How do you know where to apply nitrogen and where to avoid application?

Responses to nitrogen (N) fertiliser can vary enormously from season to season, but they can also vary a lot within the season.

The graph below shows the responses to N in seven wheat trials last year.

Figure 1.  Wheat yield responses to nitrogen fertiliser in CSBP trials conducted in 2021.

  

All these sites enjoyed a good season, but there was a big variation in the N responses.
The results from Arthur River showed we can grow big crops without N fertiliser - if soil reserves can meet the demand. Soil testing highlighted very high levels of mineral N (even after two non-legume crops), indicating a very low probability of a response to applied N.

The trial at Ogilvie matched the high yields of Arthur River, but only with N applied. Soil constraints were addressed with deep ripping and spading, and the 2 t/ha response to 84 kg N/ha represented a 250% return on investment (ROI) – even with Flexi-N at about $1,000/t.

There were also profitable responses at Corrigin and Gnowangerup, but the returns at Dalwallinu and Gairdner weren’t great.

The main takeaway is to apply N where it’s needed and hold off where it’s not. 
Where N is applied, measure how effective the fertiliser is with nitrogen use efficiency (NUE), which describes how efficiently soil N is converted into grain N.  

Figure 2. The chart below shows the NUE of the N applied in these trials.

 

The NUE at Ogilvie was exceptional – but not surprising given the removal of non-wetting and soil compaction constraints.

N recovery at Corrigin, Gnowangerup and Munglinup was 30-40% - a typical response where N fertiliser is needed. There were no major soil constraints and responses to N were profitable. 

Poor N recovery of 10% or less at Dalwallinu, Gairdner and Arthur River indicated that there was little economic reason for using N at these sites.

Much of the variation in NUE between the trials was related to the soil N stocks at each site. Sites with more soil N had lower NUEs than sites with less soil N.

These trials show that targeted N applications reap rewards. Make more efficient fertiliser decisions by considering soil test results, paddock rotations, likely constraints and yield potential when planning N applications. If there are opportunities to double NUE, there are opportunities to effectively halve the cost of the N bought.

For further information, click to view James' GRDC presentation on fertiliser use efficiency. 

 
James Easton
By James Easton
- Senior Agronomist

James has over 30 years’ experience working on soil and plant nutrition (crops and pastures).

He graduated from the University of Western Australia with a degree in Agricultural Science (Honours). He has in depth knowledge of historic fertiliser research trials and has worked closely with many growers, consultants, research institutions and farming groups over the years.

James gets a lot of satisfaction from sharing his knowledge with growers and those who support them. And he enjoys the fact that we are always learning.

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