Compaction: What is it?
Compaction is a symptom of poor soil function. The damage that is caused to soil structure by an applied force (think heavy combine or tractor) occurs when soil particles (sand, silt, clay) are pressed together. This causes a loss of pore space and aggregates.
Why should we care?
Large pores are necessary to allow water to infiltrate into the soil and allows air exchange for plant roots and soil microbes. Compacted soils have smaller pores that hold water and plant available nutrients at higher pressures, making it more difficult for plant roots to access them.
Are there different kinds of compaction?
Yes! A field can experience surface crusts, shallow (6-8”) and deep (>8”) compaction.
Surface crusts are caused by raindrops on bare soil. Residue protects the soil surface from rain drops that can hit with a force of 20 mph, causing soil aggregates to disintegrate into individual particles. Particles then move across the soil surface with runoff and settle out into pores, creating a thin solid surface. According to Jerry Hatfield, Retired Ag Research Service Director, it only takes 1/32” of an inch of crust to stop water infiltration and gas exchange.
This reduces water infiltration from future rains, creating ponding, more runoff and topsoil loss. A rotary hoe can be used to alleviate this but remember that tillage further breaks down the soil glues and pores needed by microbes and plants.
Shallow compaction/tillage pan: When a tillage implement is pulled through the soil, especially if done repeatedly at the same depth, a compacted layer can be created directly below the depth of tillage. Shallow compaction is related to the tire pressure applied to the soil. It can take 3-5 years to alleviate surface compaction.
Deep compacted layers are created by heavy axle loads (>10 ton/axle). Wet field conditions can make this situation worse. Tractors, full grain carts and combines can weigh between 18-40+ tons per axle and can create compaction down to depths of 3-4 ft, deeper than tillage equipment can reach (Daigh). This can take years to decades to alleviate.
How does compaction happen?
Poor soil structure sets the stage where soil is more likely to be impacted by rain drop impact, tire contact pressures and axle loads. Good soil structure with strong aggregates is a key defense against compaction. Without the glues to hold those aggregates together, they’re more likely to collapse into individual particles that are easier to stack and compress. Aggregates acts as tiny support columns in the soil that absorb pressure and resist compaction (Daigh). Things like over fertilization, poor surface residue and excess tillage can make things worse by creating poor soil structure.
How do I know if I have compaction?
Low yielding areas with stunted crops can indicate compaction. Use a penetrometer or wire flag to find zones of resistance. Digging a hole to look at soil structure is a great technique. Look for solid or platy structure, then look at the roots – are they making J curves or lacking fine root hairs? A slake/slump test measures aggregate stability and bulk density is a measurement of pore space vs. soil solids. All are good ways to measure compaction.
What can alleviate compaction?
Freeze thaw requires numerous cycles to address compacted layers. The number of freeze thaw cycles varies with depth. More cycles occur in the top 2-5” of soil and are more effective for shallow compaction. Soil depths deeper than 6” often only experience one freeze thaw cycle each winter. To be more effective these cycles also require sufficient soil moisture. Photo Credit: Aaron Hird
Using cover crops with different root structures (fibrous roots are great against surface compaction; tap roots are effective against deeper compaction). The increased plants and roots also feed and support a more diverse soil microbial community that builds stronger aggregates to help soil become more resilient.
Reduce axle loads and number of passes. Unload more frequently in the fall to lighten the load on the combine and grain cart. Use proper tire inflation - try to not to exceed 10 PSI tire pressure in well aggregated soils or 5 PSI in poorly aggregated soils.
Consider controlled traffic – those trafficked zones will have higher compaction, but the goal is to restrict those areas as much as possible, leaving the rest of the field. 70-90% of plow layer compaction happens on the first trip.
Are tracks the solution? How about Increased Flexion (IF) and Very High Flexion (VF) tires?
Any equipment can create compaction regardless of whether it has tracks or tires. According to multiple sources, tracks and properly inflated tires carrying the same load cause a similar amount of compaction.
According to Randall Reeder, Ag Engineer Emeritus at Ohio State University, the average contact pressure under rubber tracks is low, however, there are areas under the track where the pressure spikes and can equal or exceed those under radial tires that are inflated to low pressure. Higher tire pressure (greater than 10 PSI) can cause compaction.
Larger tires like IF and VF tires with extra sidewall flex are used by some farmers to help alleviate compaction. These tires are wider and have a longer footprint to spread the weight out compared to standard tires but come with a higher price tag.
What about tillage?
Tillage might temporarily alleviate some compaction but if the plan is to resume annual full width tillage without any management changes, you won’t see improvement. Tillage loosens soils and destroys aggregates, making it weaker with low bearing capacity. This makes it easier for the next trip across the field to cause compaction.
It all comes back to the Soil Health Principles
Remember that when it comes to compaction, management practices are critical; they can either create conditions that minimize the effects of compaction or create situations that are more prone to compaction. Following the soil health principles by keeping the soil covered with residue and a diverse community of living plants and roots are your best offense to create soil aggregates and reduce the impact of compaction.
References
Understanding Ag. Managing Compaction in Regenerative Cropping Systems with Brian Dougherty. 2023.
Upper Midwest Compaction Guide. Aaron Daigh and Jodi DeJong-Hughes. 2022. U of MN and NDSU.