The amount of residual phosphorus (P) a crop takes up is largely determined by soil moisture and soil temperature. These conditions can vary significantly across a typical three-month sowing period from April to June. This variation can impact the P fertiliser requirements of crops at sowing.
Recent trials in South Australia conducted by Sean Mason from Agronomy Solutions showed how varying seasonal conditions affected crop P requirements. The trials were performed in broad acre cropping regions during the 2017 and 2018 seasons, with three times of sowing.
As Sean explains in this GRDC Communities podcast on how the time of sowing affects Phosphorus needs, P requirements can vary significantly across sowing dates and between seasons.
At the mid-North site in 2017 (where starting soil moisture was high), low P rates (0-5 kg P/ha) were required to maximise wheat yields when sown in late April. In contrast, when the P response was assessed at the site for the mid-May sowing time, the P requirements had increased up to 50 kg P/ha depending on variety.
The trial was repeated in 2018 at a close location with very similar starting soil P levels but marginal soil moisture at sowing. The P requirements were 38-40 kg P/ha for Mace and Trojan sown in late April. When sown in mid-late May, the P requirements were similar at 41-48 kg P/ha.
This highlights that high starting soil moisture conditions at moderate temperatures can promote P acquisition from soil P reserves. This potentially places less reliance on P inputs at sowing. But if soil moisture levels are marginal, higher P input at sowing will be required. The cooler temperatures associated with later sowing times also place a higher demand on P applied as fertiliser.
Sean also presented these findings at the 2019 Agronomy Conference in Wagga Wagga, in his paper The effect of time of sowing on phosphorus requirements