ACHIEVING a boost in nitrogen fixation when growing pulses can be achieved by appropriately inoculating the seed using correct practices
Growers are being encouraged by researchers working on a Grains Research and Development Corporation (GRDC) investment, aiming to improve nitrogen fixation in pulses, to review their inoculation use to make sure they don't miss out on the full benefits.
Legume inoculants consist of living organisms (rhizobia) and exposure to dehydrating or toxic conditions reduces survival and effectiveness.
Rhizobia must be handled and applied carefully to ensure good nodulation and nitrogen fixation by the crop or pasture.
Adelaide University School of Agriculture food and wine research associate Maarten Ryder said when deciding whether to inoculate or not, some key factors include the time since the last legume crop was grown in a particular paddock, how successfully did that crop nodulate and soil acidity.
"Crop type, legume type and time since previous inoculation can affect the decision to inoculate," Dr Ryder said.
"In ideal conditions, rhizobia can survive in the soil for several seasons, however, acidic soils with a pH of less than 5.5 measured in calcium chloride can reduce the survival of rhizobia for most legumes.
"In such acidic conditions, most legumes will require inoculation each time the crop is grown."
It's important to use the correct inoculant group for the legume as each group contains rhizobia that are specific to each legume type.
Using the correct group for the type is vital to good nodulation and nitrogen fixation and nodulation failure will occur if the wrong group is used.
Dr Ryder said some pickles, or seed-applied fungicides can reduce the survival of the rhizobia on the seed.
"For example, thiram and fungicides containing thiram (P-Pickel T) are toxic to rhizobia and metalaxyl (Apron) can also be inhibitory," she said.
"To avoid issues of this type, inoculants can be applied as granule or liquid/slurry in a furrow separated from the seed.
"If inoculating seed over the top of an inhibitory seed fungicide, sow as soon as possible to minimise exposure time, ideally within six hours."
It's also important to consider using double rate inoculant if the inoculant group for that legume hasn't been used in that paddock before, when sowing into dry soil and in very acidic soils with a pH of less than 5.5.
Clean equipment and containers or tanks are also critical as rhizobia are sensitive to chemical and fertiliser residues, especially herbicide residues.
Dr Ryder said growers should not use very saline bore water or highly chlorinated water when preparing and applying peat slurry or freeze-dried inoculants.
"Use potable (drinkable) water and allow chlorinated tap water to stand in an open tub overnight to allow chlorine to disperse," she said.
"It's also necessary to not wait too long before sowing inoculated seed, growers should sow within 24 hours, or sooner if coating inoculant on to seed over some types of pickle or trace element coatings."
Rhizobia may not survive well in tank mixes with some trace elements, such as zinc and copper, and other fertilisers.
For seed inoculation, the better options are separate in-furrow liquid application or granular inoculants.
Seed coatings with trace elements and followed by inoculation can also be problematic for survival of rhizobia.
Dr Ryder said growers also shouldn't leave inoculant packets or inoculated seed in direct sunlight or in temperatures higher than 25 degrees Celsius.
"Warm to hot temperatures can have negative effects on rhizobia organisms as they have little heat tolerance, this applies to all types of inoculant in bags or packets, on inoculated seed, as liquid for furrow application or as granules," she said.
"It's also critical not to mix inoculants directly with fertilisers as some types of fertiliser can be toxic to rhizobia.
"Inoculated seed should not be left in direct contact with fertiliser for extended periods, this is particularly the case for acidic fertilisers such as mono ammonium phosphate (MAP) and super-phosphate."
The GRDC's 'Inoculating legumes: The Back Pocket Guide' can help growers navigate these issues such as crop types, nodulation assessment and information on the likelihood of crop response to inoculation.
For growers seeking a more detailed resource, 'Inoculating Legumes: A practical guide' provides insight into legumes and nodulation and how to effectively use inoculation as a key farm practice.
