Sean Mason1,2, Glenn McDonald2
1Agronomy Solutions Pty Ltd, 3/11 Ridley Street, Hindmarsh SA 5007,
2School of Agriculture, Food and Wine, University of Adelaide, Waite Road, Urrbrae SA 5064
Crop phosphorus (P) acquisition from both soil P reserves and freshly applied P is controlled by climatic conditions that drive soil moisture and temperature. Soil conditions can change significantly across a typical sowing window and therefore this potentially impacts crop P requirements. Phosphorus responses trials performed in broad acre cropping regions of South Australia across two seasons and three times of sowing showed that P requirements can alter across sowing dates and between the two 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. Comparatively when the P response was assessed at the mid-May sowing time P requirements increased up to 50 kg P/ha which fluctuated slightly with variety. When the trial was repeated in 2018 at a close location with very similar starting soil P levels on marginal soil moisture, P requirements were 38-40 kg P/ha when Mace and Trojan were sown in late April compared to 41-48 kg P/ha for the mid-late May sowing time. This highlights that high starting soil moisture conditions at moderate temperatures can promote P acquisition from soil P reserves which would potentially place less reliance on P inputs. However, if soil moisture levels are marginal there is still a reliance on P inputs. Cooler temperatures associated with later sowing times place a higher demand on P applied as fertiliser.