By H. Mark Hanna, Extension Ag Engineer; Mahdi Al-Kaisi, Agronomist and Extension Soil and Water Specialist 

Some areas have received several inches of rainfall since Sept. 1, during a time when corn and soybean water use declines significantly. This lack of water use by the plant creates saturated soil conditions susceptible to compaction this fall. High soil moisture increases soil compaction caused by field traffic and machinery. Over the past decade, the size of Iowa farms has increased, leading to larger and heavier equipment.

However, equipment size is only one factor among many causes of the soil compaction problem.

Rushing to the field when the soil is wet can increase chances for severe soil compaction, combined with the weight of equipment and traffic pattern in the field. Maximum soil compaction occurs when soil moisture is at or near field capacity because soil moisture works as a lubricant between soil particles under heavy pressure from field equipment.

Top 10 Reasons to Avoid Soil Compaction

1. Causes nutrient deficiencies
2. Reduces crop productivity
3. Restricts root development
4. Reduces soil aeration
5. Decreases soil available water
6. Reduces infiltration rate
7. Increases bulk density
8. Increases sediment and nutrient losses

Management Decisions to Minimize Soil Compaction During Harvest

1. Dedicated Travel Lanes. Many combine operators use “on-the-go” unloading into a grain cart to speed harvest. In areas that have received excessive rainfall since Sept. 1, farmers may want to consider either having dedicated travel lanes for the grain cart, or not loading a large cart to full capacity. This may not be an attractive option in high-yielding corn fields, but could be more easily implemented in soybeans where the volume of crop being removed from the field is much less. For those using grain carts with 1,000-bushel capacity or greater, axle load from the single-axle grain cart will be higher than the weight on the front combine axle. 

2. Tire Size. Properly adjusted tire size and correct air pressure for the axle load being carried is a second management tool. Larger tires with lower air pressure allow for better flotation and reduced pressure on the soil surface. Additionally, using larger tires that are properly inflated increases the "footprint" on the soil.

3. Check Soil Moisture. The most effective way to minimize soil compaction is to avoid field operations when soil moisture is at or near field capacity, thus it is important to check soil moisture conditions prior to implementing any field operation.

Most of Iowa's soils have medium textures. For these soils, a simple method of checking soil moisture is the "feel" method. Probing the top 3-4 feet with a hand soil probe to assess the field's soil moisture conditions is time well spent.

Check the soil moisture status by pushing a ribbon of soil from between the thumb and index finger. If it breaks off within 1 or 2 inches, the potential for creating compaction is less. However, if the ribbon stretches out to 4 or 5 inches, it is still too wet. The chances are good that being in the field under these conditions may cause more problems than it will solve.

Another method is to make a ball of soil 2 inches in diameter and toss it through the air. If it hangs together until impact, it has a lot of cohesiveness, is still fairly plastic and probably is too wet to work the ground. Waiting an extra day or two can reduce soil compaction significantly.

4. Reduce Tillage. Remember to hold off soil tillage operations until soil conditions are drier than field capacity and look into the benefits of conservation tillage systems.

In wet conditions, the best choice producers can make is to stay away from the field and avoid traffic on wet soil to reduce soil compaction. How you approach fieldwork after a heavy rain event can impact your soil for future growing seasons.

Related articles:

Soil Compaction and What You Can do About it
How and Why Avoid Soil Compaction

Related publications:

Building Soil Health
Iowa Soil Health Field Guide
Iowa Soil Health Assessment Card
Iowa Soil Health Management Manual