Key messages
- Nitrogen fertiliser input costs remain high and targeting nitrogen application rates to meet crop demand will help maximise gross margin returns
- Deep soil nitrogen (DSN) testing helps identify what’s already in the bank at the start of the season, enabling more refined nitrogen application decisions
- Incremented sampling provides more information than bulked tests about where nitrogen is located in the soil profile; this helps determine whether nitrogen is needed early or late in the season
Our 2025 pre-season meetings have highlighted farmer concerns about the potential for high nitrogen input costs to affect gross margins this season.
While it might be tempting to cut back on nitrogen inputs based on assumptions about what’s left over in the soil from last year’s crops, this could impact yield potential if less nitrogen is present than thought.
Nitrogen stores are likely to have been depleted over the past few seasons thanks to high grain offtakes, while drier conditions may have reduced mineralisation rates this past spring and summer. It’s also hard to estimate the nitrogen contributed by pulse crops, or how much nitrogen might be left over from frost or moisture-affected crops that yielded less than expected last season.
So how do you know what’s really in the soil?
The only way to really know how much nitrogen you have at the beginning of the season is to test. And the best way to do this is using deep soil nitrogen (DSN) tests.
Having hard data about how much is in the soil, and where it’s located in the profile, helps make decisions about nitrogen applications – this is much more reliable than relying on gut feel and takes the guesswork out of nitrogen budgeting.
It’s recommended to deep nitrogen sample at the same GPS located sites every year, as this will give an indication of changing nitrogen levels over time.
DSN testing after a pulse crop will also help determine whether nodulation was successful (or not) and provides an indication of the overall nitrogen contribution of the pulse crop to the system. Properly inoculated and nodulated pulse and legume crops growing in unconstrained soils will usually return nitrogen to the soil at an average rate of 20 kg N/ha per tonne of biomass produced — however discussions at Riverine Plains’ pre-season meetings suggest that some pulse crops didn't nodulate well in 2024, possibly due to dry conditions, poor inoculation, or undiagnosed soil acidity.
Because bulked tests combine soil samples into a single sample (i.e. 0–60cm), they can’t provide detailed information about where the nitrogen is located within the soil profile, or if it varies across a paddock. This can potentially lead to mismatches between crop demand and the timing of nitrogen applications, and could be a reason some farmers doubt their DSN results.
Incremented (segmented) tests break the samples up into depths, for example 0–30, and 30–60 cm depths, which gives a more accurate picture of where nitrogen is located in the profile. This helps when matching nitrogen inputs to crop needs; if there’s plenty of nitrogen in the surface layers, it’s likely nitrogen won’t be needed until later in the season. However, if all the nitrogen present is located further down the profile, crops will need an early application to meet demand until roots can access the nitrogen at depth.
DSN testing is ideally done from mid-late March to early April, but also often until June. While pre-sowing sampling gives an idea of what’s in the bank at the start of the season, late June sampling better accounts for fertiliser applied at or after sowing, enabling the nitrogen requirement for a target yield to be determined more precisely.
Nitrogen stores may differ across different areas of a paddock, so also consider sampling from zones identified by electromagnetic (EM) or grid soil mapping, NDVI imagery, and yield mapping, which can then be targeted for variable rate applications.
A basic nitrogen budget takes the amount of plant available nitrogen in the soil (nitrate nitrogen + ammonium nitrogen on the soil test) and adds an estimate of mineralisation (usually somewhere around 50 kg N/ha) to give total available nitrogen.
This can then be used to work out if there is enough nitrogen in the soil to meet crop demand. Wheat requires roughly 40 kg N/ha to produce 1 tonne of grain, barley 35 kg N/ha and canola, 80 kg N/ha. When determining a nitrogen budget, also consider that nitrogen use efficiency can be as low as 35-50 percent due to conditions, disease, nitrogen gas losses, leaching or immobilisation.
It’s also important to exclude the presence of a soil constraint, such as acidity, which could be responsible for poor yields, even when adequate nitrogen is supplied; if in doubt, arrange a comprehensive soil analysis.
Deep soil nitrogen testing identifies what’s in the bank and reduces the risk of yield potential being limited by low soil nitrogen. Given the high cost of nitrogen fertiliser, farmers can use DSN results to better match crop demand and supply, to maximise gross margin returns.
For further information, consult your agronomist, soil advisor or test provider.
Refining deep soil nitrogen testing to reduce environmental losses
Nitrogen balance of pulses in central and southern NSW (GRDC)