Protecting mungbean rhizobia
by Paul McIntosh, Pulse Australia
Effective nodulation of mungbeans can fix about 60 to 70 kgN/ha, sufficient to grow a 1 t/ha crop. If the rhizobial inoculant is not applied or if the bacteria are exposed to high temperatures and die, this nitrogen will need to come from residual N reserves in the soil or from a pre-plant application of urea or Big N.
The survival of Rhizobia bacteria is very low if the inoculant product is exposed directly to the hot sun and temperatures above 33–35 degrees C. It only takes a few hours of exposure to the sun and high temperatures on the back of a paddock truck or in seed bins ready for planting for high levels of mortality to occur, rendering the inoculation process ineffective.
Even if you have grown mungbeans in the paddock in the past, you simply rely on a large enough population of the mungbean strain (Group I) of rhizobia being present in the soil. Water injection into the seeding furrow using cool water of neutral pH is the best application method to promote rhizobial survival and efficacy.
Avoid mixing inoculant with liquid fertilisers. Even though liquid fertilisers are a very desirable nutrient application option, rhizobia are very sensitive to pH and direct contact with elements such as copper and zinc.
Some farmers are now adopting the practice of applying nitrogen prior to planting mungbeans to make their planting operation very smooth and staff efficient. Keep in mind that these nitrogen applications should be pre-plant and not just at planting because a wetting front is required to push this relatively mobile nitrogen deeper into the soil profile, where the roots of the young mungbean plants are developing. There is no sense applying nitrogen into dry-ish topsoil while the mungbean roots develop in a good moisture zone lower in the profile. This is particularly important in fast growing mungbean crops where there is very little time to fix a nutrient deficiency.
If you plan to use applied nitrogen rather than (or as well as) fixed nitrogen to meet the mungbean crop’s needs, a nutrient analysis of the soil profile well in advance of planting will help avoid having a sad, nitrogen-deprived mungbean crop three to four weeks after planting. There is no advantage in applying a high rate of nitrogen fertiliser and also applying inoculant, because rhizobia will only fix nitrogen effectively if the soil nitrogen levels are low. However, if your applied nitrogen remains positioned in the drier top soil layer, then seed inoculation may still be very effective.
More about rhizobia survival
Rhizobia bacteria are able to live freely in the soil without a host for a time but generally only when soil conditions, especially pH, are favourable to their survival. Rhizobia and their host legume tend to have similar pH tolerance. Inoculation is generally recommended for pulses (other than lupins) grown on soils with pH below 6 (CaCl2) or below 6.5 (in water).
In higher pH soils, rhizobia associated with all pulse crops can survive for several months without a host plant. Many growers choose to use inoculant every time they grow mungbeans to be sure that the crop will nodulate effectively and fix atmospheric nitrogen to at least meet its own needs.
High biomass crops and narrow row spacing promote increased nitrogen production through fixation, contributing more to the N-budget of the rotation.