When assessing nodules for chickpeas, plants that have nodules similar to examples A and B will not be adequately fixing nitrogen. Plants with nodule numbers similar to examples C and D will have adequate nitrogen fixation.
PHOTOS: A. Gibson
Growers are encouraged to look below the soil surface to check if legume inoculants are working.
The University of Adelaide’s Dr Maarten Ryder says the best approach is to dig up several plants over an area of about five square metres, eight to 12 weeks after sowing, then gently wash out the root system (without losing nodules during washing) and assess the level of nodulation (Table 1).
Growers should look for:
- the number of nodules and their location on the plant – nodules around the crown of the plant (where root meets shoot), where the seed and inoculant would have been, and down the taproot and along lateral roots (nodules on the crown indicate nodulation occurred early in the seedling stage, which is considered satisfactory where growers have inoculated); and
- the colour inside the nodules – a red/pink colour means the nodules are effective and they are fixing nitrogen, but white or green means the nodules are ineffective.
Dr Ryder says if growers are finding poor nodulation, they should check their inoculation strategy to ensure best management practices are followed. If both nodulation and plant performance are poor, reasons for poor nodulation need to be identified.
“Poor nodulation can cause 10 to 50 per cent 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 N being fixed, because only sophisticated scientific methods can do that, looking at the roots to determine if there has been a nodulation delay or failure is worthwhile.
“When looking at plants in the field, it is likely growers will find nodules on lateral roots as well as the main taproot. Look at all nodules when comparing the total nodule mass present to work out good or poor nodulation.
“Pulse crops which are poorly nodulated will be using more soil N than adequately nodulated crops, and fixing less N from the air.”
Poor nodulation can be caused by:
- no inoculation and low rhizobia numbers present in soil;
- poor inoculant coverage on seed;
- inoculant not being stored in cool conditions before use;
- reduced inoculant effectiveness, which can occur after mixing with certain types of seed dressings;
- inoculated seed left for more than one day before sowing; and
- crop stress, such as nutrition, waterlogging, diseases or herbicides causing root damage.
Dr Ryder says nodulation around the crown is important in supplying N for early growth but nodules on the lateral roots become important as the plant matures and sets seed. This may help growers assess whether plants are borderline in terms of their ability to adequately fix nitrogen (see photos right).
“Crops need the right rhizobia in the right numbers. While many growers may have heard the phrase ‘if in doubt, inoculate’ or ‘inoculate every year’, checking plants for adequate nodulation will help determine the success of inoculation,” Dr Ryder says.
TABLE 1 Satisfactory nodule numbers after 8 to 12 weeks of plant growth.
||Number of pink
nodules per plant
||10 to 30
|Field peas, vetch, faba beans, broad beans, lentils
||Heavier textured soils
||Crown of plant covered with nodules
||Crown is the top of the root system (where the root meets the shoot)
||More than 20
Dr Maarten Ryder
0409 696 360
Inoculating legumes: A practical guide is available at: www.grdc.com.au/GRDC-Booklet-InoculatingLegumes
A back pocket guide on Inoculating Legumes is available at: www.grdc.com.au/GRDC-BPG-InoculatingLegumes
A fact sheet on Rhizobial Inoculants is available at: www.grdc.com.au/GRDC-FS-Rhizobialinoculants
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