Across much of Kansas the soil conditions are wet at harvest time which presents harvesting challenges and delays. Another downside to wet soil conditions during harvest is the increased potential of subsoil compaction. 

Soil water content is a critical factor in soil compaction potential. Moist soils are the most susceptible to compaction. There are different types of soil compaction, but the deep compaction is the main concern at harvest time. 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.

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 very difficult to remove with tillage as it occurs at a depth that is beyond the depth of most tillage implements. For example, a moist soil can be compacted to a depth greater than 18 inches by a 10-ton axle load. Removing compaction at that depth will require more horsepower. As the depth of tillage doubles, the necessary horsepower increases by four-fold.

Much agronomic research has been conducted on subsoil compaction. The conclusions are that axle loads greater than 10 tons per axle can be very destructive to soil structure and lead to decreased crop yield potential. 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.

Harvest time is when most fields experience the heaviest loads from combines, silage harvest, and grain carts. Consider the following example:

• An empty 1,050-bushel grain cart weighs 19,700 pounds.

• A full 1,050-bushel grain cart weighs 78,500 pounds (assuming grain weight is 56 pounds per bushel).

• Assume the cart transfers about 8,000 pounds to the tractor through the wagon tongue.

• The grand total for this example is 70,500 pounds.

• If the grain cart has two axles, that equals 17.6 tons per axle.

• A 12-row combine full of corn often exceeds 20 tons per axle.

Of course, producers must traffic fields at harvest time. Two key points for minimizing compaction from heavy axle loads are to limit traffic when fields are wet, and to confine the majority of traffic to end rows when possible. Keep in mind that the first wheel pass causes 70 to 90 percent of the total soil compaction, so preventing random, unnecessary traffic routes on the field is very beneficial.

Information provided by DeAnn Presley, K-State Soil Management Specialist.

Stacy Campbell is a agriculture and natural resources extension agent for the Cottonwood Extension District. He can be reached at scampbel@ksu.edu.