The Project:

Non-wetting soils are a significant constraint to agricultural productivity in the South Coast and Great Southern regions of WA. These soils reduce water infiltration and availability to crops, causing patchy germination, poor plant establishment, and ultimately lower yields. Claying has long been recognised as an effective amelioration strategy to improve soil wetting and crop performance. However, until recently, machinery limitations resulting in higher than necessary application rates have made claying an expensive and sometimes inefficient practice.

This project, conducted between 2022 and 2024, set out to refine and optimise claying practices to improve efficiency, reduce costs, and better suit the variable soil types of the region, including sandy soils with low organic matter and gravelly soils where claying is a relatively novel approach. Four trial sites were established at Kojaneerup South, Scotts Brook (two sites), and Woogenellup to evaluate clay source quality, application rates, incorporation methods, and the subsequent impacts on soil properties, crop establishment, plant health, and grain yield and quality.

Trial Methodologies:

Kojaneerup South (2022–2024): 
Treatments tested varying clay application rates aimed to reach different target (post amelioration) soil clay percentages by incorporation to 15cm. The rates were 140 t/ha (below 5% clay content threshold), 270 t/ha (approximately 5.8% clay content) and 350 t/ha (above 5% clay content).

Scotts Brook (1) (2022–2024): 
A standard clay rate (400 t/ha) was applied to a forest gravel soil, testing different incorporation methods and depths. The incorporation methods were plozza plough (30cm incorporation), Offset chisel plough (15cm incorporation) and Speed tiller (10cm incorporation), all compared to an untreated control (no clay or tillage).

Woogenellup (2023–2024): 
Clay was applied at two rates (100 t/ha and 200 t/ha) to a gravelly sand soil and incorporated to 10cm. These treatments were compared to a tilled only treatment and an untreated control (no clay or tillage).

Scotts Brook (2) (2023–2024): 
Clay was applied at three rates (70 t/ha, 100 t/ha and 200 t/ha) to a forest gravel soil (65-70% gravel) with high organic matter, and incorporated to 15cm. These treatments were compared to a tilled only treatment and an untreated control (no clay or tillage).

Key Findings:

• Clay Content Threshold: Achieving a topsoil clay content of approximately 5% (0-15cm depth) was confirmed as the critical target for effectively alleviating non-wetting in sandy soils. Below this threshold, soils retained some water repellence, reducing plant establishment and yields.

• Clay Rates and Economic Efficiency: At the Kojaneerup South site, increasing clay rates to increase soil clay content beyond the 5% threshold was not economical. The medium rate (270 t/ha) achieved the target clay content and delivered the most cost-effective yield improvements compared to higher rates (350 t/ha). This represents a significant potential saving by reducing unnecessary additional clay applications.

• Incorporation Techniques in Gravel Soils: The Scotts Brook (1) site, characterised by forest gravels, demonstrated the challenges of clay incorporation into gravelly soils. The plozza plough, which inverts soil to 30cm, mixed clay most effectively and produced the highest initial plant establishment and yields. Shallower incorporation methods struggled to adequately mix clay with gravel, resulting in surface “clay capping” and reduced crop performance.

• Claying on Gravelly Sands: The trial at Woogenellup, on a gravelly sandy duplex, showed that even in soils with 20-35% gravel, achieving 5% clay content in the soil matrix remains important. The 200 t/ha clay rate incorporated to 10cm achieved this threshold and produced the highest yields in 2023 and 2024. Tillage alone had some positive effect on yield but was less effective than claying and tillage.

• Forest Gravel Soils: At the second Scotts Brook site, high gravel (65-75%) and organic matter (>3.5%) complicated the determination of optimal clay rates. Although clay treatments increased clay content well above 5% and improved soil wetting, there was no significant impact on plant establishment or yield in the two years observed. Seasonal conditions—dry sowing followed by heavy June rainfall—likely masked treatment effects on non-wetting. Continued long-term monitoring of the Scotts Brook sites is recommended to assess permanent benefits of claying in such soils.

Learnings for Growers:

Achieving the 5% clay content threshold remains a reliable rule of thumb for sandy soils to overcome non-wetting, and achievement of a higher clay content does not significantly increase yields (to cover cost of higher rate application). In gravelly sandy soils, clay application to achieve 5% final clay content (gravel removed) remains critical. In forest gravels, careful consideration of incorporation depth and equipment is important to ensure effective soil mixing. It is currently unclear if claying is a benefit to production on forest gravels over and above the benefits (albeit not a permanent fix) of strategic tillage. Ongoing monitoring, of claying on these soil types with high organic matter, is necessary to fully understand long-term benefits and refine best practice guidelines.

Project Background:

Clay spreading adds clay enriched (>20%) soil into clay deficient and water repellent topsoils. In very sandy soils, clay is incorporated to depths of 50cm, depending on the equipment available. The overall aim is to lift the clay content of sandy soils up to 5%. Claying topsoils reduces water repellence, water and wind erosion risk, increases water holding capacity, and has the potential to increase organic matter and carbon over time.

