Optimising Potassium Management in the West Midlands: Results, Analysis and Insights from 2024 Trial Sites

By Kate Parker, Simon Kruger & Nathan Craig, WMG

Overview

Potassium (K) deficiency remains a significant constraint to crop productivity across the sandy soils of the West Midlands region, WA. Local growers frequently note uncertainty around optimal potassium management, including application rates, placement, timing, and the capacity of various crops to recycle potassium from deeper soil layers. Responding to these concerns, the West Midlands Group (WMG) conducted detailed trials as part of the GRDC funded K Extension Project in 2024 at two locations—Dandaragan and Badgingarra—to assess different potassium management strategies and crop responses.

Dandaragan Trial Site: Investigating Crop Responses to Deep Ripping

Context and Methodology

At the Dandaragan site, the main research objective was to evaluate how different crop types (short and medium-season wheat, canola, lupin, and serradella) utilise potassium from deeper soil layers. With evidence of substantial subsoil potassium, there was interest in determining if deep ripping post-seeding could enhance crop access to these deeper reserves.

Researchers established replicated trial plots, with half deep-ripped to a depth of 60cm post-seeding and half remaining unripped. The crops were monitored closely throughout the growing season for plant establishment, soil strength, biomass production, potassium uptake, and grain yield.

Key Findings

Plant establishment results demonstrated that early post-emergent deep ripping generally reduced plant numbers initially, particularly affecting wheat and serradella. Surprisingly, canola and lupin, with lower seeding rates and larger seeds, appeared relatively unaffected.

Deep ripping markedly reduced soil strength, allowing easier root penetration down to approximately 650mm, compared with just 225mm in unripped plots. Despite these soil condition improvements, the overall biomass production benefit was less clear, with no statistically significant advantage for deep-ripped plots on average. However, specific crop species showed noteworthy trends:

• Canola and lupin biomass production improved substantially with ripping, bringing their production levels closer to wheat.
• Serradella biomass had a significant benefit from deep ripping but only by the end of the season was this trend clear.

• Mid-season and Short-season wheat notably benefited from deep ripping for grain yield, even though initial plant establishment was reduced; Medium season wheat had the highest grain yield at 3.3t/ha

Potassium Uptake Results

Interestingly, wheat (particularly mid-season varieties) emerged as the most effective at recycling potassium to the soil surface, despite assumptions that deeper-rooted crops like lupin, canola, and especially serradella would excel at accessing deeper potassium reserves. Serradella, although known for deep roots, demonstrated the lowest biomass and potassium uptake in its establishment year.

A strong relationship was observed between crop biomass production and potassium uptake across the trial (R²=0.81). This clearly indicates that the amount of potassium cycling through crops is closely tied to biomass production rather than the depth or extent of root systems alone.

Badgingarra Trial Site: Evaluating Potassium Application Rates, Placement and Products

Context and Methodology

The Badgingarra trial explored different potassium fertiliser rates (Nil, K15, K30, K45, and K75), fertiliser products (Muriate of Potash – MOP, and Sulphate of Potash – SOP), and fertiliser placement strategies (banded with seed versus below seed placement) on wheat crop establishment, growth, potassium uptake, and yield.

Key Findings

Plant establishment decreased slightly at higher potassium fertiliser rates (especially K45 and K75) when fertiliser was applied directly with seed. This was consistent with previous research indicating potential establishment risks with high potassium rates placed too close to emerging seedlings. However, despite this early setback, the crop was generally able to compensate, with minimal impact on final biomass and yield.

At Growth Stage 30 (end of tillering), biomass production was notably highest at the moderate K30 rate. Higher rates (K45 and K75) showed no additional biomass advantage. Unsurprisingly, the absence of any fertiliser (Nil Fert) resulted in significantly lower biomass, however the plant analysis showed that the potassium % in the crop was sufficient and there was no adverse effects at plant establishment. This suggests that potassium alone wasn’t limiting early growth—other nutrients were implicated and earlier plant analysis may be necessitated to catch this deficit before it affects crop growth.

