Key points

• Profitability and sustainability can be improved by including legumes in your cropping sequence
• The average legume legacy effects on canola have been put at $190 to $237 per hectare from higher grain yields and savings in urea costs (assuming a price of $600 to $1200/tonne for urea)
• Legumes that produce high grain yields or those cut for hay might not always provide a net input of mineral nitrogen due to nitrogen removal, but there are other benefits to the system

Total organic nitrogen in continuously cropped soils declines by about 2.5 per cent a year as it mineralises, CSIRO Agriculture and Food senior experimental scientist Tony Swan recently told the GRDC Northern Region Update.

Mr Swan said the only two options to stabilise the decline were to add more legumes into the system or apply more fertiliser.

He pointed to a 2012 study that predicted the increase in nitrogen fertiliser needed to maintain a wheat crop of four tonnes a hectare on red Mallee soil between 2017 and 2067, assuming organic nitrogen continues to mineralise.

The study demonstrated that if fertiliser rates increased to maintain yield as the nitrogen supplied from soil organic matter declined, fertiliser nitrogen costs as a percentage of gross margin would need to rise from nine to 10 per cent to about 14.8 per cent by 2037 and 17.5 per cent by 2067 (see Table 1).

Table 1: The increase in nitrogen fertiliser required to maintain a wheat crop of 4t/ha on red Mallee soil between 2017 and 2067 as soil organic nitrogen levels decline.

Source: Dr John Angus and Dr Mark Peoples, CSIRO, 2012.

Legume crops

When it comes to improving the amount of nitrogen fixed by legumes, Mr Swan said it hinged on several factors:

• selecting the best legume species for your soil type;
• choosing the best legume variety;
• sowing on time;
• ensuring effective nodulation; and
• maximising dry matter production to increase nitrogen-rich residue.

“Other important issues are to make sure there are no subsoil constraints, avoid planting legumes on paddocks with high levels of existing mineral nitrogen and to ensure there are no residual herbicide effects that can impact growth,” he said.

“Also consider the end-use planned for the crop – such as grazing, hay, brown manuring or grain production – because it also has a big impact on the amount of residual nitrogen left for the subsequent crops.”

Overall, he said, 80 per cent of legume crops fix more nitrogen than is removed at grain harvest, so they will leave some residual for the following crops. The amount they leave is affected by:

• the amount of rainfall during the fallow between growing seasons;
• the amount of residue left on the soil at the end of the growing season; and
• the nitrogen content and ‘quality’ of the legume residue.

“Higher rainfall between the end of the season and the start of the following growing season will result in more organic nitrogen converted to mineral nitrogen,” he said.

“The more biomass you grow, the more likely there will be a higher amount of nitrogen fixed.”

A lower carbon-to-nitrogen ratio in the crop residues also improves nitrogen mineralisation and avoids nitrogen tie-up.

Consider constraints

Mr Swan said the constraints to nitrogen fixation and legume growth were also significant considerations. “Droughts, low phosphorus, herbicide carryover and poor in-crop weed management all affect legume growth,” he said, which reduces the amount of nitrogen fixed.

Other constraints include:

• acid subsurface layers, which can kill rhizobia;
• low rhizobia numbers; and
• high soil mineral nitrogen.

“Soils with acid subsurface layers can kill rhizobia, which may mean legume crops do not fix nitrogen from the atmosphere and may even require extra urea to grow,” he said.

“Repairing your subsoil constraints before sowing legumes is paramount to maximise the nitrogen fixation that legumes provide to the farming system.”

Legacy effects

Mr Swan said 16 experiments over 26 years across South Australia, Victoria and New South Wales demonstrated brown-manured legume crops supplied an average of 60 kilograms/ha of mineral nitrogen compared to a previous cereal or canola crop (see Table 2).

Table 2: Mean estimates of soil mineral nitrogen benefits after legume grain crops (compared to cereal or canola) from 16 experiments from South Australia, Victoria and New South Wales (1990–2016).

Source: Dr Mark Peoples et al., CSIRO, 2017.

“On average, legume grain crops supply 35kg/ha of mineral nitrogen to subsequent crops,” he said.

