Gravel Soil Amelioration Crop Walk - Farmer Summary
By Kate Parker, WMG Project Officer
Site Introduction
The gravel soil amelioration trial site was established in June 2024 at Warra Warra hosted by Tim Creagh and family. Tim has been ameliorating gravel soils for the past 4 years using a Plozza plow and is looking to trial other equipment and approaches to address the issue of soil water repellence and to improve soil fertility.
The site is a shallow sand (10-20cm depth) over gravel with few large rocks evident in the gravel layer. Five treatments were implemented to understand the benefits and issues of a range of approaches, including the Plozza ‘Fanger’ plow, Nufab ripper/delver – single and double pass, compared to no amelioration (control) and current farmer practice of Plozza plowing. The Plozza plow and Fanger treatments were rolled following amelioration by a heavy steel roller, and the surrounding paddock was Plozza plowed at the same time as site establishment. The site was sown to oats in the week following amelioration.
This crop walk was an opportunity for attendees to discuss their own experiences with managing soil water repellence, with most attendees indicating that they have a soil amelioration program in place and that the Plozza plow and mouldboard plough has been the most effective methods in the past ten years. There has been few examples where there has been a re-emergence of soil water repellence. Farmers were generally attending to look at the next phase of soil amelioration to further build the productive capacity of their soils and ensure that soil water repellence does not return.
Control Plot 1
The first control treatment had a soil profile of about 20cm of sand over gravel subsoil.
Attendees noted significant weed burdens in control plots vs ameliorated plots.
‘Fanger’ Plow Treatment
The Fanger plow is a new implement developed by Ben Plozza and is designed for shallow sand over gravel soils where spading is not an option, and its intended use is to delve and mix soil through the top 30cm of the soil profile. The current design is at the concept stage with the benefit of being easier to use than the Plozza plow but with the disadvantage that it will drag up any obstacle that it finds in the soil, including large rocks.
It was discussed that a delver needs cohesive soil to slide up the delving face but the Fanger has a wider blade so doesn’t need soil to be as cohesive and therefore has the ability to mix well on dry soils. There was discussion around speed of working and power needed to pull the machine, with speed helping to add a bit of throw to mix the profile, and power being dependent on the depth of working.
Plozza Plow Treatment
Attendees at the soil amelioration crop walk were very familiar with the nature of the soil after using a Plozza plow as many have been using these machines previously.
The soil pit shows a typical Plozza plow profile with easily visible bands of topsoil being layer in the soil profile. The presence of crop roots following the bands down was also evident.
Nufab Ripper with Delving Plates (Single Pass) Treatment
There was significant interest in the use of the Nufab ripper with delving plates attached to bring up more of the subsoil to the surface. This machine is commonly used in deep, sandy soils to bring up subsoil sand and attending farmers were keen to see what could do in gravelly soils. The experienced farmers in attendance commented that it looks good when done but does take a fair bit of pulling. Tim was surprised that the tractor could pull it but ended up doing well considering it as a 420hp tractor.
Farmers were surprised in the amount of gravel that it was pulling up from the sub soil, with columns of gravel coming up the profile and lines of gravel were evident on the soil surface. Attendees discussed issues using the machine in severely rocky country and that a Plozza plow or reefinator maybe more suitable in those areas. Crop residues were mixed in but retained on the top of the soil and there was discussion on the positives of leaving some organic matter as groundcover to protect the soil.
Attending farmers were interested in following this plot to ascertain how much it addresses non-wetting considering it is not a traditional mixer of soil. Discussion on the role of this machine with other machines included having to tackle one constraint at a time such as using a mouldboard first then ripping later. The Nufab machine presents the opportunity for a one machine, one pass option to tackle multiple constraints. The Nufab treatment was seeded with minimal issues noted from Tim. The group discussed the importance of steel and rubber tire rollers to avoid developing deep wheeltracks on the paddock, and either cross working or waiting a few years and ripping on an angle to aid further soil mixing.
Control Plot 2
The second control plot for this site has shallower sand topsoil (10cm) compared to the other control plot, and with a similar gravel subsoil. There was discussion about the suitability of different machinery for various soil types. There was consensus on the following discussion points:
- This treatment would have been more appropriate for the Fanger plow as it likes very shallow topsoil
- Once gravels become wet, they become softer and this allows amelioration machinery to go deeper
- Each machine has its ‘sweet spot’ for doing the job of improving soil productivity, the role of this project should be to identify what machine for where.
- Farmers will need to know the soil type and amount of clay needed to be brought up from the subsoil, and cross reference this to which machine they intend to use to maximise the benefits of soil amelioration.
- The effect of crop residues on amelioration, whereby too much stubble can cause the machines to not work effectively and block up. This was discussed in the context that the soil amelioration process could be used to incorporate organic amendments to further improve soil health.
Nufab Ripper with Delving Plates (Double Pass) Treatment
The Nufab machine was passed over the plot twice in the same direction. Tim noted that the second pass went in much easier but also bogged down a bit. As evident in the soil pit, the second run reinforced the tine lines, bringing up a lot of gravel into the topsoil and allowing crop roots to follow down into the delve lines. Penetrometer measurements taken from the site immediately after amelioration and six weeks later showed the soils starting to firm up again across all plots, especially in the Nufab double pass plot.
The group discussed their own experiences and noted that soils tend to firm up quickly after the first year, with sands being more prone to compaction. There was also discussion about how much clay should be brought up (around 5% in the top 10 cm) and how to best gauge the depth needed depending on the soil type to achieve this. Attendees noted that the loader tracks from making the soil pit were more pronounced on the Nufab double pass plot.