Claying has become a vital amelioration tool in the Great Southern region and along the South Coast of WA. The soils in this region are typically sandy, low in organic matter, non-wetting and prone to wind erosion. Claying significantly increases grain yields on these sandy duplex soils by ameliorating one or more of these constraints (predominately non-wetting).

It has been implemented widely across the deep sandy duplex soil zones in southern WA and SA. However, the adoption of new machinery has allowed farmers to be more precise when applying clay. Prior to the adoption of large spreaders that could better tailor clay rates, farmers were utilising road working equipment, which applied large tonnages of clay that then needed to be spread or smoothed across the paddock. This resulted in farmers being unable to apply low rates of clay >300t/ha, which then resulted in a lot of time and money spent on smudging and incorporating the clay after it was applied.

Even with improved machinery/technology claying is expensive and grower strategies for determining clay spreading rates have been relatively ad-hoc and rarely quantified accurately. This project was driven by local growers and advisers who were keen to improve the efficiency of claying operations. When the project commenced, it was with the aim to improve growers’ knowledge of on-farm clay quality and refine their decision making on how much clay should be applied and what incorporation method should be used (including incorporation depth considerations).

As the project progressed through its first year, and based on the successful adoption of claying sandy soils to alleviate non-wetting, growers with sandy gravels and forest gravels facing similar non-wetting issues, were keen to assess the benefits of claying gravel soils. After encouraging preliminary results in 2022, a variation to the project was approved by GRDC to invest in further research in 2023.  The aim was to further quantify the benefits to production and to further explore what clay rates are necessary to alleviate non-wetting issues in gravel soils, with consideration of gravel content.

Under the variation (in collaboration with Southern Dirt), Stirlings to Coast Farmers (SCF) established two additional demonstrations trials in 2023, one in Woogenellup on a gravelly sand over clay, and the other in Scotts Brook on a loamy forest gravel.  The trial site at Woogenellup had two distinct zones across the demonstration site, a dense gravel (60% gravel content) on the hilltop and a sandy gravel (40% gravel content) on the slopes. It provided a great opportunity to test the impact of claying (at two clay rates) across both zones.  It should be noted that the organic matter at this site was low. The Scotts Brook site was located on a dense forest gravel with 65-70% gravel by weight. The loamy soil had a clay content of 4.8%, but also a high organic matter content of 3.68% (the main contributor to the non-wetting).

Project Objectives:

​​This project aimed to increase the efficiency of claying by improving growers’ knowledge of on-farm clay quality, refining their decision making on how much clay should be applied, and determining the best clay incorporation method (including incorporation depth).

​Given the new machinery/technology, this project also aimed to assess where efficiencies could be achieved, and what technology and rates would be optimal for farmers to adopt.

​As a later addition, the project also aimed to assess the value of applying clay to sandy gravels and forest gravels. Based on the successful adoption of claying sands to alleviate non-wetting, growers with gravel soils were keen to assess the benefits of claying gravel soils, which also have significant non-wetting issues.

​As part of the project four demonstrations trials were established, two in 2022, and two in 2023, and a series of two workshops were run (in collaboration with David Hall, DPIRD) with the aim of providing growers with the key information they needed to implement best practice.

GRDC Investment Outcome:

​By 2024, Growers spreading, or intending to spread clay, will have increased ability to assess on-farm clay sources, and have methods to better monitor clay application rates.

Project Methodology:

​​Four individual trials were established throughout the project, two in 2022, and two in 2024. Each trial, although similar in design, had a different objective and differing treatments, with the aim to demonstrate and inform farmers of best claying practice.

​Trial Designs – 2022

​In 2022, two demonstration sites, located in Kojaneerup South and Scotts Brook had clay spread in the autumn of 2022, after which they were seeded with a cereal crop. The clay was applied with contractors’ equipment (CAT Scraper) at Scott’s Brook and the farmer’s equipment (‘SPREAD-IT’ spreader) at Kojaneerup South.

​Kojaneerup South

​Site description:

​The 2022 trial site at Kojaneerup South was established on a deep sandy duplex, with sand over a compacted gravelly clay duplex layer at approx. 60cm. The topsoil (10cm) which makes up the primary non-wetting zone, had a high sand percentage (92.8%), low clay percentage (1.13%) and a low OC content (1.72). The site had been in continuous cropping for the past decade, and the host farmer had undertaken claying on other parts of his property prior to the trial commencing.

​Trial Design:

​At the Kojaneerup site, three treatments were applied, a low (140 t/ha), a medium (270 t/ha) and a high clay rate (350 t/ha), as well as a small nil clay control strip. The farmer applied 300t/ha across the rest of the paddock. All three of the treatment zones were incorporated with off-set discs to a depth of 150mm.

