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.