Pushing boundaries can be the key to achieving breakthrough gains, which is what the Department of Primary Industries and Regional Development and GRDC are seeking with a bold effort to reconstruct constrained soils … and see what happens.

A soil amelioration project is underway in the Western Australian wheatbelt in an unprecedented endeavour to determine the theoretical upper limit for crop production in water limited environments.

Researchers are re-engineering a soil profile with multiple constraints on a massive scale to see whether they can maximise crop returns by improving crop root access to deep soil moisture and nutrients.

The project is part of a $42 million co-investment between the WA Department of Primary Industries and Regional Development (DPIRD) and GRDC and is combining what has been learnt previously from single-focus soil amelioration projects and digging deep to re-engineer soil profiles.

“We are in pursuit of a quantum leap – a step change,” says project leader Dr Gaus Azam from DPIRD.

Breadth of investment

The investment builds on a wealth of WA soils research and, in particular, five years of research outcomes from the major collaborative initiative Soil Constraints – West.

This initiative delivered solutions for a range of individual soil constraints that limit crop production: water repellent soils, soil compaction, soil acidity and other subsoil constraints.

The aim of the new investment is to investigate multiple interacting soil constraints within the crop root zone through strategic combinations of soil amelioration techniques or from soil profile re-engineering.

The new large investment covers three projects. The first is managed by Dr Azam and is called ‘Re-engineering soils to improve the access of crop root systems to water and nutrients stored in the subsoil’. It focuses on the 12 million hectares of arable land covering diverse soil types. These include deep sand, texture-contrast soil (duplex) and heavy soil in the low to high-rainfall areas of WA where subsoil compaction, subsoil acidity and water repellence regularly occur together.

For example, the research site, at Mark Pearce’s property at Tarin Rock, 300 kilometres south-east of Perth, is deploying combinations of soil amelioration and amendment techniques. The aim is to assess the feasibility and potential benefit of ameliorating soils to a much greater depth than has been attempted before.

Project leader Dr Gaus Azam, centre, with host grower Mark Pearce (left) and GRDC Program Manager Dr Rowan Maddern at the Tarin Rock research site. Photo: Anvil Media

A second soils project is managed by David Hall from DPIRD and is focusing specifically on the 2.5 million hectares of sodic and transient saline soils of the low-rainfall areas of WA’s eastern grain belt. This project aims to reduce the risk and improve the profitability of grain production on these typically hostile soils. It is assessing new options for improving the capture and infiltration of rainfall through novel furrow formation and crop row management techniques.

Research assessing the potential – and financial feasibility – of targeted root zone subsoil amelioration and/or the application of soil amendments aims to give growers in these lower-rainfall regions financially viable options.

The third soils project is managed by Dr Stephen Davies from DPIRD and focuses on how WA growers can best ensure the long-term benefit and profitability of new and previously developed soil amelioration and amendment techniques.

This includes assessing options for reducing the risk of poor crop establishment and soil erosion – both commonly associated with new soil amelioration practices.

Tarin Rock site

A site at Mark’s property at Tarin Rock was selected to re-engineer soil profiles as it is representative of the problematic and complex constraints present across a wide area of WA.

This is one of the six sites that were already established before 2021 cropping seasons. The team aims to establish three more sites before the next cropping season.

“It is a shallow duplex soil where the top 25 centimetres of soil is water repellent and acidic. Below this is a 15cm sodic transition clay layer and then quite a productive, better-quality, clay at depth,” Dr Azam says.

“This sort of profile can be prone to waterlogging in the top layers and can have low cation exchange capacity – a measure of nutrient availability – further down the profile. Generally, this soil type can yield about 1.5 tonnes per hectare of barley on a 150-millimetre growing season rainfall.

Our radical renovation approach for this profile has entailed excavating to a depth of 80cm and peeling the profile back in separate layers and then ameliorating each layer with different treatments and returning them in the same order.

“Amelioration equipment at present typically operates to 20 to 40cm and usually incorporates amendments in the top 10 to 30cm, so we are doubling the depth. We are aiming to retain more moisture and nutrients at depth and improve crop root growth and access to these over the season. Earthmoving machinery has been used to re-engineer the soil profiles and we have also required quite a bit of manual labour.”

