27 February 2022, Australia: The increasing prevalence of acidic soils is hampering the expansion of most pulse crops, but doubling the inoculation rate can help improve nodulation, growth and yield while liming programs are instigated to increase soil pH.
Grain growers intending to sow legume crops in acidic soils are advised to adopt a strategic approach to inoculation to optimise nodulation.
The rhizobia that form nitrogen-fixing nodules on legumes in the E and F inoculation groups (field pea, faba bean, lentil and vetch) and N inoculation group (chickpea) are sensitive to soil acidity, especially below pH 5.5 CaCl2 (pH measured in calcium chloride). Lupin rhizobia are more tolerant of acidic soils.
Researchers with the Department of Primary Industries and Regions’ research division, the South Australian Research and Development Institute (SARDI), advise that growers can improve their chances of successful nodulation by following some simple steps.
Their recommendations have been drawn from a Grains Research and Development Corporation (GRDC) investment which aims to improve nitrogen fixation in pulses through development of improved rhizobial strains, inoculation and crop management practices.
These recommendations are being extended through a complementary GRDC investment being led by Mallee Sustainable Farming (MSF), in collaboration with SARDI, other farming systems groups, various consultants and AgCommunicators.
The nitrogen fixation process
Growing grain legumes in a crop rotation can contribute more than 100 kilograms per hectare of ‘fixed’ nitrogen to the soil for the benefit of future cereal and oilseed crops. The nitrogen fixation process in legumes has a national benefit of close to $4 billion annually in Australian cropping systems.
The nitrogen fixation process requires rhizobia (root nodule bacteria) to be present in adequate numbers in the soil to enable nodulation on the legume’s root system as the seedling grows.
In acidic soils where rhizobia for most pulses struggle, adequate levels of rhizobia are less likely to be present even if the legume was previously grown in the paddock, and rhizobia will need to be added to the seed or soil at sowing, using an inoculant.
Inoculating legumes in acidic soils
Through the GRDC investment led by MSF, an Inoculating Legumes in Acidic Soils Fact Sheet has been published. The Fact Sheet states that when sowing into acidic soils, particularly when combined with dry sowing or application of pesticides to the seed, the survival of rhizobia and the success of nodulation and subsequent performance of the pulse crop can be significantly reduced.
In areas where grain legumes are to be grown for the first time, inoculation is essential as the rhizobia will not be present in the soil. If the same pulse crop has been grown infrequently (not in the past four years), or the soil pH is less than pH 5.5, it is likely that rhizobia numbers will be low and inoculation will provide benefits.
When using inoculants, growers are applying living rhizobia. The more rhizobia that survive between inoculation and plant germination, the greater the potential for prompt and abundant nodulation.
Care must be taken if growers intend to inoculate seed which has been treated with fungicide or insecticides. Where pesticide application is necessary, the inoculant is best applied to seed and sown into moist soil as quickly as possible. Alternatively, granular inoculant may provide a better option as this reduces direct exposure of the rhizobia to the pesticide.
SARDI researcher Liz Farquharson says growers sowing into acidic soil conditions can consider a number of options to improve the chances of growing successful pulse crops.
“The first step is to measure the soil acidity and determine if it is likely to limit nodulation and crop growth,” Dr Farquharson says.
Once the soil pH is known, the following steps can be taken:
- If soil pH measured in CaCl2 is 4.5 or less, nodulation and growth of most pulses will be severely compromised. Only consider planting acid tolerant legumes, e.g. lupin, serradella and sub-clover. A liming program is strongly recommended to lift soil pH, as cereals and canola performance will also benefit.
- If soil pH CaCl2 is 4.5 to 5.0, it is unlikely that rhizobia for Group E/F (field pea, faba bean, lentil and vetch) and Group N (chickpea) will be present in adequate number, even if these crops were grown in the previous four years. Apply inoculant at double the rate (for peat inoculant applied as slurry to seed). Again, a lime program will be beneficial for most crops in coming years.
- If soil pH CaCl2 is 5.0 to 5.5, inoculation may be required even if a legume from the same inoculation group has been grown recently, as residual rhizobia may not be present in sufficient numbers for adequate nodulation. Consider applying the inoculant at double the rate if no suitable legume history. Monitor soil pH closely in coming years and consider liming.
- If soil pH CaCl2 is greater than 5.5, inoculation should only be required if a well nodulated legume from the same inoculation group has not been grown in the paddock in the previous four years. Double inoculation rate may still be beneficial if there is no legume history. If there are sufficient rhizobia in the soil, then dry sowing and seed applied fungicides are unlikely to constrain nodulation.
- The number of E/F rhizobia in the soil and the need for inoculation can be measured using a new test, PREDICTA®rNod, developed by SARDI. Tests for Group G and S and N are currently under development. The GRDC Inoculating Legumes back pocket guide also provides inoculation guidelines.
- Where inoculation is required, minimise direct contact between rhizobia and applied pesticide treatments. If sowing into dry soil with pickled seed, granular inoculants are preferable to seed applied rhizobia.
- Lime soil to lift the pH to 5.5 if planning to grow acid sensitive legumes in future years. This will also benefit other crops.
Identifying acidic soils
Dr Farquharson says soil acidity can be identified by using soil pH test kits available from gardening centres, which provide quick general guidance on soil acidity levels and whether pH varies down the soil profile. This should be backed up with analysis by commercial soil testing laboratories which measure pH in calcium chloride and are much more accurate than the garden pH kits.
“Careful soil sampling to assess variability across each paddock and to depth is important,” Dr Farquharson says.
“Traditional soil testing strategies, which sample to a depth of 10 centimetres, can sometimes mask a diagnosis of acidity. This can occur because although the average soil pH to 10cm may be neutral, there may be distinct alkaline and acidic zones within or below the top 10cm of the soil profile.”
More advice is contained in the Fact Sheet.
Acid tolerant rhizobia
Research began in 2015 to develop improved rhizobia strains with increased tolerance to soil acidity for faba bean, field pea, lentil and vetch (Group E and F inoculants).
Two elite Group E/F strains have been found to increase nodulation compared to the current inoculant strain in field trials where soil pH ranged from 4.6-5.0. It is envisaged that one of the acid tolerant rhizobial strains could be commercially available in 2022.
Soil acidification is a byproduct of productive farming systems, and soil pH will continue to drop unless neutralised with lime applications. Acid tolerant strains and double inoculation rates will help improve the reliability of pulse production on moderately acidic soils, while liming programs are instigated.