How does ameliorating gravel soils affect weed populations?
By Simon Kruger, Kate Parker & Nathan Craig
One of our main trial sites in 2024 has been on Tim Creagh’s gravel soil paddock in Dandaragan. Tim and WMG have been looking into how different soil amelioration methods perform on his gravel soils, which are increasingly becoming more water repellent. Four different soil amelioration methods, including inversion tillage with a Plozza Plow, deep ripping with delving plates (Nufab deep ripper – both single and double pass), and mixing and delving with the Plozza ‘Fanger’ have been tested across the site. While the primary focus of this GRDC funded project is to address soil water repellence, an additional benefit of soil amelioration can be the reduction of weed populations. Controlling weeds on gravel soils is notoriously difficult and although there has been research into the effects of soil amelioration on weed populations in sandy soils, data on gravel soils is limited.
The site had a high background weed population, mainly capeweed, with some ryegrass, subclover, and radish due to its long history as a permanent pasture. Visual observations of the trial site (Figure 1) indicate that the presence of weeds was having a significant effect on crop growth, and this is expected to impact on final grain yield. All four amelioration methods were highly effective in reducing weed populations immediately following amelioration, with significantly higher number of weeds present in the control plots compared to all ameliorated plots (Figure 2).
The efficacy of amelioration treatments on weed population is likely due to amelioration generally burying weed seeds well below the surface where they are not able to effectively germinate and emerge from the soil. Alternatively, lower and slower germination may also be an impact on weed growth as by the time any weeds get out of the ground, the crop has already got away and can directly compete with the weeds. Interestingly, the Fanger and Nufab machines were as effective in reducing weeds through deep mixing of the soil compared to Plozza plowing which inverts the soil and buries the weeds at the bottom of plowing depth.
Weed control with soil amelioration can be applicable to a wide range of situations. At this site we have seen the positive effect of soil amelioration on crop production, however this can also be an effective method of weed control prior to sowing pastures, or in farming systems that seek to reduce herbicide use. An important risk to consider when ameliorating is that burying weed seeds can result in inconsistent control with pre-emergent herbicides, as weeds may germinate from below the treated layer, as well as weeds not being effectively buried by a poor amelioration technique. At this site, we had professional guidance to ensure best practice soil amelioration, and this is reflected in the results.
The results from the first year have demonstrated that soil amelioration methods can have significant added benefits such as reducing weed populations. The effectiveness of these methods, particularly in burying weed seeds and reducing their emergence, highlights the potential for integrating soil amelioration into broader weed management strategies. This project will continue across 2025 and 2026 as well as expanding to new gravel and sand soil type sites across the Northern Ag region. As we explore new methods for addressing soil water repellence we will also expand on our knowledge of the additional benefits and potential challenges to soil amelioration practices, such as the effect on weed populations.
If you’d like to learn more about the overall purpose and aim of the Soil Water Repellence Project click here.
If you’re looking for an in-depth look at each of the amelioration methods used and what was discussed at the first site crop walk click here.
If you’re interested in getting more information on soils, workshops and related events click here.