Plots are six metres long by 4m wide and there are four replications of the 10 treatments, so the total area at this re-engineered experiment is about 1000 square metres.

The re-engineering and amelioration treatments were determined following extensive sampling, laboratory analysis, testing of the various soil horizons and in consultation with soil and crop science experts around the country.

“For each soil layer we undertook a series of analyses and small laboratory experiments that allowed us to determine the concentration of soil amendments, blending of layers and nutrient addition that would be optimum for retaining and supplying nutrients and water to the crop,” Dr Azam says.

“At the same time, we had to consider what was required to recover and stabilise the soil structure after such a large disturbance.”

Included in the treatments is a control, the undisturbed profile and treatments that include surface addition and shallow incorporation of nutrients and amendments.

Added to this are 10 radical re-engineered treatments to 80cm depth. The ‘gold star’ treatment, number 10, includes all the amendments and nutrients required to optimise the soil environment for the crop.

Figure 1: Original undisturbed soil profile showing constraints at various depths (a) and ‘Gold star’ treatment number 10 designed to fix pH and compaction, adding clay to sandy layers to improve water holding capacity, creating and improving soil structure by physical breaking and addition of gypsum and compost, diluting sodic layer by mixing with non-sodic clay underneath at a 1:2 ratio.

Source: DPIRD

Deep ripping machines can operate to 80cm and some delvers to 1m in depth.

“For example, in the simplest re-engineering treatment we were looking to fix the pH, compaction and to break and dilute the sodic layer by mixing with non-sodic clay underneath at a 1:2 ratio.

“For the ‘optimum’ treatment we were looking to fix not only the pH and compaction, but we also added clay to sandy layers to improve water-holding capacity, created and improved the soil structure and its stability by physically breaking and adding gypsum and compost, and by diluting the sodic layer by mixing with non-sodic clay underneath at a 1:2 ratio.”

The treatments in the field will be augmented by glasshouse studies as the research team tries to determine which re-engineering aspects and amendments are most important for optimising soil function for crop growth.

“The barley variety Spartacus CL has been grown across all treatments this year and we will be collecting at least another four years’ crop performance data from the site,” Dr Azam says.

The crop was sown by Mark using his equipment as the plots are part of his established controlled-traffic system. It will be sown to canola in 2022.

The site has soil moisture probes that are logging moisture status data every hour from each 10cm depth increment and also in situ root imaging equipment involving a 130cm-long minirhizotron combined with a portable 360° scanner.

The project will repeatedly measure the changes in soil quality parameters including pH, soil strength, cation exchange capacity, aggregate stability, water infiltration, aeration, carbon and other nutrients. Plant physiological measurements such as stomatal conductance and nutrient uptake by crops, together with crop growth, final yields and yield quality parameters will also be carried out.

Lessons to date

“We have been fortunate this year that we have received above-average growing season rainfall, which has been a windfall for establishing this re-engineered soil profile site,” Dr Azam says.

“So far, we have seen that the vegetative phase of the barley has been longer on the re-engineered treatments and the biomass two to three times larger, and root growth is also two to three times deeper.

Growers inspecting 12-week-old barley at the Tarin Rock site. Treatment eight is in the foreground, in which clay, lime, gypsum and nutrients were added in 10cm increments from 80cm to the surface, at rates determined to be optimum for crop growth. Photo: DPIRD

“We find more roots in the surface regions of the control treatments but those run out of moisture faster due to a lack of rooting depth.

This research is clearly blue sky, but if we can demonstrate value in terms of a significant gain in crop yields, machinery manufacturers and growers will build a machine to achieve this sort of re-engineering.

The project will extend and upgrade the decision support tool Ranking Options for Soil Amelioration (ROSA) financial model to incorporate the economics, and benefits of re-engineering will be a primary output of this project. This tool helps growers understand the costs and benefits of soil amelioration. This and other economic analyses will assist in determining the value for growers of the soil profile re-engineering approach.

More information: Gaus Azam, 0472 831 809, gaus.azam@dpird.wa.gov.au

Read: Exhaustive endeavour to relieve soils headache.