Compacted or not: Checking soil status

Fond du Lac — Most of Wisconsin was very wet in fall during harvest, and that increases the risk of soil compaction.

University of Wisconsin-Madison soil scientist Francisco Arriaga cautions growers not to assume there is compaction just because the harvesting equipment left ruts in the ground.

“When soil is saturated, there is a greater risk of making ruts but actually less of a chance of causing compaction,” he said. “The greatest problem is when the weight of the machinery pushes air out and pushes water between the soil particles.”

Speaking at the Fond du Lac Forage Council’s recent meeting in Fond du Lac, Arriaga said soils are most susceptible to compaction at water contents near field capacity because the proportion of soil pores filled with air and water is just right for compaction to occur. Soils with most of the pores filled with water are less susceptible to subsoil compaction because liquids are not compressible, unlike air, and can bear the load. When there is air between the particles, the pore space collapses with weight.

He suggested using a penetrometer to determine if there is a compaction problem.

“Frost will help to break up any surface compaction but not below the frost line,” Arriaga said.

If the test indicates there is no deep soil compaction, light tillage to smooth the ruts in the field may be all that is needed.

Arriaga admitted tillage takes away the soil’s ability to bear weight, but in the case of closing ruts, tillage will be the only option.

“This year with so much white mold on soybeans," he noted, "tillage may also be the best way to insure that it will not come back again in spring.”

When sub-soiling is needed to break up compaction layers, Arriaga said it is important to set up the tip of the subsoiler 2-3 inches below the lowest point of compaction depth.

“Subsoiling is expensive, and benefits should be considered against the cost,” he said. "It takes 30 to 50 horsepower per shank to pull a subsoiler."

Arriaga pointed to the benefits of cover crops such as tillage radishes for addressing compaction issues, but he noted, “It will help surface compaction problem, but the only way tillage radishes address deeper compaction is if they get in very early.”

Cover crops, including tillage radishes, could be beneficial in the long term by building healtier soil that in turn helps to prevent compaction.

While it is too late to change anything about the 2016 harvest season, Arriaga said addressing the issue early on in 2017 will be important.

“Wait for better soil moisture conditions before going onto a field,” he said. “Reduce axle loads, and maintain low equipment tire pressure.”

Arriaga also suggested establishing a traffic pattern to help contain and reduce soil damage when it is necessary to drive on a wet field. Then If there is compaction in that area, till or subsoil only the damaged part of the field.

In the long run, Arriaga said, no-tilled soil will hold up against compaction better than tilled soil.