SUM20.10: Effect of Sowing Time and Conditions on Wheat P Requirements, Northam


Recent work from South Australia has shown P requirement for optimum production to vary along with the seasonal conditions of different sowing times. This has not been investigated in Western Australia. We aim to assess the response of Scepter and RockStar wheat to increasing rates of applied phosphorus (P) when sown at different times during the sowing window in the Northam area.

Trial Details

Location: Southern Brook.

Trial Size: 2.2m x 10m, 30 treatments x 4 reps = 120 plots.

Soil Type: Loamy sand.

Crop Sown: Scepter & RockStar Wheat, sown 28/04/2020, 28/05/2020 & 15/06/2020 at 90kg/ha.

Season Rainfall: 192 mm, BOM Northam, 131 mm below long-term average.

Table 1: Pre trial soil nutrient analysis.


Figure 1: Harvest yield (bars) and grain protein (lines) presented as the mean across four replicates. Yield increases with applied P. Yields in June sown wheat were observably lower than yields in April and May sown wheat.

Figure 2: Response curves showing strong positive correlations between applied P and grain yield. The response to applied P was greater in the later times of sowing. Mitscherlich function fitted to both datasets.

Figure 3: Gross margins in both varieties show how gross margin was affected by increasing P rates at three times of sowing.

Key Messages

  • Plant growth significantly increased with applied P. Biomass was maximised at 10kg P/ha in April and May sown wheat and was not maximised until 40kg P/ha in June sown wheat.
  • Yields were higher in April and May sown wheat than in June sown wheat.
  • Yield was more responsive to P fertilizer and required higher rates the later the crop was sown.
  • Under 2020 conditions, returns were higher in the earlier sowing times. Scepter outperformed RockStar due to higher quality grain, and the highest return was $345/ha from Scepter sown in April without P.
  • Results from this trial, considering the challenging site and season conditions, warrant further investigation with potential for high impact considerations for P fertiliser management.

For further information about this trial please get in touch with local Area Manager Brayden Noble