Take home messages

• The most important driver in pulse crop nutrition is correct inoculation/nodulation.
• Application of nitrogen fertiliser to well-inoculated pulse crops did not improve grain yield and may affect nodulation and N fixation for the following crop.
• In high rainfall years of 2021 and 2022, P fertilisers could be spread and incorporated by sowing in faba beans and lupins to improve seedling emergence without affecting grain yield, but not in lentils where yield was reduced by broadcasting P fertiliser (across multiple soil types, sodic clays and acidic red sandy loam with Colwell P ranging from 44 to 97ppm).
• Nitrogen fixation is closely related to crop biomass, where for every tonne of above ground biomass, an average of 31kg/ha of nitrogen is fixed in well nodulated pulse crops.
• When calculating nitrogen benefit of pulse crops to the following crop, removal of nitrogen in the grain or hay must be considered, as different grains vary considerably in nitrogen concentrations.

Background

Pulse initiatives funded by the GRDC in NSW and Victoria have been investigating ways to improve pulse production and increase adoption of pulses in regions where grain legume growing area is typically low. FAR Australia has two sites in the Victorian project, led by Jason Brand, one in northeast Victoria at Bundalong and the other in the southern Victorian high rainfall zone (HRZ) at Gnarwarre. Both sites have a focus on faba bean agronomy and management techniques to improve grain yield.

The NSW project, led by Rohan Brill, has been focused more on the adaptability and legacy effects of pulse species to different regions of NSW.

This paper will discuss the nutritional benefits that pulse crops add to the farming system, along with the addition of nutrients to pulse crops for increased performance.

Nitrogen fixation

Nitrogen fixation was measured at 10 sites across the grain growing regions of NSW in 2021 and 2022. Peak biomass samples (around podding, before leaves start to drop) were collected from each pulse species (faba beans, lupins, field peas, vetch, chickpeas and lentils) and these samples analysed using the 15N natural abundance technique (Unkovich et al. 2008) to determine what proportion of the nitrogen in the biomass was derived from the atmosphere (and fixed by soil rhizobia). This proportion of nitrogen in above ground biomass is multiplied by a root factor (1.5 for faba bean, field pea, lupin, vetch and lentil, and 2 for chickpea) to account for below ground nitrogen in roots and nodules.

Results from this work have shown that nitrogen fixation is closely related to biomass production; more biomass produced resulted in more nitrogen being fixed by the pulse crop. For each tonne of above ground biomass produced, approximately 31.3kg of nitrogen is fixed (Figure 1).

Figure 1. Relationship between peak biomass of pulses (dry weight) and total nitrogen fixed (N-fix) across 10 trials in 2021 and 2022.

When calculating the nitrogen benefit to the system, the nitrogen removal through grain yield or fodder yield (for example, vetch hay) needs to be considered. Average nitrogen concentrations of pulse grains and vetch hay from 2021 and 2022 are included in Figure 2 and show that there is considerable variation between grain legumes, with chickpeas removing 33kg N per tonne of grain and albus lupins removing 61kg N per tonne of grain.

Figure 2. Average grain (and hay) nitrogen concentration of pulse species in 2021 and 2022 in trials in southern and central NSW.

Total nitrogen balances have varied from 2 to 343kg N/ha in 2021 and from 10 to 402kg N/ha in 2022, with an average contribution of approximately 145kg N/ha. At nitrogen prices of $700/t, the value of fixed nitrogen has ranged from $3.00/ha to $611.74/ha, with an average of $220.65/ha. This is a substantial contribution to the economics of growing pulses and, when combined with high grain prices of pulse crops, can make them a profitable part of the crop rotation.

Factors affecting nitrogen fixation

With the close relationship between biomass and nitrogen fixation, anything that impacts biomass production will likely have an impact on nitrogen fixation. Such factors can include soil constraints (namely soil acidity), poor inoculation/nodulation, sowing rate, nutrition, and disease.

Nutrition to increase pulse production

At three NSW sites in 2023, pulse species (faba beans, lupins, field peas, vetch, chickpeas and lentils) were tested under high nitrogen supply (addition of 100kg N/ha as urea 46%N fertiliser). At all three sites, there was no increase in grain yield through increased nitrogen supply. These high N scenarios have been sampled to determine N fixation, but these results are currently not available.

Trials in northeast Victoria and the Victorian HRZ have tested different nutritional treatments in faba beans to improve grain yield (Table 1). These trials have been conducted with and without inoculant to also test if poor nodulation can be overcome with increased nutrition.

Table 1: Nutrition treatments tested at Gnarwarre and Bundalong Vic in 2023. All treatments were conducted plus and minus inoculant and are in addition to a standard 80kg MAP at sowing. Phosphorus and manure were surface applied, pre-sowing, nitrogen (as urea) was applied at 4–6 node.

