It's been a few years since salinity was a buzzword in Australian agriculture.

But recent research is shedding new light on the cause of salinity in heavy textured soils, and possible ways to fight the damaging effects on crop yields.

Transient salinity is the concept of salt particles falling on to the soil from rainfall events over hundreds of years - which is diametrically opposed to the more commonly held belief that most saline soils are a result of rising water tables.

Since heavy dispersive soils are structurally unstable, clay particles clog the soil pores, reducing the rate of water infiltration and drainage. As a result, the salt accumulates in the top layers of the soil and restricts plant growth. The soil dispersion is caused by soil sodicity and alkalinity.

While the idea of transient salinity isn't new, research into ways to combat this yield constraint is finding new and exciting ways to reduce its impact.

The research

A GRDC-invested research project investigating ways to minimise the effects of transient salinity in drier years is looking for solutions, particularly for growers with these heavy textured dispersive soils.

Project leader David Hall, from the Department of Primary Industries and Regional Development (DPIRD), says the research will look at the science behind transient salinity, plus potential management strategies growers can employ to overcome the effects of this constraint on crop profitability.

"While we are still in the early stages of understanding the mechanisms that can be used to reduce the impacts of transient salinity, we hope the outcomes of this project will provide practical solutions for growers with these types of soils to implement on-farm to increase profitability," Mr Hall says.

GRDC has produced a suite of podcasts and videos focused on some of the soil amelioration strategies growers can consider using to tackle soil constraints, such as sodic soils in the podcast below and salinity.

DPIRD principal research officer Dr Ed Barrett-Lennard says dispersive soils are often found in lower-rainfall regions, such as Western Australia's eastern wheatbelt, and the expression of the effects of transient salinity are most felt in rainfall years of 200 millimetres or less.

In these drier years, he says, growers might assume lower crop yields are attributable to drought conditions, when in fact they could be a result of transient salinity caused by soil sodicity and alkalinity.

"Growers have adapted extremely well to the challenge of dryland salinity - or salinity associated with rising water tables - mainly through taking saline areas out of commercial production," Dr Barrett-Lennard says.

"Much less is known about the effects of transient salinity and its effect on cropping production."

What is well known now is that canola and barley are more salt tolerant than wheat and legumes - restricting the production of high-yielding wheat crops in these heavy textured sodic soils.

Since reducing salinity levels in the soil on a broadacre scale might be prohibitive, the research project is focusing on ways to increase water availability to the plant to mimic an above-average rainfall year, thereby diluting salt levels in the soil.

Water harvesting

Dr Barrett-Lennard says one of the water harvesting methods in the research is the creation of mounds and furrows, plus the use of plastic sheeting and other hydrophobic materials, to create water run-off on to the plants.

DPIRD engineers have designed and created a prototype machine that is pulled behind a tractor to create these mounds and furrows as part of the seeding process.

A custom-built machine creates mounds in paddocks to increase water run-off into the furrows, reducing the impact of transient salinity on crops. Photo: DPIRD

Trials run in seven locations in WA's eastern wheatbelt since 2016 have shown a positive response to both the creation of mounds and furrows, and the application of plastic sheeting to the mounds at seeding time.

In Bonnie Rock in 2017, which was a lower-rainfall year, more than double the yield response was achieved between the control (no mounds, furrows or plastic sheeting), compared with the full treatment.

While the benefits of water harvesting using mounds and furrows aren't so obvious in a softer year, Dr Barrett-Lennard says the economics of increasing production so significantly in the drier years would make this strategy worthwhile.

While the research is still in the early stages, water harvesting techniques are clearly improving yields across the board in these soil types.

Hydrophobic materials

The use of plastic sheeting and other hydrophobic materials, placed on the top of the mounds, is being trialled as one way to increase water run-off into the furrows.

"Across all of the sites, soil moisture in the furrows was greatest when the plastic sheeting was applied to both mounds either side of the furrow and, in many instances, the salinity index (indicator of the salinity of the soil solution) was lower," Dr Barrett-Lennard says.

"Greater hydration of the soil allows for salts in the soil solution to become more diluted, ensuring greater uptake of both water and nutrients by the plant."

Dr Barrett-Lennard says further research will consider the timing of the application of this hydrophobic material.

"If this material breaks down over a couple of months, and if we need these mounds to be at their most hydrophobic prior to the finishing rains in August and September, it may be more beneficial to lay the material towards the end of the season," he says.

"At this stage, this is all blue-sky research and we understand that applying it on a broadacre scale may be both practically difficult and economically unviable. But proof-of-concept research can lead to innovative solutions down the track."

Dr Barrett-Lennard says soils affected by transient salinity can be manipulated through the use of both gypsum and elemental sulphur, but careful management of this strategy is important to maintain soil structure and soil productivity.

The trials are investigating the impact of placing gypsum down at seeding time, but only in the furrows, to increase the water harvesting function of the mounds and water infiltration in the furrows.

Other research, funded by the Council of Grain Grower Organisations (COGGO), is considering the link between water harvesting and plant genotype.

Dr Barrett-Lennard says the early work is looking at breeding plants which produce fewer tillers late in the season. These late tillers take both water and nutrients away from the production of seed late in the season.

More information: Dr Ed Barrett-Lennard, 08 93683798, ed.barrett-lennard@dpird.wa.gov.au