THE application of incredibly low rates of gypsum on soils affected by transient salinity could increase barley yields by up to 28 per cent, according to data from a trial being run by the Department of Primary Industries and Regional Development (DPIRD) at the Merredin Research Station.
There are two main causes of salinity in WA - dryland salinity, which is the result of the shallow watertable caused by the clearing of native vegetation and affects maybe 10pc of farms, and transient salinity, which comes from a range of sources, including rain and dust, and builds up over time in dispersive soils.
Transient salinity was discovered at the Merredin Research Station by accident in 2010, where the soils are mostly heavy red, with alkaline at depth, and have a very low rate of water infiltration.
DPIRD principal soil scientist Ed Barrett-Lennard said growers in WA were accustomed to always thinking about the rain as fresh water, but that's not entirely the case as there are small amounts of salt in it.
"We've been able to calculate that the amount of salt stored in the top metre of soil at Merredin probably accounts for about 1300 years, on average, of salt falling in rain," Dr Barrett-Lennard said.
"There is quite a measurable amount of salt in those soils, it's not super high, but it is high enough to affect plant growth in dry years."
When it comes to plant growth, it's not the concentration of salt in the soil that determines whether it is affected, it's the salt concentration in the soil solution.
"You can double the salt concentration in the soil solution, and therefore the stress on plants, by doubling the amount of salt in the soil, but also by halving the amount of water in the soil," Dr Barrett-Lennard said.
"Because of that, the small amount of salt that we have in these soils affected by transient salinity is a problem in drier years as it is decreasing the availability of water for grain fill."
With that in mind, Dr Barrett-Lennard and his team started investigating two approaches to tackling the transient salinity problem, performing trials in 2019 and 2020.
The first approach was to use micro-water harvesting by exaggerating the furrows and mounds on the surface of the soil in order to direct all available moisture into the furrows.
With that tactic, the water concentration around the plant's roots was effectively doubled.
While the results were positive, it was really a proof-of-concept idea and currently is not commercially viable across broadacre properties.
The second approach that was tested was putting down very small amounts of gypsum in the furrow with the view to very briefly restore the hydraulic conductivity of the soil.
"That means that the salt could leech deeper into the subsoil, so instead of trying to mature with salt around the roots, the plant is trying to mature at the end of the growing season with much less salinity," Dr Barrett-Lennard said.
"The amount of gypsum applied was very low - normally when farmers apply gypsum, they put it on at one or two tonnes per hectare, whereas we were putting it on at a rate of 50kg/ha in the furrow.
"This low rate of gypsum acts like a gate - when the soil opens, the salt leaches out and when it closes, the salt is gone - but if you put on a higher rate of gypsum the soils will be open for longer and there may be other benefits associated with that."
Overall, in 2019 the yield benefit from the gypsum application in the barley crop was 28pc, while in 2020 the benefit was 20pc.
The researchers were able to show that within the first 30 days, gypsum application decreased salt concentration down to 60 centimetres by about 50pc.
Dr Barrett-Lennard said the exceptionally low rate of gypsum that was applied was similar to the amount that would have been present in superphosphate at the old traditional rate of one bag per acre.
"That led me to wonder if the movement away from super phosphate to double and triple super, therefore no small amount of gypsum in the phosphorus fertiliser, has changed the dispersion in these soils with a background of sodicity and alkalinity," he said.
"We're going to be following that up this year with more trials looking at higher rates of gypsum, to see if we can get a sense of where it peaks out.
"The extent of this issue is not yet clear but if we could achieve 10pc improvements in grain yield across 20 to 30pc of the Wheatbelt, with a soil treatment that only costs a couple of dollars per hectare, then that could be very interesting."
