Topsoil gone but nutrient loss minor: Silver lining for fire-impacted Wheatbelt farmers

Headshot of Shannon Verhagen
Department of Primary Industries and Regional Development research scientist Glenn McDonald.
Camera IconDepartment of Primary Industries and Regional Development research scientist Glenn McDonald. Credit: Supplied

Last harvest’s bin-bursting yields are still within reach for growers in the fire-ravaged Wheatbelt, with experts assuring growers the blazes would have had little impact on soil nutrients and yield potential for 2022-23.

However, farmers will need to be wary of sandblasting and consider a number of factors as they begin seeding to reduce their erosion risk.

During the Shackleton-Corrigin and Wickepin-Narrogin fires which broke out on February 6 and tore through more than 50,000ha, the region’s ground cover — be it stubble or pasture — was reduced to ash.

In the weeks that followed any breath of wind picked up the topsoil and blew it away, causing erosion and soil health concerns.

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Despite obvious topsoil losses, with sandhills forming on roadsides and paddocks exhibiting the same ripples as sand dunes, Department of Primary Industries and Regional Development research scientist Glenn McDonald said growers could take comfort it would not have effected on soil nutrients too badly.

“In term of losses, the biggest is losing the biomass from the previous year with stubble,” he said. “But generally your nitrogen loss in the soil is not large but there is some.

“Most of the organic matter will still be there, all the fire really is doing is taking out your surface material — your ground cover.

“Where it becomes a big problem is in the topsoil area, as the amount of erosion through that zone is proportional to the amount of nutrients you lose. So if you lose half of that you lose half your phosphorous.”

After the catastrophic 2015 Esperance bushfire, which claimed the lives of four people including farmer Kym Curnow, Mr McDonald studied the impact of the blaze on the soils.

While a loss of potassium was noticeable and protein levels declined, Mr McDonald said growers did not notice any yield change from burnt to unburnt country.

“Where it did change was where they’d had blowouts which took away the topsoil and took away the surface nutrients, the trace elements and phosphorous,” he said.

Erosion was the biggest risk Wheatbelt growers faced as they began seeding, Mr McDonald said, encouraging them seed into moisture rather than dry-seeding.

“Avoid driving on it at all, even driving your ute or an ATV breaks up the soil surface and the little bit of crusting you might have and triggers erosion,” he said.

“There’s a risk coming into seeding as you start to grow crops on those paddocks, that they’ll get sandblasted.

“From experience from fires around Badgingarra (in 2009) is that sometimes they had to resow their canola two or three times.”

He said it could also be seen as an opportunity to bring forward any soil amelioration planned, suggesting the lumpier the soil and more clodding, the less prone to erosion.

Bringing up gravel to the surface would “amour” the soil and act as a protectant, he said.

“Gravel is your friend,” Mr McDonald said.

“You end up with more soil available for the roots, because if you’ve had 60-70 per cent gravel content in the mid soil-subsoil, there’s only about 30 per cent the crop can grow in.

“If you flip that up and put that at the top, you’ve got more soil volume later in the year for crop growing. It’s a numbers game.”

As growers begin to bring agisted livestock back once fences are rebuilt, Mr McDonald said it was important they were not run on paddocks with high erosion potential, particularly lighter, sandier soils.

Elders Narrogin agronomist Helen Wyatt said the aim was to get cover on the ground “as soon as possible” bearing in mind moisture levels, erosion risk and weeds.

However she urged caution to “not go hell for leather” and sow all the canola early, instead spreading the risk with some changes to the rotation and sowing cereals first.

“Canola and lupins are going to be a lot more prone to damage by wind erosion and I don’t know if you’re going to want to reseed a lot of country this year as fuel is really expensive and hybrid canola seed is very hard to get and expensive,” she said.

“Cereals are going to be a lot easier to establish and they’re going to handle the sandblasting a lot quicker.

“We’ve had a great drop of rain now but if we get another three to four dry weeks that topsoil is going to evaporate a lot quicker as we don’t have any cover.

Another suggestion that came from the Esperance bushfires was sowing a canola-barley mix, she said, before removing whichever one was wanted later.

“If you’re wanting to protect hectares of really fragile country that might be an option for you,” Ms Wyatt said.

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