NSW Department of Primary Industries research agronomist Mathew Dunn said an analysis of trial data from four NSW locations from 2019–21 showed, on average, soils planted to legumes had 50kg/ha of extra mineral nitrogen than soils planted to cereals (see Table 3).

Table 3: Soil mineral nitrogen available at sowing following a range of legume crops at four NSW field sites (averaged across 2019, 2020 and 2021 sowing dates).

Source: Mathew Dunn, NSW Department of Primary Industries, 2022.

He also estimated the urea saved and the extra canola grain yield following legumes versus cereals (see Table 4).

Table 4: Urea saved and extra canola grain yield produced across four NSW trial sites following legume crops (average across four legume crop types and three seasons 2019, 2020 and 2021).

Source: Mathew Dunn, NSW Department of Primary Industries, 2022.

“We do not need to apply as much urea to target the same yield when canola follows a legume versus when canola follows a cereal,” he said. “On average, we saved 78kg/ha of urea in the following canola crop.”

He said there were also grain yield differences in canola following legumes compared with when canola followed cereals.

“We almost always saw a positive grain yield benefit following legumes even when we applied extra urea to the cereals,” he said.

“The canola grain yields following a legume were 220kg/ha, on average, more than canola grain yields following cereals.”

At a urea price of $600 to 1200/t, Mr Dunn said the benefits were location-dependent (see Table 4).

“If we factor in the extra canola grain yield at a value of $650/t, you can see – on average – we are looking at an extra $143/ha when a legume is grown before canola,” he said.

“Having a legume before canola was, therefore, worth in total an extra $237/ha in terms of extra income from both the improved canola yield and urea saved.”

Mr Dunn said under higher urea prices ($1200 or $1800/t), the legume legacy value increased to $237 or $283/ha respectively.

Other legume observations

Faba beans yield well at Urana because they tend to tolerate waterlogging better than other legume crops. Photo: Penny Riffkin

CSIRO Agriculture and Food senior experimental scientist Tony Swan said a benefit of legumes was the capacity for large-seeded legumes to emerge from two years of heavy cereal/canola stubble.

“If you have 12 tonnes of stubble sitting on the ground, a legume can be planted into the residue, potentially transforming that cover into humus if there is sufficient phosphorus and sulfur in the soil,” he said.

CSIRO Agriculture and Food’s Dr John Kirkegaard also said cereal intensity significantly affected yield in wet seasons through disease incidence and nitrogen availability.

He said grain yields at Greenethorpe were 6t per hectare in continuous wheat, while wheat sown after a double break of legume/canola yielded almost 10t/ha.

However, controlling fungal diseases in the legumes was a significant task in the wet seasons of 2020 and 2021.

In 2021, Mr Swan said 900 millimetres of rainfall at Greenethorpe assisted in Ascochyta blight spreading early throughout the season.

“Once you have Ascochyta, you just need to manage it, and it was an aggressive form of the disease,” he said. “We applied 10 fungicide treatments to the chickpeas, whereas we applied fewer treatments to the faba beans.”

NSW Department of Primary Industries research agronomist Mathew Dunn said chickpeas at Wagga Wagga required four fungicide applications in 2021 and three in 2020.

Ascochyta blight did not affect the chickpeas as early as it did at Greenethorpe and was well controlled at Wagga Wagga, leading to good grain yields.

“Faba beans have yielded particularly well at Urana on a vertosol soil with a high water-holding capacity. They tended to handle the waterlogging in 2020 and 2021 much better than other legumes,” he said.

Dr Kirkegaard said that in dry spring seasons, legumes leave about 20 to 50mm more water at depth than wheat or canola because of their shallower root systems and smaller biomass.

“Water rather than nitrogen seems to have a bigger legacy effect in drier seasons,” he said. “However, nitrogen legacies tend to be large in wet seasons.”

In average seasons, he has observed legacy effects from water and nitrogen. The trick, he said, was capitalising on that with timely sowing and effective management of the following crops.

Mr Swan said ameliorating acid soils with lime and ensuring seed treatments such as P-Pickle T did not affect rhizobia activity were critical.

“We slurry inoculate our legume seed twice and ensure we sow within four hours of inoculation,” he said. “We also buy fresh inoculum each year.”

More information: Tony Swan, tony.swan@csiro.au; Mathew Dunn, mathew.dunn@dpi.nsw.gov.au