Farmers strive to avoid soil compaction, but field conditions, season and other factors may force manure spreading at a less-than-ideal time of year.

“Compaction is never intended,” said Aaron Daigh, associate professor, University of Nebraska-Lincoln. “It’s always in the context of difficult situations – logistics, timing of rainfalls, barn clean out or to reduce manure in storage.”

Compaction is about soil macropores. “A well-structured soil that has a lot of macropores – large pores – is healthy soil with great benefits to crop production,” said Daigh. “But because of difficult decisions producers must make over the long term, the occurrence of compaction is very real. It can occur and it can persist.”

He added that soil that was once well-aggregated can become a solid mat due to increased soil density.

Daigh explained that a soil may be comprised of less than 1% macropores, but those pores contribute up to 70% of water and air movement in the ground. A one-time heavy compaction event, such as heavy machinery over the ground, can result in reduced macropores for decades, especially when compaction occurs deep in the soil profile.

Unfortunately, the trend toward larger ag machinery with heavier axle loads has contributed to compaction. “Compaction is different today than it was decades ago,” said Daigh. “Along with larger equipment sizes, soil compaction has gone deeper and deeper.”

In comparing tires to tracked tractors, Daigh said the tires on traditional tractor and manure handling equipment push into the soil, resulting in a bulb-shaped area of compaction that travels downward. However, if tractor tires are inflated to the appropriate level for field conditions – about 15 psi or lower – tires have about the same effect as tracked tractors.

“When radial tires are properly inflated, they should be applying the same stress as a track system,” said Daigh. “There’s no benefit to using a tracked tractor if pressure is properly adjusted.”

Sometimes tractor tires are not adjusted down to field conditions because driving on the road at higher speeds requires 40 to 50 psi to avoid compromising the sidewalls. Operators don’t benefit from radial tire technology if tires aren’t adjusted down to field conditions.

Daigh said that if tandem manure tanker tires are inflated to 40 psi, the topsoil at around four inches and the subsoil below 12 inches are compromised. “By the time you get to 20 inches [deep], it’s on the threshold of potentially compacting subsoil,” he said. “This is a high-risk compaction situation due to the axle loads and tire inflation rates.”

However, the same equipment with tires at 15 psi means topsoil pressure is much lower, close to the threshold and at lower risk for compaction. The subsoil is still in the medium risk category, and at 20 inches down, it’s slightly below the threshold.

“This demonstrates that you can adjust tire psi and it works well for the topsoil,” said Daigh. “But for subsoil, the psi isn’t the main story. It’s a factor, but this is where the axle load becomes important – keeping them below 10 tons per axle if soil is well-aggregated and below five tons per axle if the soil is poorly structured. These are hard goals to hit, but it’s reality if you want to minimize compaction.”

Manure and compaction

Tractors and manure equipment have gotten larger and heavier, with a higher potent for soil compaction. Photo courtesy of Richlands Dairy Farm

Measured stresses are snapshots in time and reflect when a particular piece of equipment was driven across the ground on a given day, under certain soil moisture conditions. However, as soil moisture changes, the risk for compaction changes.

Under dry conditions, compaction is minimal when equipment is driven over the soil. As soil accumulates water, it compacts until field capacity is reached, which is where it may compact the most.

“When it reaches higher than field capacity, it actually comes down a little bit,” said Daigh. “Water simply can’t get out of the way fast enough. Water is uncompressible under agricultural loads.”

Daigh explained that this is an important consideration because ponded water on a field indicates high risk for compaction and farmers should avoid field work. However, as soon as the water is no longer ponded, there’s a tendency for farmers to head to the field.

“In reality, that’s the most vulnerable conditions for compaction,” he said. “A little bit of water has drained, and particles can slip into air pockets and compact down deeper.”

Once ponded water is off the field, allow a few more days for drying for a lower risk for compaction.

Strongly aggregated soil is less prone to compaction than weakly aggregated soil. The risk of compaction can shift downward into soil based on management. Compaction can last a long time and have long-reaching consequences. Yields often drop 20% to 30% after compaction occurs.

Soil begins to recover over the next three or four years, but there may be lingering yield drops, especially in years with excess rainfall or drought. Persistent subsoil compaction, below what can be reached with deep tillage, may still be compromised a decade later. Any rutted areas in fields should be repaired to avoid additional issues.

One option for breaking up compaction is bio-drilling with roots. “It’s a multi-year effort to break up compaction,” said Daigh. “The roots have to form new channels, then there’s the time it takes for roots to rot and open up for cash crops. It usually takes several years before you see any effect.”

Another option for dealing with compaction is the natural cracking of clays during mid-summer, especially in drought. Daigh said this depends on the type of clay and the spaces between particles. Some clay types will fracture deeply below the surface of visible cracks.

Freeze/thaw cycles during winter can help with surface compaction. “Back in the day when axle loads were often 10 tons per axle or less with light equipment, it could break up compaction in topsoil,” said Daigh. “For freeze/thaws to work during winter, it takes many cycles to break up compaction.”

However, deep soil freezes once and thaws once, so winter freeze/thaw does not take care of subsoil compaction.

Deep tillage is often used for deep compaction, but Daigh said yield differences following deep ripping aren’t significant. For deep tillage to be effective, there must be a root-limiting layer that’s due to compaction. If compaction isn’t the problem, deep ripping won’t solve any problems.

“Silt loam soils do not behave well with deep ripping,” said Daigh. “That bar just smears it, and the main insult to compaction is a smear.”

If topsoil fertility is poor, deep ripping might help but only because plants can reach more nutrients.

Daigh said modern wheel loads are high and can push compaction deeper. “Avoidance is preferred over relying on alleviation methods,” he said. “Keep loads light, adjust tire pressures and make fewer field passes to avoid creating new ruts. If you have compaction, use natural alleviation (roots, cracking) to make up for what tillage can never reach.”

by Sally Colby