By Martin Murray | In The Paddock — Field Notes | Research checked 26 October 2026
A pulse crop can look like the answer to an expensive nitrogen bill. Grow chickpeas, harvest a valuable grain crop and let the following wheat enjoy the nitrogen left behind. It is an attractive story. The trouble is that the nitrogen account is more complicated than that.
Pulses do not automatically leave a huge nitrogen surplus. They need nitrogen themselves, and a substantial amount leaves the paddock in harvested grain. Some crops make a valuable contribution; others leave a much smaller balance. That is why research into breeding pulses that fix more nitrogen deserves attention—and why the benefits need careful measurement.
In this Field Notes episode, the question is whether plant breeding can improve that account without compromising the grain crop we are trying to sell.
What is actually new?
A University of Queensland-led programme is investigating improved symbiotic nitrogen fixation across chickpeas, lentils, field peas, faba beans, lupins and mungbeans. GRDC reports a $14.7 million programme, with $5.9 million from GRDC and ten collaborating organisations and breeding companies. [1]
Researchers have assembled diversity panels of 300 lines per crop, or 1,800 lines altogether. Genetic and measured-trait datasets are being used to identify useful inherited chromosome segments. The next step is testing the breeding approach in chickpeas as a proof of concept. [1]
ANU’s project record sets a goal of improving fixation by at least 10% under high and low soil nitrogen conditions. That is a research objective. It is not an announcement that growers can buy a variety delivering a guaranteed 10% saving on fertiliser. [2]
The plant still has to feed itself
Nitrogen fixation is a partnership: rhizobia in root nodules convert atmospheric nitrogen into a form the plant can use, while the plant supports them with energy. Accessing soil nitrogen is less energetically expensive. [1]
NSW DPIRD explains that fixation depends on crop growth and nitrate present at sowing. Higher soil nitrate suppresses nodulation and fixation. A pulse crop can therefore obtain nitrogen from soil as well as the atmosphere; planting a legume does not mean every kilogram of its nitrogen is newly added to the farming system. [3]
Then comes harvest. Protein-rich grain carries nitrogen off the farm. Pulse Australia highlights that much of the nitrogen accumulated by pulses is stored in grain and removed when it is harvested. [6]
The practical question is not simply whether the crop has nodules. It is how much new nitrogen entered the system, how much departed in grain or other products, and how much remains in a form the following crop can eventually use.
Three nitrogen numbers that must not be confused
| Measure | What it tells us | What it does not tell us |
|---|---|---|
| Total nitrogen fixed | Atmospheric nitrogen captured by the crop–rhizobia partnership. | How much remains after harvest. |
| Fixation minus harvested nitrogen | A simplified net contribution before other inputs and losses. | How much is immediately available to the following crop. |
| Nitrogen recovered by the following crop | The portion actually taken up in the next crop. | A universal fertiliser replacement rate for other paddocks or seasons. |
This distinction matters when assessing a new variety. A higher fixation result is encouraging, but the business outcome depends on what happens to that additional nitrogen and whether the crop remains productive.
What Australian field evidence tells us
A 2001 review by Evans and colleagues reported chickpea balances ranging from −67 to +102 kg N/ha, with a mean of +6 kg N/ha. The measure was nitrogen fixed minus nitrogen removed in grain. These historical results show that a pulse crop can have a small or even negative balance; they are not a current national average or a forecast for your paddock. [4]
More recent evidence also shows why we should not turn that warning into a blanket dismissal. Brill and colleagues’ 2024 GRDC paper reports 2021–22 trials across ten central and southern NSW sites. Estimated balances were positive across more than 50 species-by-site combinations, ranging from 2 to 402 kg N/ha. High rainfall supported exceptionally large biomass in some treatments. [5]
The study estimated below-ground nitrogen using root factors rather than measuring every root directly. These are calculated nitrogen balances, not measurements of nitrogen recovered by the next cereal. [5]
The two bodies of evidence are not interchangeable: different seasons, sites, crops and estimation methods matter. Read together, they support a more useful message: large contributions are possible, but they are not automatic. Neither a disappointing historical mean nor a standout wet-season result should become the number in every farm budget.
