Heavy-textured soils in the low-rainfall areas of Western Australia’s eastern grainbelt are challenging for cropping. These soils often come with a host of associated constraints that present physical and chemical challenges for crops, including high pH, high density, poor stability, sodicity, salinity and boron toxicity. Some of these challenges lead to water issues – including rainfall ponding, run-off and exacerbated evaporation – and, ultimately, limited water infiltration and plant availability.

Options to deal with these constraints are limited, have variable results and can be very expensive. Current GRDC and Western Australian Department of Primary Industries and Regional Development (DPIRD) projects are doing some good proof-of-concept work, particularly re-engineering the soil profile to tackle multiple interacting soil constraints (DAW1902-003RTX, led by Dr Gaus Azam) and improving water capture and availability on soils with sodicity and transient salinity (DAW1902-001RTX, led by David Hall).

Growers are also trialling a range of options – some proven, some experimental – to improve profitability on these soils. A DPIRD survey of growers that was conducted in 2021 provides insight into what growers are doing with amelioration (trying to fix the soil problems) and management (trying to live with the soil problems).

Gypsum

In the survey, gypsum was the most common amelioration strategy (used by 77 per cent of respondents). Only 42 per cent noted a positive impact on subsequent crops, highlighting that gypsum is a good amelioration option for dispersive soils that are responsive to gypsum (these soils often have too much sodium) but is not a panacea for all heavy soils.

When too much sodium is the issue, replacing it with calcium is the long-term solution. This can take a lot of gypsum (and money). Frequent applications (every two to three years) with lower gypsum rates (two to three tonnes per hectare) have a good chance of working and being more profitable by suppressing dispersion in the short term and gradually replacing sodium with calcium.

Gypsum can also help lower pH in alkaline-sodic soils, in turn improving soil stability. The calcium ions in gypsum precipitate as calcium carbonate, consuming some alkalinity. For this to work the soil needs to have a pH above about 8.3 (CaCl₂) and bullish gypsum rates are needed (at least 5t/ha).

Banding a low rate of gypsum (50 kilograms/ha) in the seed furrow might provide yield benefits, but the research is still in the early days. Of the four low-banded gypsum rate trials, one had a yield increase of 28 per cent, but the other three trials saw no notable improvement.

Where yield did improve, researchers believe the improvements stem from a small electrolyte effect – the gypsum allowing salinity to flush out of the root zone and offering good root establishment in the first few weeks. With such small rates the effect is short-term, meaning gypsum probably needs to be banded every year at seeding, particularly if seeding into a different furrow to the previous year.

Low cost makes low gypsum rates an attractive strategy, but banding granular gypsum through seeding equipment is currently less appealing. New liquid gypsum products, however, are an area of interest for researchers.

Mulch

Some growers mulch these soils, historically with straw, old hay and spilt grain, and increasingly with sand and gravel. Research near Ravensthorpe, WA, with mineral mulches is showing decent yield boosts with two to four centimetres of gravel mulch. The gravel layer reduces evaporation; glasshouse trials calculated that 2cm of gravel reduced evaporation by 61 millimetres.

Sand mulches have had less stellar results. Trials in 2020 found no difference between the control plots and sand mulched plots, but this could have been from the sand preventing evaporation during the wet August in 2020, keeping the soil waterlogged. In 2021 trials, sand mulch increased wheat yields by 0.5 to 1t/ha compared to the control. However, gypsum without sand mulch increased yields more, by 1.3 to 2t/ha.

On average, the sanding and gypsum treatments in 2021 increased wheat yields by 0.5t/ha. Sand and gravel mulch should not be incorporated. Incorporating dilutes the effect of the mineral mulch, making it less effective in reducing soil evaporation. Some minor incorporation cannot be avoided during seeding; however, this has not caused any issues.

Deep tillage

Half the survey respondents indicated they are using some form of deeper tillage, such as deep ripping, on these heavy soils but only one-third noted a positive impact from ripping.

Research says ripping responses are variable and can even result in a yield penalty, especially if more hostile soil is brought to the surface. Where there is an improvement in year one, the effects can be short-lived. If the soil is dispersive and gypsum responsive, unless gypsum is applied in conjunction with ripping, the soil will disperse the next time it rains, then set hard again.

Non-cracking clay in Western Australia’s southern wheatbelt. These soils are typically dense, poorly drained and can be dispersive. Photo: MySoil

Many growers have reported success from tilling heavy soils for years, though there are few clear answers behind the cause of improved productivity. The short-term benefits could come from a more friable soil surface to seed into and, therefore, better crop emergence and more nitrogen mineralisation. It might also be from creating preferential pathways for water in spring, something being researched at Bencubbin, WA, as part of the re-engineering soils project (DAW1902-003RTX).

These trials are attempting to solve the issue of disconnected subsoil and topsoil water. In heavy soils, more summer/autumn rain ponds on the surface and evaporates than infiltrates into the soil.

