Soil compaction occurs when soil particles are pressed together, limiting the space for air and water. The results are decreased permeability, moisture and nutrient stress, and the reduced exchange of gases. The amount of soil water is a critical factor in soil compaction potential. Moist soils are the most subject to compaction. There are different types of compaction, but at harvest time, deep compaction is the type that tends to occur.
Deep compaction is related to the maximum axle load, and is not reduced by distributing the weight across more tires or larger tires. Deep compaction is nearly impossible to remove with tillage as it occurs at a depth that is beyond the depth that most tillage implements can reach. A moist soil can be compacted to a depth of greater than 18 inches by a 10-ton axle load.
Much agronomic research has been completed on subsoil compaction, with the conclusion being that axle loads greater than 10 tons per axle can be very destructive. These yield effects will be most severe in a dry year, and less so in a wetter year, since soil strength increases as soils dry.
At harvest is when most fields experience the heaviest loads from combines, silage harvest, and from grain carts. Consider that the weight of a 1,050 bushel grain cart is approximately 19,700 lbs when empty. Filled with 1,050 bushels of grain, weighing approximately 56 lbs per bushel, add 58,800. That’s a total of 78,500 lbs. Assume that the grain cart transfers about 8,000 lbs to the tractor through the tongue of the wagon. The grand total for this example is 70,500. If the grain cart has two axles, that equals 17.6 tons per axle. A 12-row combine full of corn exceeds 20 tons per axle.
Producers must, of course, traffic fields at harvest time. Two key points for minimizing damage from heavy axle loads are to limit traffic when fields are wet, and to confine the majority of traffic, to the extent possible for the operation, to end rows. Keep in mind that the first pass of a wheel causes 70 to 90 percent of the total compaction, so preventing random traffic routes on the majority of the field is very beneficial.
DeAnn Presley, Soil Management Specialist