Field Research & Agronomy

PHOSPHORUS AND POTASSIUM RESPONSES

This trial emphasized the need for sub soil sampling for nutrients such as potassium. Some Western Australian soils have higher K levels at depth or accumulate applied K over time, in the subsoil.

At this site, K levels are deficient in the top-soil but increase to satisfactory levels to 30cm. Responses to K, both biologically and in gross margin, were limited to 10kg/ha K.

There were large responses to applied phosphorus.

Fir further details of this trial contact your local Area Manager or our field research team on research@summitfertz.com.au

RESULTS:

Considering the below average growing season rainfall, this trial generated respectable yields of between 2 and 3.6 t/ha.
There was a significant response to phosphorus  but not to potassium  under these conditions.
Soil test data suggested this trial would respond more to potassium than phosphorus with Colwell K levels of 25 to 52 ppm down to 30cm depth and phosphorus levels from 18 to 28 ppm with a PBI of under 50.
A response to potassium fertiliser is generally expected when Colwell K soil analysis results are below the nominal 50 mg/kg threshold.
Similarly, adequate soil P is considered for Colwell P levels above 15-20 mg/kg.
However, the importance of fresh P applied at sowing has consistently been demonstrated across many trials in the eastern wheatbelt.

Other trials have contrarily shown responses to K fertiliser when Colwell K levels are well above 50 mg/kg.
This highlights some issues of inconsistency in Colwell K soil analysis results and their use in predicting plant responses to K fertiliser application. Work on valid alternative soil K tests and crop response relationships is required.

Plant sampling taken on the 23rd August demonstrated a strong biomass response to P and a slight increase to 10 and 20 kg/ha of K.
Similarly at harvest, a consistent P response to 15 kg/ha was measured although the K response had diminished to be insignificant.

Notable is the performance of the NPKS compound fertiliser Vigour®,with lower total K than adjacent P treatments but contains all the plant nutrients in every granule for efficient plant uptake

The highest yielding treatment was 125 kg/ha of Vigour®,supplying 15P and 15K generating 3.64 t/ha.

Average yield and gross margin response curves suggested phosphorus was optimised at 15 kg/ha and potassium at 10 kg/ha.
The slight response to 10 kg/ha of K was enough to cover the extra fertiliser cost whilst still increasing gross returns.
Under these conditions, despite the minimal K response there was no loss in returns relative to the nil treatment from the application of K up to 20 kg/ha.

CONCLUSIONS:

• There was a significant yield response to phosphorus but not to potassium under these conditions.
• The highest yield and returns were generated by 125 kg/ha of Vigour supplying 15P and 15K (3.64 t/ha).
• Average yield and gross margin were optimised at 15kg/ha of P and 10 kg/ha of K with no significant difference between method of K application.
• It is important to ensure K levels are not being mined by applying at least maintenance K fertiliser, therefore safeguarding future crops.