Crops can access some rain after the season break but often hit a dry layer of soil that lies above deeper subsoil moisture. This causes shallow crop roots and a less resilient crop. Connecting the topsoil and subsoil water will give the crop a chance to grow deeper roots, accessing moisture and previously untapped nutrient stocks.

The trials are ripping with existing tillage equipment – a Paraplow or Agroplow – to create channels in the soil to physically connect the topsoil and subsoil water. Ameliorants including gypsum, phosphorus, nitrogen and humates aim to improve deep root growth and soil structure.

One of the trials is also using a summer crop followed by a cereal or a taprooted crop in winter to create biological pathways for the crops from the second season onwards. Getting plant roots growing into mechanically or biologically made preferential pathways will start to add organic matter, which will help maintain the longevity of the pathways. These trials are only working with non-dispersive or mildly dispersive soil (soils with an exchangeable sodium percentage less than six).

Early observations indicate the Paraplow works better than the Agroplow because it creates channels without bringing up subsoil clods or having the topsoil fall into the channels. Root mapping in 18 soil pits has shown deeper and more roots in soils that were broken up with the Paraplow. One potential issue on these soils is that if a crop does establish well, it is at risk of extracting the limited water too early and droughting itself if there are not late rains.

Furrow management

For some growers, creating a better micro-environment in the furrow to improve crop establishment is the main strategy. Capturing water in the furrow can help leach transient salinity and aid organic matter retention that helps improve soil stability, infiltration and early root growth.

Options being trialled to create, maintain and nurture furrows include:

• seeding into or near the previous year’s furrow when the season has a dry start and there is limited stubble from the previous year. Too much stubble leads to stubble flow problems and poor seeding depth control. If there is a wet start to the season (so water harvesting is not as important) sowing interrow is less risky and easier for stubble flow;
• harvesting high and retaining stubble to reduce rainfall splash damage of the furrow and reduce evaporation;
• working on the same lines and not grazing paddocks to reduce damage to furrows and protective stubble; and
• experimenting with low doses of gypsum (as described in the gypsum section above).

Cropping strategy

Opportunistically cropping paddocks in better seasons (more than 150mm rainfall) is one way to minimise the risks associated with problematic heavy soils. Deciding if a season is good enough to crop is usually based on the amount of summer/autumn rainfall and stored soil moisture.

Cropping after fallow is another option because it reduces reliance on in-season rainfall, as well as the fallow phase being a popular way to control weeds. Wheat and barley are the most-popular crop choices on these soils. From the survey, Scepter is the most common wheat variety, and Spartacus CL the most popular barley variety.

Biological re-engineering by growing deep-rooted crops might also help create preferential pathways in the soil. Dr Azam’s team working on re-engineering soils is also investigating crop options to enhance deep root growth.

Last year, the trial grew legumes, perennial species and different types of canola. There was also a fallow treatment in the 2021 season. This year, the whole site is sown to barley, which should show which crops offer the most benefits for follow-on cereals.

Paddocks that are consistently unprofitable are often consigned to grazing or even abandoned. Twelve per cent of the growers in the survey had withdrawn cropping paddocks with heavy soils because they were unproductive.

Grazing

According to the survey, these soils are grazed just as much as other soil types. The likelihood of grazing increases as the area under crop decreases. All surveyed growers who crop less than 3000ha include grazing and/or livestock in their rotation on heavy soil paddocks; compared to 63 per cent of growers who crop between 3000 and 5999ha and 33 per cent of growers who crop 6000ha or more.

The benefit of grazing is that it is lower-cost and lower-risk (from insufficient rainfall and frost) than cropping. However, animals trampling across the paddock can adversely affect soil stability, particularly of previous years’ furrows.

Fertiliser strategy

The survey revealed 67 per cent of growers use different fertiliser rates on heavy soils than on better and more consistently performing soil types. Growers are cost-sensitive because of the seasonality/seasonal risk associated with heavy soils. They tend to use less fertiliser upfront at seeding (40kg/ha MAP is common) and only top-up with in-season nitrogen if the season is looking good.

Their strategy de-risks the soil type, letting seasonal conditions drive yields rather paying for fertiliser – which may or may not deliver – to try and force them.

References and further reading

Fulwood J. ‘Precision gypsum placement provides hope for sodic soils.’ GroundCoverä, Issue 148, September-October 2020.

Precision gypsum placement provides hope for sodic soils.

Hall D, Daw P, Burrell L, Parker W and Munir R. ‘Mineral mulches improve water relations and yields on sodic clay soils.’ GRDC Update Paper.

Knights S. ‘The great dig to re-engineer soils.’ GroundCover, Issue 156, January-February 2022.

Research Solutions 2021. ‘Mitigation strategies for the management of high pH soils in low rainfall regions of the wheatbelt.’ Report for the Department of Primary Industries and Regional Development, May 2021.