The primary goal of this pilot study was to establish whether the scattered reports of K deficiency in broad-acre crops growing on heavier soils of south-eastern Australia are indicating an emerging issue for the grains industry in these areas and hence an opportunity for growers to improve their productivity and profitability. If the issue were to be confirmed, then the project had secondary goals of:

1. investigating whether existing soil and plant testing thresholds were still relevant as diagnostic tools for crops in current farming systems,
2. assessing the efficacy of application strategies for treating K deficiency in current farming systems, and
3. ensuring that growers understand the value of soil and plant testing to diagnose K deficiency and at least 60 growers will be able to economically assess the best practice application to remediate K deficiencies and manage K levels long term to avoid future deficiency.

While this pilot study did not include an extensive survey of the issue (so we can not be certain about the true extent of K deficiency), it did find that K deficiency is present in cropping on heavier soils of south-eastern Australia. It also found that K deficiency can cause substantial losses in productivity in a range of crops when it occurs and that it may not yet be a widespread problem. However, it is reasonable to predict that its extent and severity will increase in the future because K is exported from farms in commodities and without additions of K, soil reserves of K can only decline with time (Ma et al 2022). More attention to this disorder is warranted for cropping in south-eastern Australia, including more research into detecting and managing K deficiency in current farming systems.

This project has identified several gaps in knowledge which currently make detection of the problem unreliable and its correction not fully effective or at minimised cost. These gaps and opportunities are:

1. The extent and severity of K deficiency in crops growing on heavier soils of south-eastern Australia is uncertain but likely to be increasing. A more thorough assessment of how widespread and severe the problem is currently across the cropping zone of se Australia would scope the size of the issue more accurately.  A range of soil types were present in the trials reported here but not frequently enough to establish whether K deficiency was more likely on some than others.  Identifying which soil types are more prone to the problem would help management of K deficiency.
2. The current thresholds for the Colwell K soil test are not appropriate for the heavier soils of se Australia. A high priority would be to develop robust thresholds for this region. These studies could include assessments of other soil testing approaches which might be more suitable for cropping heavier soils in this region.
3. Broad-leaved crops behaved differently to wheat in this study.  More knowledge about the relative sensitivities of crop types to K deficiency would be useful, especially with respect to thresholds for different crops in the Colwell K soil test.
4. Subsoil levels of Colwell K vary widely in the heavier soils of se Australia.  Their impact on K deficiency and its management need to be defined.  This information needs to be incorporated into interpretations of soil testing for K reserves.
5. Banding of MOP was far more effective at correcting a severe K deficiency than broadcasting and foliar K was not effective but there are still large gaps in our knowledge on how to correct K deficiency cost-effectively in modern farming systems.
   1. Are sources of K other than MOP competitive in effectiveness and cost ?
   2. Will changing timing and sources of K improve the effectiveness of foliar applications. ?
   3. How much K can be applied with the seed on heavier soils without reducing emergence ?  Does the type of seeder affect these rates ?
   4. What is the residual benefit of soil-applied K ?
   5. Do different crops respond the same to applications of K ?
6. On heavy soils, where K deficiencies may be exacerbated by slow cycling between pools and/or slow movement of K in soil solution, investigating the role of soil microbiology in K nutrition would be valuable.
7. The relationship between in-season growth and grain yield was less clear for break crops than it was for wheat. Better understanding of the impact of K deficiency early in the season on the grain yield of break crops would be useful.