Key messages

1. Soil potassium (K) levels have been declining for a long time and represent a widespread potential to limit cereal and canola yields in WA, especially on sandy soils with low cation exchange capacity.
2. Depending on the yield environment and crop and soil K supply, banding 10-20kg K/ha of a compound K fertiliser at seeding proved to be a profitable annual K strategy in many CSBP trials. However, including higher initial application rates on very K deficient soils can be a more profitable and sustainable K strategy in the longer-term, and avoids further soil K decline.
3. Potassium can move down the profile in leaching environments, but a longer-term CSBP trial on Mingenew sandplain showed that K may be recovered from deeper soil layers in a wheat-lupin rotation.

Aims

Growers need to be aware that managing short-term financial risk by minimising K input costs is likely to lead to under-application of K fertilisers, long-term soil K decline and reduced productivity. Maintenance of soil fertility lowers the long-term risk of reduced agricultural productivity. Therefore, this paper compares shorter- and longer-term K strategies to determine profitable and sustainable K applications that improve soil K fertility.

Introduction

Colwell K values for the median topsoil (0–10cm) and subsoil (10–30cm) have declined in WA cropping and pasture areas since 2000 in from about 100 to 75mg/kg and 60 to 40mg/kg respectively (Figure 1). The 2010–2015 trend is supported by data from Harries et al (2019), who also calculated negative K balances. Insufficient soil K will not only reduce crop production and profitability, but also reduce the efficiency of all other fertilisers, particularly nitrogen.

Figure 1. Median Colwell K values are declining in top- and subsoils across the WA grainbelt

More recent soil K values (2020–2023) from the CSBP laboratory show similar K value distribution maps (Figure 2) when compared with previously presented maps (Ma et al 2022). The maps show K deficiency occurring not only sandy soils, but now also on some loamy soils in the central and southern wheatbelt. Growers are advised to take soil samples to check their paddocks for K deficiencies pre-season. On paddocks with high stubble loads it is recommended to sample after some summer rain, so that K leached from the stubbles into the soil can be accounted for.

Figure 1. Median Colwell K values are declining in top- and subsoils across the WA grainbelt

More recent soil K values (2020–2023) from the CSBP laboratory show similar K value distribution maps (Figure 2) when compared with previously presented maps (Ma et al 2022). The maps show K deficiency occurring not only sandy soils, but now also on some loamy soils in the central and southern wheatbelt. Growers are advised to take soil samples to check their paddocks for K deficiencies pre-season. On paddocks with high stubble loads it is recommended to sample after some summer rain, so that K leached from the stubbles into the soil can be accounted for.

Figure 3. Comparing the relative yield (RY) response, the average ROI ($/ha) and average K use efficiency (KUE in kg grain/kg K applied) to three treatments (top-dressing pre-seeding, banded at seeding and top-dressing post-seeding) at 30kg K/ha (left) and 60kg K/ha (right). Note: Colwell K (mg/kg) values were: 15 or less in the top 50cm at Wyalkatchem, 20 or less in the top 40cm at Jaloran, 40 in 0-10cm and less than 30 in the subsoil at York.

When growing canola or lupins (crops that are more prone to fertiliser toxicity) top-dressing (pre-seeding) or split applications are recommended, especially for high rates like 30 or 60kg K/ha.

Longer-term CSBP trials indicate that capital applications of K beyond removal rates can be more profitable and sustainable. The Bolgart trial, where the only banding rate at seeding was 15kg K/ha, recommended to apply annually 60kg K/ha top-dressed pre-seeding for higher profitability. This site is on a sand over gravel soil with Colwell K values of 15–25 mg/kg in the top 40cm. Wheat yields at that site were 2.0–3.5t/ha and barley yielded up to 5.4t/ha in 2016. The Cascade trials, where the only banding rate at seeding was 13kg K/ha, indicated an annual application rate of 30kg K/ha would be most profitable when applied top-dressed pre-seeding. This was on a slightly less K deficient soil (Colwell K of 21–27mg/kg in top 50cm). Wheat and barley yielded 4–5t/ha and canola about 2–2.5t/ha.

At a very K deficient site near Mingenew (Colwell K<10mg/kg from 0 to 50cm), on a deep sandy soil, the best strategy was a high capital application (top-dressed pre-seeding) followed by three years of lower K rates (13kg K/ha), either banded at seeding for wheat or top-dressed pre-seeding for lupins. The initial application increased K at depth. Lupins can take up more and deeper K than wheat and this may have played an important role in this trial. The lupins yielded 2–2.5t/ha, which was better than the 1–1.5t/ha wheat yields at this site.

Conclusion

Growers are encouraged to review their K fertiliser strategy. It is recommended to test for K deficiency, to monitor soil K supply and, depending on results and past paddock management, to apply a suitable K strategy. Affordability of K fertilisers has now returned to historical levels. Thus, increasing K applications could correct declining soil K levels in paddocks that demand higher rates for maximum returns.

References

Ma, Q., Bell, R.W., Scanlan, C.A., Neuhaus, A. (2022). Long-term rundown of plant-available potassium in Western Australia requires a re-evaluation of potassium management for grain production: a review. Crop & Pasture Science, 73, 981-996.

Harries M, Flower KC, Scanlan CA (2021) Sustainability of nutrient management in grain production systems of south-west Australia. Crop & Pasture Science 72, 197–212.

Roberts, T.L. 2007. Right product, right rate, right time and right place... the foundation of best management practices for fertilizer. IFA International Workshop on Fertilizer Best Management Practices (FBMPs), 7-9 March 2007, Brussels, Belgium. International Fertilizer Industry Association (IFA), Paris, France

Acknowledgments

The authors would like to acknowledge the CSBP Field Research team and the CSBP Soil & Plant laboratory team for their hard work supporting this research as well as our collaborating growers who are generous with their land and assistance.

Contact details

Andreas Neuhaus
CSBP Fertilisers
Ph: 0408910070
Email: andreas.neuhaus@csbp.com.au