SUM19-06: Factorial Nitrogen x Phosphorus Rates in Scepter Wheat, Merredin


To evaluate the response of wheat to nitrogen (N) and phosphorus (P) fertilizer application at a loamy sand site near Merredin.

Trial Details

Location: Korbel, Merredin (Paddy Hooper).

Trial Size: 64 plots.

Soil Type: Loamy Sand.

Crop Sown: Scepter Wheat, 82kg/ha, 15/05/2019.

Table 1: Pre-trial soil nutrient analysis.


Following a sharp finish due to a hot, dry September, yields across the trial were low and available nitrogen converted to grain protein rather than yield (Figure 1). Average yield across all treatments was 1.24 t/ha. Despite seasonal weather limitations, both N (p

Figure 1: Individual treatment yield presented as the mean across 4 replicates. There is a significant increase in yield and protein with applied N and P. Yield (bars), grain protein (lines).


Figure 2: All yield responses to P and N rate applications. Mitscherlich function fitted to both graphs.


Figure 3: Grouped mean gross margins for applied P and N. Highest returns from applied 12-18kg P/ha and 20kg N/ha. Profits decreased at the higher N rates due to minimal yield and quality improvements and higher fertilizer cost.

The most profitable treatment was 12kg P/ha 25kg N/ha and with a return of $355/ha. An additional $50/ha was made by selecting 12kg P/ha and $15/ha from applying 25kg N/ha compared to applying no N or P.

Key Notes

  • The importance of adequate nutrition, especially P, for crop production regardless of rainfall conditions was demonstrated.
  • Merredin experienced a decile 2 growing season rainfall in 2019, with significant dry periods during [1] early April to mid-June, [2] most of July and [3] all September, severely impacting plant vigour and yield.
  • Yield ranged between 1.02-1.24 t/ha and was severely limited by the low in-season rainfall and the hot dry September.
  • The most profitable treatment under 2019 conditions was $355/ha after 12kg P/ha 25kg and N/ha was applied.

Contact your local Area Manager Kobus Marais for more details on this trial, 0427 766 508.