WHEN it comes to managing soils post fire, wind erosion is generally the biggest risk, with sandier soil types the biggest danger.
Speaking at the Farming After Fire forum hosted in Wickepin and Corrigin on March 31, Department of Primary Industries and Regional Development research scientist Glenn McDonald said anything which disturbed the soil surface could enhance erosion, such as livestock and driving vehicles.
He said it was important to preserve any roots or crop stumps and avoid disturbing them, with experiences from other fire events showing this residual anchored crop material was important in minimising wind erosion.
"Rain can help a crust to form, which armours the soil surface a bit," Mr McDonald said.
"Soils with lots of gravel or stones are less likely to have significant erosion compared with soils that don't.
"There may be situations where creating an uneven soil surface by tillage helps but this should only be tried if there is no anchored crop residue and severe erosion as any tillage can destroy the fragile soil aggregates and increase the erosion risk."
While rain can have its advantages, heading into autumn, water erosion also poses a risk.
In the lead up to seeding, there are several factors to consider, with the most critical being there may still be erosion events as the crop germinates meaning there is a risk of emerging crops being damaged by sandblasting.
Cereals are more able to cope with this than broadleaf crops, such as canola and lupins.
Mr McDonald said as a result, strategies around what to do with things like fertilisers and herbicides if reseeding is needed and should be thought about.
"Maybe longer coleoptile varieties could be used in case the seeding furrow gets filled in or adjust seeding depth," he said.
"Also, if erosion is likely then higher seed rates may be required to compensate for any seedling death."
Seeding burnt areas as soon as the soil is moist from rain will increase the speed that ground cover is produced, while dry seeding burnt areas should be avoided as it will increase the risk of erosion.
Generally the only organic matter that has been lost is the surface residues and stubbles.
Most of the soil organic matter will still be in the soil, unless there has been erosion that has carried the organic rich topsoil away, however it's the same for all nutrients in the topsoil.
If 20 per cent of the topsoil was lost, then about 20pc of the soil organic matter and nutrients has probably gone with it.
Apart from at the surface, all the microbes and other soil critters will still be in the soil but without the crop residues and less evaporation of rain at this time of year, soil water is likely to be higher.
Mr McDonald said the nutrient loss would be effectively the same story as for organic matter.
"The loss of last seasons crop residue means that the nitrogen in that material has also been lost," he said.
"But this is only a small proportion of the total organic nitrogen available to the next crop and for most nutrients, the erosion of topsoil is the cause of nutrient loss after fire."
While many soil amelioration techniques can increase the wind erosion risk, with good soil moisture it may be a good opportunity to undertake some strategic soil amelioration.
Soil water repellence is unlikely to have changed after the fire and without the crop residues, repellence is likely to be more of a problem this season.
On top of that, many soils could do with some lime, especially if there has been some soil erosion.
"If rollers/packers are removed, many amelioration techniques can leave an uneven soil surface which can help reduce erosion," Mr McDonald said.
"As with any soil amelioration process, the soil constraints should be adequately assessed and measured.
"After doing this it is much easier to develop a plan for amelioration techniques that address as many constraints as possible, such as soil water repellence, subsoil acidity, compaction, etc."
There are many costs associated with fires, especially for infrastructure such as buildings, machinery and fences.
Assuming erosion was not significant, there's only a small agronomic cost of the fires to cropping rotations, apart from the loss of one or two seasons of stubble and surface crop residue.
"Experience from other fire events suggests crop yields this season shouldn't be affected and there may be other effects such as reduced crop diseases because all of the disease inoculum has been burnt off," Mr Mc Donald said.
"There will be a larger impact on livestock and pasture enterprises where the loss of stockfeed could result in major management decisions such as stock confinement, hand feeding and stock care."
Overall, any significant decisions should be discussed with agronomists, consultants and/or other growers.
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