Growers get the nod to check legumes for nitrogen fixation
Author: Sharon Watt | Date: 16 Aug 2016
South Australian grain growers are being encouraged to assess their legume crops for nodulation levels.
Dr Maarten Ryder, senior extension officer with the University of Adelaide’s Nitrogen Fixation Program, says late winter and early spring is the best time to sample crops.
“It’s important to check crops to see whether adequate nodulation is occurring. And that applies to growers who have used inoculants and those who haven’t,” says Dr Ryder, whose research is supported by the Grains Research and Development Corporation (GRDC).
Grain legume crops (such as pulses) and pasture legumes initiate a symbiotic relationship with rhizobia bacteria, to form nitrogen-fixing root nodules. This nitrogen (N) fixation process has a national benefit of close to $4 billion annually in Australian cropping systems.
As an example, a recent trial showed that inoculated faba beans in Western Victoria yielded 2.7 tonnes/hectare (1 t/ha more than the uninoculated crop) and added 180 kilograms/ha of extra fixed N to the soil.
Growers can treat seed prior to sowing with inoculants containing live bacteria to stimulate nodulation and the N-fixation capacity of legumes. It is important to match the correct inoculant group to each legume.
Dr Ryder says for farming systems to derive maximum benefits from legume N-fixation, optimal nodulation is necessary.
“Sampling legume root systems for evidence of good or poor nodulation is a recommended management strategy, regardless of whether the legume seed was or was not inoculated,” Dr Ryder says.
SA growers have extra incentive to check for nodulation in their crops this year – FREE Eyre and the GRDC, along with Agbyte and Profarmer Australia, have launched the second annual “Show us Ya Nods” challenge for the State’s pulse and pasture producers.
Initiated in 2015, the challenge sets out to find SA’s best nodulating legume crop or pasture and provides free, expert feedback from GRDC-funded researchers to all participants.
GRDC Southern Manager Grower Services, Craig Ruchs, says legume crops and pastures deliver an incredibly cost-efficient, alternative form of N, as well as acting as weed and disease breaks for cereal crops and generating income and enterprise diversity benefits for growers.
“The importance of adequate nodulation is well recognised by the GRDC. In fact, improving the survival and viability of rhizobia in current systems was one of the issues recently identified as a high priority by the GRDC’s Southern Regional Cropping Solutions Networks,” Mr Ruchs said.
“GRDC-funded research, development and extension has delivered a wealth of insights and information on inoculating legumes and nodulation, and we encourage initiatives, such as this challenge, that better inform growers and advisers around how to ensure we realise the benefits of N fixation by legume crops as an alternative source of N.”
To enter the Show Us Ya Nods challenge, participants apply online at www.freeeyre.com.au and are then sent a competition instruction pack.
Dr Ryder says assessment of root systems provides growers with valuable insights about what is occurring below the soil surface. The best approach is to:
- Collect three samples of about 10 plants from three different spots within a paddock (suggested to be 20 metres, 60 metres and 100 metres in from the edge), putting each sample of 10 plants in a separate bucket;
- Carefully wash off the soil in a bucket of water and rinse the roots to remove the remaining soil (soak for up to 30 minutes for heavy soil);
- Score each sample for the percentage of plants adequately nodulated and work out an average of scores for the three sampling locations.
Growers should look for:
- The number of nodules on a plant: the desirable number of nodules varies for different legumes, (for example 50 per plant is adequate for field pea, vetch and faba bean) and to some extent between soil types (lower numbers per plant are found on lighter soils);
- The location of nodules on the plant: where growers have inoculated seed, expect to see more nodules around the crown of the plant (where root meets shoot). These will boost the early growth of seedlings. Non-inoculated legumes will have nodules spread over the root system on crown, taproots and laterals, if rhizobia are already present in the soil;
- The colour inside the nodules: a red/pink colour means the nodules are effective and are fixing nitrogen. White or green nodules mean they are ineffective.
Dr Ryder says if poor nodulation is apparent, growers should check their inoculation strategy to ensure best management practices are being followed. If both nodulation and plant performance are poor, reasons for poor nodulation need to be identified.
“Poor nodulation can cause 10 to 50 percent yield loss in pulse crops, not to mention the lower potential nitrogen benefits to following crops,” he says. “While a visual assessment will not indicate the actual level of nitrogen being fixed, which requires sophisticated scientific methods, looking at the roots to determine if there has been a nodulation failure or delay is worthwhile.
“When looking at plants in the field, it is likely growers will find nodules on lateral roots as well as the main tap root. Look at all nodules when comparing the total nodule numbers in order to work out good or poor nodulation. Pulse crops that are poorly nodulated will be using more soil nitrogen than adequately nodulated crops, and fixing less nitrogen from the air.”
Nodulation assessment will indicate whether this year’s inoculation has been successful or whether troubleshooting is necessary. It will also tell a grower if non-inoculated legume should be inoculated next time it is grown in that paddock.
Poor nodulation can be caused by: no inoculation where low rhizobia numbers are present in soil; incorrect inoculant group or inoculant not being stored in cool conditions before use; inoculant effectiveness that has been reduced after mixing with certain types of seed dressings or liquid in furrow treatments (trace elements, pesticides, fertilisers or organic amendments); inoculated seed left for more than one day before sowing; and crop stress, such as nutrition, waterlogging, diseases or herbicides causing root damage.
To view GRDC GroundCover TV videos of Dr Ryder explaining legume nodulation field sampling, sample preparation and sample scoring, please visit http://www.grdc.com.au/GCTV17-LegumeNodulationPlaylist.
Maarten Ryder, University of Adelaide
Sharon Watt, Porter Novelli
GRDC Project code: UA000138
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