Wheat after chickpeas: How much nitrogen does the legume break actually deliver?

By Simon Kruger and Chanel Duggan, WMG

The 2025 chickpea crop at Jim Hamilton’s property near Moora came through a difficult season and still recorded near-complete biological nitrogen fixation. The proportion of nitrogen derived from the atmosphere sat at or near 90% across all treatments, with total nitrogen fixed estimated between 59 and 83kg N/ha depending on treatment. The paddock averaged 2.34t/ha across the whole area, following a failed April establishment, a reseed in June and prolonged waterlogging through winter.

Fixation of that order raises a question the chickpea season alone could not answer. How much of that nitrogen becomes available to the following cereal, and what does that mean for the rate a grower applies?

Post-harvest soil sampling in January 2026 gave an early indication. Nitrate-N in the top 10cm of the profile was 10mg/kg, declining to 4 to 5mg/kg below that, with ammonium-N low throughout. Most legume-fixed nitrogen is held in crop residues and root biomass rather than sitting in the soil as mineral nitrogen immediately after harvest. How much of it becomes plant available during the critical growth stages of the following wheat crop is difficult to predict on any given soil type or in any given season.

In 2026 the paddock is in wheat, which gives WMG and Jim the opportunity to track that question directly, under commercial conditions, on the same heavy grey clay that hosted the chickpeas.

Why this paddock

The site is in the same 42ha paddock that carried the 2025 chickpeas, which is intentional. Keeping it in the same location means the nitrogen response data can be read against the 2025 fixation results and the January 2026 soil nitrogen baseline, rather than having to account for a different soil type or management history. The paddock was in wheat in 2023 and 2024, so the legume break is a single season sitting between cereal phases.

What the site is testing

Calibre wheat was sown on 15 May 2026 at 90kg/ha. The site is structured as four unreplicated demonstration strips running the full length of the paddock, each approximately 400m long and 36m wide, matching Jim’s seeder and sprayer width. Strips run parallel to the direction of sowing, which keeps the boundaries clean when the GS30 application is made by sprayer.

The four treatments span a range of total seasonal nitrogen rates, from a base only control through to a rate intended to sit above the likely agronomic optimum.

All strips received the same base application of liquid nitrogen down the tube at seeding, delivering 32kg N/ha. This is consistent with Jim’s standard practice on this soil type. The heavy grey clay reduces leaching risk relative to the sandier soils more common across the broader West Midlands region, which makes a larger single application at seeding appropriate. A second application is made by sprayer at approximately GS30, with the rate varied by strip to reach the target total for each treatment. The control strip receives no second application.

The control at 32kg N/ha asks what the paddock can produce on the residual nitrogen from the legume phase alone. The low N strip sits below Jim’s typical seasonal rate and tests whether a reduced application is sufficient when following chickpeas on this soil. The typical N strip reflects Jim’s normal wheat program and provides the commercial reference point. The high N strip is intended to sit above the likely agronomic optimum, giving the site the headroom to show where the response flattens.

Keeping the application timing consistent across all strips, with only the GS30 rate varying, means any differences at harvest can be attributed to nitrogen rate rather than to timing or application method. This is a demonstration on a working commercial farm rather than a replicated small plot experiment, and the results will be read with that in mind. Yield analysis will draw on the middle 12m harvest pass from each strip to reduce edge effects between adjacent treatments, with spatial variability assessed using Jim’s yield map data at harvest.

Season to date

The 2026 season opened below average on rainfall across the Moora district. Total rainfall from January through May reached 92.2mm against a long-term average of 128.1mm for the same period. April was the exception, recording 32.6mm against a long-term average of 23.6mm. May came in at just 17.6mm compared with the historical average of 55.7mm. By 16 June a further 44.8mm had fallen at the nearest DPIRD weather station, bringing the running 2026 total to 137mm.

Rainfall (mm) at Moora DPIRD weather station. LTA = long-term average.

The dry May was a feature across much of the northern agricultural region and will be a factor in how this season develops at the site.

Crop establishment

Plant establishment counts were conducted across all four treatment strips on 16 June 2026, approximately four weeks after seeding. At the time of counting, all strips had received only the base application at seeding. No GS30 topdress had been applied, so the counts reflect the agronomic starting point from which nitrogen response will be assessed rather than any treatment effect.

Mean plant density ranged from 158 to 185 plants/m² across the four strips, all comfortably above the GRDC-recommended optimum population range of 110 to 150 plants/m² for the yield expectations typical of the Moora district. Establishment was good and reasonably uniform across the paddock. Some within-strip variability was noted, consistent with the clay-dominated soil type and the normal variation in a commercial sowing operation, and that will be taken into account when the yield map data is interpreted at harvest.

What is being measured

The GS30 topdress applications were yet to be made at the time of writing. Biomass sampling and plant tissue testing will follow at GS30, with a second biomass assessment planned for peak biomass in September or October. Harvest yield, protein and screenings data will be collected by Jim at the end of the season, with post-harvest soil nitrogen sampling to follow in January 2027. A gross margin comparison across the four treatments will be worked through at the end of the season, using nitrogen input costs and harvest grain prices.

The site is a single season on one soil type at one location. The heavy grey clay at Jim’s property has different nitrogen dynamics to the pale deep sands and sandy duplexes that dominate much of the West Midlands cropping area, so the results will be most directly relevant to growers farming similar heavy country in the Moora district.

What comes next

Results from the GS30 monitoring will be reported through WMG’s enewsletter and website as the season progresses. The full dataset, including yield, protein, screenings, biomass, tissue tests and post-harvest soil nitrogen, will be available in early 2027 and will be read alongside the 2025 chickpea fixation data to give a two-season picture of the legume nitrogen cycle at this site.

Growers farming heavy country in the Moora district who have chickpeas or other pulse crops in their rotation and want to discuss nitrogen management in the following cereal are welcome to contact WMG at extension@wmgroup.org.au.

This trial is part of the Grain Legumes Project (GGA2110-002SAX), led by the Grower Group Alliance and funded by GRDC. WMG thanks Jim Hamilton for hosting the trial and for his continued involvement across both the 2025 and 2026 seasons.