As winter sowing dates get later a crop’s potential response to Phosphorus (P) fertiliser rises, at the same time absolute yield potential is falling. This raises the question of what adjustments might need to be made to P fertiliser rates.
Plant physiology indicates that later sown crops should be more responsive to P fertiliser. Later sown winter crops usually grow more slowly. Their smaller root systems restrict P uptake from the soil compared to an early sown crop with a more developed root system, in contact with a greater volume of soil. Actual trial results suggest response to higher rates of P fertiliser at late sowing is uncertain.
In a trial run in the Southern Region in 1993 wheat crops sown later in the season (June, July) did not require more P fertiliser applications to reach maximum yield in terms of yield response or grain quality. A three week delay in sowing after mid/late May resulted in a 0.3t/ha (Murtoa) and 0.5t/ha (Birchip) yield penalty. Further delay in sowing (to July) caused similar decreases in yield. No consistent yield or grain quality response to the rate of P applied was seen. Colwell P was 32ppm at Birchip and 13ppm at Murtoa.
A NSW DPI trial (1988 – 89) at Cowra (37ppm Colwell P) and Condobolin (7ppm Colwell P) confirmed the lower yield potential of later sown crops compared to those sown earlier – with or without P inputs. The actual P fertiliser required to reach yield potential was low for the early sown crops at both sites (>2kg/ha). The late sown crops needed 11.5 kg P/ha at Cowra and 32.5kg P/ha at Condobolin to reach their lower yield potentials.
Managing the P budget
In the longer term P inputs should be in balance with the amount of P exported out of the paddock in grain and stubble. Only a small proportion of P fertiliser is taken up by the crop in the year it is applied. Except for in P deficient conditions the current crop relies more on the longer term soil P resource than the recently applied fertiliser.
What about deep sowing?
Relative response to P based on sowing date is not the only issue to consider! In the northern grains region there are a lot of chickpeas currently being sown. The planting window is ideal but surface moisture is low. Chickpeas are being sown in furrows up to 250mm deep. The application of starter fertiliser including P with this seed will be crucial to ensure they get off to a good start. It’s common for P fertilisers to be applied to the surface soil, so even if the paddock’s P status is adequate there can be little available for the seedling when sown deep.
Late sown grain is not ideal to keep for seed, when a high grain P content is desirable. More P in the seed helps with seedling vigour. Early sown crops accumulate much greater P within both plant and grain compared to late sowings even when P fertiliser is applied to the late crop.
Australian Journal of Experimental Agriculture Effect of sowing date on the uptake and utilisation of phosphorus by wheat (cv. Osprey) grown in central New South Wales
Online Farm Trials Phosphorus rate by timing in wheat – 2003
Mark Conyers, NSW DPI.
Photos courtesy of the GRDC and Stephen Gibson.