None of the nutrition treatments at either site increased grain yield when tested under standard inoculated scenarios. However, in the absence of inoculant, or in a situation where we haven’t achieved satisfactory nodulation, the addition of nitrogen and phosphorus fertiliser gave an increase in yield that brought it up to the same level as the standard inoculated control (Figure 3), suggesting we do have some ability to make up for poor biological activity.

One consistent message from both projects has been the importance of inoculation. Across all species that have been tested under plus and minus inoculant (chickpeas, faba bean and vetch), there has been significant yield responses of up to 3t/ha. Even where the paddock has had a history of the same inoculant group, we have seen increases from applying inoculant to the seed.

Figure 3. Influence of inoculation and nutrition treatment on faba bean grain yield (t/ha) and harvest biomass (dry weight t/ha), Gnarwarre, Vic 2023. Yield P value=0.03, Harvest DM=not significant. Bars with different letters denotes statistical differences.

Phosphorus management

It is well known that phosphorus (P) is a macronutrient required for crop growth and its requirements are often early in the season. In most crops, P fertiliser is applied in-furrow at sowing, so the plant has easy access early in the season. However, high rates of P fertiliser placed close to the seed can cause toxicities to seedlings and reduce crop establishment (Figure 4).

Figure 4. Effect of phosphorus rate (applied as single superphosphate) and placement on PBA Samira faba bean establishment at Ganmain in 2021. Seed – fertiliser sown in furrow with seed, IBS – fertiliser broadcast and incorporated by sowing (Colwell P 76mg/kg, pH (CaCl2) 5.3, 0–10cm). Bars with different letters denotes statistical differences.

By surface spreading P fertiliser prior to sowing and then incorporating by sowing (IBS), we have increased crop safety by creating distance between the fertiliser and the seed. This has also increased sowing efficiency in large-seeded legumes, such as faba beans, where the fertiliser bin can be used as a second seed bin. Broadcasting P fertiliser has been tested on three pulse species (lupins, lentils and faba beans) within the NSW project in 2021 and 2022. From these trials, there was no yield difference between P spread pre-sowing and P placed with the seed for lupins and faba beans, but lentils had a  yield loss when P was spread and incorporated by sowing (Table 2). Crop safety (establishment) was improved in all species by spreading and incorporation by sowing, however this benefit wasn’t greater than placing the fertiliser with the seed in terms of grain yield.

Table 2: Effect of phosphorus rate and placement (applied as MAP) on plant establishment (21 June) and grain yield of lentils and lupins at Barellan in 2021 (red sandy loam pH (CaCl2) 4.5, Colwell P 97ppm, 0–10cm).

Conclusion

The fundamental ingredient for successful pulse crops has been getting good inoculation and nodulation of the crop. In the absence of inoculant, there has been occasions where we have been able to make up for the lack of biological nutrient supply with artificial fertiliser applied early in the season, however application of N to a well nodulated crop did not increase yield and in some cases reduced yields.

Good supply of phosphorus is required by pulse crops but, depending on the species, the placement of P fertiliser may not be critical. Lupin and faba bean crops were able scavenge P broadcasted pre-sowing and incorporated by sowing, whereas lentils required P fertiliser to be placed in-furrow.

Nitrogen fixation capabilities of pulse crops are responsible for providing a large proportion of the nitrogen requirements of the pulse crop itself and for continuing to provide nitrogen to the farming system in subsequent years. The addition of nitrogen fertiliser to pulses did not increase grain yield and the effects of high nitrogen on N fixation are yet to be determined however past research has shown soil nitrate suppresses legume nitrogen fixation (Farquharson et al 2022).

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. Thanks to the wider project team at AgGrow Agronomy, Frontier Farming, Grain Orana Alliance, Brill Ag, and FAR Australia for their contributions and to the many grower co-operators allowing this work to take place.

References

Farquharson EA, Ballard RA, Herridge DF, Ryder MH, Denton MD, Webster A, Yates RJ, Seymour NP, Deaker RJ, Hartley E, Gemmel LG, Hackney B, O’Hara GW. 2022. Inoculating Legumes: Practice and Science, Grains Research and Development Corporation, Australia

Unkovich M, Herridge D, Peoples M, Cadisch G, Boddey R, Giller K, Alves B, Chalk P (2008) Measuring plant-associated nitrogen fixation in agricultural systems – Part 4. (ACIAR: Canberra)

FAR Australia research results

NSW pulse agronomy development and extension project – 2021 summary of field trial results.

Contact details

Tom Price
FAR Australia
Mulwala, NSW
0400 409 952
tom.price@faraustralia.com.au