The Threat of Deep Compaction

Soil compaction is considered by farmers as a prevalent problem! A problem to be avoided if possible. A problem to be fixed with rotation or through tillage if necessary. An evil that is sometimes impossible to avoid!

There are several main theories on how to avoid or manage soil compaction 1) confine traffic to permanent lanes within the field providing traffic-free zones for crop production, 2) avoid wheel traffic on wet soils, 3) use equipment with lower axle weights, 4) increase the size of the "foot print" by employing radial tires, larger tires, more tires, or tracks, and 5) reduce tire inflation pressures. There has been a considerable effort on the part of equipment manufacturers to do at least two of the above, increasing tire size and reducing inflation pressures. This generally results in lower ground contact pressures and therefore less compaction (and fewer ruts!).

An Ohio State University compaction study indicated that a four wheel drive tractor (articulate) operating at inflation pressures of 6 (rear) and 7 (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. The use of larger foot print tires, with inflation pressure adjusted correctly for axle load, is an important tool in reducing compaction risks!

What gets lost in this discussion is that soil compaction at depths below the surface few centimeters, is a function of total axle weight. High axle load is the main culprit in causing compaction below the normal depth of tillage. We get particularly concerned with deeper soil compaction because of the difficulty and expense involved with eliminating it through tillage, and because of the persistence that it displays in compacted fields. Studies over the past 20 years have demonstrated that sub-soil compaction can be very persistent, and that beneficial effects attributed to freeze-thaw cycles in correcting this sub-soil compaction were significantly over estimated. On fine textured soils in Minnesota and Quebec researchers have shown that sub-soil compaction persisted for more than 6 years following the compaction event!

Try to keep these high axle loads (combines, manure tankers, grain carts) off wet fields. A tall order! If possible, wait for frozen ground. Otherwise, count on soil compaction that neither time nor tillage will remove easily. Isn't farming fun!

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