Harvest can remove a sizeable share
The NSW trial paper reported approximately 33 kg N per tonne of chickpea grain, compared with 40–45 kg N/t for faba beans, field peas and lentils. Albus lupins were around 60 kg N/t. These are trial averages, not fixed grain specifications. [5]
Using the chickpea figure as an illustration, a 2 t/ha grain crop would export about 66 kg N/ha. That calculation tells us the size of the harvest withdrawal; it does not tell us how much nitrogen the crop fixed. We still need the other side of the account.
A simple example of what better fixation could change
Hypothetical example—not research results or a fertiliser recommendation. Suppose a crop fixes 100 kg N/ha and grain harvest removes 80 kg N/ha. The simplified balance is +20 kg N/ha.
Now suppose fixation rises to 110 kg N/ha while grain removal remains at 80 kg N/ha. The balance becomes +30 kg N/ha. That is ten additional kilograms of nitrogen entering the account, and a larger proportional change in the remaining balance.
However, if the improved crop also exports more nitrogen in grain, the retained gain will be smaller. If residues release nitrogen when the next crop cannot use it, recovery may be limited. The example excludes other inputs and losses and deliberately holds grain removal constant. It demonstrates the arithmetic, not the outcome of the breeding programme.
It also explains why a 10% research target must not be translated into ten percentage points of atmospheric nitrogen uptake, a 10% yield increase, or 10% less fertiliser. Those are different claims requiring different evidence.
How breeding could improve the partnership
The proposed strategy combines favourable chromosome segments, known as haplotypes, using genomic prediction to help choose crosses. Think of selecting a complementary set of useful traits across parents, then testing whether their offspring perform as predicted. [1]
For growers, the attractive prospect is adding nitrogen performance to the traits already considered when choosing a variety. But selecting for one trait does not remove the need to test the complete crop. A line that fixes more nitrogen yet yields poorly, matures at the wrong time or struggles with disease may not be commercially useful.
When could growers see a variety?
ANU records its project period as 15 April 2024 to 30 June 2028. GRDC describes the wider programme as five years. These are separately attributed research descriptions, not confirmed seed-release dates. [1] [2]
Research materials still need to translate into competitive varieties, with evidence that performance survives outside the environments used to select them. The sources reviewed do not establish a commercial availability date for improved varieties arising from this work.
The questions I would ask before putting it in a farm budget
What improved? Ask whether the result is a greater proportion of nitrogen from the atmosphere, more total kilograms fixed, a better post-harvest balance, or measured uptake by a following crop.
Compared with what? The relevant benchmark is a competitive commercial variety grown under comparable conditions. A large advantage over a weak experimental line may have little relevance to a buying decision.
Where did it work? Look for replication across seasons, soils and starting nitrogen levels. One successful treatment is a reason to investigate, not a complete regional recommendation.
What happened to grain performance? Compare yield, quality, maturity, disease response and harvestability alongside fixation. Count any additional seed or management costs.
Did the next crop benefit? A rotation-level claim should be backed by rotation-level measurements. Soil testing and subsequent crop performance are more useful than assuming the entire residual balance becomes available immediately.
Why this is worth watching
The strongest case for this research is that it tackles a trait with potential value beyond the pulse crop itself. A reliable increase in atmospheric nitrogen capture could change the economics of a rotation. That makes it worth pursuing, even if the eventual benefit is less dramatic than a headline about free fertiliser.
My test is straightforward: does the improved variety deliver competitive grain performance and a repeatable benefit across the farming system? Until those results are demonstrated, better fixation belongs in the category of promising research rather than a guaranteed saving.
If two pulse varieties yielded the same, what evidence of additional nitrogen value would persuade you to choose one over the other?
Sources and further reading
[1] GRDC GroundCover — Stacking the genetics of nitrogen fixation in pulses
[2] ANU — Genetic initiative to transform symbiotic nitrogen fixation in Australian pulse crops
[3] NSW DPIRD — Nitrogen benefits of chickpea and faba bean (2015)
[6] Pulse Australia — Environment
General information, not paddock-specific advice. Research targets, historical observations and hypothetical calculations are identified separately. Featured artwork is AI-generated and illustrates the episode; it does not depict a research trial.
