CSIRO’s Dr Gupta Vadakattu says 50 per cent of microbial populations in soils are found in the top five centimetres. Soils affected by fire can have their microbial buffer against diseases weakened.
While some plant pathogens may be reduced following extreme fire events such as the Pinery fire in South Australia’s lower north region last year, growers affected by fire are being urged to monitor nutrient levels and soilborne disease pathogens in the lead-up to and during the 2016 growing season.
Fire removes carbon and nutrients from crop residues – the food source for microbes – reducing microbial activity in the soil. CSIRO principal research scientist Dr Gupta Vadakattu says 50 per cent of microbial populations in soil are found in the top five centimetres. If the heat from the fire is strong enough to reach that depth then it would reduce microbial populations and weaken the microbial or natural buffer against disease.
“Following a fire, the risk of disease from stubble-borne pathogens such as fusarium and take-all as well as other leaf diseases will be reduced,” he says. “Depending on the intensity of the burn, the effects on soil-borne pathogens such as Rhizoctonia and nematodes may be limited.”
If the microbial buffer is weakened, it will take less pathogen inoculum to cause more impact.
“A small amount of inoculum will have greater disease impact than it would in a normal situation,” Dr Vadakattu says.
Reduced microbial populations will also have an effect on nutrient availability for following crops. Cultivation has been necessary for many fire-affected growers with sandy soils to bring clay to the surface and reduce drift, but this, along with the lack of stubble, will accelerate nutrient mineralisation in the soil and increase the potential for leaching with heavy rain.
“Because there is no stubble-associated microorganisms tying up nutrients from the soil or fertiliser, growers may see more mineral nitrogen or available nutrients at sowing,” Dr Vadakattu says. “However, with the recent rain there is potential for those nutrients to be leached.
“If you compare a paddock that is burned against one that is unburned, the recent rains will have moved the nitrogen down the profile in a burned paddock, whereas that nitrogen would be tied-up with the stubble in an unburned paddock.”
Dr Vadakattu is encouraging growers to do soil tests before they plan their fertiliser regime for the 2016 season.
“For growers looking to grow legumes following a fire, they should look at inoculating the seed with the appropriate rhizobia because rhizobia populations can be reduced from the heat,” he says.
“With the fire affecting the usual microbial processes, this season more than ever, growers will need to test soil and closely inspect crops throughout the growing season.”
Dr Gupta Vadakattu, firstname.lastname@example.org
GRDC soil testing for crop nutrition fact sheet