Trial harvesting at Nathan Lawrence's Northam farm after the low rainfall season of 2020.

The trend towards increased farm size and continuous cropping has resulted in many growers now having to manage large seeding programs. Depending on soil moisture availability, seeding typically takes place anywhere from April to June. That's a three-month sowing window where a range of conditions including soil temperature can change remarkably.

Within that time frame is the mind-set that given sufficient moisture for seed germination and crop emergence, early sown crops have higher yield potential. As a general rule, late sown crops have a lower yield potential.

And yet, despite these perceived yield differences there is little evidence to suggest that WA growers adjust seeding fertilizer rates, in-particular phosphorus (P), in response to the sowing window and changing soil conditions.

There may well be a good reason for that. Local data detailing crop P requirement and response for different sowing times in the WA Wheatbelt has been hard to find.

Indeed, it has not been generated for new, higher yield potential varieties, or, for varieties with different maturation times which may be important if they are sown early.

If growers had to make an educated guess, a reasonable assumption for P seeding rates would be to follow seasonal trends for other nutrients such as nitrogen.

Following that rationale, high potential early sown crops would require more P. Later sown crops with lower yield potential could have their seeding P rate cut.

Results from two recent Summit field trials however have challenged this theory and revealed a lot more about P requirements in two very contrasting rainfall seasons.

The 2020 trial at Northam was on Nathan Lawrence’s farm in a low rainfall year and the 2021 trial was on Rod Dempster’s Meckering property in a season with substantially higher than average rainfall.

The findings support similar studies in South Australia that demonstrate when wheat is sown in cooler temperature soils late in the sowing window, it is more difficult for it to acquire P. Hence, reliance on supplementary P becomes greater.

Opportunity may exist for growers sowing wheat in April and early May, when conditions of good soil moisture occur, to decrease P inputs while still achieving near maximum growth and yield potential.

Shifting those saved P resources to crops sown later in the season may have a proportionately greater impact on increasing yields and contributing to an overall improvement in yield and returns across a cropping program.

This approach would of course rely on informed management and making use of the residual soil P pool, which the trial data also showed can be depleted by higher P export in early-sown crops. Contact your Summit Area Manager to discuss the tools we have available within our inSITE program to make you better prepared to make these decisions.

Video Presentation

Watch Recording of Summit Field Reearch Manager Dr Mark Gherardi presenting at the GRDC Grains Research Update, Perth 2022.

We would like to acknowledge the GRDC and the 2022 Grains Research Updates – Perth event for providing this recording.

Read the full research Paper presented at the GRDC 2022 Perth Updates here.

A summary of this paper can be found on p3-5 in our Autumn 2022 Broadacre newsletter.