Heavy Equipment and Soil Compaction
By Greg Stewart, OMAFRA Corn Specialist

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Can large tires and low inflation pressures solve all your problems?

Surveys indicate that corn producers view soil compaction as a prevalent problem. I say it's a problem to be avoided if possible, and alleviated through tillage if necessary.

There are several theories on how to avoid or manage soil compaction. These include:

• Confine traffic to permanent lanes within the field, to provide traffic-free zones for crop production;
• Avoid wheel traffic on wet soils which are susceptible to compaction;
• Use equipment with lower axle weights;
• Increase the size of the foot print by employing radial tires, larger tires, more tires, or tracks, and
• Reduce tire inflation pressures.

In fact, we have seen a considerable effort on the part of equipment manufacturers to do increase tire size and reduce inflation pressures. This generally results in lower ground contact pressures and less soil rutting and compaction.

A compaction study done at Ohio State University has been fairly well publicized, in which, the four-wheel-drive tractor (articulate) operating at inflation pressures of six (rear) and seven (front) psi resulted in less soil compaction than two belted track tractors. However, when this same tractor was operated at inflation pressures of 24 psi it caused the worst compaction in the experiment. Certainly the use of larger foot-print tires with inflation pressure adjusted correctly for axle load – has been an important tool in reducing compaction risks.

What might get lost in this discussion is that soil compaction, particularly at depths below the surface few centimeters, is also a function of total axle weight. Large tires with low inflation pressures cannot eliminate the potential for very heavy axles to cause compaction. Rutting or smearing may indeed be reduced by this type of running gear, but pressure distribution under these large tires can still negatively affect soil structure. A recent study (see Table 1) conducted in Quebec compared several different manure tanker configurations and pointed out that switching to larger tires and lower inflation pressures did not significantly reduce soil compaction as measured by changes in the bulk density of the soil.

The underlying concept here can be somewhat counter intuitive, so I suggest you consider the following analogy. If you drive your 10-tonne combine over an old bridge with some rotten boards on top, your tires might do some damage but the bridge stays in tact. If your drive your 20-tonne combine over that same bridge, but it has very wide tires, you may not damage the boards but the bridge collapses. The analogy is not perfect but serves to remind us that total axle weight is a critical component in assessing a vehicle’s potential to cause soil compaction.

Soil compaction can be a yield-limiting, expensive problem. Efforts should be made to avoid causing it in the first place. Proper tires with correctly adjusted inflation pressures can significantly reduce surface compaction. Keeping axle loads to a workable minimum is your best insurance against causing compaction in the deeper soil layers.

TABLE 1. The effect of manure tanker tire size and inflation pressure on ground pressure and changes in soil density after traffic. (Y. Bedard, S. Tessier, C. Lague, Y. Chen and L. Chi, Laval University)
Axles	Tire Size	Inflation Pressure (psi)	Axle Load (tonnes)	Ground Pressure (psi)	Pre-Traffic Bulk Density (22.5 cm deep)	Post-Traffic Bulk Density (2.5 cm deep)
Tandem Tandem	21.5L - 16.1 28L - 26	32 20	7.7 7.7	30 20	1.34 g/cm3 1.34 g.cm3	1.41 g/cm3 1.40 g/cm3

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