Summit - Long Term K Trial Results

By Summit Fertilizers

A casual conversation between Brett Beard from Summit and Badgingarra grain producer Rohan Marriott, sparked a conversation that resulted in a potassium (K) trial that would run over five seasons.

Rohan’s questions on K nutrition were focussed mostly to which soils would respond and how much they would need. Although the conversation revealed there were more questions under the surface.

Would there be any agronomic benefit or financial difference between MOP and SOP? And would the lower salt index of SOP be of any benefit?

Would the placement of those fertilizers impact on crop emergence, early growth or final yield?

Rohan kindly made available to Summit a sandy soil type on his farm to run a long-term K trial designed to better understand:

• The importance of K placement (with or banded below seed);
• K source (MOP or SOP);
• Response to five K rates (0, 15, 30, 45, and 75 kg/ha);

Those K rates were applied every year on the same plots meaning they were cumulative with an aim of observing soil run-down, maintenance or build-up of K over time.

Rohan’s crop rotation and rainfall ended up being:

• 2020 wheat (decile 2)
• 2021 wheat (decile 10)
• 2022 lupins (decile 10)
• 2023 canola (decile 2)
• 2024 wheat (decile 4)

The trial has now been concluded and as would be expected, yield responses varied with the seasons and crops, and revealed some very interesting patterns.

Yield

At this site, 30kg/ha was the annual K rate that resulted in the highest combined crop yield over the five years (Figure 1). This yield trend was true in four of the five seasons, where higher rates of K were not utilised by the crops to produce yield.
The combined harvest yield across the five seasons for MOP and SOP was similar, indicating that any potential issues with chloride in the soil with MOP were not enough to affect yield over the term of the trial.

Returns

Increasing K up to 30kg/ha resulted in increased gross margins, which then dropped off at higher rates due to increasing fertilizer cost and no yield benefit. While SOP did produce a slightly higher average yield over the five years, this was not enough to compensate for its extra cost.

Across the five seasons MOP was more profitable than SOP by $174/ha.

Fertilizer placement

Plant establishment was not affected by fertilizer treatments in most seasons.
In 2022 however, lupins did have a lower emergence with MOP compared to SOP. Nevertheless, this did not result in a yield penalty at the end of a very good rainfall season.

Soil K status

Soil K rundown was evident in the lower rates of K and the nil fertilizer treatment.