Strategic Farming: Let's talk crops! session January 12 covered compaction

by Angie Peltier, Extension educator - crops

Nothing reveals soil compaction better than crops grown during a historic drought, as Minnesota experienced throughout the 2021 growing season. 

 During periods of drought or excessive soil moisture when plants are under stress, compacted soil adds an additional source of plant stress. 

Jodi DeJong-Hughes, UMN Extension water quality educator and Aaron Daigh, associate professor of soil physics and hydrology with the Department of Soil Science at North Dakota State University, joined UMN Extension educator Angie Peltier for a wide-ranging discussion of how compaction happens, how to avoid compacting soil in the future and how to remediate compaction that has already occurred. This was the second of the 2022 Strategic Farming: Let’s talk crops! webinars in this series.

Watch a recording of this webinar: Strategic Farming 2022: Let's talk soil compaction

How compaction happens

Soil is largely made up of sand, silt and clay particles and organic material that are held together by root exudates and soil microbes to form aggregates. Between these aggregates is pore space for water and air. The better aggregated your soil is, the better the soil will resist compaction - to a point. 

Stress from something very heavy on the soil (ex. cattle, farm equipment) causes aggregates from higher up in the soil profile to be pressed down against neighboring aggregates, which are then pressed down into aggregates those below them and so on, reducing the size of these hard won pore spaces. Compaction will tend to be more severe closer to the soil surface where the pressure was applied and lessen in severity deeper into the soil profile.

Soil moisture and compaction 

Compaction is very dependent on soil moisture as wetter soils tend to be weaker than drier soils. While a well-aggregated, healthy soil that is dry is able to resist some weight, water acts as a lubricant, weakening the soil and allowing soil particles to more easily glide past each other. The compacted soil that impacted crops during the 2021 drought may have been the result of operating field equipment during the historically wet autumn of 2019.  

Soils can become too wet to compact. When the pore space is filled with water, a soil will deform, rut, and have other smearing issues, instead of compacting. 

In addition to wheel traffic, other causes of compaction include down pressure of tillage implements and tillage that reduces the crop residue on the soil surface. Less surface residue allows raindrops to cause crusted soil, another type of compaction. 

Compaction can cause significant and long-term impacts on crop productivity and is capable of reducing crop yields by as much as 30 to 50%.

What does larger scale equipment have to do with compaction?

Over the years as the number of farms has decreased and the acreage of individual operations has increased, equipment has gotten larger to get all field work done in a timely fashion and with fewer personnel. With larger equipment comes more weight being carried on each axel. The greater the axel load, the deeper into the soil profile that compaction goes. As these farming trends march along, the severity of compaction increases and the costs of and time it takes to remediate the compaction increase and the feasibility of doing so at depths below 20 inches decreases.

But doesn’t freezing and thawing help to remediate compaction?

People in northwest Minnesota like to talk about how their soils are “very forgiving” and as a result folks need not worry as freeze and thaw cycles tend to remediate compaction. There are several freeze and thaw cycles in Minnesota each year, but it takes dozens of freezing and thawing cycles to help alleviate compaction, and only in the top 5-6 inches of soil.

What are some ways to avoid compaction?

Soil aggregation is the best way to defend against compaction. Reducing tillage and diversifying the types of plant roots that are grown in a soil, makes for a healthier, better aggregated soil, but this sort of soil takes some time to build. In addition to reducing compaction risk, a well aggregated soil will allow better water infiltration and retention and a less impeded path in which crop roots can grow.

Tillage also destroy aggregation if one chooses the wrong tillage implement or to practice more aggressive tillage than is actually needed. In general, points and shanks lift the soil and separate it along its natural planes, while disking is going to break up many more aggregates and smear the soil.

Tire size and inflation can have a large impact on compaction. In general, the larger the tire, the larger the area that the weight is spread out onto and the more axels the better. While it is important to have well-inflated tires for road travel, following tire manufacturers’ in-field tire inflation pressure recommendations (which will be lower than road pressure) is an important way to reduce compaction risk.

Controlling traffic on fields, or limiting the wheel traffic to as small of a field area as possible, will also help to reduce the overall percentage of a field where crops must contend with compaction.

For an in-depth discussion, see UMN Extension's Soil compaction.

Fielding audience questions

Jodi DeJong-Hughes and Aaron Daigh answered many audience questions that were either asked live during the webinar or posed when attendees registered for the series, including: Do tracks or tires cause more compaction? What about duals on tractors or combines? How do some of the new vertical tillage implements on the market do as relates to compaction? How do the salts in some of our soils affect compaction? Is there a specific cropping system that can reduce the risk of compaction? How does one figure out how deep a soil is compacted to come up with an action plan?  View the recording for the complete discussion.

Join the webinar series

University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2022.

Finding sufficient livestock feed supplies was a significant challenge during the historic 2021 drought, leading some to bale corn that had originally been intended for grain production. Join us this week as we have a conversation with Drs. Joe Lauer, UW-Madison corn agronomist, and Luiz Ferraretto, UW-Madison ruminant nutritionist, as we compare and contrast how to best manage corn intended for grain and livestock feed purposes.

For more information and to register, visit z.umn.edu/strategic-farming.

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!