Legumes, including pasture legumes, are estimated to fix almost three million tonnes of nitrogen a year in Australia.
PHOTO: Brad Collis
Inoculation of legumes with rhizobia can deliver substantial nitrogen inputs, but inoculants need to be used strategically and with a risk/benefit approach to maximise the nitrogen delivered.
Care is also needed where the survival of rhizobia can be compromised, such as when dry sowing, in acid soils and when adding fertilisers and pesticides.
These are the key take-home messages derived from a 2013 survey of growers’ use and understanding of inoculants.
While the inoculation of legumes with rhizobia has become a standard practice, it can often be improved by fine-tuning practices.
Many growers follow the dictum ‘if in doubt inoculate’, but this blanket approach can lead to unnecessary inoculation or scepticism if the results are not as expected. This is why a targeted approach to inoculation and nitrogen management is needed. This includes consideration of soil type, legume species and inoculation history.
Also, changing practices, such as the trend towards early (dry) sowing in some regions, is taking us into new territory for recommendations about rhizobial inoculation. Another important practical issue is the compatibility between rhizobial inoculant and fertilisers and seed-applied pesticides and additives.
N fixation benefits
Legumes (crop and pasture) are estimated to fix almost three million tonnes of nitrogen a year in Australia – worth about $4 billion. This amount of fixed nitrogen contributes about half of the estimated six million tonnes of nitrogen that is required annually for grain and animal production on Australian farms.
However, the contributions made by legumes vary considerably with species (Table 1) and with the situation (soil type, seasonal rainfall and crop management).
Crop legumes, on average, fix 110 kilograms of nitrogen per hectare annually, including nitrogen in the grain (Table 1).
However, the range is large, varying in individual paddocks from close to zero to more than 400kg of N/ha depending on the conditions.
Nitrogen fixation generally increases with crop biomass, therefore good agronomic management leading to optimal legume growth will favour higher nitrogen inputs.
In southern Australia, legume growth is strongly influenced by the amount of water that the crop or pasture can access. Management practices that optimise water use efficiency and keep soil nitrate levels low will favour legume growth and nitrogen fixation.
TABLE 1 Estimates of the amounts of N fixed annually
by crop legumes in Australia.
||% of crop N requirement fixed
||Shoot dry matter (t/ha)
||Shoot N (kg/ha)
||Root N (kg/ha)
||Total crop N (kg/ha)
||Total N fixed* (kg/ha)
* Total N fixed = per cent N fixed X total crop N
SOURCE: primarily Unkovich et al, 2010
When, where, how?
There is a low chance of response to grain legume or pasture inoculation where:
- there has been a recent history of inoculation with the appropriate rhizobia;
- the soil pH is more than 6 (in CaCl2); and
- grain yields and pasture production have been sufficient.
In these situations, inoculation every four years or so will be adequate because soil rhizobial populations will generally be maintained. After four years there is more likelihood of a response to inoculation because lower numbers of rhizobia will remain in the soil, so a top-up with the potent commercial inoculant strain may be beneficial.
If the legume species (or another that uses the same rhizobia) has not been grown in the previous four years, or soil conditions are hostile, then the probability of a response to inoculation is much higher.
This is the case where acid-sensitive legumes (for example, field peas and beans) are sown into acid soils (pH 5.5 or less in CaCl2). In these situations, it will be prudent to inoculate every time a crop is sown because rhizobial populations tend to diminish quickly under these soil conditions. The exception to this acid soil rule are lupins because both lupins and their rhizobial strain are well adapted to acid soils.
Where a crop such as chickpeas, which have a very specific rhizobia requirement, is grown for the first time, inoculation is essential as there will be no background presence of suitable rhizobia.
To check for ‘sufficient nodulation’ growers and/or consultants are encouraged to dig up several plants over a five metre by five metre area two to three months after sowing, gently wash out the root systems and look at the amount of nodulation. A visual check will show if a reasonable number of nodules is present and well distributed or whether there has been a nodulation delay or failure.
Carefully breaking open nodules will reveal if they are a pink or reddish colour; this shows that the nodules are active, whereas green or white nodules are inactive. Nodulation failures are usually difficult and expensive to fix, so it is worth paying attention to the guidelines.
Common inoculation issues
Can I sow inoculated seed into dry soil?
Sowing inoculated seed into dry soil is not recommended where a legume crop is sown for the first time. However, where a legume has been used frequently and the soil is not hostile to rhizobia, the risk of nodulation failure resulting from dry sowing is much less.
Rhizobial formulations that are applied in-furrow, such as granules or peat suspended in liquid, are placed deeper in the soil and will have a better chance of survival.
Can I mix inoculated seed with fertiliser and trace elements?
Rhizobium biologists recommend against mixing inoculant with fertilisers that are acidic (such as superphosphate).
We suggest either separating the inoculant and the fertiliser or, if possible, doing small-scale tests if you are considering mixing inoculum with fertilisers and micronutrients. Some micronutrient preparations are acidic and therefore likely to cause reduced nodulation effectiveness.
Tanks should be cleaned before they are used for rhizobial inoculum. Placement of the fertiliser or trace elements away from the rhizobial inoculum (for example, in-furrow below the seed) is recommended.
If molybdenum is required as a seed treatment (Mo is sometimes needed for optimum nodulation, especially in acid soils), then molybdenum trioxide or ammonium molybdate should be used, not sodium molybdate which is toxic to rhizobia.
The detrimental effects of mixing inoculants and fertilisers are often only realised when a nodulation problem appears in a paddock that is otherwise responsive to inoculation. These situations can then be very difficult to rectify during the season.
Can I mix rhizobial inoculant with pickled seed or pesticides?
Some combinations of rhizobia with some pickles and pesticides appear to perform well, whereas others kill rhizobia. The GRDC booklet Inoculating legumes: a practical guide (www.grdc.com.au/GRDC-Booklet-InoculatingLegumes) lists the compatibility of different rhizobia groups with seed-applied fungicides. Pickled seed can be coated with rhizobia (except soybeans and peanuts), but the time interval between inoculation and sowing should be less than six hours. The use of granular inoculants or liquid inoculation into furrows can reduce this impact by separating the pickled seed from the inoculant.
The following mixtures are NOT compatible with peat, liquid and freeze-dried inoculants:
- chemicals containing high levels of zinc, copper or mercury;
- fertilisers and seed dressings containing sodium molybdate, zinc and manganese;
- fungicides such as Sumisclex® or Rovral®; and
- insecticides containing endosulfan, dimethoate, omethoate or carbofuran.
Dr Maarten Ryder,
08 8313 1098
For related GRDC resources visit the Ground Cover Direct segment in this issue of Ground Cover.
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