soils tested from SA and Victoria had very low numbers of pea rhizobia (<100 rhizobia/g soil) and therefore have a reasonable chance of being responsive to inoculation (Table 1).

By comparison, there were more than 40,000 pea rhizobia per gram in some soils. By examining the relationship between soil characteristics, agronomic practices, environmental data and the number of pea rhizobia in the soils it was possible to identify the most important factors associated with survival of the rhizobia.

These include low soil pH and the frequency of high summer temperatures between November and April - both factors make it difficult for pea rhizobia to survive over summer and reduce nodulation potential by autumn, close to the time of sowing. The checklist (Table 2) can be used as a guide to understanding when the inoculation of field pea is advisable.

In soils that contained adequate numbers of rhizobia, their symbiotic effectiveness was measured. Symbiotic effectiveness indicates how well the soil rhizobia fix nitrogen in comparison to the strain of rhizobia used in commercial inoculants for field pea.

In 20 to 30 per cent of soils across SA, Victoria and WA, rhizobia were found to be less effective, having less than 70 per cent of the nitrogen fixation capacity of the commercial strain. In these soils, inoculation may offer some means to overcome suboptimal nitrogen fixation by the soil rhizobia.

This is particularly the case for large-seeded pulses, where high numbers of rhizobia (50,000 per seed) can be introduced with good inoculation practice.

With all inoculants, it is important to remember that the rhizobia are living organisms and can be adversely affected by heat, chemicals and drying out, so need to be stored and applied according to the specific instructions of the manufacturer.

Optimising the nodulation of pulse crops and making sure the nodules contain effective rhizobia ensures biological nitrogen fixation is sufficient to meet plant needs and in doing so provides the best opportunity to improve soil nitrogen levels.

It is worth keeping in mind that it is difficult to gauge how good the symbiosis is even if the plants are dug up and inspected for nodulation. A few pink nodules on the roots are a good sign, but often fall well short of what might be possible (see Figure 2).

Inoculation with peat-based rhizobia inoculants is still relatively inexpensive (about $7 a hectare) and cost effective, especially in light of the potentially large nitrogen benefits that can be captured.

Furthermore, there is a rapidly expanding range of inoculant formulations on the market (for example, freeze-dried, granular), which may offer ease of use in application and in future may provide the opportunity to further increase inoculation rates and improve the chances of getting a response to inoculation.