The damage is permanent once high axle loads have compacted lower subsoil to the point where it has a hardpan layer.
That’s one of the conclusions reached by Penn State researchers after participating in 20 soil compaction studies throughout the northern latitudes of Europe and North America, some of which ran for 12 years.
The network of soil scientists reached three main conclusions:
- compaction in topsoil relates only to ground contact pressure
- compaction in the upper layer of subsoil relates to both ground contact pressure and axle load
- compaction in the lower subsoil relates only to axle load
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They found that compaction in the lower subsoil cannot be eliminated by deep tillage, frost-thaw cycles or wet-dry cycles.
A 10-ton axle load will cause subsoil compaction at 20 inches and deeper. Swedish researchers participating in the study found that even a six-ton axle load causes significant compaction at 20 inches.
Researchers found the arbitrary cut-off point for avoiding deep compaction is an axle load of five tons or less. Significant surface compaction will still occur at that point, but it can be addressed with tillage.
Producers can determine axle loads by dividing gross weight of the implement by the number of axles upon which it sits.
If the total loaded weight of the vehicle is 20 tons and it rides on two axles, then the axle weight is 10 tons, enough to cause significant subsoil compaction.
Contact pressure is different than axle load. It is the actual pressure exerted by a tire or rubber track on the top of the soil surface, expressed in pounds per sq. inch.
Surface contact pressure is one or two pounds higher than tire pressure in most agricultural applications because of stiffness in the tire. Lower surface contact pressure results in less topsoil compaction but does not reduce subsoil compaction.
A more accurate way to determine surface contact pressure is to calculate the load per tire, expressed in pounds, and then divide that weight by the area of tire/soil contact footprint, expressed in square inches. This provides the approximate surface contact pressure.
The commonly held assumption that rubber tracks treat topsoil better than round rubber tires was put to the test by Ohio State University researchers who compared a 310 horsepower tracked tractor to a 350 h.p. tractor with duals.
The tire tractor caused the most compaction when the duals were inflated to 24 psi and the least compaction when tire pressures were dropped to six psi. That meant the rubber tracked tractor caused more compaction than the tire tractor with low psi.
“Tracks offer some advantages such as a long but narrow contact area. Tracks are known to provide better traction than tires,” the Ohio study said.
“However, very low average contact pressures under a track does not tell the whole story. The belt is flexible and there are pockets of high pressure under the axles of the belt that can be as high as those under a tire-mounted tractor.
“Each axle in a track represents a pass over the soil that causes a little more compression. Tracks increase the dwelling time of the load on the soil, which increases compaction.”
Ohio researchers concluded that tracks were no better than tires when it comes to compaction.
They also noted that larger diameter tires lengthen the footprint, thus lowering surface contact pressure without increasing the area of the field subjected to traffic, as duals or triples will do.
Researchers recommend driving implements faster to shorten the load dwelling time, staying off wet fields and limiting the field subject to traffic by using repeatable designated traffic lanes.
Scientists at the University of Minnesota defined the ideal non-compacted soil as comprising 45 percent minerals, five percent organic matter and 50 percent free pore space. The pore space should be half filled with air and half filled with water.
Air pockets disappear, water flow is impeded and proper root development becomes impossible as implements compact pore space by pushing soil particles closer together,.
For more information, visit pubs.cas.psu.edu/freepubs/pdfs/uc186.pdf and www.blu-jet.com/soilcompaction.htm.
