There are two widespread myths about compaction in northern states, says Jodi DeJong-Hughes, University of Minnesota Extension educator/soils.

The first is that freeze-thaw cycles will alleviate a majority of soil compaction created by machinery. The second is that deep tillage or subsoiling break up deep compaction.

Neither is true — at least not anymore, DeJong-Hughes says.

Alhough the soils in the northern U.S. freeze to depths of 3 feet or more, they don’t freeze and thaw more than once below the first few inches of soil.

“The belief that freeze-thaw cycles will loosen compacted soils may have developed years ago when compaction would have been relatively shallow because machinery weighed less, and grass and legumes were grown in the rotation,” DeJong-Hughes says.

Both heavy axle loads and wet soil conditions increases the depth of compaction in the soil profile. Tractors weighing less than 10 tons an axle usually keep compaction in the top 6-10 inches, which can be alleviated by tillage. A majority of the smaller tractors weigh under 10 tons an axle. However, full combines, slurry tankers, 500-hp tractors and grain carts weigh between 20 and 40 tons an axle. Whether equipped with tracks or tires, they can create compaction as deep as three feet on a moist soil.

Since 3 feet is well below the depth of normal tillage, the compaction is more likely to persist compared to shallow compaction that can be largely removed by tillage, DeJong-Hughes says.

“While deep tillage (greater than 15 inches) is capable of shattering hard pans created by wheel traffic, it has not been proven to increase yield consistently or for long periods of time,” she says. “In the Midwest, research results have shown few positive yield responses to subsoiling, and when they occur, are variable and relatively small. It is difficult to accurately predict the effects from subsoiling because of differences in soils, degree of subsoil compaction, soil moisture, future traffic, weather conditions, and differences in the crop grown and in tillage methods.”

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