As farm populations have declined over the past few decades, the size of farms and farm equipment has increased. The heavy equipment is increasingly causing subsoil compaction. Near-surface compaction is undesirable, but it can largely be ameliorated through ordinary tillage operations. Subsoil compaction, on the other hand, is nearly impossible to ameliorate. Subsoil compaction becomes much more likely when axle weights rise above 4.5 Mg (5 tons). These axle loads are not uncommon. Large 4-wheel drive tractors may have axle weights of 7 Mg. Eight-row corn combine harvesters have axle loads of 15 Mg on the front and 5 Mg on the rear. Large sugar beet wagons, grain carts, manure spreaders, and spray wagons can carry loads of 30 Mg on a single axle. Contrary to what was previous thought, freeze/thaw cycles are not effective at eliminating deep compaction. Researchers in Minnesota measured a yield effect of a single compaction event a decade after the event. (Lindstrom and Voorhees 1994). The rise in subsoil compaction may be making widespread and largely permanent changes to the quality of some agricultural soils.
Lindstrom, M.J., and W.B. Voorhees. 1994. Responses of temperate crops in North America to soil compaction. In: B.D. Soane and C. Van Ouwerkerk, eds. Soil Compaction in Crop Production. Elsevier, New York.