By Kate Parker, WMG Project Officer
Plant tissue testing can sometimes throw up numbers that don’t seem to add up. At a local wheat trial site, tissue samples taken at GS30 (stem elongation) showed that some treatments had about five times more nitrate nitrogen than others — yet the total nitrogen levels across treatments were almost the same. This raised a question that many farmers might also ask when looking at their own test results: how can nitrate-N be so different while total N hardly changes?

The Trial Site
This work took place at the Minty’s property in Dandaragan, where 2025 marked the first year of a soil water repellence project trial site. The site is being used to look at how different mechanical tillage treatments affect crop establishment, nutrient uptake, and soil condition. The GS30 plant analysis provided an early opportunity to see how nitrogen was behaving under these contrasting management approaches.
Nitrate-N and Total N: What’s the Difference?
Nitrogen in plants comes in more than one form. Nitrate (NO??) is the main form taken up from the soil, especially after fertiliser or mineralisation. But once it’s inside the plant, most nitrate doesn’t stay as nitrate for long. The plant converts it into amino acids and proteins, which is where the real growth benefit comes from.

A tissue test that measures nitrate-N is picking up what’s sitting in the plant waiting to be converted. A total nitrogen test includes both the nitrate and all the organic forms, so it shows the whole picture. That’s why two samples can have very different nitrate levels but similar totals — it depends on the balance between how much nitrate has just been taken up and how quickly it’s being turned into protein.
What We Saw in the Trial
At GS30, plant tissue samples were collected from five different tillage treatments: the untreated control, a Plozza plow, a Plozza–Delver–Spader (PDS) combination, a Horsch Tiger, and a Plozza–Delver–Tiger (PDH) combination. When the results came back, the control, Plozza, and PDS plots all had much higher nitrate-N levels than the Horsch Tiger treatments. Yet total nitrogen was steady across the board, sitting between 35 and 47 kilograms per hectare.


To help make sense of this, it’s useful to look at how much of total nitrogen is actually made up of nitrate. The chart below shows the proportion of nitrate-N compared to other nitrogen forms for each treatment. Even where nitrate was “five times higher,” it still made up less than two percent of total nitrogen.

The likely reason for these differences lies in how the tillage affected mineralisation and plant uptake. The Plozza and Spader treatments encouraged an early flush of nitrate by mixing organic matter more aggressively and stimulating microbial activity. Plants took up a lot of nitrate quickly, but at GS30 hadn’t yet converted it all into proteins. The control plots, though undisturbed, also showed high nitrate. Here the effect was probably due to mineralisation concentrated in the surface layer where roots were active, combined with slower growth and assimilation. By contrast, the Horsch Tiger and the PDH combination, which disturbed the soil less aggressively, appeared to support a more gradual release of nitrate. This steadier supply allowed nitrogen to be assimilated more consistently into organic forms and distributed across greater plant biomass, leading to lower tissue nitrate despite similar total nitrogen.
Conclusion
For anyone looking at their own plant test results, the key message is that nitrate-N and total N tell you slightly different stories. A high nitrate number might not mean the crop has more nitrogen overall; it might just reflect a recent flush from fertiliser or mineralisation, or slower assimilation into proteins. A lower nitrate result doesn’t necessarily mean the crop is short on nitrogen if total N is where it should be.
Results at GS30 are a useful check, but they’re only a snapshot in time. They are best interpreted alongside soil tests, crop observations, and seasonal conditions.
A field event is planned for this site on the 20th of August, which will be a good chance to see the paddock firsthand and discuss these results in more detail.














































