Some places have received significant rainfall over the summer and the last week, causing soil moisture to be high in some areas. Soil moisture content can lead to the threat for soil compaction. Understanding principles of surface and subsoil compaction can help reduce undesirable effects.

Surface compaction (less than 12 inches deep) is caused by high contact pressures. Road tires inflated to 100 psi and iron wheels cause very high contact pressures. Surface compaction can cause very high yield losses the year immediately following the act. 

In no-till, yield losses due to surface compaction can be dramatic, although they bounce back quickly. Research from Kentucky has shown that long-term no-till soils will recuperate from most surface compaction without tillage within one year due to high biological activity common to no-till management. 

Tillage does not completely alleviate surface compaction. In a series of international compaction trials in which soil was moldboard plowed, average yield loss was 15% due to the combined effect of subsoil and surface compaction the first year after compaction. In some cases (especially clay soils) yield losses amounted to 50% in the first year. Using flotation tires or tracks can help reduce surface compaction.

Subsoil compaction (greater than 12 inches deep) depends on axle load, not on contact pressure. This means that reducing soil contact pressure by using flotation tires or tracks will not reduce subsoil compaction, although it helps to reduce surface compaction and sinkage. 

If you traffic soil that is really too wet with axle loads of 10 tons or higher, you’re likely causing subsoil compaction below 20 inches. These high axle loads are becoming more common as farm equipment increases in size. At Ag Progress Days, there was a 1300-bushel grain cart with two axles — the weight of the grain alone is 36 tons, so a 20-ton axle load is a realistic expectation with this type of equipment, and subsoil compaction is therefore an increased risk. 

Research has shown that subsoil compaction is there to stay. Freeze-thaw and wetting-drying cycles will not remove this compaction, nor will biological forces such as earthworms, roots, or microbial activity. Subsoilers usually don’t go this deep, nor can they completely alleviate this subsoil compaction (and at considerable cost).

Research conducted in seven European and North American countries showed a 5% yield decrease due to subsoil compaction that lasted longer than 10 years. Although you don’t traffic 100% of the field with high axle loads in one year, you will probably get there in three years, so a 5% yield decrease due to subsoil compaction is not unrealistic. 

The key to subsoil compaction avoidance is to reduce axle load.

Besides managing your equipment to reduce surface and subsoil compaction, your soil management also affects the ability of the soil to resist and recuperate from compaction. Eliminating tillage, building organic matter content, and maintaining a living root system in the soil are ways to increase resistance to compaction, while a highly active soil biology helps alleviate the effects of compaction. Tillage should only be relied on as a means of last resort.

Keys to avoid soil compaction are:

  • Stay off the field until soil has dried out sufficiently
  • Reduce surface compaction by using flotation tires, duals, or tracks — remember to use lowest allowable inflation pressure (at least  less than 35 psi) in flotation tires or duals
  • Avoid subsoil compaction — keep axle loads below 10 tons by either reducing load or increasing number of axles
  • Avoid trafficking the entire field by respecting traffic lanes
  • Use cover crops — their root system makes soil resist compaction better
  • Use no-tillage — surface organic matter accumulation makes soil resist compaction better
  • Use long-term no-tillage — the increased biological activity creates a soil that is perforated with macropores while the soil matrix remains firm