Australian Mungbean Association
Australian-grown mungbeans have quality written all over them!
By Kerry McKenzie, PulseAg Consulting
It is widely known that as a pulse crop, mungbeans have the ability to fix their own nitrogen when the correct rhizobia bacteria are supplied. It is also well known that mungbeans don’t fix very much nitrogen, however there are ways to ensure this is maximised.
To grow a 1 t/ha mungbean crop requires approximately 70 kg/ha of nitrogen, if there are no other limiting nutrients. This nitrogen can be supplied to the growing plant from soil reserves, applied fertiliser or nitrogen fixed from the atmosphere.
While most people don’t like the job of inoculating mungbeans at sowing time, getting it right can provide the nitrogen that the crop requires and ensure maximum yields.
The correct rhizobia strain (Group I) can be found in two formulations – the well-known peat based inoculant and as a freeze dried formulation.
Both of these product can be applied as a slurry to the seed or water injected during the seeding process. Both work well so the choice is down to what suits your situation best, taking into account time constraints at planting, logistics and equipment available.
The inoculation process starts from the time you pick up the inoculant from your preferred supplier. Keep the inoculant cool and out of direct sunlight, refrigeration is even better but do not freeze the inoculant. Clean all containers, tanks and lines used for mixing and delivery of inoculant. Contaminants such as zinc and other heavy metals will kill rhizobia bacteria very quickly. Always use potable water when mixing with inoculant. The water should be ‘soft’ and as close to a neutral pH as possible.
Keep seed that has been treated with the inoculum slurry as cool as possible to ensure maximum rhizobia survival, and plant into moist soil. While the manufacturers’ state that peat inoculum can be on seed for up to 24 hours and freeze dried product 6 hours, I suggest that growers should only treat the quantity of seed that can be planted in a 6–8 hour period, and that treated seed needs to be kept in the shade in the paddock. High ambient temperatures at planting time reduces the length of time that rhizobia can be expected to survive prior to planting.
Even coverage of the seed is required and application must be at the manufacturer’s recommended water rates. There are many methods for applying slurry to seed, including mixing small batches in cement mixers, a shovel on concrete floor, or through an auger when seed is transferred. Whatever the method used, ensure you have the ability to calibrate your application equipment to know the amount and / or flow rate of the slurry mixture and the weight of seed it is being applied to.
If using an auger to treat seed the slurry can be dispensed direct into the hopper or spray nozzles mounted in the flight. To reduce potential damage to the seed coat ensure the auger is slow moving and not too steep. Other inoculant applicators are available that attach to the end of augers and apply the slurry to the falling seed.
Peat inoculum can be applied with a low pressure applicator. The auger mixes the inoculant with the seed. Photo: P Munns
If water injection is the preferred method, as for slurry, the water needs to be potable, close to neutral pH and free of chemicals and heavy metals. Any tanks and lines carrying the inoculant mix need to be clean and free from any chemical residues.
If using peats for water injection, either mix the peat into a dilute slurry or place it into a porous bag (calico bag or fine muslin / cheesecloth or nylon stocking) before adding the bag to the tractor mounted tanks. Freeze dried product should be dissolved in a bucket first, with its protectant, before adding to tanks.
Peat inoculants are a fine milled product and readily disperse in water, despite this it may be necessary to add inline filters to ensure extraneous matter does not block the injection system.
Recommended application water rates are a minimum of 50 L/ha and preferably 100 L/ha. The stream of water needs to be applied with or below the seed. if the soil trench fills with soil before the inoculant comes in contact with the seed, no nodules will be formed.
Calculate your application rate based on the recommended amount of seed treated per packet/vial of inoculant and the planting rate. For example, if a larger peat packet treats 500 kg of seed and the planting rate is 25 kg/ha, this will treat 20 ha and if applied at 100 L/ha, one packet will require 2000 L of water.
It is always recommended to follow up in the paddock after the crop has emerged to check the effectiveness of the inoculation job. It takes around 4–6 weeks for the formation of good numbers of nodules that can be seen easily by the eye. This is the time to assess of level of nodulation achieved.
To assess nodulation, remove plants from the soil using a shovel or garden fork and wash soil from the roots using a bucket of water. Pulling plants by hand may rip nodules off the roots and not give a true indication of nodule numbers.
