Is the freeze-thaw effect a myth?

Steve Larocque conducting a presentation.

What if the notion that the freeze-thaw action of icy winter weather gives Prairie farmers a free pass on soil compaction problems turns out to be wishful thinking?

If so, the implications should be enough to send a chill down a big-iron-loving farmer’s spine.

“We often say that we don’t have to worry about soil compaction, because Mother Nature is going to fix it for us,” Marla Riekman, a land management specialist with Manitoba Agriculture, Food and Rural Development told the recent annual workshop of the Manitoba-North Dakota Zero Tillage Association.

“Maybe she does, and maybe she doesn’t. It depends on where you are, and how deep we’re talking.”

The freeze-thaw effect for breaking up compacted layers has been a part of the established farm orthodoxy for decades, but Riekman cited a bundle of new research that suggests that the freeze-thaw effect only works in the top two to six inches.

When looking down farther to the nine-inch depth, the real heavy lifting in heavy clay soils may depend more on wet-dry cycles that cause cracks to form.

“We freeze deep, sure, but it’s actually the multiple freeze-thaw events that actually break up the compaction, not just a single frost,” she said.

Evidence of the long-lasting impact of soil compaction can be found in wagon ruts dating from the 1870s that can still be seen criss-crossing the Prairies.

“You’d think that if freeze/thaw works, that after 100 years you wouldn’t be able to see that trail,” said Riekman, who added testing on one of them showed a 50 per cent reduction in air and water infiltration in the tracks even though it was only used for three years.


Helping Mother Nature out in the case of soil compaction is largely a case of preventing it in the first place, she added.

Soil compaction can be caused by excessive or untimely tillage as well as wheel traffic. Telltale signs are roots that reach out horizontally instead of straight down, ponding after heavy rains and crops that flop over due to poor root development.

In theory, avoiding field work when the soil is wet is the key to preventing compaction. But in practice, farmers are under enormous pressure to get seed in the ground, wet or not.

But Riekman said contrary to popular belief, the unsightly ruts that form when the soil is extremely wet don’t result in serious compaction because the water in the pore spaces of soil particles prevents their closure.

The real damage occurs when the soil is moist. At such times, the larger pores can be flattened because they are filled with air. It’s easy to believe there’s no serious impact at such times, because the “lips” that the tire tracks create are barely visible.

“This is where I get yelled at because I know that we are not able to stop this. But this is when we need to limit activities as much as we can and limit the weight,” said Riekman.

Tests at the Prairie Agricultural Machinery Institute found that properly inflated duals were more effective at reducing compaction than 24-inch and 36-inch rubber tracks, but overinflated duals were the worst offender, she said.

Stick to the tracks

Steve Larocque, an Alberta farmer and owner of Beyond Agronomy, an independent consulting firm, believes that he has the solution.

Larocque said that he has seen dramatic improvements in water infiltration rates on his 1,000-acre farm on heavy clay soils north of Calgary after switching to a controlled traffic, RTK-based, no-till system with inter-row seeding.

“If you look at the equipment we run now and have run for decades, it’s 30,000 to 60,000 pounds on four tires,” said Larocque.

Instead of randomly travelling all over the field and leaving telltale tire tracks that “retard” crop growth and affect yields, he has gone to great lengths and even extensive modifications to match up his axle and equipment widths so that the tires only travel on the same paths, or “tramlines,” year after year, with a 30-foot drill, a 30-foot combine and a 60-foot sprayer that all run on 121.5-inch centres.

By restricting wheel traffic to the same ruts, he has cut the compacted area down from anywhere from 40-50 per cent of his land to just 17-20 per cent.

After several years of avoiding random wheel traffic, he has seen improvements in soil quality and tilth, but the most dramatic impact has been in terms of water infiltration rates.

“It offers a more resilient system. When the heavy rains come, you can absorb a lot more water and a lot faster,” he said.

Increased infiltration rates prevent roots from drowning out. Also, no hardpan layer means that in dry years, the roots can reach down deep to tap extra supplies of moisture. In his area, where an average year sees only 12 inches of rainfall, subsoil reserves are often the key to good late-season crop development.

Controlled traffic farming also helps with highly precise inter-row seeding in tall stubble, he added. With less compaction, his seed openers seldom drift from side to side, and the soil flows around them “like butter,” he said.

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