How does crop species influence subsoil potassium cycling?
By Nathan Craig, Kate Parker & Simon Kruger
Grain growers across Western Australia are well aware of the importance of potassium (K) as a key nutrient that can limit grain yield potential, particularly in sandy soils. To better understand this issue locally, a crop nutrition survey was conducted as one of the first steps of the GRDC funded K Extension Project in 2023 to assess the potassium balance of 10 paddocks on a variety of soil types. Two key findings emerged:
- Significant amounts of potassium were present in the subsoil at most sites, suggesting that crops could benefit from accessing a larger volume of soil for potassium.
- Current fertilisation practices on these paddocks were insufficient to meet the potassium removal rates expected from crops, underscoring the need to adjust fertiliser strategies to ensure adequate potassium availability for optimal crop growth.
Building on these findings, WMG has been investigating the ability of five different crop species to access potassium from deeper soil layers to recycle K up to the soil surface. As a side investigation, this site also includes the impact of early post-emergent deep ripping on crop growth and the availability of K in the soil profile.
The Yathroo site (Figure 1) is a replicated trial with duplicate treatments for +/- early post-emergent deep ripping to 60 cm. Five crop species were grown in each scenario, including short and medium season wheat, lupin, canola, and serradella. The potassium levels in each crop were sampled at the start of stem elongation (GS30, 6/8/24) for cereals and canola, at the start of flowering for lupin and serradella (17/9/24), and again at maturity for serradella (24/10/24). The aim of this trial was to determine the amount of potassium (kg/ha) contained in the plant biomass produced by each crop to understand the ability of each crop species to recycle potassium.
Potassium uptake was greatest in the mid-season wheat (Wheat-M) and lowest in the serradella treatments for both wheat anthesis and end-of-season timing for serradella (Figure 2). Canola and lupin were intermediate in their uptake of K compared to all other crop species. The impact of deep ripping on K uptake was significant, with ripped treatments generally having approximately 20 kg/ha more potassium in plant biomass than unripped treatments (Figure 3).
The same trend for potassium uptake was evident in the biomass production for each crop species, where mid-season wheat had the highest biomass and serradella had the lowest biomass (Figure 4). Canola and lupin again had intermediate biomass production; however, lupin biomass production was similar to short-season wheat (Wheat-S). There was also a significant effect of deep ripping on biomass production, with all ripping treatments having higher biomass than non-ripped treatments. This confirms that there is a strong relationship between plant biomass and K uptake (R²=0.81), indicating that about 80% of the variation in K uptake can be explained by the variation in biomass production (Figure 5).
This study has found that the uptake of potassium in the soil is not directly related to the crop species but rather to the amount of biomass produced by each crop during the season. Any crop that can maximise biomass production in a given year is likely to maximise the uptake of K from the soil, particularly from deeper layers, and recycle this back to the soil surface. K recycling can also be influenced by other factors, such as deep ripping where crops can better access subsoil layers, or the length of the growing season.
Our understanding of K recycling at this site is limited to the total amount of K uptake by crops, as more accurate estimates of K uptake from each soil layer (particularly deeper layers) were not individually measured. From the K balance survey in 2023, it was known that the site had a low amount of potassium in the 0-30 cm layer and greater levels in the 30-90 cm layer, suggesting that most of the potassium taken up is likely to have come from the deeper soil layers to satisfy crop needs.
Wheat is known to be a high biomass crop and often contributes a significant amount of stubble to be recycled back into the soil. This is the main pathway through which most of the potassium taken up by the crop returns to the soil. In reality, much of the potassium in the stubble is contained in the plant cells as soluble potassium, which is easily leached out with significant summer rainfall (>15 mm). This highlights the importance of spreading stubble at harvest across the full harvest width to ensure that potassium is returned in an even distribution across the paddock for future crop use.
Deeper-rooted crops like canola, lupin, and serradella did not appear to bring up more potassium compared to wheat. Surprisingly, serradella performed poorly in terms of bringing up potassium from the soil in plant biomass, despite its reputation for effectively doing so due to its deep root system. In this study, serradella did exhibit a high percentage of K concentration compared to other crop species but this appeared to have little impact on K recycling (data not presented). Serradella has also been noted as being less efficient at accessing soil K in deep sandy soils compared to a shallower duplex soil. This study highlights that while serradella has the potential to increase potassium levels at the soil surface through its biomass, its effectiveness is largely dependent on achieving good biomass production. It should be noted that serradella’s ability to uptake potassium was limited by the low plant populations typically observed in the first year, with better results expected in the second year. The season was also very short, with the season break occurring on 1 June and the trial site being sown on 7 June, and a dry spring period (data not presented) limiting serradella growth at either end of the season. Future work comparing regenerating serradella to annual crop species would provide a better understanding of potassium uptake.
If you’d like to learn more about the overall purpose and aim of the K Extension Project click here.
If you’re looking for an in-depth look at the topics discussed at the Yathroo site crop walk, check out the WMG Farmer Summary here.
If you’re interested in getting more information on crop nutrition, workshops and related events click here.