Peak biomass and potassium uptake confirmed that the K30 rate appeared optimal for total seasonal potassium uptake, providing no advantage at higher rates. Placement method and fertiliser product type (MOP vs SOP) did not significantly impact biomass or potassium uptake.

A clear correlation between biomass production and potassium uptake was evident (R²=0.58), reinforcing findings from the Dandaragan site. This suggests the key to effective potassium utilisation lies in ensuring adequate nutrition across all nutrients to maximise biomass growth, rather than relying solely on potassium rate or product.

The grain yield of all treatments where potassium fertiliser was applied (K15, K30, K45, K75) were all statistically similar in grain yield compared to the Nil K control treatment (Figure 4). While not significant, there was a trend that grain yield was maximised where 30 kg K/ha (K30) was applied, and all treatments had a higher grain yield compared to the Nil Fert treatment.

Practical Implications for West Midlands Growers

Soil Testing and Monitoring

• Regular comprehensive soil tests, including subsoil layers, are recommended to understand potassium reserves and formulate fertiliser strategies. The results from this trial indicate that monitoring potassium levels down to 90cm to detect reserves available deeper in the profile may be beneficial.

Deep Ripping Considerations

• Growers should consider deep ripping where compaction limits crop productivity, particularly for wheat varieties and potentially lupin and canola. However, serradella may not immediately benefit in respect to biomass until late in the season.
• Early post seeding ripping shown in this trial site indicate some benefits thus presenting growers with an alternative to pre-seeding ripping if conditions and timing are not optimal. Care should be taken when considering this method in respect to crop type, the window between seeding and deep ripping, and the type of deep ripping machine used.
• It’s important to carefully evaluate the economic trade-offs of deep ripping. In soils where potassium is abundant at depth and surface compaction limits root growth, deep ripping can offer longer-term benefits, although immediate yield responses may not always be clear or consistent.

Potassium Application Strategies

• Moderate potassium application rates around 30 kg K/ha appear optimal, balancing improved biomass production and cost-effectiveness. Higher rates showed diminishing returns, particularly on sandy soils similar to those at Badgingarra. This does not necessarily mean the fertiliser would be wasted; as the crop only takes up what it needs in season, the excess fertiliser would reside in the soil and could either be lost through leaching and volatilisation or has the potential to be used in the future as shown in principles such as N-banking. The trial site this past year did not collect data on long-term soil reserves of fertiliser however future trials are being planned to assess the efficacy of N-banking strategies for soil types and rainfall patterns common in the West Midlands region.
• Placement of potassium fertiliser away from the seed row is recommended where higher rates are used, mitigating the risk of poor early establishment. However, modest reductions in plant numbers observed at higher seed-placed potassium rates had minimal impact on final biomass and yield, indicating some flexibility.

Potassium Cycling connection to Biomass

• In the Dandaragan Trial, wheat emerged as the most effective in recycling potassium compared to other crops largely due to its high biomass production. Growers might leverage this characteristic strategically by focusing on optimising biomass production.
• This trend was also evident in the Badgingarra trial with Potassium uptake coinciding with biomass production and not so much fertiliser strategies.

Conclusion

The 2024 trials provide valuable insights into potassium management within the West Midlands farming region. While these results are promising, they underscore the importance of carefully balancing potassium rate, placement, crop choice, and soil management practices for optimal crop performance. Growers should remain cautious in extrapolating the trial findings broadly, recognising the influence of seasonal variability and specific site conditions.

The West Midlands Group will continue investigating these key questions, building a stronger evidence base for potassium management strategies that enhance productivity and sustainability. Farmers are encouraged to engage with ongoing research, share experiences, and adopt tailored potassium management approaches to suit their specific farm context.

Further information and ongoing updates from the K Extension Project can be found on the project page.