Assess the nodules on the roots firstly by their location, high numbers of nodules close to the crown of the plant indicate that nodules have formed from the applied inoculant, nodules formed away from the crown may indicate infection has occurred from rhizobia in the soil. This is less accurate if water injection has been used as applied rhizobia will be along the entire seed trench.
It is not only the number of nodules that matters, but also whether they are effectively fixing nitrogen. This can be determined by cutting open nodules – a working nodule will be pinkish or red on the inside, and if the inside of the nodule is white or green then they are not fixing nitrogen from the atmosphere.
A strong pink colour inside the nodules indicates that the rhizobia are actively fixing nitrogen. Photo: J Howieson
If the crop is showing signs of nitrogen deficiency or crop yellowing and there are only low numbers of nodules formed, there is no way of introducing rhizobia to the crop at this stage and nitrogen from fertiliser may need to be applied. If this scenario occurs, give consideration to what may have gone wrong with the application technique or environmental conditions at treatment/planting, so refinements can be made next season.
Inoculants definitely need to be applied if mungbeans have not been planted in the paddock previously (there may be some exceptions to this in the Burdekin area where native rhizobia that will infect mungbeans are often present).
Even if mungbeans have been planted in the paddock before there is no way to determine if they have survived. If they have survived but are located deeper in the profile than where the seed is sown then infection may happen much later in the crop’s development and the potential amount of nitrogen fixed will be reduced. Rhizobia that have survived in the soil can swap genes with other soil bacteria, making them less effective at either forming nodules or fixing nitrogen compared to the commercial strains.
Using inoculants is relatively inexpensive and applying every time you plant is good insurance to maximise the potential of the crop.
Mungbeans are considered ‘lazy’ fixers of nitrogen. They will generally only meet their own requirements and not leave much residual nitrogen. However, there are ways to maximise the amount of nitrogen the crop fixes.
After getting the application and planting right, the next big inhibitor of nitrogen fixation is the available soil nitrogen in the soil. As it is a symbiotic relationship between the plant and bacteria there is a cost to the plant in supporting the rhizobia; it is much less taxing to get the nitrogen freely from the soil if it is there. If mungbeans are planted into a long fallow block there may already be more than the 70 kg/ha N required to grow a 1 t/ha mungbean grain crop available in the profile. Under such circumstances the crop will have no additional requirement for N so no nodules will be formed, even if the rhizobia are present. Nitrogen fixation will be higher if the crop is planted into a paddock with inherently low nitrogen.
The next major factor that will influence fixation is the amount of crop biomass produced. To grow larger plants more nitrogen is required, therefore, to fix more nitrogen grow more plant. This can be achieved through optimal agronomic husbandry – ensuring adequate supplies of other nutrients, managing pests and diseases, ensuring adequate soil moisture etc. Trial results from the Queensland Pulse Agronomy Initiative (GRDC, QAAFI and DAF research project) showed that planting the crop on narrow rows (50 cm or closer) can increase biomass production, and previous trials by Dr Nikki Semour (DAF) have shown higher amounts of nitrogen fixed when planted at 30 cm row spacing compared to 90 cm .
Total plant nitrogen fixed by three mungbeans varieties at two row spacings. Satin II fixed more nitrogen than Crystal and Jade-AU when averaged over 30 cm and 90 cm row spacings. When all varieties were combined the 30 cm row spacing fixed more nitrogen.
Additional information from the Pulse Agronomy project show a straight line relationship between biomass production and grain yield. The harvest index across many trials and treatments resulted in a harvest index of 0.3, i.e. for every tonne of biomass produced, mungbeans will produce 300 kg of grain. Therefore, whatever agronomic practices adopted to grow more grain will also achieve the additional benefit of fixing more nitrogen and potentially leave some nitrogen available for the subsequent crop.
To grow a successful mungbean crop there are many variables to consider and it all starts with getting the basics right with paddock selection, crop nutrition, variety, planting rate and row spacing among others. With inoculation a very important part of the process where there is no coming back from poor inoculation processes; if low/no nodules form there is no ‘free’ nitrogen available to grow your mungbean crop.
Getting the basics right with good inoculation is a cheap insurance policy that will give your crop the best chance of meeting its full potential.