WithTheGrain: Effective inoculation of pulses is a numbers game

Author: | Date: 04 Apr 2018

University of Adelaide Senior Agronomy Lecturer Dr Matthew Denton says peat slurry inoculants are very effective in increasing nodulation, but are sometimes overlooked for options such as liquid or granular inoculants, which are easier to apply.

Peat slurry inoculation is performing consistently well in providing rhizobia in good numbers to promote legume performance and nitrogen fixation across all zones of the southern region.

University of Adelaide Senior Agronomy Lecturer Dr Matthew Denton says peat slurry inoculants are very effective in increasing nodulation, but are sometimes overlooked for options such as liquid or granular inoculants, which are easier to apply.

“While peat slurry is usually an effective inoculant, it can be quite time-consuming and a bottleneck for larger sowing programs,” he says.

Another disadvantage of peat inoculants is that fertiliser and fungicides applied on the seed can kill the rhizobia. In comparison, clay-based granules or liquid inoculants can be separated from the seed.

Peat slurry technology has been around for a long time and while granules and liquid inoculants show much promise in terms of ease of use, they are not always as effective.

Dr Denton says putting these potential drawbacks aside, the key to the success of peat slurry inoculants is that they typically contain a greater number of rhizobia when applied at recommended rates which is the driver for greater nodulation, compared with other formulations tested across multiple trial sites (figure 1).

Data point graph showing the  mean levels of nodulation  for rhizobia inoculation by peat slurry, Peat granule with seed, peat granule below seed, attapulgite granule with seed, attapulgite below seed, Bentonite granule with seed and Bentonite below seed across multiple trial sites.
Figure 1: Mean nodule numbers for rhizobia inoculation using either peat slurry inoculants or with different granular inoculant treatments applied with or below the seed. Data are means of 37 replicated field experiments conducted in SA, Victoria and southern NSW. Background nodulation for un-inoculated plants was 4.3 nodules per plant (geometric mean for the 37 experiments), shown as a dashed line. Error bars are 1% least significant difference intervals; if these overlap for a pair of treatments they are not significantly different at the 1% level. Data from Denton et al. (2009).

“Economically, peat makes sense because peats contain much larger numbers of rhizobia per gram than granular inoculants,” Dr Denton says.

“That’s key in terms of getting results, as you need high numbers of rhizobia on or near the seed to improve nodulation and ensure pulses are fixing nitrogen and providing residual nitrogen for your next crop.”

Research has shown pulses are estimated to fix 40 to 136 kilograms of nitrogen per hectare on average, worth about $220 million each year to Australian farming systems. Pulse crops provide an inexpensive and sustainable source of fixed nitrogen that underpins high value pulse production and reduces fertiliser nitrogen requirements for following cereal and canola crops.

Dr Denton says there can be a tendency to overlook inoculation when sowing pulse crops, however he advises careful consideration, especially new pulse growers.

“In some cases inoculation will not improve nodulation because of good background rhizobial numbers in the soil,” he says.

“This often occurs where the same pulse was recently grown in the paddock and the soil is neutral in pH.  But where inoculants fail for whatever reason, there are often no easy ways to rectify the situation.”

Improved pulse varieties and inoculants with better-adapted rhizobia for varying soil conditions will expand pulse production into new areas, such as the acid soils of the high-rainfall zone (HRZ) and the alkaline soils of the low-rainfall zone (LRZ).

Dr Denton says with no current commercial tests available for estimating rhizobia numbers in soils, new growers should definitely inoculate when sowing pulses into acidic soils (less than pH 6.0 measured in calcium chloride) or if there has not been a well-nodulated pulse crop of the same type grown in that paddock within the last four years.

“With good prices for lentils and chickpeas in the last few years, there’s a lot more growers planting these crops for the first time in new areas. In these situations forgetting to inoculate could easily result in a crop failure,” he says.

Inoculant efficacy is also being tested by the trend towards sowing crops early in the season into dry soil, as farmers try to optimise large sowing programs and take advantage of the first opening rains.

Dr Denton says understanding how dry sowing impacts the performance of different rhizobia formulations and application methods is a question often asked by growers. At this stage more data is required to help growers make sound decisions on the use of inoculants when dry sowing. He hopes to conduct research in the near future to better understand the effects of dry soil conditions on inoculant survival and efficacy.

“Rhizobia applied in peat slurries are sensitive to desiccation in dry soils and while there have been limited independent studies in Australia, manufacturers of clay-based granule inoculants claim that rhizobial survival, nodulation, and nitrogen fixation are superior to peat inoculants when sown into dry soil,” he says.

GRDC research codes: UA00138, DAS00128

More information

Matthew Denton, 08 8313 1098, matthew.denton@adelaide.edu.au

Useful resources

GRDC Inoculating Legumes: A Practical Guide

University of Adelaide - Useful resources for legume growers

GRDC Update Paper: Best options for optimal performance of rhizobial inoculants

GRDC Project code: UA00138, DAS00128