Inoculants need knowledge and tactics
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.
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.
Dr Maarten Ryder,
08 8313 1098
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GRDC Project Code UA00138