​Scotts Brook

​Site Description:

​The initial trial site at Scotts Brook was established in a  sandy gravel over clay, with the clay layer lying approximately 25cm below the soil surface. The topsoil at Scotts Brook had a high sand percentage 91%, a high gravel percentage of between 15% and 25%, and a relatively high clay percentage 4.4% (for a non-wetting soil), it also had a high OC percentage 3.5-5%. The trial site has been part of a 5-year cropping rotation, which included a legume. Prior to this the paddock was in pasture for 2 years.

​Trial Design:

​At Scotts Brook, a consistent rate of clay was applied over the site (400 t/ha) with CAT scraper over which different incorporation methods were implemented; plozza plough, speed tiller and off-set chisel plough (Figure 2). These treatments incorporated to differing depths of 30, 15 and 10cm respectively.  Untreated (nil clay or incorporation) zones were left in between claying runs.

​Trial Designs – 2023

​Woogenellup

​Site description:

​The Woogenellup trial site was established on a shallow sandy gravel duplex over a compacted sandy gravel layer at approximately 30cm below the soils surface. The topsoil had a high sand content 94.2%, a low clay content 1.4%, and a low OC .9-1.4%. The trial area had been in continuous crop for the previous decade.

​Trials Design:

​At the Woogenellup trial site (Figure 3), two clay rates (100t/ha and 200t/ha) were assessed over the two distinct soil type zones (60% gravel and 40% gravel) and incorporated by speed tiller to a depth of 10cm. The two controls included a tilled only (with speed tiller) zone and a nil clay and nil incorporation zone.

​Scotts Brook 2

​Site description:

​The second Scotts Brook trial site was established on a forest gravel soil with a high gravel percentage of 50-60%, a relatively high clay percentage for a non-wetting soil 4.8%, a sand percentage of 88%, and a high organic carbon content of 7.8%. The area had been in a 7-year cropping rotation, prior to that it was in long term pasture.

​Trial Design:

​The Scotts Brook (2) trial design included three clay rate treatments (70t/ha, 100t/ha and 200t/ha). The clay across all treatments was incorporated with a speed tiller, to a depth of approximately 15cm. Again, the two control treatments included a tilled only treatment and a nil clay, nil incorporation treatment.

​Monitoring and Measurement

​The following measurements were undertaken in 2022 and 2023 across both newly established sites.

• ​Baseline soil sampling 0-10cm and 10-30cm (incl. gravel, clay and OM content)

• ​Baseline water repellence assessments (MED test)

• ​Clay source – clay fraction determination

• ​Plant establishment counts

• ​Follow-up MED testing after clay application

• ​Biomass Assessment

• ​Soil strength at field capacity (somewhat difficult in gravel)

• ​Grain yield (farmer machinery) and quality

​The 2022 and 2023 established were continuously monitored through to the end of the 2024 growing season, with the following measurements taken.

• ​NDVI

• ​Plant establishment

• ​Grain yield and quality

Handy Claying Tips:

Claying has become a vital amelioration tool within the Great Southern region on WA, where the soils are typically sandy, low in organic matter and have a prevalence for non-wetting issues. Claying has shown to be most effective at increasing yield performance on sandy duplex soils, where the clay is incorporated in to soils with an extremely high sand percentage in the top 30-50cm of the soils profile, to lift the overall clay content within crucial area of the soil profile. However, there is an increasing desire form farmers to investigate the potential for claying to increase yield potential and alleviate non-wetting issues on other soil types such as the forest gravels and loamy sands common to this area.

Claying as an amelioration technique is the process of adding clay enriched (>20%) subsoil into clay deficient and water repellent topsoils. Claying topsoils reduces repellence, water and wind erosion risk, increases water holding capacity, and has the potential to increase organic matter.

The best practice for alleviating non-wetting and repellence issues is to target a clay fraction of 5% clay in the topsoil. However, there are quite a few variables that impact the ability of a grower to achieve this benchmark. Firstly, it is unlikely that a paddock has a consistent soil type, let alone a consistent soil texture. To establish how much clay a grower should spread to achieve the best practice benchmark, you need to know three key details:

1. What is the current clay percentage in the soil?
2. What is the clay percentage in the product you plan to spread?
3. What depth will you incorporate the clay in the soil?

To determine your current soil’s clay percentage, you need to take soil samples from a representative area in the paddock and to the planned depth of clay incorporation. Ideally, growers would take multiple soil samples within the paddock at different depths to your target incorporation depth.

You then need to determine the percentage of the clay fraction of the “clay” you are going to spread. “clays” can range anywhere from 15 to 40%, which will significantly impact the amount of clay that need to be applied to achieve best practice results.

Lastly you need to determine what implement you will use to incorporate your clay, and to what depth that will incorporate the clay too? Implement selection will ultimately be based around what the farmer has available to them, and what the desired outcome of the claying is. For example if you are looking to alleviate non-wetting then it is unlikely that you will need to incorporate your clay as deep as you would if you were looking to improve water holding capacity.

Once you have selected your implement and you know your incorporation depth you can use the following formula to determine how much clay is required to shift your top soil form its current clay % to 5% clay.