Wildfires pose threat to fields’ crop residue

Keaton Donahue of Biggar, Sask., is still weighing his options on what to do with 320 acres of sooty black, charcoal-crusted soil.

A brush fire pushed by big winds quickly tore through fields of wheat stubble on May 16.

“I’m still playing my cards. I don’t know what I’m going to do with it yet. I was going to put canola down and now I’m not sure,” he said.

He said he plans to seed the rest of his 4,300 acres first.

Donahue is not alone. Several farmers in the hilly area had their land burned during the fire, which also consumed a stack of 400 hay bales and a couple of uninhabited farmyards.

The wildfire hazard is typically the highest across the Prairies in late April through May, when crop residue and grasses have extremely low moisture after snow melt.

Saskatchewan’s Wildfire Management Branch reports that most fires are human caused.

As of May 22, 42 wildfires were reported in Saskatchewan with one still active north of Prince Albert.

Before Donahue puts a seed in his blackened earth, he said he’ll consult with an agronomist to figure out how to compensate for missing nutrients.

“I’ve lost a whole year of straw off that field, so now I’m going to have to either put more fertilizer going forward for a few years or something to kind of make it back up,” he said.

About 10 years ago, Dennis Bulani of RACK Petroleum in Biggar was hired to complete a report on what the estimated economic losses were from the impact of fire on a field near that town.

Much of the scientific data came from a South Dakota State University study involving a controlled residue burn to determine nutrient loss.

“They burned straw and then measured what was still in the ash. Ninety-eight to 100 percent of the nitrogen was lost, 75 percent of the sulfur, 21 percent of the phosphate and 35 percent of the potash,” he said.

Mineral nutrients like phosphorus and potassium may be retained in the ash but, depending on the terrain, most of it blows off the field.

After a burn, the previous three years of crop residue needs to be factored in and using a pea, wheat and canola rotation, Bulani determined total losses could average two to three tons per acre of residue.

“If you assume there’s three crop residues on there, your losses could be between $60 to $90 an acre of actual nutrient fertility,” he said.

After the fire, he said producers should stick to their crop rotation, but establishing a higher residue crop like a cereal as soon as possible is advisable to hold down the soil.

But returning soil to its former nutrient levels could take up to six years.

“Especially if you’re soil building and direct seeding and leaving your residue on top to create all the things that happened with healthy soil. So you’ve lost a lot of years of building soil back,” he said.

Estimates show a burned field will lose 0.1 percent of organic matter, which is capable of holding up to 1.75 inches of available water for crop growth.

“So in dry seasons, that amount of water might result in an additional three bushels an acre of crop,” he said.

“It drives home the point about how valuable that component of your soil is. That’s the thing you can’t replace in one year. We can buy the fertilizer but the 0.1 percent organic matter lost from the burn takes years to rebuild.”

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